Note: Descriptions are shown in the official language in which they were submitted.
CA 02714847 2010-09-16
Background of the Invention
Mane attempts have been made for providing method and devices to reduce
optical transmission to desired low \alue for high incident power. and to keep
a good
transmission for lovN incident power. I he main target of these attempts is to
protect
sensitive optical sensors. components and human eyes from laser damage. This
kind of
devices has mane other applications. such as for laser power regulation or
stabilization.
for optical data signal level restoration and so on ("Handbook of Optics",
Vol. IV. 2d ed..
edited by M. Bass. J. M. Enoch. E. W. V. Stryland. W. L. Wolfe. McGraw-Hill.
New
York. 2001. p. 19.1). If these devices have enough low limiting threshold,
they can even
be used for non-laser light. such as to view the object in dazzling background
or emitting
strong light, thus to act as sun visor for night driving. welding helmet and
the like.
Since advantages of simplicity. high speed, compactness. and lo~N cost. the
passive optical limiters attract more interests than the active control ones.
There are
various principles oi' passive optical limiting. They are nonlinear
absorption, scattering,
refraction. and photorefraction, photosensitivity. etc ("Handbook of Optics",
Vol. IV. 2d
ed.. edited by M. Bass. .1. M. Enoch. F. NV. V. Stryland. W. L. Wolfe. McGraw-
Hill. Nety
1
CA 02714847 2010-09-16
York. 2001, p. 19.2-19.9; .1. S. Shirk. "Protecting the War Fighter's Vision
in a Laser-
Rich, Battlefield Environment'. Optics & photonics News, Vol. 11. No. 4, p.19-
23, 2000).
The nonlinear absorption is produced by intensive irradiance. It absorbs light
energy and reduces the high light power. The nonlinear absorption includes
reverse
saturable. two-photon and free carrier absorptions and relates to
semiconductors, organic
and organornetallic materials. The nonlinear scattering is due to strong light
induced
creation of scatter centers. These centers scatter the light rays of the
transmitted beam and
attenuate the high light power. The nonlinear scattering materials usually
consist of a
material with large refractive nonlinearity and a linear material which is
suspended in the
former or inversely. The indexes of two materials are matched normally. At
high
intensities, the indexes are not matched again and the materials become
scattering. These
materials include nonlinear photonic crystals and carbon-black suspensions,
etc. The
nonlinear refraction is generally caused by high intensity induced thermal
change of
refractive index. Usually the largest index change is at the beam focal point
by heating
and following material expansion, resulting in beam defocusing. Such self-
defocusing
bends the light rays from their transmission directions and reduces the high
light power.
A typical nonlinear refraction material is semiconductor. Photorefractive
materials
change their index when exposed to light. and their index change is in
proportion to the
intensity gradient of light. They include photorefractive crystals. such as
Lithium Niobate.
The photosensitive materials change their optical property, such as
polarization, when
illuminated by light. A typical material is liquid crystals. The liquid
crystals have an
irradiance-dependent birefringence, which is used for optical limiting.
In order to get ideal limiting results, the used materials must have large
nonlinear
optical absorption, scattering, refraction, or photorefractive, photosensitive
coefficients,
and intensity of the stimulating light must be high. Since the said
coefficients of existing
materials are not large enough, the power of incident light must be high.
There is a
commonly used method to raise the stimulating light intensity and so to lower
the
limiting threshold. It is to focus the incident beam by lens. Beam focusing
raises the
intensity greatly. However. due to optical diffraction, the shrink of waist
size of the
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CA 02714847 2010-09-16
focusing beam has a limitation. In visible region. for a laser beam with
excellent space
coherence. the minimum waist size is larger than a few microns. Therefore, the
available
optical limiters are just effective for high or relatively high light powers.
New method
and devices for limiting low power light, such as low power CW laser and even
non-laser
light are needed.
Summary of the Invention
The goal of this invention is to create a method for passive optical limiting,
and to
provide limiter structure designs based on this method. This method can lower
the optical
limiting threshold. Furthermore. this method can be used to construct various
types of
optical limiters. including nonlinear optical absorption, scattering,
refraction. along with
photorefraction. photosensitivity and so on, thus to satisfy different optical
limiting
requirements. These optical limiters have characteristics of simple structure,
ordinary
elements and compact size. They not only can he installed in various optical
equipments
and systems as a component. but also can be made as a portable goggle type of
device for
individual use.
The said method of the invention is based on forming and utilizing the
standing
waves formed by spherical partial reflecting, circlewise cut amplitude
dividing, or
circlewise cut partial reflecting the beams with different incident angles
from whole field
of view. These standing waves are formed in the nonlinear optical absorption.
scattering.
refraction, or photorefraction, photosensitivity material, and are made have
intensity
distribution with periodic continuous multiple layer structure.
Speaking in detail, the parallel beam with each incident angle from field of
view
is converged by lens or mirror. The converged beam is then divided into two
separate
beams by spherical partial reflection, circlewise cut amplitude division, or
circlewise cut
partial reflection (beam dividing may be before beam converging). Afterwards.
two
separate beams are guided to travel along opposite or near opposite
directions, and are
connected at their focal points in the nonlinear optical absorption,
scattering. refraction.
3
CA 02714847 2010-09-16
or photorefraction. photosensitivity material. When using spherical partial
reflection, two
meeting beams are overlapped entirely. When using circlewise cut amplitude
division. the
solid angle of one of each pair of divided beams is circlewise cut by a
diaphragm to make
remain area of the solid angle can be overlapped entirely by another divided
beam of the
same pair. When using circlewise curt partial reflection, the solid angle of
each
transmitted beam passing through the partial reflecting coating and output
(compound)
lens is circlewise cut by a diaphragm to only let the beam part emerging from
the entirely
overlapped area pass. In these entirely overlapped areas. the standing waves
are formed.
These standing waves have intensity distribution with periodic continuous
multiple layer
structure. Realize same process for all parallel beams with different incident
angles from
the whole field of view. Then utilize the features of the formed standing
waves, including
higher light intensity at anti-node. ununilormity of intensity distribution.
large intensity
gradient, periodic or quasi periodic continuous multiple layer structure of
intensity
distribution, and so on, to lower the optical limiting threshold and enhance
the optical
limiting performance.
In other words, the above said method is to create two identical focal curved
surfaces or focal planes in the nonlinear optical absorption, scattering,
refraction, or
photorefraction, photosensitivity material. and to make them coincide with
each other. Or
the above said method is to create two identical image fields for all parallel
beams with
different incident angles from field of view in the nonlinear optical
absorption. scattering,
refraction. or photorefraction. photosensitivity material, and to make them
coincide with
each other.
Every pair of two separate beams must have same frequency, same polarization,
and appropriate amplitude ratio (the ratio value depends on concrete limiter
structure and
is discussed underneath). If an incident beam has a relatively wide bandwidth,
its two
separate beams must have same spectral distribution. Thus a special optical
interference
field containing many standing waves is formed in the above said optical
limiting
material, that is, the nonlinear optical absorption, scattering, refraction,
or photorefractive,
photosensitive material. Each standing wave corresponds to one parallel beam
from the
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CA 02714847 2010-09-16
field of view, and has a periodic continuous multiple layer structure. For an
incident
beam with a relatively wide bandwidth. if the phase difference between its two
separate
beams at their connected focal points is near zero, a thin quasi periodic
continuous
multiple layer structure may still be formed in the optical limiting material
due to optical
interference.
It is known that in a standing wave consisting of two beams with almost equal
amplitude value, the light intensity at anti-nodes is 4 times as high as the
light intensity of
each component beam. Because the coefficients of the above said optical
limiting
materials rise with increase of light intensity, and most of them are
nonlinear, the
standing wave will enhance these effects around its anti-nodes. In addition.
the respond
time of these effects usually reduces with increase of the light intensity, so
the standing
wave will shorten the respond time of these effects too in some situations.
For nonlinear
optical scattering and refraction materials, the ununiformity of intensity
distribution in
standing wave (the light intensity varies from zero at a node to a maximum at
an anti-
node in a distance of 4) will make distributions of scattering centers and
refractive index
become more un-uniform. It will enlarge optical scattering and self-
defocusing.
Furthermore. for nonlinear optical refraction materials, the standing wave
will make the
index distribution have a period continuous multilayer structure. This
structure satisfies
Bragg reflection requirement just for the beam induced it ("Handbook of
Optics". Vol. III,
2d ed., edited by M. Bass, J. M. Enoch. E. W. V. Stryland, W. L. Wolfe. McGraw-
Hill,
New York, 2001. p. 22.1-22.4). Thus, a mirror like multilayer reflector is
formed. This
mirror like reflector is much more efficient than the normal interference
pattern or grating
for optical limiting. The interference pattern or grating used for optical
limiting is
induced by two-beam coupling in the nonlinear refractive or photorefractive
materials.
Using such interference pattern or grating to limit the light is like using a
lattice to block
light. The light leaks from the holes of the lattice and deteriorates the
limiting result.
However. the standing wave with continuous multiple layer structure can work
like a
multilayer reflector or mirror with no holes. It will limit the intensity of
the incident light
greatly. At last, in the standing wave the light intensity gradient is
extremely large
CA 02714847 2010-09-16
because the intensity changes from about zero to maximum within a short
distance of .
This extremely large intensity gradient will dramatically increase the
limiting results of
the effects depending on the light intensity gradient. such as in the photo
refractive
crystals and photosensitive liquid crystals.
The said method of the invention is realized through setting up optical
focusing,
dividing, cutting. deflecting, and adjusting paths by using appropriate
combinations of
ordinary optical elements. including lens, mirror. prism, diaphragm, etc, and
using the
above said optical limiting materials. The preferred embodiments of limiter
structure
design for realizing the said method will be described in the underneath.
Obviously. these
embodiments are not the all limiter structures which can be designed based on
the said
method. By using existing optical design knowledge, these embodiments may be
alternated, modified to fit different practical needs. Furthermore, basing on
the said
method and using the existing design knowledge, the other limiter structures
may be
worked out. Therefore, the applicant of this invention reserves the right of
all alternatives.
modifications. and equivalent arrangements of the limiter structure
embodiments
described underneath. The applicant also reserves the right of any optical
limiter structure
designs which base on the said method.
The aforementioned aspects and advantages of the invention will be appreciated
from the following descriptions of preferred embodiments and accompanying
drawings
wherein:
Fig.] is the schematic optical structure of the limiter with spherical partial
reflection as the first preferred embodiment of the limiter according to the
invention.
Fig.2 is the schematic optical structure of the container used for the limiter
shown
in Fig. I.
Fig.3 is a schematic diagram illustrating reflected non-parallel beam by
spherical
partial reflection. A special standing wave is formed as interference.
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CA 02714847 2010-09-16
Fig.4 is a schematic diagram illustrating reflected parallel beam with tilting
incident angle by spherical partial reflection. A special standing wave is
formed as
interference.
Fig.5 is the schematic modified optical structure of the limiter with
spherical
partial reflection shown in Fig. I. A layer for avoiding self-defocusing and a
pre-limiter
are added.
Fig.6 is the schematic modified optical structure of the container shown in
Fig.2.
A layer for avoiding self-defocusing and a pre limiter are added.
Fig.7 is another schematic modified optical structure of the limiter with
spherical
partial reflection shown in Fig.l. The optical limiting material filled in the
container is
nematic liquid crystals.
Fig.8 is a schematic diagram illustrating nematic liquid crystal director
alignment
in relaxed phase state (left tiled row). and in the reorient state driven by
electric field
induced via standing wave (right tilted row).
Fig.9 is a schematic diagram illustrating the ratio of the cross section areas
separated by interval of 4 for the focusing beam.
