Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method and Apparatus for Scanning Optical Delay
Line
(01) The present invention relates in general to optical interferometric
systems, and in particular to scanning optical delay lines of an
interferometric
system.
(02) Interferometric systems are deployed in a wide and growing number of
applications. Typically, interferometric systems involve two arms, a beam
splitter and a beam combiner. A beam of light incident the beam splitter is
divided in two: one part of the beam is directed down each of the arms. The
two parts are then recombined at a beam combiner. If the parts of the beams
are out of phase with respect to each other, they will destructively
interfere,
resulting in an attenuated recombined beam. if the parts of the beams are in
phase, they will constructively interfere, and the recombined beam will
maintain (substantially) the power of the incident light beam. If the incident
light beam emanates from a broadband source with a finite coherence length,
interference phenomena only occur if the path length difference between the
two arms is smaller than the coherence length. Typically, one of the two
arms, the reference arm, is set to a desired path length, using a scanning
optical delay line for example, to investigate a sample placed in the other
arm,
the sample arm, at a given path length position. In many applications the
optical path length of the reference arm is made to vary with a pre-
established
periodic manner. Based on the interference observed in the recombined
beam, a feature in the sample can be determined i) within an accuracy of a
fraction of wavelength if the phase information is used, or ii) with an
accuracy
of the coherence length if only the coherence properties are investigated.
Accordingly, interferometric systems are used in many situations for pulse
autocorrelation, ranging, profiling, and imaging, among many other
applications.
(03) Important parameters for scanning optical delay lines are: a scan
range, i.e. a distance over which the optical path length of the reference arm
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varies, a scan velocity i.e. a rate at which the optical path length of the
reference arm may be varied, a duty cycle that determines the fraction of time
over which the scanning optical delay line provides a usable, controlled,
variation in optical path length, and a linearity of variation of the optical
path
length. The first three parameters determine a scanning repetition rate of the
optical scanning optical delay line, i.e., the number of cycles of the
periodic
variation required per unit of time to achieve a specified data output rate.
The
linearity directly impacts a quality (e.g. signal-to-noise ratio (SNR)) of an
optical output of the interferometric system. Additional parameters to take
into
account in the design of an optical scanning optical delay line are dispersion
effects, polarization effects, and optical power loss. Dispersion and
polarization effects can impact the precision of OCT measurements, but can
be corrected using known mechanisms. Optical power loss is an additive
property that limits the optical path length and number and kind of optical
devices that can be included in the arms and still obtain a detectable signal
(i.e. a signal of a high enough quality). For the mass production of scanning
optical delay lines and for continuous use in medical or industrial
environments, important additional criteria are the ease of alignment of the
interferometric system and the beam, and the robustness, i.e. an ability for
adequate alignment to be maintained in spite of vibrations or other motion of
the beam, or the interferometric system.
(04) Development of scanning optical delay lines has been an active field of
research recently, especially in the field of Optical Coherence Tomography
(OCT) where systems providing high resolution, real-time (high data rate)
imaging are required. Recently developed scanning optical delay lines for
OCT measurements inherit from all the developments previously performed in
the other application fields and thus provide a good overview of the current
state of the art. A detailed review of scanning optical delay lines for OCT
measurements has been recently published by Andrew M. Rollins and Joseph
A. Izatt (in Handbook of OCT, edited by B.E. Bouma and G. J. Tearney,
published by Marcel Dekker Inc., New-York, 2002, p.99).
(05) OCT measurements are generally performed with a scan range of a
few millimeters, and require a repetition rate of at least a few kilohertz to
allow
real-time imaging. Typical OCT scanning optical delay lines are continuously
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scanned, and retroreflecting, meaning that the light is delivered and
collected
by the same optics. The optical scanning optical delay lines used in OCT can
be categorized in five categories:
~ linear translation of retroreflective elements;
~ galvanometer-mounted elements;
~ uniformly rotating elements;
~ optical fiber approaches; and
~ use of a diffraction grating.
(06) The simplest design of a scanning optical delay line is obtained from
the mechanical translation of a retroreflective element, as taught, for
example
by Huang et al., in Science, 254,1178 (1991 ). Other simple systems are
based on a galvanometer-mounted retroreflector as taught by Izatt et al., in
IEEE Selected Topics Quantum Electron, 2,1017 (1996). For scanning
ranges of the order of a few millimeters like those usually required in OCT,
such systems are limited to repetition rates of the order of 100 Hz, which is
too low for real-time imaging. Additionally, such systems also require
acceleration and deceleration of a given mass impacting robustness and
linearity. Higher repetition rates can be obtained with a galvanometer in a
resonance mode, but at the cost of a higher nonlinearity and lower duty cycle.
(07) Higher stability and higher repetition rates can be obtained from the
use of uniformly rotating elements since high-speed rotating motors with high
rotation stability are commercially available. Examples of such designs are
the use of the reflection from the side of a multi-segment CAM (as taught in
US Patent 6,191,862 to Swanson et al.) or from the surface of a helicoidal
mirror (US Patent 5,907,423 to Wang et al.). These can attain high repetition
rates in the kHz range, good linearity, and high duty cycles. Unfortunately
such designs require careful machining and alignment.
(08) Another design relies on the use of rotating parallel mirrors (US Patent
6,243,191 to Fercher). It requires a careful assembly to ensure the
parallelism
of the mirrors, but once assembled, this unit is very easy to align. High
repetition rates are achievable, however the system taught by Fercher suffers
from non-linearity and a low duty cycle. Still further examples are based on
the use of a cube or octagon rotating around its center-of-mass (US Patent
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6,144,456 Chavanne et al.), on the use of an ensemble of prisms on a rotating
disc on a rotating belt (US Patent 6,407,872 Lai et al.), or on the use of a
rotating parallelogram prism [Giniunas et al., Applied Optics, 38, 7076
(1999)].
