Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED COUPLING OF LIGHT FROM A
SMALL ARC LAMP TO A LARGER TARGET
Field of the Invention
The invention is in the field of systems for collecting and
condensing electromagnetic radiation and coupling that radiation
into a target.
Background of the Invention
It has long been a goal in the field of fiber optic
technology to develop a system for more efficiently collecting
and condensing electromagnetic radiation from an incoherent light
l0 source, approximated by a point source. Conventional systems
have attempted to direct radiation originating from a
conventional incoherent light source into a small spot size
without an accompanying decrease in radiation flux.
Commonly, two approaches have been taken in the development
of such systems. The first involves the use of condensing lenses
between the light source and target. Such condenser lenses
typically have several drawbacks in that they often are
relatively costly, space consuming, inherently difficult to
align, and they create chromatic and spherical aberrations. The
other common approach is the use of ellipsoidal reflecting
mirrors. These reflecting systems are also very costly, and they
have the inherent drawback that they cause a natural
magnification of the image resulting in a reduction in the flux
density to the target.
The most common prior art system involves a parabolic
reflector used together with a lens as shown in Figure 5. The
parabolic reflector 9 forms the housing of the lamp 1 with
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surfaces coated with either aluminum or silver. The gas is
sealed into this housing using a window. The arc of the lamp is
placed at the focus of the parabola which causes the output beam
to be comprised of parallel rays. A reflective coating of
aluminum or silver reflects radiation from UV through visible to
infrared. As a result, for applications like medical
illuminations, a visible filter is needed to filter out unwanted
UV and infrared radiation. Usually, a transmission filter is
used which can not be made with sharp cutoff of wavelength. The
resulting output therefore is comprised of more than the
desirable amount of UV and infrared radiation. When a reflective
filter is used, the distance between the lamp and the focusing
lens has to be increased to accommodate the filter. This reduces
the coupling efficiency of the system. To couple the light into
an output device 7, such as an optical fiber bundle, a focusing
lens 10 is typically used to redirect the paral~~~l beam into a
small spot. The output numerical aperture from the lens is
matched to the numerical aperture of the fiber bundle to achieve
the maximum possible coupling efficiency. Due to the intrinsic
nature of the combination of the parabola and focusing lens, the
magnification of the arc onto the bundle is not constant over the
whole aperture. As a result, the output spot size is always
larger than the arc of the lamp itself. This mechanism results
in a decrease in the maximum possible brightness or flux
intensity at the focusing point. Together with aberrations
created by the focusing lens, such systems produce an output with
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a spot size significantly larger than the arc gap, and a
distribution which is non-uniform.
Figure 6 illustrates another common configuration for
focusing output from an arc lamp into a fiber bundle. In this
case, the arc of the lamp is placed at one focus of the
ellipsoidal reflector 3 with the electrode placed along the major
axis of the ellipsoid. The output fiber 7 is placed at the
target 6 which is located at the other focus along the major
axis. The size of the ellipsoidal surface and the distance
between the two foci determine the numerical aperture of the
output beam. Due to various paths for light to go from one focus
to the other, the magnification is not constant for all rays .
As a result, the output spot size at the other focus is usually
a few times larger than the arc itself. This inherent
magnification again reduces the brightness of the arc.
U.S. Patent No. 4,757,431 to Cross et al., the specification
of which is herein incorporated by reference, discloses a
collecting and condensing system which utilizes an off-axis
spherical concave reflecting system to enhance the amount of flux
density at the target point over previous ellipsoidal reflecting
systems. The layout of such a system is shown in Figure 7. This
system, while allowing increased flux density derived from its
inherent 1:1 image magnification at the target spot, has the
drawback in that its flux concentration efficiency decreases with
the linear off-axis distance between the target 6 and the arc
lamp 1. Any attempt to limit such flux loss by minimizing the
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off-axis displacement is constricted by the physical size and
shape of the illumination source and target or fiber optic output
device 7. U.S. Patent No. 5,430,634 to Baker et al., the
specification of which is also herein incorporated by reference,
discloses a variant of the off-axis reflecting system as
disclosed in U.S. Pat. No. 4,757,431 wherein a concave toroidal
reflector is employed in place of the concave spherical reflector
4.
