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Claims  |
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What is claimed is:
1. A system for performing optical imaging on a sample comprising:
an optical radiation source;
a reference optical reflector;
a first optical path leading to said reflector;
a second optical path leading to said sample, said second optical path
terminating in a probe module, said probe module including means for
controlling the transverse position on said sample at which imaging is
being performed, said sample position being selectively changed by said
means for controlling to scan the sample in at least one transverse
dimension;
means for applying optical radiation from said source through the first
optical path to said reflector and through the second optical path
including said probe module to the sample;
means for controlling the longitudinal range with the sample from which
imaging information is being obtained;
means for combining reflections from the reflector received through the
first optical path and reflections from the sample received through the
second optical path, the resulting combined optical output having optical
interference fringes;
means for detecting said output; and
means for processing the detected output to obtain a selected image of the
sample.
2. A system as claimed in claim 1 wherein said optical radiation source is
a short coherence length optical source, wherein said means for
controlling longitudinal range controls the relative lengths of said
optical paths in accordance with a predetermined velocity profile having
an instantaneous velocity V at each point on the profile, wherein
interference fringes occur at length matched points on the two optical
paths, and wherein said optical output has an instantaneous modulating
frequency.
3. A system as claimed in claim 2 wherein said modulating frequency
includes a Doppler shift frequency at a frequency f.sub.D
.about.NV/.lambda., where .lambda. is the wavelength of the radiation
source.
4. A system as claimed in claim 1 wherein there is a bandwidth requirement
to compensate for predominant low frequency noise for the system and for
system, aliasing, wherein the velocity V is not sufficient to result in a
Doppler shift frequency f.sub.D which is sufficient to meet said bandwidth
requirement wherein said longitudinal range controlling means includes
means for causing an additional modulation at a frequency f.sub.M for at
least one of said optical paths, and wherein said means for processing
includes a demodulator which demodulates for a modulating frequency which
is a selected combination of f.sub.D and f.sub.M.
5. A system as claimed in claim 4 wherein said additional modulation
causing means includes at least one acousto-optic modulator (AOM) in at
least one of said optical paths.
6. A system as claimed in claim 5 wherein there are two AOM's in said at
least one path with f.sub.M being the difference frequency shift caused by
said AOM's.
7. A system as claimed in claim 1 wherein said transverse position
controlling means includes means for moving a probe module at the end of
the said second optical path in at least one dimension substantially
perpendicular to the direction in which optical radiation is applied to
the sample.
8. A system as claimed in claim 7 wherein said means for moving moves the
probe in two directions perpendicular to the direction of optical
radiation.
9. A system as claimed in claim 1 wherein said probe module includes means
for directing the optical radiation at a transverse position on the
sample, and wherein said transverse position controlling means includes
means for optically changing said transverse position in at least one
dimension generally perpendicular to the direction in which the optical
radiation is applied to the sample.
10. A system as claimed in claim 9 wherein said means for optically
changing changes the transverse position in two directions perpendicular
to the direction for optical radiation.
11. A system as claimed in claim 9 wherein said means for optically
changing includes at least one movable mirror in the optical path of the
radiation for steering the radiation at an angle dependent on mirror
position.
12. A system as claimed in claim 11 wherein said at least one mirror is
movable in two orthogonal directions for steering the radiation in a
direction which varies in two dimensions.
13. A system as claimed in claim 11 wherein there are two mirrors
successively spaced along said optical path, said mirrors being movable in
different generally orthogonal directions.
14. A system as claimed in claim 9 wherein said means for optically
changing includes at least one of an electro-optic or acousto-optic beam
deflector.
15. A system as claimed in claim 9 including means for rotating said mirror
for changing its pitch to effect circular scanning.
16. A system as claimed in claim 9 wherein said probe module is a mechanism
for scanning internal channels.
17. A system as claimed in claim 16 wherein said probe module includes a
rotating mirror for transversely directing the second optical path
radiation.