Fig.10 is a schematic diagram illustrating two special standing waves formed
in
the limiter shown in Fig. 11, which correspond to the parallel normal incident
beam.
Fig.lI is the schematic optical structure of the limiter with circlewise cut
amplitude division as the second preferred embodiment of the limiter according
to the
invention.
Fig.12 is a schematic diagram illustrating two special standing waves formed
in
the limiter shown in Fig. 11, which correspond to the parallel tilting
incident beam.
7
CA 02714847 2010-09-16
Fig.].') is a schematic diagram illustrating two special standing waves formed
in
the limiter shown in Fig. ll. which correspond to the non-parallel and normal
incident
beam.
Fig.14 is the schematic optical structure of another limiter with circlewise
cut
amplitude division as the third preferred embodiment of the limiter according
to the
invention.
Fig. 1 is the schematic left side view, taken on the dash line 122-122 in
Fig.14 of
the third preferred embodiment of the limiter according to the invention.
Fig.16 is the schematic optical structure of the limiter with circlewise cut
partial
reflection as the fourth preferred embodiment of the limiter according to the
invention.
Fig. 17 is the schematic modified optical structure of the limiter with
circlewise
cut partial reflection shown in Fig.] 6. The solid angle of the entirely
overlapped beam
part is widened by enlargement of input lens aperture.
Fig.l8 is a schematic diagram illustrating the special standing wave formed in
the
limiter shown in Fig. 16. which corresponds to the non-parallel tilting
incident beam.
Fig. 19 is the first schematic image erecting scheme for first preferred
embodiment
shown in Fig. I of the limiter according to the invention. Two right-angle
prisms are used.
Fig.20 is the schematic top side view. taken on the dash line 176-176 in Fig.]
9 of
the first schematic image erecting scheme for the first preferred embodiment
of the
limiter according to the invention.
Fig.21 is the second schematic image erecting scheme for the first preferred
embodiment shown in Fig. 1 of the limiter according to the invention. An
Abbe's prism is
used.
Fig.22 is the third schematic image erecting scheme for the first preferred
embodiment shown in Fig. I of the limiter according to the invention. An
inverting lens is
used.
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CA 02714847 2010-09-16
Fig.23 is the fourth schematic image erecting scheme for the first preferred
embodiment shown in Fig.1 of the limiter according to the invention. Two right-
angle
prisms are used. With position change of input compound lens, the field of
view is
increased.
Fig.24 is the schematic top side view, taken on the dash line 221-221 in
Fig.23 of
the fourth schematic image erecting scheme for the first preferred embodiment
of the
limiter according to the invention.
Fig 25 is the first schematic image erecting scheme for second preferred
embodiment shown in Fig. 11 of the limiter according to the invention. An
inverting lens
is used.
Fig.26 is the second schematic image erecting scheme for the second preferred
embodiment shown in Fig. 11 of the limiter according to the invention. A right-
angle
prism is used.
Fig.27 is the schematic top side view, taken on the dash line 256-256 in
Fig.26 of
the second schematic image erecting scheme for the second preferred embodiment
of the
limiter according to the invention.
Fig.28 is the first schematic image erecting scheme for fourth preferred
embodiment shown in Fig.16 of the limiter according to the invention. Two
right-angle
prisms are used.
Fig.29 is the schematic top side view, taken on the dash line 272-272 in
Fig.28 of
the first schematic image erecting scheme for the fourth preferred embodiment
of the
limiter according to the invention.
Fig.30 is the second schematic image erecting scheme for the fourth preferred
embodiment shown in Fig.16 of the limiter according to the invention. An
Abbe's prism
is used.
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CA 02714847 2010-09-16
Fig.31 is the third schematic image erecting scheme for the fourth preferred
embodiment shown in Fig.16 of the limiter according to the invention. An
inverting lens
is used.
Fig.32 is the fourth schematic image erecting scheme for the fourth preferred
embodiment shown in Fig. 16 of the limiter according to the invention. Two
right-angle
prisms are used. With position change of input lens, the field of view is
increased.
Fig.-33 is the schematic top side view. taken on the dash line 324-324 in
Fig.32 of
the fourth schematic image erecting scheme for the fourth preferred embodiment
of the
limiter according to the invention.
Fig.34 is the fifth schematic image erecting scheme for the fourth preferred
embodiment shown in Fig.16 of the limiter according to the invention. Four
mirrors are
used.
Fig.35 is the schematic top side view, taken on the dash line 350-350 in
Fig.34 of
the fifth schematic image erecting scheme for the fourth preferred embodiment
of the
limiter according to the invention.
Fig.36 is the schematic detailed structure of the solid material 342 used in
the fifth
schematic image erecting scheme for the fourth preferred embodiment in the
Fig.34.
Fig.37 is the schematic detailed structure of the container 342 filled with
liquid
material used in the fifth schematic image erecting scheme for the fourth
preferred
embodiment in the Fig.34.
Detailed Description of the Invention
Fig.] illustrates the schematic optical structure of first preferred
embodiment of
the optical limiter according to the invention. A converging input compound
lens
comprising lenses 10 and 18 focuses a parallel incident beam into a solid
above said
optical limiting material 12. The focal point of the beam falls on the back
surface 14 of
CA 02714847 2010-09-16
the solid material. The back surface 14 is spherical and its sphere center is
at optical
center of the input compound lens. There is a partial reflecting coating with
an intensity
reflection ratio of. such as 90%, on the back surface 14. Thus most of the
incident light
returns hack against its incident direction. A small part of the beam passes
the output
compound lens comprising lenses 16 and 20. and is converged to become a
parallel beam
again. The complemental lens 18 is used to curve the image field of the lens
10. Unlike
the normal correction. here the focal curved surface of the lens 10 needs to
be spherical,
and to match the back surface 14 of the solid material. In fact, any simple
lens has a
curved image field with concave focal surface tilting to the lens optical
center, and there
is a mature aberration correcting technology for modifying such field
curvature
("Handbook of Optics". Vol. I. 2d ed.. edited by M. Bass, J. M. Enoch, E. W.
V. Stryland,
W. L. Wolfe. McGraw-Hill, New York, 2001, p. 33.1-33.6). Similarly, a
complemental
lens 20 is used to make the focal curved surface of the lens 16 to match that
of the input
compound lens. and two focal curved surfaces are coincided. Because how to
modify the
focal curved surface of a lens may adopt the existing technology, here it is
not discussed
in detail.
If using liquid above said optical limiting material, the liquid material is
placed
into a transparent container 22 shown in Fig.2. In this situation, the said
partial reflecting
coating is on the inside back surface 24 of the container. The shape and
location of the
inside hack surface 24 of the container are same as those of the back surface
14 of the
solid material.
In order to reduce energy loss and arbitrary light disturbance. all other
optical
surfaces of the lenses, the solid material, and the container are coated with
high
antireflection films for the desired wavelength range. The other optical
surfaces of the
solid material and the container are spherical too and have a common sphere
center with
their partial reflecting surfaces.
Thus, each of the reflected beams has same frequency, same polarization,
roughly
equal amplitude" and opposite direction with its corresponding incident beam.
The phase
difference between the reflected beam and its corresponding incident beam is
zero at
11
CA 02714847 2010-09-16
reflecting coating and is fixed within their coherence length. For a beam with
narrow
bandwidth, especially for a laser beam, its coherence length AL is long
according to the
equation
AL A~ (1)
where A is the central wavelength, A2, is the bandwidth ("Handbook of Optics",
Vol. I.
2d ed.. edited by M. Bass, J. M. Enoch, E. W. V. Stryland, W. L. Wolfe, McGraw-
Hill,
New York. 2001. p. 4.3. p.11.8). As a reference, for a beam with the bandwidth
of full
visible region from 0.4 pm to 0.76 m, its coherence length is 1.6 m. For a
beam with
the bandwidth being 1% of the full visible region. its coherence length is 160
[till. The
coherence length of most laser beams is much over 1 Omm. Thus, for a narrow
bandwidth
beam especially a laser beam, a long conoid standing wave is formed in the
traveling path
of the incident beam, which overlaps the incident beam completely within the
coherence
length. In this spacial interference field, light intensity distributes in the
manner of
continuous multiple concentric spherical layers, and the interval between
contiguous
layers is just half of the wavelength of the incident beam.
A special attention must be given to reflection phase change. In normal light
reflection, there is a phase change of 180 for one of two polarization
components, which
depends on the refractive indexes of the media on two sides of the reflecting
interface and
the incident angle ("Handbook of Optics". Vol. 1, 2d ed., edited by M. Bass.
J. M. Enoch,
E. W. V. Stryland, W. L. Wolfe, McGraw-Hill, New York, 2001. p. 5.1-5.7).
These two
polarization components will interfere with the incident beam and form two
standing
waves. They intervene to each other with interval of 4 . If amplitudes of two
components
are same or almost same, the distribution of the compounding intensity will
become
smooth as superposition of two standing waves. The periodic multilayer
structure of
intensity distribution will disappear or almost disappear. Therefore, measures
must be
adopted to eliminate it.
12
CA 02714847 2010-09-16
How to change or eliminate the phase difference between two polarization
components is also an intensive study topic in Optics, and has got plenty
results. The
simplest way is to insert a half-wave plate made of birefringent crystal into
the traveling
path of the incident light at a suitable position ("handbook of Optics". Vol.
I. 2d ed.,
edited by M. Bass, J. M. Enoch. E. W. V. Stryland. W. L. Wolfe, McGraw-Hill,
New
York. 2001, p. 5.22-5.25), such as at the input end of the limiter. Thus, one
polarization
component of the incident beam, such as the extraordinary ray. travels slower
or faster
than the ordinary ray by half wave length. resulting in a phase difference of
180 being
produced. When these two components are reflected, another phase difference of
180 is
produced. making the total phase difference become 0 or 360 . However, as a
convergent beam enters the reflective coating with different angles. each
extraordinary
ray splits into another two polarization components. The polarization
direction of one is
parallel with the incident plane. and another is perpendicular with the
incident plan. Thus,
part of the extraordinary ray does not have the same phase change as the other
part. The
same thing happens for the ordinary ray. As result, not all reflected rays
have 0 or 360
phase difference with their corresponding incident rays. It means two standing
waves are
still formed. However, by choosing indexes of used limiting material and
reflecting
coating. and crystal axis direction of the half-wave plate properly, the
intensities of two
standing waves may differ large, resulting in a periodic distribution of the
compounding
intensity with relatively large difference between the anti-node and node. In
addition,
half-wave plate is wavelength dependence. For different wavelengths, it
retards phase
differently from 180 . This makes the length of conoid standing wave reduced,
or in an
exact word, the intensity distribution of the standing wave is modulated.
A better way is to deposit special constructed reflecting coating on the solid
material back surface 14 or the container inside back surface 24. Usually such
coating has
multilayer structure. Such as metal/dielectric interferometer mirrors, total-
internal-
reflection phase retarders with 1, 2. or even 4 internal reflections, and so
on (`Handbook
of Optics"', Vol. I. 2d ed., edited by M. Bass, .1. M. Enoch, E. W. V.
Stryland, W. L.
Wolfe, McGraw-Hill, New York. 2001, p. 42.98-42.100). These mirrors and
retarding
films can maintain phase difference of being or near 0 or 360 for parallel
and
13
CA 02714847 2010-09-16
perpendicular polarization components in the vicinity of the designed incident
angle,
typically 0 or 45 . Furthermore, they permit wavelength to change within a
certain range.
Another thing needing consideration is the shift of the image field of input
compound lens. The real light source or object is not located at infinity
distance. The
moving of light source or the object. or the light sources or objects at
different distances
will make the image field shift or thicken. The lens equation is
1 = l + -1 (2)
.S';
f S'
where f is the lens focal length. s and s' are distances from object and image
to lens
optical center respectively ("Handbook of Optics", Vol. I. 2d ed., edited by
M. Bass, J. M.
Enoch, E. W. V. Stryland, W. L. Wolfe. McGraw-Hill, New York. 2001. p. 1.55).