These designs suffer from one or more of the following: low-duty cycle,
nonlinearity, difficult alignment, and lack of robustness.
(09) Some designs are based on the use of fibers. One such approach is
based on the stretching of a fiber winded around a piezoelectric plate or
cylinder whose expansion induces an scanning optical delay line in the fiber,
as in Tearney et al., Optics Letters, 21,1408 (1996). Such a design can
achieve high scanning rates but suffers from high power requirements, poor
mechanical and temperature stability, and induced birefringence effects.
(010) A scanning optical delay line based on the use of a diffraction grating
was first proposed by Kwong [Kwong et al., Optics Letters, 18, 558 (1993)]
and later improved by Tearney [Tearney et al., Optics Letters, 22, 1811
(1997)] which was patented (US Patent 6,282,011 ). The design involves a
"double-pass" optical arrangement usable in retroreflective configuration,
which makes the already complex setup even more so. The optical alignment
is delicate because many parameters must be considered simultaneously:
beat frequency, distance from a focal point of lenses, dispersion
compensation, and optical delay. Mechanical stability may be exceedingly
difficult for use in an industrial environment or for achieving high accuracy.
The optical path length is fairly long (requiring a considerable coherence
length of the incident light beam) and the number of optical components
makes the design difficult to miniaturize. Furthermore an amplitude of the
output signal varies as the mirror moves away from the focal point, posing
another constraint on the design.
Summary of the invention
(011) Accordingly it is an object of the invention to provide a scanning
optical
delay line providing a good performance in terms of repetition rate,
linearity,
and duty-cycle. As such, the scanning optical delay line may be suitable for
application in the context of OCT measurements, but its application is not
limited to that field.
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(012) In accordance with an aspect of the invention, a scanning optical delay
line is provided that includes an optical path element rotated about an axis
that is directed generally orthogonal to an incidence line in order to vary an
angle between the incidence line and a front of the optical path element. The
structure rotates substantially uniformly, so that no angular acceleration or
deceleration is applied during normal operation. A constant angular velocity
improves robustness and longevity of the scanning optical delay line. The
optical path element provides a substantially linearly varying optical path
length for an incident beam received along the incidence line as a function of
angle. Naturally the line of incidence intersects a circular arc swept by any
point on the optical path element during a fraction of each cycle of rotation.
It
is during a part of this (first) fraction of the cycle that the optical path
element
intersects the incidence line at a range of angles and radial offsets that
provides the substantially linearly varying optical path length. Outside of
this
fraction of the cycle the line of incidence does not meet the optical path
element.
(013) The incidence line extends between a beam source and a reflector that
reflects a beam transmitted on the incidence line outside of the first
fraction of
the cycle onto a reinsertion line. The reinsertion line passes a similar
distance
from the axis of rotation as the incidence line so that in use the reinsertion
line
defines a second fraction of the cycle during which the reflected input beam
is
inserted into the optical path element. As will be appreciated by those of
skill
in the art, the reflector may include one or more surfaces at which the beam
may be redirected by reflection, total internal reflection or refraction.
(014) First and second ends for the optical scanning optical delay line are
provided for receiving a beam of light transmitted through the optical path
element during the first and second fractions of the cycle, respectively. The
ends may be retrorefiectors, or transmission elements.
(015) Reinsertion of the optical beam into the optical path element aims at
increasing the duty cycle by reusing the beam when it is not intercepted by
the optical path element along the incidence line. The beam is redirected by
the reflector towards the optical path element in a direction substantially
orthogonal to, and at a distance from, the rotation axis such that the optical
path length is again varied upon rotation. In some configurations the beam
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can be reinserted more than once, thereby further increasing the duty cycle
and repetition rate. Additionally the reflector and ends of the scanning
optical
delay line can be positioned in such a way that the center of the scan range
can be different for each reinsertion. Consequently, at each revolution of the
optical path element, scanning ranges centered on different path length
values can be covered, which effectively increases a scanning depth of the
apparatus.
(016) In certain embodiments of the invention, the optical path element
includes two planar parallel reflectors arranged to enclose a transmission
medium in the shape of a parallelogram prism. The parallel planar reflectors
are oriented in a direction substantially orthogonal to the axis of rotation
to
form side walls of the parallelogram optical path element. In some
embodiments, the parallelogram optical path element is defined by two
parallel mirrors that enclose air, and in other embodiments the parallelogram
optical path element is defined by a solid prism of a given refractive index.
If
the solid prism is used, side walls of the solid prism may be metallized to
ensure total reflection. The set of faces of the solid prism used for
refraction
and reflection are substantially parallel. The degree of parallelism required
for
the good operation of the scanning optical delay line can currently be
obtained
with commercially available elements.
(017) It should be noted that a confusion of language exists in relation to
the
term 'prism' in that it is commonly taken to mean both a geometrical form
(i.e.
a shape of a class of regular solids), and an optically dispersive medium.
Herein 'parallelogram prism' is used to refer to the geometrical form that has
a
surface that consists of parallel top and bottom parallelogram bases that are
interconnected by rectangular faces, expressly without the presumption that
the parallelogram prism is a solid, dispersive, medium. In contrast, the term
'prism' as used herein refers to a solid dispersive medium, which in the
context of the invention assumes the configuration of a parallelogram prism.