Tapered rods and cones are commonly incorporated into the
input light post of endoscopes for maximizing the collection of
light from a large diameter source and transforming the collected
light into a smaller spot size and a larger numerical aperture.
Typically these configurations are highly inefficient because the
cone length is too short for optimizing the transformation both
spatially and angularly. U.S. Patent No. 5,729,643 to Hemlar et
al. discloses the use of a tapered optical fiber having an input
core diameter which tapers down to a smaller output diameter in
order to focus light into a smaller spot size.
As shown by U.S. patent 5,680,257 to Anderson, beam integrating
optics using lenses and conical integrators and reflectors to
condense light into a small spot size with increased angular
divergence are also known in the art. All of these previous
systems, however, necessarily produce an increase in the
numerical aperture of the light. Therefore, such systems are
inefficient when used to couple light into an optical fiber.
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The resulting increase in numerical aperture, or divergence, of
the light during efforts to decrease the spot size causes a large
portion of the collected light to exceed the numerical aperture
of an output optical fiber placed at the image point. Thus, a
5 considerable proportion of incident light at the image point
cannot be transmitted by the fiber. There remains a need in the
art for improvements in coupling light from light collecting and
condensing systems.
Summary of the Invention
In the field of the present invention, the incoherent light
from the arc lamp 1 is generally desired to be imaged onto a
target 6, such as the end of a single fiber or fiber bundle.
Coupling of the light from the condensing and collecting systems
into an optical fiber is optimized when the numerical apertures
of the reflector or condenser lens and the fiber optic target are
equal. As a rule, the numerical aperture of the light output
from the fiber will be the same as that of the reflector/lens
system or the fiber, whichever is smaller. This is because an
optical fiber can generally be said to have an intrinsic
numerical aperture which represents the highest propagation angle
a beam of light can have and be completely contained within the
optical fiber. Any time the light passing through an optical
fiber exceeds the numerical aperture of the fiber, leakage of
light will occur. This fact becomes very important whenever an
optical fiber is bent, typically causing a localized decrease in
the fiber's effective numerical aperture. Thus, it is desirous
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to have high flux density light passing through an optical fiber
which has a numerical aperture less than the numerical aperture
of the fiber.
On the other hand, to redirect the maximum amount of light
flux from the arc lamp onto the target spot requires use of a
primary mirror with as large a numerical aperture as possible.
Commonly the high numerical aperture light from the mirror/lens
will be larger than that of the fiber optic or fiber optic bundle
at the target spot. Because of the transmission limits described
above, this means a significant portion of the light reaching the
target will not be transmitted by the output fiber and will be
lost.
The invention improves upon prior art for coupling light
into large diameter targets. It provides a mechanism for
coupling the light of high numerical aperture into an
intermediate optically transforming device such that the light
collected from the lamp from any condensing and collecting system
is transformed into an output having a smaller numerical aperture
and larger spot size for efficient coupling to the input end of
a large diameter single fiber or fiber bundle matched in diameter
and numerical aperture. The net result is higher efficiency and
output relative to prior art systems coupling light into the same
target.
Tapered rods and cones are commonly incorporated into the
input light post of endoscopes for maximizing the collection of
light from a large diameter source and transforming the collected
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light into a smaller spot size and a larger numerical aperture.
Typically these configurations are highly inefficient because the
cone length is too short for optimizing the transformation both
spatially and angularly.
Tapered hollow tubes with reflective interior surfaces are
also commonly used to ~~funnel" light into a small spot size from
a source. Such hollow tapered tubes work like a funnel in that
they have an aperture at either end, one aperture being larger
than the other. The tube takes light in at the larger aperture
and smoothly condenses it by reflection inside the conical
surface into a small spot size and larger divergence when it
leaves at the smaller aperture. These types of optical devices
are commonly incorporated in LCD projectors, DMD projectors, and
the like.
Another species of light guide with specific applicability
to the present invention is a specialized form of the tapered
hollow tube known as the compound parabolic concentrator, or
"CPC." CPCs are like the tapered hollow tube, but their interior
reflective surfaces are parabolic, or curved. Such paraboloid
surfaces have been found to be effective in concentrating light
emitted from a large source at a distance into a small spot size.