For
example, if f= 5 cm. and object distance s = 4 in, 10 m, 100 m and 1000 m. the
image
distance s'= 5.06329 cm , 5.02513 cm, 5.0025 cm. and 5.0003 cm respectively,
resulting
in an image field shift or thickening of 0.6299 mm.
First this is not a problem for the applications with fixed light source, such
as in
some optical equipments and systems, because the input compound lens may be
moved
or specially designed to let its image field fall on the reflecting coating.
Second, for the
most important application - the protection from laser damage. such as in the
battlefield
or laboratory, it is not a problem too. It is known that the extremely high
brightness of the
laser beam originates its extremely small divergence angle, that is, almost
all of laser
beams (for ones still with high brightness) are very good parallel beams.
Therefore,
almost all of the laser beams entering the limiter can be focused on the
reflecting coating
well no matter how far they come from. Of course, for the strong non-laser
light sources
located at finite distances. the problem of image field shift or thickening
must be
considered.
This problem can be solved by the said method of the invention pleasantly. See
Fig.3, a non-parallel beam comes from a light source at a finite distance.
After converged
by input compound lens, it becomes beam 26. its focal point does not fall on
the
14
CA 02714847 2010-09-16
reflecting surface 28. that is, not on the back surface 14 of the solid
material or the inside
back surface 24 of the container. Its virtual focus is at point 30. This beam
is reflected by
the surface 28 and focused at point 32. It then diverges into beam 34, and
interferes with
the beam 26. It is known that in an interference field of two spherical waves
with same
frequency. polarization. amplitude. fixed phase difference. and within the
coherence
length, a conoid standing wave with intensity distribution of periodic
continuous
multilayer structure can still be formed along the connecting line of two
sphere centers
("Handbook of Optics". Vol. 1. 2d ed.. edited by M. Bass. J. M. Enoch. E. W.
V. Stryland,
W. L.. Wolfe. McGraw-Hill. New York. 2001. p. 2.11-2.14). These layers may not
be
spherical, but are still continuous within a certain solid angle. The angle
value depends on
the distance between two sphere centers. There are continuous multiple layers
of intensity
distribution between two sphere centers too. They are hyperboloids of
revolution and are
continuous within the whole coherence length. In brief, for the strong non-
laser light
beam comes from a finite distance, if its bandwidth is narrow enough, and its
source is
not located too near to the limiter, the standing wave with periodic
continuous multilayer
structure is still formed. This standing wave overlaps its corresponding
incident beam
completely within the coherence length. Of course, the output compound lens is
designed
to be moveable to adapt to object distance change.
As the reflecting coating has a high reflectance. the brightness of the
objects in
field of view is reduced for observer. Therefore the input compound lens is
designed to
have larger aperture and longer focal length than those of the output compound
lens. Thus,
the cross section area of the output beam is smaller than that of the input
beam, which
compensates the intensity loss for the objects. For example, if the radius of
the input
compound lens is 3.16 times the radius of the output one, the output intensity
will be 10
times as high as that of the input one. The diameter and focal length of the
input
compound lens depends concrete applications, usually is from 0.5 cnl to 50 cm
and 1 cm
to 100 cm respectively. The thickness of the used solid above said optical
limiting
CA 02714847 2010-09-16
material, or the container filled with liquid above said optical limiting
material is from I
m to 5 cm.
Thus, different types of optical limiters can be constructed by using
different solid
or liquid materials with nonlinear optical absorption. scattering, refraction,
or
photo refractive, photosensitive effects as the solid material 12 in Fig. I.
or as the liquid
material filled in the container 22 in Fig.2.
First, the optical limiting process of the limiter using solid or liquid
nonlinear
optical absorption materials is described here. Making a solid nonlinear
absorption
material, such as a glass containing semiconductor nanocrystals with two-
photon
absorption, as the material 12, which has the same shape and reflecting
coating as the
material 12. or filling the container 22 with a liquid nonlinear optical
absorption material.
such as the organic dye solution with reverse saturable absorption. Such type
of limiter is
built. If a high power laser beam enters the limiter, it is focused on the
reflecting coating
36. that is, the partial reflecting coating 14 in Fig. I or 24 in Fig.2, at
point 38 as shown in
Fig.4. and is reflected partially. As mentioned above, a special standing wave
forms in
the traveling way of the incident beam and overlaps the incident beam
completely within
the coherence length.
Supposing the incident beam has the amplitude of unit 1. the amplitude of
reflected beam is 0.9 because of 90% intensity reflectance. Then the maximum
compounding intensity is (1 + 0.9~ = 3.797 at anti-node. and is (i - 0.9
0.0026 at
node. Thus the light intensity at anti-node increases to about 4 times as high
as the
intensity of the original incident beam. The intensity at node drops from I to
0.0026. As
the intensity drop amount at node is about quarter of the increasing amount at
anti-node.
the net and cumulate intensity amount in total absorption range is raised. The
optical
absorption coefficients of used materials increase with the rise of light
intensity
nonlinearly, thus the absorption increasing at anti-node is over 4 times.
Therefore. the
high power light will be absorbed more than that in the situation without the
standing
wave. Thus. comparing with the limiter under same working conditions, that is.
same
16
CA 02714847 2010-09-16
limiting material and same light intensity, the limiter invented here has
lower optical
limiting threshold.
For a non-laser incident beam located at a finite distance, if its bandwidth
is
narrow enough. as shown in the Fig.'), a special standing wave with periodic
continuous
multilayer structure of the intensity distribution is formed too. Thus
similarly as
mentioned above, the high power light is absorbed more than that in the
situation without
the standing wave, making the limiter invented here have lower optical
limiting threshold.
Second, the optical limiting process of the limiter using solid or liquid
nonlinear
optical scattering materials is described in sequence. Making a solid
nonlinear optical
scattering material, such as a nonlinear photonic crystal with fluence
dependent
transmission as the material 12, which has the same shape and reflecting
coating as the
material 12, or filling the container 22 with a liquid nonlinear optical
scattering material,
such as the black carbon suspension with the scattering induced by heating
carbon
particles. Such type of limiter is built.
Many nonlinear optical scatterings depend on high intensity induced
ununiformity
of optical properties of the material, such as the distribution ununiformity
of refractive
index and density. For example, a typical nonlinear photonic crystal consists
of ordered
array of tiny holes or channels in a glass. These holes or channels are filled
with a
nonlinear material whose refractive index initially matches that of the glass.
The high
intensity makes the index matching lost, resulting in strong scattering and
reduced
transparency. In the black carbon suspension, high intensity produces plasma
formation
and scattering from the bubbles induced by a rapid heating of absorbing carbon
particles.
Similarly as mentioned above, if a high power laser beam enters the limiter,
it is
focused on the reflecting coating and reflected partially. Then the special
standing wave
is formed. It has been known that in such a standing wave, the intensity is
increased to
3.797 times at anti-node. and reduced to 0.0026 times at node if intensity
reflectance is
90%. Thus, the scattering increases nonlinearly at anti-node with intensity
rise. Since the
intensity drop at node is about quarter of the intensity increase at anti-
node, the net and
17
CA 02714847 2010-09-16
cumulate intensity amount in total scattering range is raised. It will
increase the total
scattering amount. Furthermore. the distribution of the index changed
holes/channels or
density changed bubbles becomes un-uniform too. Such ununiformity is large
because it
is from almost zero to over 4 in a very short distance of A , which will cause
additional
4
scattering. Thus, the high power light is scattered more than that in the
situation without
the standing wave. Comparing with the limiter under same working conditions,
that is,
same material and same light intensity, the limiter invented here has lower
optical
limiting threshold. Same thing is for the non-laser beam, if it has enough
narrow
bandwidth and is not located too near to the limiter.
Next, the optical limiting process of the limiter using solid or liquid
nonlinear
optical refraction materials is described underneath. Making a solid nonlinear
optical
refraction material, such as semiconductor ZnSe exhibiting both of nonlinear
absorption
and refraction effects, as the material 12, which has the same shape and
reflecting coating
as the material 12, or filling the container 22 with a liquid nonlinear
optical refraction
material, such as pararosanilin dye in liquid media with third-order nonlinear
refraction
(G. Vinitha, & A. Ramalingam, "Third-order optical nonlinearities and optical-
limiting
properties of a Pararosanilin dye in liquid and solid media", Laser Physics.
Vol. 18, No. 9,
P. 1070-1073, 2008). Such type of limiter is built.
When a high power laser beam enters the limiter, it is focused on the
reflecting
coating 14 or 24 and reflected partially. Then the special standing wave is
formed.
Because the intensity reflectance is 90%. the maximum compounding intensity at
anti-
3
node is 1460 times as high as that at node since .797 - =1460 as mentioned
above. Thus
0.0026
the induced refractive index change at anti-node is much larger than that at
node (it may
happen first that the light energy is absorbed much more at anti-node than at
node). As a
result, the distribution of refractive index exhibits periodic continuous
multiple layer
structure. The interval between every two contiguous layers is - (see Fig.4).
This kind of
structure is very like periodic multilayer reflector consists of two
materials. which has
18
CA 02714847 2010-09-16
very high reflectance if the index difference of two materials is large and
especially the
number of periods is large. For a periodic multilayer reflector consists of
two materials
which refractive index is homogeneous, the highest reflectance occurs whenever
nõ dõ
n,, d,, are each equal to an odd multiple of 4 , where nõ , n,, , dõ and d,,
are indexes of two
materials A. B. and thicknesses of the layers made of A and B respectively.
The
maximum intensity reflectance for normal incident is given by
n /n+(n/n J- 0)
where n,,, . n, and N are indexes of two contiguous media beside the reflector
and number
of periods respectively ("Handbook of Optics", Vol. I, 2d ed., edited by M.
Bass, J. M.
Enoch. E. W. V. Stryland, W. L. Wolfe, McGraw-Bill, New York, 2001, p. 42.34-
42.42).
In the standing wave, the refractive index is not homogeneous in each layer.
The related
calculation is somewhat different. However, the intensity reflectance of such
standing
wave will he very high. One reason is that the standing wave has very large
intensity
gradient, another reason is that getting a large number of periods N is easy
in the
standing wave.
For a beam comes from a finite distance, some more things need to be
considered
compared with the situations using nonlinear absorption and scattering. It is
known that a
conoid standing wave may also be formed if the focal point of the beam falls
behind the
reflecting coating. And a periodic continuous multilayer structure is produced
as shown
in Fig.3. However, the value of the solid angle and the length of the conoid
standing wave
depend on the distance between points 30 and 32. and the beam coherence
length. Unlike
nonlinear absorption and scattering, the layers in the multilayer structure
must be
continuous. Otherwise, light reflectance is reduced obviously like a
multilayer reflector,
that is, a mirror breaks at somewhere. Therefore. short distance between
points 30 and 32,
and long coherence length are preferred.
19
CA 02714847 2010-09-16
In order to obtain these conditions, the nonlinear optical refractive
materials with
(In do
minus index change, that is. with minus or -- , are better candidates, where
do .
dT dl
dT and dl are increments of refractive index, temperature and intensity
respectively.
Thus, the distance between points 30 and 32 is reduced or even the focal point
of the
beam falls in front of the reflecting surface. As indicated above, the layers
between the
centers of two spherical waves in an interference induced multilayer structure
are
continuous within the whole coherence length. Thus, the distance between two
sphere
centers is allowed longer. Using bandpass filter, the bandwidth of incident
beam may be
reduced. To get white light feeling, the filter having three narrow
transmission regions for
tree primary colors may be used.