(018) The incidence and reinsertion lines are separated from the axis of
rotation by a distance that provides for intersection of an acute corner of
the
parallelogram optical path element and not an obtuse corner of the
parallelogram optical path element during the rotation. In other words, the
incidence and reinsertion lines are separated from the axis of rotation by a
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distance intermediate one half a major diagonal of the parallelogram, and one
half a minor diagonal of the parallelogram. In such configuration, a beam
input on the incidence or reinsertion line enters a front of the parallelogram
optical path element, reflects off each of the side walls once, and exits the
parallelogram optical path element at a back of the parallelogram optical path
element in a direction parallel to the incidence or reinsertion line for a
significant part of a fraction of the cycle of rotation of the parallelogram
optical
path element.
(019) The fact that the optical path length of an input beam, as it traverses
the parallelogram optical path element is independent of the position it hits
the
front of the parallelogram optical path element (as long as the beam meets
the front of the prism within a range of angles and positions at which it
undergoes internal reflection off of each of the side walls exactly once), and
therefore depends only on an angle between the front and the incidence or
reinsertion line, can provide a distinct advantage in the context of this
invention. The position independence can significantly improve a robustness
of the system and facilitate alignment because specific alignment with respect
to the incidence and reinsertion lines are not necessary.
(020) In certain embodiments of the invention, a plurality of parallelogram
optical path elements arranged in rotational symmetry around an axis of
rotation are used to further improve a duty cycle of the scanning optical
delay
line. In these embodiments the parallelogram optical path elements are
arranged so that a beam exiting the back of one parallel to the incidence or
reinsertion line on which it entered, does not encounter any of the other
parallelogram optical path elements.
(021 ) Rotation of the parallelogram optical path elements around an axis not
centered on its centroid provides additional freedom in the choice of
parameters that can be selected to improve the angular range over which the
optical beam intercepts the structure and exits parallel to its initial
direction,
for example. It also provides freedom to reduce the nonlinearity of the
optical
scanning optical delay line while maintaining a duty cycle. The duty cycle is
also improved by the number of parallelogram optical path elements used.
This embodiment can provide a high sampling rate making the system on par
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with high-end state-of-the-art scanning optical delay lines, but has greater
robustness, and ease of alignment.
(022) To further improve robustness, some embodiments include a
synchronization system for time gating an output of the optical scanning
optical delay line. The synchronization system may include a sensor that
identifies an angular velocity and position of the one or more parallelogram
optical path elements. To achieve a higher accuracy, each front of the
parallelogram optical path elements) that intersect the reinsertion and
incidence lines can be characterized and the angular position is used to
indicate which of the calibrations to apply to each coherence sample. One
calibration for each face of the parallelogram optical path element at which
the
beam is incident, for each insertion line is ideal. Independent calibration of
each insertion increases robustness and ease of alignment of the optical
system since all the parallelogram optical path elements do not need to be
placed perfectly in the same rotation symmetric orientation or the
parallelogram optical path element does not have to rotate about its exact
centroid, and the shape of the parallelogram optical path elements) doles)
not have to be perfect. Small differences in the angle of incidence can be
accounted for by appropriate time-gating, and small differences in dimensions
of the parallelogram can be accounted for by the use of independent
calibration curves. The calibration curves may relate the angular position of
a
rotating surface that holds fast the parallelogram optical path element(s), to
the optical path length. If, for some reason, the rotating surface becomes
deformed, or the parallelogram optical path elements move after long-term
use, changes to the calibration curves can be readily determined to ensure
the precision of the optical scanning optical delay line over time.
Alternatively,
because commercial prisms can be bought with very close dimensional
tolerance, the same calibration curve can be used for each insertion line,
provided appropriate time-gating is performed.
(023) In addition to achieving efficiency on par and even exceeding current
state-of-the-art scanning optical delay lines, the invention can provide
improved ease of alignment and robustness, parameters that are desirable for
mass-production and long-term problem-free use.
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(024) One advantage of using a prism as the parallelogram optical path
element is improved linearity, and one advantage of using parallel mirror
configuration of the parallelogram optical path element is a reduction in
dispersion. Dispersion can also be minimized by appropriate selection of the
material of which the prism is fabricated.
Brief description of the drawings
(025) A better understanding of the operation and advantages of the
invention is afforded by the detailed description and the following drawings,
in
which a common set of references numerals are identified:
(026) FIG. 1 is a schematic plan view of an optical path through a prism
mounted for rotation about its centroid;
(027) FIG. 2 is a graphical representation of optical path length lp as a
function of an angle of incidence in accordance with the embodiment of
FIG. 1;
(028) FIG. 3 is a graphical representation of a variation of a path length
difference with the angle of incidence ( dl p l dB ) in accordance with the
embodiment of FIG. 1;
(029) FIG. 4 is a graphical representation of a range of angles of incidence
over which the transmitted beam exits a prism as a function of separation of
an incidence line from an axis of rotation L;n;
(030) FIG. 5 is a graphical representation of an optical path length variation
resulting from a usable angular range as a function of the distance L;n:
(031 ) FIG. 6 is a graphical representation of a percentage of variation of
the
derivative dh /dB over the usable angular range as a function of the distance
Lin:
(032) FIGs. 7a and 7b are two schematic plan views of an embodiment of a
scanning optical delay line using a single prism rotating around its centroid
showing insertion on an incidence line, and a reinsertion tine respectively;
(033) FIGs. 8a and 8b are two schematic plan views of an embodiment of a
scanning optical delay line with five prisms distributed along the
circumference of a disk showing insertion on an incidence line, and a
reinsertion line, respectively:
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(034) FIG. 9 is a graphical representation of optical path length as a
function
of angular position B of a prism for the embodiment shown in FIGs. 8a,b:
(035) FIG. 10 is a graphical representation of a variation of the derivative
dl~ /d8 as a function of the angular position B of a prism for the embodiment
shown in FIGs. 8a,b:
(036) FiG. 11 is a schematic plan view of an embodiment of a scanning
optical delay line using a single pair of parallel planar mirrors rotating
around
its centroid showing insertion on an incidence line, and a reinsertion line,
respectively;
(037) FIG. 12 is a schematic plan view of the embodiment of FIGs. 8a,b with
the addition of a synchronization detector; and
(038) FIG. 13 is a schematic side view of the synchronization detector of
FIG. 12; and
(039) FIGs. l4a,b is a schematic partial side view of the embodiment of
FIG. 7a,b and an alternative double pass embodiment.