Therefore, CPCs find common application in collecting solar rays
for heating or generating electricity. For such applications,
the input end of the CPC has a larger cross section than the
output end, and light emitted from the output end has a much
larger numerical aperture.
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Solid glass CPCs can also be configured to produce similar
results. Alternatively, a lens can be employed as the light
guide. As shown in U.S. patent 5,680,257, lenses are commonly
employed to condense light into a small target spot. Again, such
use necessarily results in an increased NA, or divergence, of the
light.
A light guide in the form of a single tapered cladded rod
or cone, a tapered fused fiber optic bundle, a reflective tapered
hollow tube, a compound parabolic concentrator, a negative lens,
or a combination thereof, placed at the image point of the system
can maximize the transmission of light through the final fiber
optic target. The present invention makes use of such prior art
devices as a light guide by utilizing it in a manner reverse to
their typical manner of use. The above devices are positioned
whereby the incident light directed from the optical collection
system, such as from any of the aforementioned prior art systems,
is increased in spot size and decreased in angular distribution
to maximize the amount of light ultimately collected and able to
be transmitted through a fiber optic device.
Brief Description of the Drawings
Fig. 1 is a schematic illustration of one embodiment of the
present invention using an off-axis toroidal concave reflector
as the primary collector.
Fig. 2 is a schematic illustration of one embodiment of the
present invention using an off-axis ellipsoidal concave reflector
as the primary collector.
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Fig. 3 is a schematic illustration of one embodiment of the
present invention using an on-axis extended ellipsoidal concave
reflector as the primary collector.
Fig. 4 is a schematic illustration of one embodiment of the
present invention showing a negative lens being used as a light
guide.
Fig. 5 is a schematic illustration of a prior art condenser
and collector system employing a parabolic concave reflector and
focusing lens.
Fig. 6 is a schematic illustration of a prior art condenser
and collector system employing an ellipsoidal concave reflector.
Fig. 7 is a schematic illustration of a prior art condenser
and collector system employing a toroidal concave reflector with
the source and target located in an off-axis relationship.
Detailed Description of the Invention
Embodiments of the invention generally are comprised of a
short arc lamp 1, as shown in the figures. Suitable arc lamps
include lamps producing arc gaps of up to about 8 mm, including
but not limited to Xenon, Mercury, Mercury-Xenon, AC metal
halide, and DC metal halide type lamps ranging in power anywhere
from 100 to 500 watts. Experiments have indicated that
acceptable results have been achieved using 1 mm, 1.5 mm, 2 mm,
3 mm, and up to 6 mm arc gaps from 100 and 500 watt Xenon and 250
and 270 watt metal halide arc lamps.
The arc lamp 1 is used in conjunction with any known primary
collection system. Figure 2 illustrates one embodiment of the
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invention where an off-axis ellipsoidal concave reflector 2 is
used as the primary collector. Figure 3 illustrates another
embodiment of the invention where an on-axis ellipsoidal concave
reflector 3 is used as the primary collector.
5 Figure 1 illustrates one preferred embodiment of the
invention where an off-axis spherical concave reflector 4 is used
as the primary collector. In any of the above embodiments, a
retro-reflector 5 may be employed to increase light flux to the
primary collector 2, 3, or 4, as shown in Figures 1, 2, and 3,
10 respectively. The primary collector and retro-reflector 5 can
optionally be coated with dielectric material, aluminum, or
silver for circumstances where a specific wavelength of light is
desired to be collected or where broad band electromagnetic
radiation is so desired. For example, where the radiation is to
be used for purposes of illumination with visible light, the
mirror can be coated with a mufti-layer dielectric:: coating that
reflects only the visible light and rejects the UV and IR
radiation. The output would be a visible light only having a
color temperature dependent upon the source, such as a xenon lamp
with color temperature on the order of 6000 degrees Kelvin. Such
light output is particularly suitable for visual applications
such as in surgical illumination.
The light from the lamp 1 is directed by the primary
collector 2, 3, or 4 to a target spot 6. In prior art systems
as depicted in Figures 5, 6, and 7, a light transmitting output
device 7 is placed at the target spot 6. In the present
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invention, a device 8 for transforming the numerical aperture and
spot size of the collected light, or "light guide," is placed at
the target spot to transform light to a spot size and numerical
aperture matched to that of the output device 7. For example,
Figures 1 and 7 differ by light guide 6 which enables the
collected light to be more efficiently inputted and transmitted
through fiber optic 7, thereby, increasing the amount of usable
light at the distal end of fiber optic 7.