Unlike the normal limiting using nonlinear optical refraction effect, the self-
defocusing must be avoided, which destroys the formation of the special
standing wave.
One method is to build an additional layer of linear material on top of the
reflecting
coating as shown in Fig.5 and Fig.6. In Fig.5 and 6, 40 and 46 are the added
layers. and
42 and 48 are the reflecting coatings. In these added layers, the beam
intensity is allowed
to be very high. Because these layers are linear optical materials. the self-
defocusing does
not happen. The thickness of these layers depends on the possible power range
of the
incident beams, and the values of the material nonlinear refractive
coefficients. An
additional optical limiter 44 in Fig.5 or 50 in Fig.6 may be inserted into the
optical path to
pre-limit the incident power below a certain value. In some cases, two or more
than two
additional optical limiters may be inserted into the optical path to get
larger power pre-
limiting. The refractive index of the layers 40 and 46 are selected to near or
match that of
the used solid or liquid nonlinear optical refractive materials, or the
partial reflecting
coating. Thus, the beam focal point still falls on the partial reflecting
coating, and
additional reflection phase change is eliminated, or the amount of the
reflection part with
undesired phase change is reduced.
Therefore, the high power light will be limited much more than that in the
situation without the standing wave. Thus, comparing with the limiter using
same optical
CA 02714847 2010-09-16
refractive material and under the same incident intensity, the limiter
invented here has
much lower limiting threshold.
Next. the optical limiting process of the limiter using solid or liquid
photorefractive materials is described underneath. Making a solid
photorefractive
material, such as lithium niobate crystal doped with Fe. as the material 12,
which has the
same shape and reflecting coating as the material 12, or filling the container
22 with a
liquid photorefractive material, such as dye-doped liquid crystals (lam Choon
Khoo, Min-
Yi Shih, M. V. Wood. B. D. Guenther. Pao Hsu Chen. F. Simoni. S. S.
Slussarenko, 0.
Francescangeli, L. Lucchetti, "Dye-doped Photorefractive Liquid Crystals for
Dynamic
and Storage Holographic Grating Formation and Spatial Modulation", Proceedings
of
IEEE, Vol. 87, Issue 11, P. 1897-1911, 1999). Such type of limiter is built.
When a parallel light beam enters this limiter, the special standing wave is
produced. As emphasized above, in the standing wave and if the intensity
reflectance is
90 10. the maximum compounding intensity at anti-node is 1460 times as high as
that at
node within a very short distance 4 . It corresponds to a very large intensity
gradient.
Most of photorefractive crystals. including photorefractive liquid crystals
are intensity
gradient dependence. Thus the material refractive index changes much larger at
anti-node
than at node. As a result, the distribution of refractive index forms a
periodic continuous
multilayer structure. This structure satisfies the Bragg reflection
requirements. So in the
material, a periodic multilayer reflector is formed. Because the index
difference between
anti-node and node may be large. and the number of periods is large too
because of large
intensity gradient and large photorefractive coefficients of the materials,
this kind of
reflector has very high reflectance, and so a very low limiting threshold.
Most of' the details about structure design and limiting process of this type
of
limiters are like those discussed for using nonlinear optical refraction
material, such as
inserting additional limiters to pre-limit high light power. Here they are not
repeated. As
photorefractive materials have very large photorefractive coefficients, there
is a
21
CA 02714847 2010-09-16
possibility to use these limiters to limit non-laser light. In the following,
this possibility is
discussed, with using these limiters as the sun visor for driving at night as
an example.
It is difficult to view the object in blazing background, such as the road
ahead for
drivers facing oncoming car light at night. Most cars use filament or arc
headlamps with
power around 50 w. For a typical low beam headlamp. its beam cross section
areas at the
distances between 20 m and 30m from the car are about 10 in x 3 m,
corresponding to a
intensity of 0.167 mw/cm' The light intensity increases when the car comes
nearer.
Supposing the light energy of the beam distributes in the whole visible region
from 0.4
pun to 0.76 pm. In order to get enough long coherence length, a bandpass
filter with a
bandwidth of 5110 of the whole visible region is used. The bandwidth central
is at
wavelength of 0.55 m. According to the above coherence length calculation
equation (1),
the passed beam has a coherence length of 16.8 pm. In addition, According to
the above
lens equation (2), when the car is at the distance of 20 in, the image field
shift is 125 m.
When this beam is reflected from the partial reflecting coating, the initial
phase
difference between the incident beam and the reflected beam is zero. Thus a
short special
standing wave with a thickness of half coherence length, that is, 8.4 m is
formed. In this
conoid standing wave, there are 30 reflecting layers because 8.4 pm/ (0.5 x
0.55 pin) _
30.
n ln-(n/n
The above multilayer reflector equation (3). that is, R,,,,,,
_
n /n, +(nõln)
may be used here for a rough estimation. Choosing N = 30, n = 1.5 17,= 1 and
17,=1.5.
then for n,,=1.501, 1.51, 1.55. 1.6, 1.65 and 1.7. the R,,,,, is 0.048. 0.146.
0.688, 0.946,
0.991 and 0.9985 respectively. As reference, if choosing n = 1.5 n, = 1, nõ
=1.5, n,, =1.51,
but N is 30, 100, 300 and 1000, the R is 0.146, 0.4899, 0.952 and 0.999995
respectively. By focusing the incident beam from the car. the beam intensity
may be
increased far over -mw/cm ' within its coherence length range. The
photorefractive
sensitivity is from about -mw/em ' for the photorefractive materials
("Handbook of
Optics". Vol. II, 2d ed.. edited by M. Bass, .1. M. Enoch, E. W. V. Stryland,
W. L. Wolfe,
22
CA 02714847 2010-09-16
McGraw-Hill, New York, 2001, p. 39.12). Thus to get intensity reflectance over
90% for
the incident beam from a car is possible. For limiting blazing light from a
car, an intensity
drop of 90% is already useful. One may visualize the scene as a 50 w lamp
becomes 5 w
one.
Using bandpass filter reduces the brightness of the objects further. In the
case of
the filter transmission width is 5% of the visible region, the passing energy
is about 10%
of that in the visible region (the light energy has an un-uniform distribution
within the
visible region, and the maximum value is at 0.55 m). This may be compensated
by
reducing the reflectance of the partial reflecting coating or increasing the
aperture size of
the input compound lens. Using bandpass filter with three transmission regions
around
tree primary colors can increase the objet brightness and produce white color
feeling.
Therefore, like the limiters discussed above, this type of limiters can limit
the high
power light much more than that in the situation without the standing wave.
Comparing
with the limiter using same photorefractive material and under the same
incident intensity,
the limiter invented here has much lower optical limiting threshold, and
provides the very
precious possibility for limiting non-laser light.
At last, the optical limiting process of the limiter using solid or liquid
photosensitive materials is described underneath. Making a solid
photosensitive material
as the material 12, which has the same shape and reflecting coating as the
material 12, or
filling the container 22 with a liquid photosensitive material, such as dye-
doped nernatic
liquid crystals (I. C. Khoo, M. V. Wood, M. Y. Shih, and P. H. Chen,
"Extremely
Nonlinear Photosensitive Liquid Crystals for Image Sensing and Sensor
Protection",
Optics Express. Vol. 4. No. 11. p. 432-442, 1999). Such type of limiter is
built.
Similarly, the structure design is for forming a special standing wave in the
path
of the incident beam. and for utilizing the features of this standing wave to
limit the beam
power. As many details about structure design and limiting process for this
type of limiter
are like those discussed for the above limiters, including phase change
eliminating, high
power pre-limiting, and so on. These details are not discussed repeatedly.
Only some
23
CA 02714847 2010-09-16
special considerations about the structure design for using nematic liquid
crystals is
described underneath.
Two polarization filters 52 and 54 are inserted into the optical path as shown
in
Fig.7. One is in front of the container 56. that is, the container 22. and
another is behind
the container 56. The alignment of the liquid crystals in the container is
chosen so that its
relaxed phase is a twisted one, the twist angle may be one of 45 , 90 180 ,
200 and
270 ('Handbook of Optics". Vol. II. 2d ed.. edited by M. Bass, J. M. Enoch,
F. W. V.
Stryland, W. L. Wolfe, McGraw-Hill. New York, 2001, p.14.14). Here,
considering the
90 twist angle. In order to get this relaxed phase alignment, two inside
surfaces of the
container 56, including the completed partial reflecting coating, are treated
using existing
technology, such as rubbing or tilting deposition. Probably. an additional
transparent
layer needs to be deposited on top of the completed partial reflecting coating
for getting
the relaxed phase. In this situation, the refractive index of the additional
layer must be as
same as or near that of the liquid crystal or the reflection coating. Thus,
the additional
phase change can be avoided, or the amount of the reflected light with
undesired phase
change can be reduced. The directions of two polarization filters are crossed
(oriented at
90 to one another) and match the liquid crystals alignment. The direction of
the polarizer
52 may be selected arbitrarily in the plane perpendicular to the primary
optical axis of the
limiter. Here, making the direction of the polarizer 52 vertical and that of
the polarizer 54
horizontal. The alignment of the liquid crystals needs to match the polarizer
directions.
Thus the beam passes through the first polarizer 52, and then its polarization
direction
rotates 90 following the liquid crystal directors, and passes through the
second polarizer
54. The most of the beam reaching the partial reflecting coating is reflected
hack, and
returns into the liquid crystals again. If the beam power is enough low, the
returned beam
rotates its polarization direction reversely following the liquid crystal
directors, and
passes the first polarizer. In Fig.8, the left tilted row indicates the
schematic director
alignment in the relaxed phase. Sheets 58 and 60 are the first and second
polarization
filters respectively.
24
CA 02714847 2010-09-16
If beam power is high, as described above, in the formed standing wave the
maximum compounding intensity at anti-node is 1460 times as high as that at
node within
a short distance - , if the intensity reflectance is 90%. It is a very large
intensity gradient.
and is much larger than that produced in a focusing beam which was utilized by
I.C.
Khoo and etc (see above reference). In a focusing beam, the light intensity
gradient is
proportional to the ratio of the beam cross section areas separated by a
certain distance.
From Fig.9, one can see that in a focusing beam, the ratio of the cross
section areas
separated by interval of is equal to 'R2 , R, = R, + AR and sin 0 = (4 x OR)/
A . The
4 yrR,_ -
angle 0 is determined by the f-number N / of the input compound lens (N f/d,
where f
and d are focal length and diameter of the entrance pupil of the input
compound lens).
Usually, N 1 is larger than 1, thus 0 is less than 26 . Substituting 0 = 23.57
, thus sin 0 =
0.4. the ratio becomes I + 0.:A + 0.1), . Even at the waist of the laser beam,
the radius
R1 R,
of the waist is larger than several m (" I-landbook of Optics", Vol. IV. 2d
ed., edited by
M. Bass, .l. M. Enoch. E. W. V. Stryland. W. L.,. Wolfe. McGraw-Hill. New
York, 2001. p.
28.22). Supposing R, A . the ratio is equal to 1.21. Thus. the intensity
gradient in the
special standing wave is more than 1207 times as high as that in the focusing
beam even
with a large solid angle of 47 .
Such high intensity gradient will reorient the directions of the liquid
crystal
directors. The intensity changes along the light traveling direction
periodically. It
generates a photo-induced periodic space charge distribution in the liquid
crystals. and
produces a periodic space electric field distribution. This periodic space
electric field
reorients the liquid crystal directors, and making them form a periodic
alignment.
Because the electric field strength is very large in each periodic interval,
the most of the
directors are aligned parallel to the light traveling direction. The right
tilted row in Fig.8
indicates the schematic reoriented alignment of the directors. It just shows a
very short
CA 02714847 2010-09-16
range of , and the number of the liquid crystal directors in that range is
much less than
the real number. The length of a typical liquid crystal molecule is 30
angstroms.