Detailed description of the preferred embodiments
(040) The invention provides a scanning optical scanning optical delay line
for an interferometric system. The scanning optical scanning optical delay
line uses reinsertion to provide a higher duty cycle and/or greater linearity,
in
an application that can provide a high scan rate for optical coherence
tomography applications.
(041) In the context of this invention, it should be noted that arrangements
of
optical devices, mechanical devices etc. are inherently imperfect. When
Applicant refers to geometric idealizations lines, planes, directions,
orthogonality, planar surfaces, parallel lines, etc., these are only achieved
in
limited approximation in operative embodiments, and the person of ordinary
skill will understand that these terms are only intended to be limiting within
reasonable limits.
THEORY
(042) FIG. 1 illustrates a prism 10 rotating in an x-y plane around an axis 11
passing through its centroid. The axes x and y define coordinates (lower case
letters) in the reference frame of the prism 10. The axes X and Y (capital
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letters) define coordinates in a reference frame of the laboratory. The
angular
position of the prism 10 is determined by an angle 8 between the axes x and
X, defined positive in the counterclockwise direction from the X axis. The
prism 10 is characterized by a characteristic angle 8p, dimensions b and c, a
height d (not in view) and a refractive index nP. The faces of the prism 10
are
identified as l0a,b,c,d,and e. Face 10a is serving as a front, face 10c is
serving as a back, and faces l0b,d are serving as side walls that provide
internal reflection of an incident beam 1. The prism 10 also has a top
parallelogram base 10e and a bottom (not in view) parallelogram base that
has the same shape as the top parallelogram base 10e.
(043) In such a configuration, the incident beam 1 propagating at a fixed
distance L;~ from the rotation axis 11, the exiting optical beam is parallel
to its
initial direction when the prism 10 is oriented in a specific, relatively
small,
angular range if the beam is incident at a range of distances L;~ that varies
between one half a minimum diagonal dm and one half a maximum diagonal
dM of the parallelogram bases from the rotation axis 11. It will be understood
herein that the rotation axis 11 is perpendicular to a plane in which the
beam 1 is transmitted, and that accordingly the distance to the rotation
axis 11, is a distance between the nearest points on a line the beam 1
follows,
and the axis 11.
(044) This angular range is covered twice per revolution as the front and
back faces (l0a,c) alternate during rotation. Outside the allowable range of
angular values, one of two events occurs: the beam exits in a direction
different from its initial direction, or the beam is not intercepted by the
prism.
(045) It will be appreciated by those skilled in the art that it is only when
the
optical path length is substantially linearly varying that the optical delay
path is
operating, and outside of the angular range, the light is not useful for
correlation. The duty cycle of the optical scanning optical delay line is
therefore tied to twice the angular range in this embodiment. Additionally,
limited linearity of the resulting scanning optical delay line is possible at
the
expense of shortening the duty cycle. The nonlinearity is evaluated below by
computing the variation in percentage of the variation of optical path length
with the incidence angle ( dln l dB ) over the range covered by the delay
line.
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(046) The beam 1 first encounters a face 10a with an incidence angle y, and
is refracted with an angle z According to Snell's law:
z = arcsin lsin( y) / hp ~ (1 )
(047) After a single reflection on each of faces 10b and 10d, the beam 1 is
refracted again through face 10c and exits parallel to its initial direction
and at
a distance Lost from the rotation axis 11. The angles y and z in FIG. 1 are
defined positive in a counterclockwise direction from a normal of the front
face
1 Oa. For the case depicted in FIG. 1, 8 < 0 , y > 0 , and y = -8 . The
optical
path length IP relative to that in absence of the prism, is given by the
expression:
cos(2z)sin(6 ) 2
cos(z) + b sin(Bp ) cos(z + 9P ) - c cosy) + c + b cos(z + BP )P tan() ( )
(048) The optical path length iP in Eq. (2) only depends on the properties of
the prism 10 and on the orientation of the prism relative to the incoming beam
1. It is independent of the entry point of the beam 1, as long as the beam 1
is
intercepted by the prism 10, and exits the prism 10 parallel to its initial
direction after undergoing two internal reflections. Because of this entry
point
independence a scanning optical delay line can be made that provides robust
operation, and easy alignment.
(049) In most of the embodiments discussed herein, the prisms are rhombic
prisms (i.e. having sides of equal length), chiefly because of their
availability.
However as the equation 2 shows, any prism having the shape of a
parallelogram prism (i.e. for any values of b, c, and 8p) can be used.