Different optical devices may suitably serve as the light
guide 8 in embodiments of the present invention.
The use of a tapered cladded rod as the transforming device
8 in the current invention provides for optimal transformation
of the light's angular distribution. The spatial distribution
conversely is not optimized because the output from a cladded rod
is typically not uniform and is comprised of concentric rings of
light. However, if the final output device 7 is a randomized
fiber bundle, the light is scrambled at the output of the fiber
bundle and there is no negative consequence of having an input
that is nonuniform in spatial profile. A tapered fused bundle
can alternatively be used as the light guide, but a tapered fused
bundle is less efficient in the transmission of light to the
final target for the same length of glass as a tapered rod.
However, the output from the tapered fused bundle is spatially
randomized and more uniform. Therefore, the light from a tapered
fused bundle acting as the transforming device 8, or light guide,
is more readily coupled into a large diameter single fiber to
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produce a uniform output from the single fiber. If a shorter
fused bundle taper is used, the overall transmission loss can be
minimized. A fused bundle taper provides better spatial
uniformity for a shorter length because the small diameter,
typically less than 80 microns, of the individual fibers of the
fused bundle taper transform the angular and spatial profile
within approximately 30 diameters of the individual fiber.
A tapered hollow reflective tube, or a CPC, can also be
employed as the light guide in embodiments of the present
invention. The smaller aperture of the tube or CPC would be
placed at the target spot such that the light is transformed to
a NA and an output diameter approximately equal to that of the
output device.
This class of light guides can be coated to reflect only
certain wavelengths of light, such as with a multi-layer
dielectric coating. A coated tapered hollow reflective tube or
coated CPC would then provide the user with the ability to filter
unwanted light if the collecting and condensing system employed
did not have this capability. The output from a tapered hollow
reflective tube and a CPC normally has a non-uniform spatial
profile.
Another embodiment of a light guide of the present invention
is depicted in Figure 4. A negative lens 11, when used as the
light guide, redirects rays rs and r6 to the output device 7 such
that the rays are deflected more to the normal of the target spot
surface. In the preferred embodiment, a lens with a leading flat
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face is used. This deflection results in more efficient coupling
due to the smaller NA and larger spot size. Negative lenses used
as the light guide, like tapered cladded rods and tapered fused
bundles, can act as filters of unwanted wavelengths of light.
When using a lens as the light guide, it should be noted that the
light emanating from the guide will be spatially non-uniform and
may contain spherical aberrations . Use of a negative lens in
combination with a fused bundle will improve the spatial
uniformity.
In alternative embodiments of the present invention, a fused
bundle or cladded cylindrical rod having an NA and diameter
similar to that of the output device can be placed between the
light guide and output device such that the light from the light
guide is transferred through the rod or bundle to the output
device. A design incorporating either of the two would have
advantageous practical implications. The spatial profile from
the output of a fused bundle of optical fibers is uniform, even
if the input profile was non-uniform. Therefore, when a light
guide which produces a non-uniform spatial profile is used, such
as, for example, a tapered cladded rod, a negative lens, or
tapered hollow reflective tube, such a fused bundle can provide
a uniform input to the fiber optic output device. A cladded rod
used for such a purpose would be especially advantageous if the
fiber optic output device was particularly sensitive to heat, and
therefore needed to be removed from the heat present at the
target spot.
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One skilled in the art will appreciate that the particular
type of light guide employed in embodiments of this invention
will vary according to the purpose and the particulars of the
output device and condensing system, including; whether light
filtering is desired, whether the fiber optic output device is
particularly sensitive to heat, whether a uniform spatial profile
is necessary, and whether the specific system has size
restraints.