Therefore. 40 to 50 liquid crystal molecules may align in the range of a
quarter of
wavelength of 0.55 m. In each periodic interval of electric field, the most
directors are
aligned parallel to the field direction as high field strength. In this state,
the liquid crystal
directors do not reorient the polarization direction of the light, so the
light polarized at the
first polarizer is absorbed at the second polarizes. and the beam is blocked.
Very few
directors located around anti-nodes and nodes are aligned perpendicularly to
the field
direction. As the lengths of the ranges with almost zero field at anti-node
and node are
much less than the wavelength, the directors in these ranges do not affect the
light
reorientation.
In the work did by I. C. Khoo and etc (see above reference), the dye-doped
liquid
crystals can limit incident CW laser beam with low power of several mw. They
used
focusing lens to produce light intensity gradient in the liquid crystals.
Because the light
intensity gradient produced by standing wave is much higher than that produced
by a
focusing lens, the limiter invented here will provide much lower optical
limiting
threshold, and provides great possibility for limiting non-laser light, such
as being used as
the sun visor for driving at night.
Fig.] I illustrates the schematic optical structure of second preferred
embodiment
of the optical limiter according to the invention. A converging input lens 62
focuses the
parallel incident beam into a partial reflecting mirror 64. The light
intensity reflectance of
the mirror 64 is 37.5%. The transmitted part of the beam enters a right-angle
prism 66.
After one total internal reflection, this transmitted part is reflected by
another partial
reflecting interface 68, and goes into a solid above said optical limiting
material 70. 70
may also be a transparent container filled with a liquid above said optical
limiting
material. The focal point of the transmitted part of the beam falls on the
middle plane of
the solid material 70 or the container 70. The light intensity reflectance of
the interface 68
is 60%. The reflected part of the incident beam from the mirror 64 enters a
penta prism
72 ("Handbook of Optics', Vol. 11, 2d ed., edited by M. Bass, .1. M. Enoch, E.
W. V.
26
CA 02714847 2010-09-16
Stryland, W. L. Wolfe. McGraw-Hill, New York, 2001, p. 4.13-4.14). After two
total
internal reflections (the penta prism has larger refractive index), this
reflected part goes
into the solid material 70 or the container 70 from an opposite side. Then,
this reflected
part travels through the partial reflecting interface 68 and is converged by
output lens 74
to become a parallel beam again. The right-angle prism 75 has same refractive
index with
the prism 66, and is connected to the prism 66 without air gap. The focal
point of this
reflected part of the beam falls on the middle plane of the solid material 70
or the
container 70 too, and is connected to the focal point of the transmitted part
of the beam.
The optical path lengths of the transmitted part and the reflected part are
made to be same
when their focal points meet at the middle plane of the solid material 70 or
the container
70. Realize the same process for all parallel incident beams with different
incident angles
from the field of view. Thus two focal planes are created and are coincided
with each
other in the solid material 70 or the container 70. In other words, two
identical image
fields for all parallel incident beams with different incident angles from
total field of view
are created. and are coincided with each other in the above said optical
limiting material.
If a laser beam is incident, two special standing waves arc formed in the
solid
material 70 or the container 70 as shown in Fig. 10. In Fig. 10. 76 are the
connected focal
points of the transmitted and reflected parts of the incident beam, and 78 is
the focal
plane of the input lens 62. In the Fig.] 0, the reflected part of the beam is
overlapped by
the transmitted part of the beam completely. It means that the incident high
power beam
can be limited effectively. However. for the incident beam which traveling
direction is
not along the primary optical axis of the device. its two divided beam parts
can't overlap
each other completely when they meet again. Thus in the un-overlapped area the
special
standing wave is not formed, resulting in the un-overlapped part of the beam
is not
limited and leaks out the limiter. For a high power beam, even a fraction of
its energy
passing the limiter directly may cause a serious damage. If this problem
persists, the
related limiting scheme is only effective for limiting the beam with single
incident
direction, and so becomes no use in practice. The desired scheme is one that
can limit
high power beams with any possible incident angle from the total field of
view. Limiting
the beams with different incident angles is much more difficult than limiting
the beam
27
CA 02714847 2010-09-16
just with single incident angle. Furthermore, the scheme for limiting the
beams with
different incident angles needs to be simple and convenient. without losing
the simplicity
and compactness of the passive device.
By circlewise cutting the solid angle of one of two divided beams, this
problem
can be solved ideally. In Fig. 11. a diaphragm 80 is used to circlewise cut
the solid angle
of the reflected part of the tilting incident beam. As result, the passing
part of the tilting
incident beam is overlapped well as shown in Fig.12. In Fig.12, the reflected
part 82 of
the tilting incident beam has a reduced solid angle cut by the diaphragm,
which is smaller
than that of the transmitted part 84 of the same beam. Thus the special
standing wave is
formed in this overlapped solid angle. As result, all of the output lights
through the lens
74 have been treated by the limiting process. In Fig.12, 86 is the focal plane
of the input
lens 62, 88 is the primary optical axis of the limiter. The diaphragm size is
chosen in a
way that the reflected part of the tilting beam reaching the edge of the
desired image field
may be overlapped effectively by the transmitted part of the same beam.
During the limiting process, some lights of the transmitted part of the beam
will
return to the right-angle prism from the solid material 70 or the container
70, and go out
through the lens 74. Its origins are related to the structure of the limiter
and the optical
effect utilized, such as back scattering from induced scattering centers,
reflection from
the generated multilayer reflectors" and so on. These noise lights degrade the
function of
the limiter. In order to block these noise lights, a quarter-wave plate 90 and
a polarizer 92
are used. The polarization direction of the polarizer 92 is placed at 45 to
the optical axis
direction of the quarter-wave plate 90. Thus, the linear polarization light
emerging from
the polarizer 92 is divided into two polarization components when passed
through the
quarter-wave plate. These two polarization components combine to become a
circular
polarization light. When this circular polarization light returns back by
reflection,
scattering and so on, it passes through the quarter-wave plate and becomes
linear
polarization light again. but with a further 90 rotation of the polarization
direction
("Handbook of Optics", Vol. 1. 2d ed.. edited by M. Bass. J. M. Enoch, E. W.
V. Stryland.
28
CA 02714847 2010-09-16
W. L. Wolfe. McGraw-Hill, New York, 2001, p.5.22-5.26). Thus, the returned
light can't
pass the polarizer 92 again.
However. the two polarization components produced by the polarizer 92 and
quarter-wave plate 90 has a 90 phase difference, which reduce the intensity
gradient of
the standing wave formed in the solid material 70 or the container 70 (see
above
discussions about reflection phase change as the reference). Therefore,
another polarizer
94 and a three-quarter-wave plate 95 are used. The polarization direction of
the polarizer
94 is parallel to that of the polarizer 92, and is at 45 to the optical axis
direction of the
three-quarter-wave plate 95. Thus. the linear polarization light emerging from
the
polarizer 94 is divided into two polarization components when passed through
the three-
quarter-wave plate 95. These two polarization components have a 270 phase
difference.
When these two polarization components traveling from left to right meet two
polarization components traveling from right to left, the two components which
directions are parallel to the polarizer direction have zero phase difference.
The two
components which polarization directions are perpendicular to the polarizer
direction
have 360 or 0 phase difference. Therefore, the standing wave formed in the
solid
material 70 or the container 70 has largest intensity gradient.
To block the noise lights returning to the right-angle prism from the solid
material
70 or the container 70. and to get largest intensity gradient of the standing
wave may
adopt other methods according to the existing optical design technology.
In order to eliminate or reduce the reflection phase changes caused by
reflecting
surfaces, interfaces of the mirror and prisms, special constructed reflecting
coatings
maintaining phase difference of being or near 0 or 360 for parallel and
perpendicular
polarization components introduced above are deposited on these surfaces and
interfaces.
For decreasing cost, some of these surfaces and interfaces may not have these
special
reflecting coatings if the induced cumulated reflection phase change between
two
polarization components is, equates with or near 0 or 360 . For example, the
two
reflecting surfaces of the penta prism may not have these special coatings.
29
CA 02714847 2010-09-16
In order to reduce energy loss and arbitrary light disturbance, all optical
surfaces
of the lenses, prisms, mirror, wave-plates, polarizers and the solid material
or the
container, excepting above said reflecting surfaces and interface, are coated
with high
antireflection films for the desired wavelength range.
Thus, each of the reflected beam parts from the mirror 64 has same frequency,
same polarization, roughly equal amplitude, and opposite or near opposite
direction with
its corresponding one of the transmitted beam parts through the mirror 64. The
phase
difference between each pair of the reflected and transmitted parts is zero at
middle plane
of the solid material 70 or the container 70, and is fixed within their
coherence length.
Thus, for a narrow bandwidth beam especially a laser beam, two long special
standing
waves are formed in the traveling paths for each other. The passing part
through the
diaphragm of the reflected part of the incident beam is overlapped by the
transmitted part
of the same beam completely within the range of the coherence length. In this
spacial
interference field, light intensity distributes in the manner of multiple
continuous layers,
and the interval between two contiguous layers is just half of the wavelength
of the
incident beam.
For a non-laser light beam which source is located at finite distances, if its
coherence length is enough long, two standing waves may still be formed like
shown in
Fig.13. In Fig.13 the focal points of the reflected and transmitted parts of
the incident
beam are located at 98 and 100 respectively. Of course, the output lens is
designed to be
moveable to adapt to object distance change.
As partial reflecting. diaphragm cutting and polarizer filtering, the
brightness of
the objects in field of view is reduced for observer. Therefore the input lens
62 has larger
aperture and longer focal length than those of the output lens 74. For
example, if the light
intensity reflectances of the mirror 64 and interface 68 are 37.5% and 60%
respectively,
the diaphragm cutting ratio is 33%, as the transmittance of the polarizer is
50%, the ratio
of output/input energy is 5%. Thus. making the radius of the input lens is
4.47 times the
radius of the output lens, the output intensity will be 20 times as high as
that of the input
one. The diameter and focal length of the input lens depend on the concrete
applications.
CA 02714847 2010-09-16
Usually they are from 0.5 cm to 50 cm and 1 cm to 100 cm respectively. The
sizes of
mirror and prisms are matched with the input lens. The thickness of the used
solid above
said optical limiting material, or the container filled with liquid above said
optical
limiting material is from 1 m to 5 cm. In order to reduce the weight, the
prisms may be
replaced by appropriate combinations of mirrors.
Thus, different types of optical limiter can be constructed by using solid or
liquid
materials with different nonlinear optical absorption, scattering, refraction,
or
photorefractive. photosensitive effects as the solid material 70 in Fig. 11,
or as the liquid
material filled in the container 70 in Fig.11. The details about structure
modification and
limiting process discussion for these limiters, such as adding polarizers for
liquid crystals
material. adding additional pre-limiters, one may see the above descriptions
for the first
embodiment shown in Fig.], '15. 6 and 7 as the reference.
Fig. 14 illustrates the schematic optical structure of third preferred
embodiment of
the optical limiter according to the invention. A converging input lens 102
focuses the
parallel incident beam into an abbe's prism 104 ("Handbook of Optics", Vol.