(050) The conditions for Eq. (2) to apply can be expressed in allowable range
of values for the coordinates xo and x3 of the entry and exit points in the
reference frame of the prism. The coordinate x3 is given by:
cos(2z)sin(Bp)
x3 = xo - c tan(z) - b cos(z) cos(z + 6 p ) ~ ( )
(051) Conditions on the xo and x3 coordinates are:
t b - c + c tan(2) < x o < 1 b + c (4)
2 tan(9 ~ ) 2 tan(6 n )
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- 1 b + c + c tan(2') < x3 < 1 b - c (5)
2 tan(8~) 2 tan(8~)
(052} FIG. 2 shows the variation in optical path length as a function of the
angle y for a beam 1 propagating at a distance L;~= 3.5 mm from the axis of
rotation 11 for a prism with np= 1.5, c= 5 mm, b=7.07 mm, and 6p=45°.
The
optical path length is evaluated relative to that in absence of the prism. A
negative value of -5 mm for the path length indicates that the beam does not
intercept the prism, while a negative value of -2.5 mm indicates that the beam
is intercepted by the prism 10 but does not exit parallel to the incoming
beam.
It will be appreciated that for a range of angles between about -30°
and about
27° there is a monotonic rise in optical path length, from about 8.5 mm
to
about 16.5 mm.
(053) FIG. 3 illustrates the derivative of the optical path length as a
function
of the angle y. Ideally, this variation would be constant, indicating that the
optical path length varies linearly with the angular position 8. However, FIG.
3
clearly shows that the variation in the derivative is quite substantial for
the
specific case if one considers the whole angular range available.
(054) As noted above, the optical path length I~ varies only with the angle
between the beam 1 and the front face 10a, and not with the distance L;~, but
the distance L;~ determines the angular range over which the beam enters and
exits the prism correctly. We thus now consider the prism rotating around its
center of mass for various distances of the incoming beam 1 from the rotation
axis.
(055) FIG. 4 is a graph showing transmission properties of the incident beam
as a function of angular position 8, and separation (L;n) from the rotation
axis 11. In the blank region, the prism 10 does not intercept the beam 1. In
the darkest region the beam enters the prism 10 and exits parallel to its
initial
direction. The intermediate (light gray) region corresponds to the case where
the beam 1 is incident the prism 10, but the beam exits in a direction
different
from its original direction. This happens if a different sequence of internal
reflections occurs. The graph represents the properties of a prism with
parameters np=1.5, c=5 mm, b=7.07 mm and 6p=45° rotating around its
centroid.
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(056) FIG. 5 schematically is a graph illustrating a variation in optical path
length scan range resulting from the rotation of the prism for the various
values of L;". A largest scan range is obtained for L;~=3.5 mm.
(057) FIG. 6 illustrates the percentage of variation in derivative
([(dlp/d8)maX
(dl~,/d8)n,;n]~(dIF,/d0)r~aX for each value of L;~. It shows that reasonable
variations
(less than 10%) are obtained for values of L;n slightly smaller than 6 mm.
FIG. 5 also shows that for those values of L;~, the variation in optical path
length is rather small.
(058) In the case of the prism, if the parameters that maximize the optical
path length variation are chosen (L;"= 3.5 mm), we obtain a duty cycle of 35%
with a nonlinearity of 31 %.
APPLICATION
(059) In accordance with the invention, improved duty cycle, linearity of
variation, and/or scan range of a scanning optical delay line are provided.
This is accomplished by reuse of the angular range by reinsertion of the
beam.
(060) FIGs. 7a,b schematically illustrate a first embodiment of the invention
showing how multiple insertions of the beam may be achieved. A prism 10, is
mounted for rotation about an axis 11 passing through its centroid,
orthogonally to parallelogram top and bottom bases of the prism 10, as in
FIG. 1. The optical scanning optical delay line assembly also includes two
mirrors 17,18 each for reflecting (by 180°) beams passing through the
prism 10 at two ranges of angular positions, and a third mirror for reflecting
the beam from a line of incidence 12 with the prism 10 over a first range of
angles, and a reinsertion line 13 that intersects the prism 10 over a second
range of angular positions.
(061) In FIG. 7a, a beam 15a exits an optical coupler 16 along a line of
incidence 12. Incidence line 12 is directed orthogonally to the rotational
axis 11 from which it is offset by the distance L;n, as defined in FIG. 1. The
distance L;~ is intermediate one half a major diagonal (dM) of the prism 10
and
one half a minor diagonal (dm) of the prism 10 so that during rotation the
prism 10 periodically intersects the line of incidence 12. The beam 15a
propagates towards the prism 10. As shown in FIG. 7a, the beam 15a is
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incident on the prism 10, is twice reflected, and exits toward a reflective
surface 18, in a direction parallel to the first line of coincidence. The
reflective
surface 18 may be a retroreflector, or a mirror that is disposed in a
direction
perpendicular to the line of incidence 12. The reflective surface 18 extends
in
the X direction a range of distance to cover L;~ + L°~t from the
incidence
line 12. While L;~ is a constant, it will be appreciated that L°~t
varies with 8.
Reflective surface 18 reflects beam 15a to retrace the same path. As such,
reflective surface 18 is an end of the scanning optical delay line. In other
embodiments the scanning optical delay line is of a transmission type, and
instead of retroreflecting the beam, the end serves to couple the beam 15a
with a sample beam 15a for coherence measurement. In the illustrated
retroreflective embodiment, however, the beam 15b again passes through
prism 10, and is finally collected by the optical coupler 16. The optical path
length traversed by the propagating optical beam is related to the orientation
of the prism 10.