For maximum collection of light at the target spot 6 in the
embodiment shown, two conditions are preferred: (i) the input
diameter of the target spot 6 must be at least two (2) times the
length of the arc gap to assure collection of greater than 800
of the total light at the target spot 6 and (ii) the numerical
aperture ("NA") of the primary collection system at the target
spot 6 should be maximized. The latter is accomplished by using
a primary collector with the largest possible NA.. However the
output device 7, such as a single fiber or a fiber bundle, may
have a lower NA than the NA of the primary collector. For
example, the light at the target spot coming from the primary
collector might have an NA from 0.7 to 0.8, and the output fiber
or bundle about 0.5 which is typical of fiber bundles. This
mismatch in NA, if the light is coupled directly to the output
fiber, will result in a large loss of light and the generation
of unwanted heat. In the preferred embodiments of this invention
discussed below, a transforming device 8, in the form of tapered
fused bundles and tapered cladded rods, transforms the large NA
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light emerging from the primary collector into a smaller NA as
shown by rays rl and r2 in Figure 1.
From basic optics, if the diameter of a light guide is
increased along its length through tapering of the glass, the
5 angle of illumination, 8, will decrease, and thus the numerical
aperture, will also decrease. Therefore, by tapering a fused
bundle or cladded rod from a smaller input area into a larger
output area, the angle of illumination is adjusted to match that
of the output device 7. In terms of angle of illumination, 8,
10 diameter of the fiber optic cross section, d, and numerical
aperture, NA, the inherent relationship:
NA1 x dl = NA2 X d2 ( 1 )
where NAi = sin ( 6i/2 ) ( 2 )
applies. In the present invention, as depicted in Figure 1,
IS relationships (1) and (2) are being manipulated by the light
guide to optimize the amount of light for the NA and diameter of
the fiber optic output device.
The output of the lamp 1 is imaged to the target spot 6
using any known means, such as a spherical concave, toroidal, or
ellipsoidal primary mirror systems. In the preferred embodiments
of the invention, best results are obtained by having a 1:1
imaging system, such as the prior art off-axis configuration
shown in Figure 7, because of the increased flux density it
provides at the target. For light collecting and condensing
systems that do not produce a 1:1 image of the light source such
as an arc lamp, the spot must be small compared to the size of
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the target to incorporate the advantages of this invention as
described below. In general, the type of light collection and
imaging system is used is often determined by the size and
dimensions of the target, the size and type of light guide, or
the diameter and type of fiber optic output device, and all of
their respective numerical apertures.
The light collection and imaging system illustrated in
Figure 1 utilizes a concave toroidal reflector in off-axis
configuration and produces approximately a 1:1, or unmagnified,
image of the arc. However, because of inherent optical
aberrations of such a 1:1 imaging system, maximum collection
efficiency is achieved if the input cross section diameter of the
optically transforming device, or light guide, is two to three
times the size of the arc gap of the lamp. To collect as much
total light as possible, the numerical aperture of the off-axis
reflector is made as large as possible. For example, in an off-
axis system like that in Figure 1, the NA is typically designed
to be about 0.7, which produces a cone of light having an
approximately 90 degrees solid angle. A larger numerical
aperture system is possible and is only limited by the mechanical
layout of the components. This angle is indicated as 61 in
Figure 1. To increase the output further, a retro-reflector 5
is placed behind the lamp directly opposite the primary mirror.
The retro-reflector will reflect light back through the lamp and
focused through the arc, increasing the luminosity collectible
by the primary mirror and increasing the total output at the
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image point, the location of the target spot 6. To allow maximum
coupling of light into a plastic fiber without damage, such as
by heat for example, a fused bundle can be placed between the
tapered cladded rod or tapered fused bundle and the input of the
plastic fiber. If a tapered cladded rod is the optical
transforming device, the fused bundle also facilitates scrambling
the transmitting modes so as to produce a more uniform output for
coupling to a single plastic fiber or a fiber bundle.
Aside from the use of a single large target, the invention
also facilitates the more efficient coupling and transmitting of
high intensity light through multiple fibers as the target. This
not only includes a fiber bundle of hundreds or thousands of
small diameter fibers, approximately 50 microns in diameter, but
also bundles of larger fibers that can transmit sufficient amount
of light for use in applications ranging from surgical
illumination to commercial display lighting. As with a single
fiber target, a multiple fiber target comprised of glass, quartz
or plastic single fibers can be coupled directly, or, depending
on the output from the particular type of light guide used,
through intermediary fused bundles for minimizing damage to the
fiber target. Typical fiber optic output devices can vary from
a fiber bundle, comprised of small diameter optical fibers
typically less than 80 microns diameter, to a single large
diameter fiber optic typically made of plastic. For a target
with multiple fibers where each fiber has a cross-sectional area
of A(f), the total number of fibers of in the bundle is
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necessarily less than the number obtained by dividing the cross
sectional area of the output of the bundle by A(f).