II, 2d ed.,
edited by M. Bass. J. M. Enoch. E. W. V. Stryland, W. L. Wolfe, McGraw-Hill,
New
York. 2001, p.4.7), and reaches the partial reflecting interface 106. The
light intensity
reflectance of the interface 106 is 37.5%. The transmitted part of the beam
enters another
reflecting interface 108, and then is reflected by a pair of mirrors l 10 and
112. In Fig.14.
the mirror 1 12 is behind the mirror 110, and the two mirrors are
perpendicular to each
other, with a structure like a roofing prism (see Fig.15, which is a schematic
left side
view, taken on the dash line 122-122 in Fig.14). These two mirrors invert the
beam, and
deviate the beam to the mirror 114. The intensity reflectances of the
reflecting interface
108, mirrors 110, 112 and 114 are 100%. After further reflection by partial
reflecting
mirror 116, this transmitted part goes into a solid above said optical
limiting material 118.
1 18 may also be a transparent container filled with a liquid above said
optical limiting
material. The intensity reflectance of the partial reflecting mirror 116 is
60%. The focal
point of the transmitted part falls on the middle plane of the solid material
118 or the
container 118. The reflected part of the incident beam from the interface 106
enters two
31
CA 02714847 2010-09-16
roofing surfaces of the Abbe's prism, and is inverted and deviated to the
reflecting
interface 108. The reflections on two roofing surfaces are total internal
reflections
("Handbook of Optics' Vol. 11. 2d ed., edited by M. Bass. J. M. Enoch, E. W.
V.
Stryland. W. L. Wolfe. McGraw-Hill, New York, 2001, p. 4.7-4.8). Then this
reflected
part goes into the solid material 1 18 or the container 118 on an opposite
side. Afterwards,
this reflected part travels through the partial reflecting mirror 116 and is
converged by
output lens 120 to become a parallel beam again. The prism 122 has same
refractive
index with the Abbe's prism 104, and is connected to the prism 104 without air
gap. The
focal point of the reflected part of the beam falls on the middle plane of the
solid material
118 or the container 118 too, and is connected to the focal point of the
transmitted part of
the beam. The optical path lengths of the transmitted part and the reflected
part of the
beam are made to be same when their focal points meet at the middle plane of
the solid
material 118 or the container 118. Realize same process for all parallel
incident beams
with different incident angles from the field of view. Thus two focal planes
are created
and are coincided with each other in the solid material 118 or the container
118. In other
words, two identical image planes for all parallel beams with different
incident angles
from field of view are created and are coincided with each other in the solid
material 118
or the container 118.
If a laser beam is incident, two special standing waves are formed in the
solid
material 118 or the container 1 18 same as shown in Fig. 10. For limiting a
tilting incident
beam well. a diaphragm 126 is used to cut the solid angle of the reflected
part of the
tilting incident beam. As result. the passing part through the diaphragm of
the tilting
incident beam can he overlapped well as shown in Fig.12. Also, the diaphragm
size is
chosen in a way that the passing part of the reflected part of the tilting
beam reaching the
edge of the desired image field may be overlapped completely by the
transmitted pat of
the same beam.
In order to block the noise lights returning from the solid material 118 or
the
container 118, a quarter-wave plate 128 and a polarizer 130 are used. The
polarization
direction of the polarizer 130 is placed at 45 to the optical axis direction
of the quarter-
32
CA 02714847 2010-09-16
wave plate 128. Thus, the light becomes linear polarization light first and
then circular
polarization light when passes the polarizer 130 and quarter-wave plate 128.
When this
circular polarization light returns back by reflection, scattering and so on,
it passes
through the quarter-wave plate and becomes linear polarization light again
with a 90
rotation of the polarization direction, and thus can't pass the polarizer 130.
In order to
compensate 90 phase difference between two polarization components produced
by the
polarizer 130 and quarter-wave plate 128, another polarizer 134 and a three-
quarter-wave
plate 132 are used. The polarization direction of the polarizer 134 is placed
parallel to
that of the polarizer 130. and is at 45 to the optical axis direction of the
three-quarter-
wave plate 132. Thus. the two polarization components passing through the
polarizer 134
and the three-quarter-wave plate 132 have a 270 phase difference. When the
reflected
part light traveling from left to right meet the transmitted part light
traveling from right to
left, the two components which polarization directions are parallel to the
polarizer
direction have zero phase difference, and the two components which
polarization
directions are perpendicular to the polarizer direction have 360 or 0 phase
difference.
Therefore, the special standing wave formed in the solid material 1 18 or the
container
118 has largest intensity gradient.
To block the noise lights returning to the mirror 116 from the solid material
118
or the container 1 l 8. and to get largest intensity gradient of the standing
wave may adopt
other methods according to the existing optical design technology.
In order to eliminate or reduce the reflection phase changes caused by
reflecting
surfaces, interfaces of the mirrors and prisms, special constructed reflecting
coatings
maintaining phase difference of being or near 0 or 360 for parallel and
perpendicular
polarization components introduced above are deposited on these surfaces and
interfaces.
For decreasing cost, some of these surfaces and interfaces may not have these
special
reflecting coatings if the induced cumulated reflection phase change between
two
polarization components is, equates with or near 0 or 360 . For example, the
two roofing
surfaces of the Abbe's prism, the two reflecting surfaces of the mirrors 110
and 112. may
not have these special coatings.
33
CA 02714847 2010-09-16
In order to reduce energy loss and arbitrary light disturbance, all optical
surfaces
of the lenses, prisms, mirrors. wave-plates, polarizers and the solid material
or the
container, excepting above said reflecting surfaces and interface. are coated
with high
antireflection films for the desired wavelength range.
Thus, each of the reflected beam parts from the interface 106 has same
frequency,
same polarization, roughly equal amplitude, and opposite or near opposite
direction with
its corresponding one of the transmitted beam parts through the interface 106.
The phase
difference between each pair of the reflected and transmitted parts is zero at
middle plane
of the solid material 118 or the container 118, and is fixed within their
coherence length.
Thus. for a narrow bandwidth beam especially a laser beam, two long special
standing
waves are formed in the traveling paths for each other. The passing part
through the
diaphragm of the reflected part of the incident beam is overlapped by the
transmitted part
of the same beam completely within the range of the coherence length. In this
spacial
interference field, light intensity distributes in the manner of multiple
continuous layers.
and the interval between two contiguous layers is just half of the wavelength
of the
incident beam.
For a non-laser light beam which source is located at finite distance, if its
coherence length is enough long, two special standing waves may still be
formed like
shown in Fig.13. Of course, the output lens is designed to be moveable to
adapt to object
distance change. As partial reflecting. diaphragm cutting and polarizer
filtering, the
brightness of the objects in field of view is reduced for observer. Therefore
the input lens
102 has larger aperture and longer focal length than those of the output lens
120. For
example. if the light intensity reflectances of the interface 106 and the
surface 116 are
37.5% and 60% respectively, the diaphragm cutting ratio is 33%, as the
transmittance of
the polarizer is 50%. the ratio of output/input energy is 5%. Thus. making the
radius of
the input lens is 4.47 times the radius of the output lens, the output
intensity will be 20
times as high as that of the input one.
The diameter and focal length of the input lens depend on the concrete
applications. Usually they are from 0.5 cm to 50 cm and I cm to 100 cm
respectively.
34
CA 02714847 2010-09-16
The sizes of mirror and prisms are matched with the input lens. The thickness
of the used
solid above said optical limiting material 118, or the container 118 filled
with liquid
above said optical limiting material is from I pm to 5 cm. In order to reduce
the weight,
the Abbe's prism 104 may be replaced by suitable combination of mirrors, and
without
using the fitting prism 122. Vice versa, the mirror pairs of 110 and 112, 114
and 116 may
be replaced by a roofing prism and a right-angle prism respectively to
increase device
stability and assembling simplification.
Thus, different types of optical limiter can be constructed by using solid or
liquid
materials with different nonlinear optical absorption, scattering, refraction,
or
photorefractive, photosensitive effects as the solid material 118 in Fig.14,
or as the liquid
material filled in the container 118 in Fig.14. The details about further
structure
modification and limiting process discussion for these limiters, such as
adding polarizers
for liquid crystals material, adding pre-limiters, one may see the above
descriptions for
the first embodiment shown in Fig. 1, 2. 5, 6 and 7 as the reference.
Fig.16 illustrates the schematic optical structure of fourth preferred
embodiment
of the optical limiter according to the invention. A converging input lens 136
focuses the
parallel incident beam into a solid above said optical limiting material 138.
138 may also
be a transparent container filled with a liquid above said optical limiting
material. The
focal point of the beam falls on the back surface 140 of the solid material
138 or the
inside back surface 140 of the container 138 (the container is not drawn in
detail here). A
partial reflecting coating with light intensity reflectance of, such as 90%,
is coated on the
back surface 140 of the solid material 138 or the inside back surface 140 of
the container
138. Unlike the first preferred embodiment, the surface 140 is plane here, and
is located
at the focal plane of the lens 136. Thus the most part of the beam returns by
partial
reflection. The returned part of the beam interferes with the incident beam.
The
transmitted part of the incident beam goes to the lens 142, and is converged
to become a
parallel beam again.
In order to eliminate or reduce the reflection phase changes caused by partial
reflection, the special constructed reflecting coating, which maintains a
phase difference
CA 02714847 2010-09-16
of being or near 0 or 360 for parallel and perpendicular incident
polarization
components introduced above, is deposited on the back surface 140 of the solid
material
l 38 or the inside back surface 140 of the container 138.
If a laser beam is incident, most part of the beam returns back into the solid
or
liquid limiting material as shown in Fig.16. If the beam has a tilting
incident angle, the
returned beam can't overlap the whole incident beam, resulting in part of the
returned
beam interferes with part of the incident beam. That is, the standing wave is
not formed
in whole solid angle of the incident beam. The un-overlapped part of the
incident beam
will not be limited, and then leaks out the lens 142. This problem is also
solved by using
a diaphragm 146. Place the diaphragm at a proper position. At that position,
only the
overlapped part of the incident beam, that is, the part interfered with the
returned part of
the beam can pass. In addition, at that position the overlapped part can pass
with
maximum amount. The position and the aperture size of the diaphragm usually
can be
determined by drawing an optical path chart. By enlarging input lens aperture
size, the
ratio of the overlapped part of the beam to the whole of the beam can be
increased. It is
shown in Fig.17. The locating position and aperture size of the diaphragm 150
are also
changed to match input lens enlargement.
As returned part of the beam has same frequency, same polarization, roughly
equal amplitude, and zero phase difference with the incident beam at the
reflecting
coating, the special standing wave is formed in their interference area. In
the standing
wave, the light intensity distribution exhibits a periodic continuous
multilayer structure.
As described above. such type standing wave can produce enhanced light
absorption and
scattering in nonlinear optical absorption and scattering materials. It can
also produce
high and extremely high reflection in nonlinear optical refraction and
photorefractive
materials by self-formed multilayer reflector. And at last, it can produce
extremely large
light blocking in photosensitive nematic liquid crystals. Of course, two
polarizers must be
used, and two inside surfaces of the container 138 must be treated to produce
relaxed
phase when using photosensitive nematic liquid crystals, which is like that
described for
36
CA 02714847 2010-09-16
the first embodiment in Fig.7. And pre-limiters may be used in some situations
to pre-
lower the power of incident beam.
Thus only the part of the beam which has been treated by the optical limiting
process can pass through the limiter.
In order to reduce energy loss and arbitrary light disturbance. all optical
surfaces
of the lenses, the solid material 138 or the container 138 filled with liquid
material.
excepting the said partial reflecting coating. are coated with high
antireflection films for
the desired wavelength range.
For a non-laser light beam which source is located at finite distance, if its
coherence length is enough long. the special standing wave may still be formed
like
shown in Fig. 18. In Fig. 18. the virtual focus of the tilting incident beam
152 is at point
154. The tilting beam 152 is reflected by the reflecting coating 156, that is.
the back
surface 140 of the solid material 138 or the inside back surface 140 of the
container 138,
and focused at point 158. It then diverges into beam 160. It is known that in
an
interference field of two spherical waves with same frequency, polarization,
amplitude,
fixed phase difference. and within the coherence length, the special standing
wave with
intensity distribution with periodic continuous multilayer structure can be
produced
between two sphere centers, or outside the region between two sphere centers.