(062) When the angular position of prism 10 is such that the prism 10 does
not intersect the line of incidence 12, the beam 15a becomes available for
reinsertion into the prism 10. This is depicted in FIG. 7b where the same
prism 10 has been rotated by 90° in a counter-clockwise direction. The
beam
15a first follows the incidence line 12 past an obtuse corner of the prism 10,
and then is redirected by mirror 19 towards onto a reinsertion line 13. The
reinsertion line 13 is separated from the rotational axis 11 by L;~, and is
directed orthogonally to the rotational axis 11, and accordingly the
reinsertion
line 13 is equivalent to the incidence line 12 up to a phase offset. As shown
in
FIG. 7b, the beam 15a passes through the prism 10 exiting parallel to the
reinsertion line 13. The beam 15c is reflected by a surface 17 that is
disposed
to retroreflect the beam 15c causing it to retrace its path through the prism
10,
and along the reinsertion line 13, to the incidence line 12.
(063) For the embodiment depicted in FIGs. 7a,b, good characteristics have
been obtained for the scanning optical delay line with a material of high
refractive index, for example, using a ZnSe prism with dimensions b= 4.24
mm, c= 3 mm, 0p= 45° and with a refractive index of 2.46 at 1310 nm. It
will
be appreciated that this prism 10 has a different refractive index than the
CA 02524242 2005-10-21
previous examples, resulting in greater linearity. If the first line of
incidence
passes a distance of L;~= 2.8 mm from the rotation axis, a scan range of 4
mm, a duty cycle of 65% and a nonlinearity of 11 % can all be produced.
(064) A second embodiment of the invention uses off centroid rotation which
improves the selection of the range of angles the fine of incidence makes with
a front face of the prism. By rotating off centroid, only one surface is used
as
the front surface, and consequently there is no alternation of front and rear
surfaces to double the number of times the beam is inserted in the prism, per
cycle. Accordingly multiple prisms may be used to improve the duty cycle.
(065) An example of the second embodiment is schematically illustrated in
FIGs. 8a,b. The scanning optical delay line includes five prisms 31-35 fixed
on a rotating disc 25. Each of the prisms 31-35 is oriented in a rotationally
symmetric manner so that they all provide substantially the same range of
angular variances with respect to lines of incidence 27 and reinsertion 28.
Basically this embodiment has two additional parameters for optimization: a
radial distance R of a centroid of the prism from the rotational axis; an
angle
8o between a radial line from the rotational axis through the centroid of the
prism, and a front face of the prism.. In any case the prisms are arrayed with
an acute corner radially distant the axis of rotation, so that the incidence
27
and reinsertion 28 lines intersect an arc swept by the acute corner. Once the
parameters R, 8o b, c, 9P and np are chosen, a maximum number of prisms,
L;~, an angle between the incidence 27 and reinsertion 28 lines, and positions
of reflecting surfaces 36,38 for reinsertion, can be chosen to optimize the
duty
cycle and linearity of the optical scanning optical delay line.
(066) FIG. 8a shows a scanning optical delay line with five prisms 31-35,
fixed to a disc 25, rotatabie around the center of the disc 26. Each prism 31-
35 has a center of mass at a radius R from the center of the disc 26. The
orientation of each prism 31-35 is determined by an angle 8o that the front
face 10a of the prism makes with respect to a radial line passing through the
center of mass of the prism, 9o being defined positive in a counterclockwise
direction from the radial line. Surrounding the disc 25 are a plurality of
mirrors 36, 37, 38. The mirrors 36-38 are oriented to reflect beams as
described below.
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(067) An angle 8 is defined between a radial line from the center 26 of the
disc 25 passing through the centroid of the prism and the X-axis, the angle B
being defined positive in a counterclockwise direction from the X-axis. While
FIGs. 8a,b illustrate a specific embodiment where 5 prisms are used, it will
be
appreciated that different numbers of prisms could be used as long as the
paths through each prism doesn't intercept another prism" which happens
when the prisms are too densely disposed.
(068) A beam 21 a exits an optical coupler 20 that both delivers and collects
light from an interferometric system. The beam 21 a propagates towards the
delay path assembly. During a part of the cycle of rotation where one of the
prisms (i.e. an active prism 31 ) intersects the incidence line 27, as shown
in
FIG. 8a, the beam 21 a is intercepted by prism 31 and exits parallel in
direction
to the insertion line 27 toward mirror 36,. Mirror 36 is aligned in such a way
that the reflected beam 21 b follows the exact inverse path as beam 21 a.
Accordingly the mirror 36 is perpendicular to the beam 21 a but displaced in
the X direction to accommodate for the lateral displacement of the beam. The
beam 21 b again passes through prism 31, and is finally collected by the
optical coupler 20. The optical path length traversed by the propagating
optical beam 21 depends on the instantaneous orientation of the prism.
(069) When the beam 21 a is not directly intercepted by the prism, it becomes
available for reinsertion into the disc 25. This is depicted in FIG. 8b where
the
same delay path assembly is shown rotated by 108° in a counter-
clockwise
direction. The beam 21 a first crosses the disc 25 without intercepting any
prism, and is therefore redirected by mirror 37 onto reinsertion line 28.