Other embodiments of the present invention can be directed
toward directing light to a plurality of fibers as the fiber
optic output device wherein each fiber is typically greater than
0.1 mm diameter and less than 5 mm in diameter. This further
embodiment of the invention provides for a distributed fiber
optic lighting system wherein maximum light through each fiber
optic is achieved by transforming the numerical aperture of the
l0 light collection system to match that of the individual fiber
optics. In addition, the use of either a tapered fused bundle
or a tapered cladded rod in conjunction with a fused bundle
provides a nearly uniform output for coupling approximately the
same amount of light into each individual optical fiber in the
output bundle.
Intrinsically, a tapered cladded rod is mor« efficient in
overall transmission than a tapered fused bundle. On the other
hand, a tapered cladded rod requires a longer length to transform
the NA completely than a tapered fused bundle, and requires a
much longer length to scramble the modes of the rod. That is,
the taper-length of a cladded rod, required for both changing the
NA and scrambling the modes to produce a uniform output, is
substantially longer than that required to change the NA only.
The small diameter of the individual fibers in a tapered fused
bundle, typically less than 80 microns, transform the angular and
spatial profile within approximately 30 diameters of the
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individual fiber. By contrast, a tapered cladded rod requires
a much longer length to change both the numerical aperture and
to produce a spatially uniform output.
Since relative to a cladded rod, a fused bundle is less
efficient, the application of either as the light guide in
embodiments of this invention will depend upon the dimensions of
the fiber optic output device and the layout of the primary
collector system. The final numerical aperture and overall
efficiency for the transformation in the case of either a tapered
fused bundle or tapered cladded rod is determined according to
simple optical geometry, and varies according to the taper angle
and length over which the taper occurs.
Given that the light source for each embodiment has a broad
spectral output, wavelength discrimination is achieved in the
invention through the use of dielectric coatings applied to the
primary reflector of the light collection system and/or to either
the input or output surface of the light transforming devices.
Example 1
Employing an off-axis imaging system having a 1:1
magnification, such as the one depicted in Figure 1, impacts upon
the choice of the rest of the components. Because the primary
mirror has a large angle of collection, the target image
inherently experiences astigmatism and other optical aberrations
which cause the image to be necessarily larger than the size of
the arc gap. Maximum collection efficiency is achieved in a 1:1
imaging system if the input diameter of the optically
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transforming device 8 is two to three times the size of the arc
gap of the lamp and the input numerical aperture of the
transforming device is similar to the numerical aperture of the
of the incident light at the target spot. In Figure 1, the NA
5 of the off-axis imaging system is approximately 0.7 and the NA
of the optical transforming device is 0.66 or larger.
If the output device is 12 mm diameter single core plastic
optical fiber with a 0.6 NA, any imaging system that produces a
small focused spot that is less than approximately 6 mm would be
10 suitable. For the 1:1 magnification present in an off-axis
imaging system, a lamp with an arc gap of approximately 3 mm
would be suitable to assure at least 80o collection of light at
the target spot taking into account optical aberrations in the
system which blur the image.
15 In general terms for this embodiment, the diameter, d3, of
the output fiber optic device should be approximately equal to
or greater than the output diameter, d2, of the tapered light
guide and the input diameter of the tapered light guide, dl,
which is less than d2 and d3, must be approximately 2 times the
20 length of the arc gap (or roughly equal to the inherent image
spot size to arc gap ratio for some other type of imaging system
of unspecified magnifying properties). In addition, the NA of
the output fiber optic, NA3 should be about equal to NA2, the
output NA from optical transforming device 8, and the input NA1,
greater than NA2, should be similar to that of the light
collection system to produce optimum overall efficiency. In
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addition, the taper angle and length of element 8 is determined
by equation 1.