Between
two sphere centers, the produced layers are hyperboloids of revolution and are
continuous
within the whole coherence length. Outside the region between two sphere
centers, the
continuous layers are just within a certain solid angle which axis is along
the connecting
line of two sphere centers. The value of the solid angle depends on the
distance between
two sphere centers. Therefore, if the bandwidth of the tilting incident beam
is narrow
enough, and the beam is not from a position located too near to the limiter,
the special
standing wave having continuous periodic multiple layer structure is still
formed. Of
course. the output lens is designed to he moveable to adapt to object distance
change.
As partial reflecting, diaphragm cutting, the brightness of the objects in
field of
view is reduced for observer. Therefore the input lens may have larger
aperture and
37
CA 02714847 2010-09-16
longer focal length than those of the output lens 142. For example, if the
light intensity
reflectance of the partial reflecting coating is 90%, the diaphragm cutting
ratio is 50%.
the total ratio of outputlinput energy is 5%. Thus, making the radius of the
input lens is
4.47 times the radius of the output one, the output intensity will be 20 times
as high as
that of the input one.
The diameter and focal length of the input lens depends on the concrete
applications, usually. they are from 0.5 cm to 50 cm and 1 cm to 100 cm
respectively.
The size of output lens is matched with the input lens. The thickness of the
used solid
above said optical limiting material 138, or the container 138 filled with
liquid above said
optical limiting material is from I m to 5 cm. In order to reduce the device
size,
especially in the case of using short focal length lens, the input and output
lenses may
adopt Fresnel lenses.
Thus, different types of optical limiter can be constructed by using solid or
liquid
materials with different nonlinear optical absorption, scattering, refraction,
or
photorefractive, photosensitive effects as the solid material 138 in Fig.] 6,
or as the liquid
material filled in the container 138 in Fig.16. The details about further
structure
modification and limiting process discussion for these limiters, one may see
the above
descriptions for the first embodiment shown in Fig. 1, 2, 5, 6 and 7 as the
reference.
One important application of optical limiter is to protect human eyes. Such
kind
of application needs the limiter to provide erect image for observer,
including providing
erect image for some imaging optical instruments, with performing ideal
optical limiting
function. The preferred embodiments of the limiter structure design providing
erect
image will he described in the underneath. The principle of these designs is
using lens,
prism, mirror and so on to invert the images limited by the above described
limiters, that
is, above four preferred embodiments of the of the optical limiter according
to the
invention. By using existing optical design knowledge. these embodiments may
be
alternated, modified to fit different practical needs. The applicant of this
invention
reserves the right of all alternatives, modifications, and equivalent
arrangements of the
limiter structure embodiments described underneath.
38
CA 02714847 2010-09-16
In the following descriptions, the stress is laid on inverting inverted image.
The
structure design and optical process about light power limiting is omitted.
For detailed
consideration about light power limiting, one may refer the above descriptions
of related
preferred embodiment of the optical limiter according to the invention.
Fig.19 shows first schematic image erecting scheme for the first preferred
embodiment shown in Fig.l and 2 of the optical limiter according to the
invention. In
Fig.19. the input compound lens consisting of lens 162 and 164 converges the
parallel
incident beam. The image is inverted 180 in vertical plane by right-angle
prism 166, and
is inverted 180 in horizontal plane by right-angle prism 168. Afterwards, the
image is
selectively limited by the solid above said limiting material 170 or the
container 170
filled with liquid above said limiting material, and goes out through the
output compound
lens consisting of lens 172 and 174. The partial reflecting coating of the
solid material
170 or the container 170 is located at the focal curved surface of the input
compound lens.
In order to eliminate or reduce the reflection phase changes caused by the
reflections on prism surfaces. the special constructed reflecting coatings.
which maintains
a phase difference of being or near 0 or 360 for parallel and perpendicular
polarization
components introduced above, are deposited on these surfaces. Considering the
phase
changes on surfaces of each right-angle prism are complementary, these special
coatings
may not be needed. This scheme is like the optical structure of one most
popular
binoculars with double right-angle prism design. Thus, plenty of modification
designs for
binoculars with double right-angle prism design may be adopted to improve the
imaging
performance of this optical limiter. Fig.20 is a schematic top side view,
taken on the dash
line 176-176 in Fig.19.
Fig.21 shows second schematic image erecting scheme for the first preferred
embodiment shown in Fig.] and 2 of the optical limiter according to the
invention. In
Fig21, the input compound lens consisting of lens 176 and 178 converges the
parallel
incident beam. Then the beam enters an Abbe's prism. The beam is inverted 180
in
vertical plane by reflections on two roof surfaces 180 and 182 (182 is behind
180), and is
inverted 180 in horizontal plane by reflections on surfaces 184, 180. 182 and
186 in
39
CA 02714847 2010-09-16
sequence. Afterwards, the beam is selectively limited by the solid above said
optical
limiting material 188 or the container 188 filled with liquid above said
optical limiting
material, and goes out through the output compound lens consisting of lens 190
and 192.
The partial reflecting coating of the solid material 188 or the container 188
is located at
the focal curved surface of the input compound lens.
In order to eliminate or reduce the reflection phase changes caused by the
reflections on prism surfaces, the special constructed reflecting coatings,
which maintains
a phase difference of being or near 0 or 360 for parallel and perpendicular
polarization
components introduced above- are deposited on these surfaces. Considering the
phase
changes on each pair of surfaces. that is. the roof surfaces 180 and 182, the
surfaces 184
and 186 are complementary. these special coatings may not needed. This scheme
is like
the optical structure of another most popular binoculars with Abbe's prism
design. Thus,
plenty of modification designs for binoculars with Abbe's prism design may be
adopted
to improve the imaging performance of this optical limiter.
Fig.22 shows third schematic image erecting scheme for the first preferred
embodiment shown in Fig.1 and 2 of the optical limiter according to the
invention. In
Fig.22, the objective lens 194 focuses the parallel beam on its focal plane
196. The input
compound lens consisting of lens 198 and 200 converges the divergent beam from
the
plane 196 on its spherical image surface 202, that is, the back surface of the
solid above
said optical limiting material 204 or the inside back surface of the container
204 filled
with the liquid above said optical limiting material. The partial reflecting
coating is on the
surface 202. The beam is selectively limited by the solid limiting material
204 or the
liquid limiting material in the container 204. The beam then goes out through
the output
compound lens consisting of lens 206 and 208. During this process, the image
is inverted.
and then inverted again becoming an erect image.
Fig.23 shows fourth schematic image erecting scheme for the first preferred
embodiment shown in Fig.] and 2 of the optical limiter according to the
invention. In
Fig.23, the right-angle prism 210 inverts beam 180 in vertical plane. Then
the input
compound lens consisting of lens 212 and 214 converges the parallel incident
beam. The
CA 02714847 2010-09-16
beam is inverted 180 in horizontal plane by another right-angle prism 216.
Afterwards,
the beam is selectively limited by the solid above said optical limiting
material 218 or the
container 218 filled with liquid above said optical limiting material, and
goes out through
the output compound lens consisting of lens 220 and 222. The partial
reflecting coating of
the solid material 218 or the container 218 tilled with liquid material is
located at the
focal curved surface of the input compound lens.
In order to eliminate or reduce the reflection phase changes caused by the
reflections on prism surfaces. the special constructed reflecting coatings,
which maintains
a phase difference of being or near 0 or 360 for parallel and perpendicular
polarization
components introduced above, are deposited on these surfaces. Considering the
phase
changes on surfaces of each right-angle prism are complementary, these special
coatings
may not be needed. This scheme uses double right-angle prism design. So it
keeps the
imaging advantages of one most popular binoculars, and plenty of modification
designs
for this kind of binoculars may be adopted to improve the imaging performance
of this
optical limiter. Fig.24 is a schematic top side view, taken on the dash line
221-221 in
Fig.23.
In order to reduce the device weight or to adapt different focal length of the
input
compound lens, two right-angle prisms may be replaced by two pairs of mirrors,
such as
two pairs of mirrors A, B and C, D. The locations and directions of the mirror
pairs of A,
B and C, D are same as four reflecting surfaces of two prisms 210 and 216
respectively.
The input compound lens may be placed between A and B, or B and C, or C and D,
or
even after D. In every situation, the spherical focal surface of the input
compound lens
coincides with the back surface of the solid material 218 or the inside back
surface of the
container 218. Thus, the limiter can have different field of view.
Fig 25 shows first schematic image erecting scheme for the second preferred
embodiment shown in Fig. 1 1 of the optical limiter according to the
invention. As most
details of the structure design are same as those described for Fig.I 1. here
only difference
between Fig.25 and Fig. 1 I is indicated. Moving output lens 74 to make output
beam from
interface 68 focused on the image plane 226. Chen another lens 230 is used to
converge
41
CA 02714847 2010-09-16
the divergent beam from plane 226 becoming a parallel beam again. Thus, the
inverted
image is inverted again to become erect image.
Fig.26 shows second schematic image erecting scheme for the second preferred
embodiment shown in Fig. l1 of the optical limiter according to the invention.
The
converging input lens 232 focuses the parallel incident beam into the partial
reflecting
mirror 234. The transmitted part of the beam enters the right-angle prism 236.
After two
total internal reflections, this transmitted part goes into the solid above
said optical
limiting material 238, or the container 238 filled with a liquid above said
optical limiting
material. The focal point of the transmitted part of the beam falls on the
middle plane of
the solid material 238 or the container 238. Then. this part travels through
the partial
reflecting interface 241. passes another right-angle prism 252, and then is
converged by
output lens 254 to become a parallel beam again. The light intensity
reflectance of the
interface 241 is 60%. The reflected part of the incident beam from the mirror
234 enters
the penta prism 240. After one total internal reflection, this reflected part
is reflected
partially by the interface 241, and then goes into the solid material 238 or
the container
238 from an opposite side. The fitting prism 256 has same refractive index
with the prism
240, and is connected to the prism 240 without air gap. The focal point of the
reflected
part of the beam falls on the middle plane of the solid material 238 or the
container 238
too, and is connected to the focal point of the transmitted part of the beam.
The optical
path lengths of the transmitted part and the reflected part are made to be
same when their
focal points meet at the middle plane of the solid material 238 or the
container 238.
Realize the same process for all parallel incident beams with different
incident angles
from the field of view. Thus, the image is inverted by two right-angle prisms
in vertical
and horizontal planes, and becomes erect image when output.
In Fig.26, 242 and 244 are polarizer and quarter-wave plate. 246 and 248 are
another polarizer and three-quarter-wave plate, and 250 is the diaphragm. They
are used
for blocking returned light. eliminating reflection phase change and
circlewise cutting the
solid angle of the transmitted part of the beam. Fig.27 is a schematic top
side view, taken
on the dash line 256-256 in Fig.26.
42
CA 02714847 2010-09-16
The above third preferred embodiment shown in Fig.14 of the optical limiter
according to the invention provides already an erect image, so there is no
modification for
it.
Fig.28 shows first schematic image erecting scheme for the fourth preferred
embodiment shown in Fig. 16 and 17 of the optical limiter according to the
invention. In
Fig.28. the input lens 260 converges the parallel incident beam. The beam is
inverted
180 in vertical plane by right-angle prism 262, and is inverted 180 in
horizontal plane
by right-angle prism 264. Afterwards, the beam is selectively limited by the
solid above
said optical limiting material 266 or the container 266 filled with liquid
above said optical
limiting material, and goes out through the output lens 268 to become a
parallel beam
again. 270 is the diaphragm, which only let the beam part overlapped by the
reflected part
of the incident beam pass (see the description related to the Fig.16 and 17).