Prism
31 is in position on the reinsertion line. The mirror 37 is at an angle with
respect to the incidence line 27 so that beam 21 a is directed along the
reinsertion line 28 passing a same distance from the center of the disc 26 as
the initially launched beam 21 a. Beam 21 a exits prism 31 parallel to its
direction prior entering the prism 31, and is reflected 180° by mirror
38. The
reflected beam 21 c follows the reciprocal path of beam 21 a to finally be
collected by the optical coupler 20. As shown in FIG. 8b, reinsertion at a
given prism occurs 108° after the direct insertion. This reinsertion
could have
occurred at other angles like 36°, 180°, 252°, or
314°, while providing similar
17
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performance. For a different arrangement of prisms, the possible angles
would also be different.
(070) In the embodiment presented in FIGs.Ba,b, during the period of
rotation of the disc 25, each prism is used twice: once on each of the
incidence and reinsertion lines. The parameters of the scanning optical delay
line can be chosen to optimize the duty cycle and linearity of the scanning
optical delay line. BK7 is found appropriate for operation around 1.3 Nm with
a bandwidth of a several tens of nanometers. Using commercially available
BK7 prisms (np= 1.5037 at a wavelength of 1310 nm), with the dimensions c=
mm, b= 7.07 mm, 6p= 45°, we can use the results presented in FlGs. 3
and
4 as a guideline. To optimize the duty cycle, the beam 21 a should intercept
each prism over angular ranges of about 36° (360/2n, where n is the
number
of prisms). From FIG. 3 it is determined that to minimize the nonlinearity,
the
angular range should be centered on an angle y= 17°. From FIG. 4, the
beam
should enter each prism when the beam is at a distance of about 3.5 mm from
the center-of-mass of the prism (i.e. R + 3.5 mm from center 26), and exit
when it is a distance a little over 6 mm (i.e. R + 6 mm from center 26). The
optimal configuration is obtained for a radius R = 14 mm, and an orientation
of
each prism of 60=-35°, and a value of L;~= 17.5 mm.
(071) A graphical representation of the resulting optical path length
variation
for a single prism as a function of the angle 8 for one of the prisms is shown
in
FIG. 9. The optical path length in FIG. 9 corresponds to a single-pass through
the prism, and therefore illustrates half the total path length between the
exit
and reentry in the optical coupler 20 in FIG. 7. The use of one half the total
path length is standard in the field of interferometry where the sample arm
will
also be in a retroreflecting configuration, as is common in optical coherence
tomography. The prism is active between angles y from 0.1 ° and 34.1
°.
Outside this angular range, an optical path length value of -5 mm corresponds
to the case where the beam does not intercept the prism, the beam is thus
available for reinsertion or to be intercepted by the preceding or following
prism. The resulting duty cycle of the scanning optical delay line can thus be
more than 90%.
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(072) FIG. 10 graphically illustrates the variation of the derivative dlpld9
over
the angular range, showing that the nonlinearity is small, i.e. less than 6%.
Accordingly, the embodiment of FIGs. 8a,b can provide an improved linearity
of the optical path length as a function of angle y, by using an angular range
corresponding to a most linear portion of the curve.
(073) Furthermore it will be noted that a sampling rate of more than 8,000
samples/s with a 50,000 rpm rotating motor is possible. These numbers are
on par with high-end state-of-the art scanning optical delay lines but improve
over the prior art in terms of ease of alignment and robustness.
(074) FIG. 11 is a schematic top plan view of a scanning optical delay path
that includes the disc 25 and prisms 31-35 of FIGs. 8a,b, with the addition of
a
marking system. FiG. 12 shows an active part of the marking system and
scanning optical delay path in an elevation view. Like reference numerals
identify like features of the delay path assembly, and descriptions of these
are
not repeated here. An optical source 40 emits a beam 41 of light into the
disc 25. The beam of light 41 is focused to gather light at a distance of the
slit 42. The beam 41 meets front walls of an adjacent one of the prisms 31-35
(e.g. prism 34 as shown), depending on an angular position of the disc 25.
The angle of incidence of the beam 41 on the front wall ensures that
sufficient
light is reflected from the face of the adjacent prism. At specific angular
positions of the disc 25, like the one depicted in FIG. 11, the optical beam
41
is reflected from the face at a specific angle that passes through a narrow
slit 42 and is detected by a detector 43.
(075) As can be better seen in FIG. 12, the beam 41 from source 40 is
directed to the prism at an angle from the plane of the disc 25 and as are the
slit 42 and detector 43. This configuration allows beam 41 to hit the upper
part of the prism 34, ensuring that only the reflection from the front face is
sent to the detector. A refracted part of beam 41 enters the prism and is
partly reflected internally by the other faces but is not sent back towards
the
detector, to avoid spurious detections that could degrade the quality of the
synchronization signal. The source 40 and detector 43 are shown at different
radial positions in FIG. 11, but since they are at different height, they
could be
put one on top of the other to provide a more compact system. Because the
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beam 41 is reflected by a revolving prism, the angular velocity is twice that
of
the prism thus providing a very precise synchronization signal. This precision
is enhanced by the use of a very narrow slit, and a highly focused beam 41.
Additionally, the synchronization signal is produced from a detector signal
from the detector 43, and the detector signal can be fitted to a function,
such
as a Gaussian function, to determine more precisely a center, to further
increase the precision. Finally, the system can be positioned relative to the
scanning optical delay line such that the synchronization signal is detected
in
a dead time of the scanning optical delay line (i.e. during a time outside of
the
duty cycle), to avoid interference with the scanning operation.