Example 2
Given that the maximum collection efficiency of this
invention depends on both the collecting and condensing/imaging
optics and the design of the light guide, there is a family of
configurations or preferred embodiments that will increase the
amount of collected light transmitted through a fiber optic
target depending on the size of the target. In an off-axis
configuration, to obtain higher collection efficiency from the
primary mirror requires that the effective NA of the primary
mirror be increased. However, by increasing the solid angle over
which light is reflected to the target, some rays will be
magnified and some will be demagnified instead of imaged 1:1 as
shown in Figure 2. For example, the ray r3 as shown in the
figure has the reflection point on the mirror closer to the lamp
1 than the target spot 6 and this will give a magnified image on
the target. Ray r9, as shown, has the reflection point at the
mirror closer to the mirror than the lamp, will give a
demagnified image. The overall image size, composed of the sum
of all the rays, will increase the overall spot size from 1:1.
To compensate for the increase in image size requires that the
diameter of the input of the tapered rod or fused bundle be
increased to maximize collection efficiency and should typically
be somewhere between about 2 to about 3 times the length of the
arc gap of the light source. Therefore, a non-uniform imaging
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off-axis optical system such as that in Figure 2 having partial
magnification of up to 3:1 with a lamp arc gap of 2 mm would
produce a target spot diameter of approximately 6 mm, assuming
no aberrations and require a 6 mm input for the tapered rod.
Example 3
Another way to collect and utilize light over a larger
collection angle is to use an extended elliptical reflector as
shown in Figure 3. Using this configuration, the majority of the
light is collected by the reflector, but the magnification is not
1:1. Typically, such a configuration will have a magnification
of no less than 3:1. The NA of light at the target in this case
is still too large, about unity, to be coupled into a large
diameter target, such as a fiber bundle or a large single plastic
fiber each with an NA of about 0.5 to 0.6. In prior art systems
incorporating elliptical collecting and condensing reflectors,
the reflector is truncated and does not inclu~.'.e the bolded
portion of the reflector 3a in Figure 3. Light from the bolded
portion in prior art systems cannot be used because the light
collected from the high NA portion would be of too high an NA and
will not couple into typical fiber targets having NA typically
around 0.6 or smaller. In this embodiment of the invention,
transformation of a high NA light to lower NA with a tapered
cladded rod or tapered fused bundle such as the light guide 8
allows additional light flux, transformed from higher to lower
NA, to be coupled into the fiber optic target. Again, the input
diameter of the tapered light guide would have to be larger than
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the arc gap of the source, typically at least three times larger,
for such a configuration.
Example 4
Light transmission through the target is optimized if the
output numerical aperture of the tapered light guide is less than
the numerical aperture of the fiber optic output device. The NA
of the output optical fiber is related to the input NA of the
tapered light guide by relationship (1), and the input NA of the
tapered light guide is typically equal to or less than that of
the optical collecting and imaging system. The length of the
tapered optical transforming device is determined by ratio of
input and output NA's of the device and whether a fused bundle
or cladded rod is tapered. In either case, the input NA of the
tapered light guide must be at least equal to the NA of the
primary collector system at the target spot for maximum
collection efficiency at the target.
For example, a 5-inch long tapered cladded rod is used as
a light guide. The tapered cladded rod has an input diameter of
about 2.5 mm and an output diameter of about 4 mm. This rod
transforms light with an input NA of about 0.7 (such as from a
primary collection system as described in Example 1) to an output
NA of about 0.45. This output light couples efficiently to an
output fiber optic bundle having a 5 mm diameter and NA of 0.5.
Compared to a cladded rod having no taper, the increase in output
through the output fiber bundle is about 15o and can be increased
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further by dielectrically coating the input and output ends of
the taper with an anti-reflection coating.
Example 5
In another embodiment, a tapered fused bundle having an
input end diameter of about 6 mm and an output end diameter of
about 10 mm is used to couple light from a small arc lamp into
a large optical fiber core, approximately 12 mm in diameter.
Compared with a fused bundle without taper, the output from the
optical fiber core increases by 220.
The invention having been thus described, it will be
apparent to those skilled in the art that the embodiments of the
invention may be varied and modified in many ways without
departing from the spirit and scope of the invention. Therefore,
any and all such modifications are intended to be included within
the scope of the following claims.