The partial
reflecting coating of the solid material 266 or the container 266 is located
at the focal
plane of the input lens.
In order to eliminate or reduce the reflection phase changes caused by the
reflections on prism surfaces, the special constructed reflecting coatings,
which maintains
a phase difference of being or near 0 or 360 for parallel and perpendicular
polarization
components introduced above, are deposited on these surfaces. Considering the
phase
changes on surfaces of each right-angle prism are complementary. these special
coatings
may not be needed. This scheme is like the optical structure of one most
popular
binoculars with double right-angle prism design. Thus, plenty of modification
designs for
binoculars with double right-angle prism design may be adopted to improve the
imaging
performance of this optical limiter. Fig.29 is a schematic top side view,
taken on the dash
line 272-272 in Fig 28.
Fig.30 shows second schematic image erecting scheme for the fourth preferred
embodiment shown in Fig.16 and 17 of the optical limiter according to the
invention. In
Fig.30, the input lens 280 converges the parallel incident beam. Then the beam
enters an
Abbe's prism. The beam is inverted 180 in vertical plane by reflections on
two roof
surfaces 282 and 284 (284 is behind 282), and is inverted 180 in horizontal
plane by
43
CA 02714847 2010-09-16
reflections on surfaces 286. 282. 284 and 288 in sequence. Afterwards, the
beam is
selectively limited by the solid above said optical limiting material 290 or
the container
290 filled with liquid above said optical limiting material, and goes out
through the
output lens 292 to become parallel beam again. 294 is the diaphragm, which
only lets the
beam part overlapped by reflected part of the incident beam pass. The partial
reflecting
coating of the solid material 290 or the container 290 is located at the focal
plane of the
input lens.
In order to eliminate or reduce the reflection phase changes caused by the
reflections on prism surfaces, the special constructed reflecting coatings,
which maintains
a phase difference of being or near 0 or 360 for parallel and perpendicular
polarization
components introduced above, are deposited on these surfaces. Considering the
phase
changes on each pair of surfaces, that is. the roof surfaces 282 and 284, the
surfaces 286
and 288 are complementary, these special coatings may not be needed. This
scheme is
like the optical structure of another most popular binoculars with Abbe's
prism design.
Thus. plenty of modification designs for binoculars with Abbe's prism design
may be
adopted to improve the imaging performance of this optical limiter.
Fig.31 shows third schematic image erecting scheme for the fourth preferred
embodiment shown in Fig.16 and 17 of the optical limiter according to the
invention. In
Fig.31. the objective lens 300 focuses the parallel beam on its focal plane
302. The input
lens 304 converges the divergent beam from the plane 302 on its image plane
307, that is,
the back surface of the solid above said optical limiting material 306 or the
inside back
surface of the container 306 filled with the liquid above said optical
limiting material.
The partial reflecting coating is on the back surface 307 of the solid
material 306 or the
inside back surface 307 of the container 306. The beam is selectively limited
by the solid
limiting material 306 or the liquid limiting material in the container 306.
The beam then
is converged and goes out through the output lens 308 to become parallel beam
again.
310 is the diaphragm. During this process, the image is inverted, and then
inverted again
becoming an erect image.
44
CA 02714847 2010-09-16
Fig.32 shows fourth schematic image erecting scheme for the fourth preferred
embodiment shown in Fig.16 and 17 of the optical limiter according to the
invention. In
Fig. 32. the right-angle prism 312 inverts beam 180 in vertical plane. Then
the input lens
314 converges the incident parallel beam. The beam is inverted 180 in
horizontal plane
by another right-angle prism 316. Afterwards. the beam is selectively limited
by the solid
above said optical limiting material 318 or the container 318 filled with
liquid above said
optical limiting material. The beam is converged and goes out through the
output lens 320.
322 is the diaphragm. The partial reflecting coating of the material 318 or
the container
318 filled with liquid material is located at the focal plane of the input
lens. Fig.33 is a
schematic top side view, taken on the dash line 324-324 in Fig.32.
In order to eliminate or reduce the reflection phase changes caused by the
reflections on prism surfaces, the special constructed reflecting coatings.
which maintains
a phase difference of being or near 0 or 360 for parallel and perpendicular
polarization
components introduced above, are deposited on these surfaces. Considering the
phase
changes on surfaces of each right-angle prism are complementary. these special
coatings
may not he needed. This scheme uses double right-angle prism design. So it
keeps the
imaging advantages of one most popular binoculars, and may adopt plenty of
modification designs for binoculars with double right-prism design to improve
the
imaging performance of this optical limiter.
In order to reduce the device weight or to adapt different focal length of the
input
lens, two right-angle prisms may be replaced by two pairs of mirrors, such as
two pairs of
mirrors A. B and C. D. The locations and directions of mirrors A, B and C. D
are same as
four reflecting surfaces of two prisms 312 and 316 respectively. The input
lens may be
placed between A and B, or B and C, or C and D. or even after D. In every
situation, the
focal plane of the input lens coincides with the hack surface of the solid
material 318 or
the inside back surface of the container 318. Thus, the limiter can have
different width of
field of view.
At last, Fig.34 shows fifth schematic image erecting scheme for the fourth
preferred embodiment shown in Fig.16 and 17 of the optical limiter according
to the
CA 02714847 2010-09-16
invention. In Fig.34, the mirrors 330 and 332 invert parallel incident beam
180 in
horizontal plane. Then the mirror 334 inverts the beam 90 in vertical plane.
Afterwards,
the input lens 338 (see Fig.35) converges the parallel incident beam, and
another mirror
336 inverts the beams 90 in vertical plane further. An optical pre-limiter
340 pre-limits
the beam from the mirror 336 to lower its power under a certain value. 342 is
a solid
above said optical limiting material. 342 may also be a transparent container
filled with
liquid above said optical limiting material. The back surface of the solid
material 342 or
the inside back surface of the container 342 coincides with the focal plane of
the input
lens 338. The detailed structure of the solid material 342 or the container
342 is shown in
Fig. 36 or 37. Afterwards, the beam is limited selectively by the solid
limiting material
342 or the liquid limiting material filled in the container 342. The beam then
is converged
and goes out through the output lens 344. 346 is the diaphragm, which is used
to cut the
unlimited part of the tilting incident beams through the output lens. The
partial reflecting
coating is on the back surface of the solid material 342 or the inside back
surface of the
container 342. Fig.35 is a schematic top side view, taken on the dash line 350-
350 in
Fig.34.
In order to eliminate or reduce the reflection phase changes caused by the
reflections on mirror reflecting surfaces, the special constructed reflecting
coatings,
which maintains a phase difference of being or near 0 or 360 for parallel
and
perpendicular polarization components introduced above, are deposited on these
surfaces.
Considering the phase changes on each pair of surfaces, that is. mirrors 330
and 332 or
334 and 336, are complementary, these special coatings may not be needed. This
scheme
is like that of binoculars using double right-angle prism design. So it keeps
the imaging
advantages of one most popular binoculars, and the plenty of modification
designs for
binoculars with double right-prism design may be adopted to improve the
imaging
performance of this optical limiter.
The mirrors 330 and 332 may be replaced by a right-angle prism. In order to
adapt
different focal length of the input lens, the input lens 338 may be placed
between mirrors
330 and 332, or 332 and 334, or even after 336. In every situation, the focal
plane of the
46
CA 02714847 2010-09-16
input lens must coincide with the back surface of the solid material 342 or
the inside back
surface of the container 342. Thus, the limiter can have different width of
field of view.
Fig.36 is the detailed structure of the solid material 342. It is made of a
solid
above said optical limiting material. A partial reflecting coating is
deposited on its back
surface 360. The light intensity reflectance of the reflecting coating is from
20% to 98%.
in the case described in Fig.34, the reflectance is 90%. The reflecting
coating is plane,
and is located at the focal plane of the input lens 338. The reflecting
coating is the special
constructed reflecting coating, which maintains a phase difference of being or
near 0 or
360 for parallel and perpendicular polarization components introduced above.
If needed.
on top of the reflecting coating. an additional layer may be deposited to
avoid self-
defocusing of the high power beams, which happens around the beam focal points
(this
layer is not drawn in Fig.36). The refractive index of this additional layer
must be same
as or near that of the material 342 or the reflecting coating under it to
eliminate the
induced reflection phase change or reduce the amount of the light with induced
reflection
phase change. In any situation, the reflecting coating is kept to be located
at the focal
plane of the input lens 338.
Fig.37 is the detailed structure of the transparent container 342. It is
filled with a
liquid above said optical limiting material. A partial reflecting coating is
deposited on its
inside back surface 362. The light intensity reflectance of the reflecting
coating is from
20% to 98%, in the case described in Fig.-34, the reflectance is 90%. The
reflecting
coating is plane, and is located at the focal plane of the input lens 338. The
reflecting
coating is the special constructed reflecting coating, which maintains a phase
difference
of being or near 0 or 360 for parallel and perpendicular polarization
components
introduced above. If needed, on top of the reflecting coating, an additional
layer may be
deposited to avoid self-defocusing of the high power beams, which happens
around the
beam focal points (this layer is not drawn in Fig.37). The refractive index of
this
additional layer must be same as or near that of the liquid material filled in
the container
342 or the reflecting coating under it to eliminate the induced reflection
phase change or
47
CA 02714847 2010-09-16
reduce the amount of the light with induced reflection phase change. In any
situation, the
reflecting coating is kept to be located at the focal plane of the input lens
338.
For using photosensitive nematic liquid crystals. two polarizers 364 and 366
are
placed on both sides of the container. The alignment of the nematic liquid
crystals in the
container is chosen so that its relaxed phase is a twisted one, the twist
angle may be one
of 45 , 90 , 180 . 200 and 270 . Here, considering the 90 twist angle. In
order to get
this relaxed phase alignment. two inside surfaces of the container 342,
including the
completed partial reflecting coating, are treated using existing technology,
such as
rubbing or tilting deposition. Probably, an additional transparent layer needs
to be
deposited on top of the completed partial reflecting coating for getting the
relaxed phase.
In this situation, the refractive index of the additional layer must be as
same as or near
that of the liquid crystal or the reflection coating. Thus, the additional
phase change can
be avoided, or the amount of the reflected light with undesired phase change
can be
reduced. The directions of two polarization filters are crossed and match the
liquid
crystals alignment. The direction of the polarizer 52 may be selected
arbitrarily in the
plane perpendicular to the primary optical axis of the limiter. Here, making
the direction
of the polarizer 364 vertical and that of the polarizer 366 horizontal. The
alignment of the
liquid crystals needs to match the polarizer directions.
Us each of the above described preferred embodiments offering erect image,
including those shown in Fig.14. Fig.19, Fig.21. Fig.22, Fig.23, Fig.25.
Fig.26, Fig.28,
Fig.30, Fig.31, Fig.32 and Fig.34 as an optical limiter. Then mount a pair of
two identical
such optical limiters side-by side and align them to point accurately in the
same direction.
and allow the viewer to us both eyes when viewing distant objects, the
portable optical
limiter for individual use to protect observer eyes are built. Most of them
are sized to he
held using both hands. The location positions of the output lens are moveable
to let
observer to view the objects at different distances.
In all of the above described preferred embodiments, the input lenses, the
output
lenses and the objective lenses may he modified by using additional
compensation lens to
correct their aberrations. In addition, in all of the above described
preferred
48
CA 02714847 2010-09-16
embodiments, the light intensity rellectances of the partial reflecting
coatings, partial
reflecting surfaces and the partial reflecting interfaces may be changed
according to the
concrete requirements of the applications, for example, from 10% to 98%.
49