(076) In certain embodiments, the marking system can determine which of
the prisms 31-35 is detected. This can be accomplished in two ways: the
detected reflection from each prism may have a different amplitude caused by
imperfections in positioning of the prism; or by variations in the reflective
properties of the faces of the prisms that were intentionally created. As a
result, at each revolution of the disk, five signals of different amplitudes
are
detected by detector 42 and this information can be used to identify which
prism is active under direct insertion or under reinsertion at a given moment.
It
will be noted that the number of signals detected correspond to the number of
prisms, which is five in the current example. It will be appreciated that in
alternative embodiments a different number of samples could also be taken,
and that these samples could be associated with apertures or markings on the
disc 25, one or more attachments to the prisms, etc. It is advantageous to
use the front face for detection so that if one of the front faces is moved,
the
marking system can declare misalignment.
(077) If each of the prisms is identified by the marking system, the detector
can send a synchronization signal to a detection and analysis system, which
can then identify intervals of a coherence signal output by the interferometer
that correspond to a sample (i.e. time gating of the interferometer output),
and
can apply a corresponding calibration for each sample. As will be evident
each sample is produced by a corresponding one of the prisms, produced
along either the incidence or reinsertion lines. As there may be slight
differences in L;~ between the incidence and reinsertion lines, it may be
preferable that there be one calibration for each prism along each insertion
CA 02524242 2005-10-21
line. Accordingly the synchronization signal permits accounting for small
departures from ideal positioning in prism positioning during assembly. This
increases the precision of the scanning optical delay line.
(078) Alternatively, the synchronization of the scanning optical delay line
can
be performed by any approach that includes but is not limited to optical,
electrical, mechanical, and magnetic systems. The use of synchronization
signals to trigger the detection system is well known to those skilled in the
art.
(079) The embodiments of FIGS. l3a,b illustrate the replacement of the
prism 10 with an alternative parallelogram optical path element that consists
of two parallel planar mirrors 82,83. The equation of optical path length for
such a parallelogram optical path element is represented with equation 2
where the index of refraction nP is set to one. The features of the
parallelogram optical path element that are constant between these two
embodiments are the parallel side walls, the effectively parallel front and
rear,
the constant index of refraction of the optical medium enclosed between the
two parallel walls, and the fact that over a range of angular positions and
L;~,
the optical path length is substantially invariant of L;~.
(080) Parallel mirrors 82 and 83 are fixed to a plate 84 that is adapted to
rotate around an axis passing through a centroid 81 of the mirrors. In
operation the embodiment of FIGs. l3a,b the beam is transmitted in the same
manner as that of FIGs. 7a,b except that there is no refraction of the
incident
beam as it enters and leaves the parallelogram optical path element in
accordance with the instant embodiment. Consequently the detailed
description of the path is not repeated here.
(081) For a pair of mirrors 4.24 mm long separated by a distance of 3 mm
defining an angle 6p between a front of the parallelogram optical path element
and mirror 83, and with a line of incidence at a distance L;~= 2.8 mm from the
centroid 81, the scan range is 2.7 mm with a duty cycle of about 40% with a
nonlinearity of about 40%. The performance of such a scanning optical delay
line is poorer than for the previously described embodiments, but it
advantageously avoids dispersion due to the propagation in the material from
which the prisms are made.
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(082) A still further embodiment can be obtained by replacing the prisms in
the multiple prism assembly of FIGs. 8a,b with parallelogram optical path
elements consisting of pairs of parallel mirrors. Again, this would be a good
choice if one wants to avoid dispersion in the material from which the prisms
are made. There will be a similar improvement in linearity and duty cycle by
taking advantage of off-centroid rotation of the parallelogram optical path
elements in combination with reinsertion.
(083) FIG. 14 schematically illustrates a further alternative embodiment of
the
invention that provides a double pass system. The principal advantage of the
double pass embodiment is a depth of the scan is doubled. FIG. 14a
schematically illustrates a profile view of parts of the scanning optical
delay
line of FIG. 7a,b. In contrast, FIG. 14b schematically shows the addition of
an
offset reflector 84 consisting of a pair of mirrored surfaces that meet to
form a
square edge. As the square edge is substantially orthogonal to rotation
axis 11, the offset reflector 84 effects substantially no offset in the plane
of the
incidence and reinsertion lines. This minimizes any difference in the optical
path length traveled by the beam 15 during the two passes through the
prism 10. The second path through the prism 10 ends with a retroreflecting
mirror 18 as before, however retroreflecting mirror 18 is moved to a position
above the optical coupler 16. While a second mirror 17 is not in view, it will
be
appreciated by those skilled in the art that it too is replaced by an offset
reflector for similar operation.
(084) It will be appreciated by those skilled in the art that multiple passes
can
equally be effected by other reflections that take the same or different paths
through the prism 10. Furthermore the same double pass configuration is
equally applicable to the embodiment of FIGs. 8a,b.
(085) While the invention is described for a retroreflective-type scanning
optical delay line, it will be evident to those skilled in the art that the
same
scanning optical delay line could equally be used in a transmission
configuration scanning optical delay line by replacing retroreflective ends
with
transmission elements.
(086) It will further be noted that while an advantage of the illustrated
embodiments include that the reflection of the beam from the line of incidence
to the line of reinsertion is performed by a single mirror, in other
embodiments
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it may be necessary to use reflections off 2 or more surfaces to insert the
beam on the reinsertion line.
(087) It will be appreciated by those skilled in the art that a "double pass"
configuration can be implemented using the proposed optical delay line for
effectively doubling the optical path delay. For example, a double pass
configuration may be implemented by going through the prism at different
height levels along the rotation axis. The change in height level may be
realized by a set of mirrors such as a corner retroreflector.
23