18. A system as claimed in claim 17 wherein said probe module includes an
outer sheath, an inner sheath rotatably mounted within said outer sheath,
means for directing second optical path radiation through the inner
sheath, and means movable with the inner sheath for focusing said
radiation at a selected position on the internal channel, said selected
position varying as the inner sheath is rotated.
19. A system as Claimed in claim 18 wherein said focusing means is a mirror
mounted to rotate with said inner sheath and to reflect radiation passing
through the inner sheath in a selected direction beyond an end of the
outer sheath.
20. A system as claimed in claim 16 wherein said probe module includes an
outer sheath, a bundle of optical fibers mounted in said sheath, means for
optically connecting the second optical path to a first end of a selected
one or more of said optical fibers, said means for optically connecting
including means for controlling the optical fibers to which the second
optical path is connected, and means for establishing a selected
transverse position on the sample for each of said optical fibers and for
optically connecting a second end of each optical fiber to the
corresponding selected transverse position.
21. A system as claimed in claim 16 wherein said first and second optical
paths are in the form of first and second optical fibers respectively; and
wherein said probe module includes an outer sheath, means for securing a
distal end of the second optical fiber to an inner wall of said sheath,
said means including means for moving said distal end toward and away from
said wall, and means for optically connecting said distal end to the
sample, said means for optically connecting establishing a selected
transverse position on the sample for each position of said distal end
relative to said wall.
22. A system as claimed in claim 9 wherein said first and second optical
paths are in the form of first and second optical fibers respectively; and
wherein said transverse position controlling means includes means for
translating the distal end of said second optical fiber.
23. A system as claimed in claim 1 wherein said longitudinal position
controlling means includes means for controlling the length of the second
optical path by controlling the spacing between the probe module and the
sample.
24. A system as claimed in claim 1 wherein said longitudinal position
controlling means includes means for periodically altering the length of
the first optical path, resulting in periodic changes in the depth
position in the sample for a length matched point of the second optical
path; and
wherein said probe module includes means for controlling the depth focus
for the module in the sample so that the depth focus is maintained
substantially at said length matched point as said point is periodically
changed.
25. A system as claimed in claim 24 wherein said probe module includes at
least one focusing lens for radiation received from said second optical
path, and wherein said depth focus controlling includes means for moving a
focusing lens in the direction of the radiation passing therethrough to
control focus depth.
26. A system as claimed in claim 1 wherein the rates at which said
longitudinal position controlling means and said transverse position
controlling means are moved are such that points at all longitudinal
ranges of interest are scanned for a given transverse position on the
sample before the transverse position controlling means causes the probe
module to initiate scanning at a new transverse position.
27. A system as claimed in claim 1 wherein the rates at which said
longitudinal position controlling means and said transverse position
controlling means are moved are such that points at all transverse
positions to be scanned in at least one dimension are scanned at a given
longitudinal range in the sample before the longitudinal position
controlling means causes scanning at a new longitudinal range to be
performed.
28. A system as claimed in claim 1 wherein said transverse position
controlling means includes means for performing a two dimensional
transverse scan at a longitudinal position in the sample determined by
said longitudinal position controlling means.
29. A system as claimed in claim 1 wherein images taken during a given scan
of the sample through all longitudinal ranges and transverse positions may
have spurious intensity variations; and
including means for performing a plurality of scans on said sample, and
means for averaging said scans to compensate for said intensity
variations.
30. A system as claimed in claim 1 wherein said sample is a biological
sample.
31. A system as claimed in claim 1 wherein said source is frequency
modulatable spectrally coherent optical source, wherein said longitudinal
position controlling means includes means for modulating the frequency of
the source output, said interference resulting in a signal having a
frequency proportional to the difference between the first an second path
lengths, and wherein said means for processing includes means for
converting said signal into imaging information.
32. A system as claimed in claim 1 including a plurality of first and
second optical paths, there being an optical radiation source at the
proximal end of each path, a reference reflector at the distal end of each
first optical path, and a transverse point on the sample at the distal end
of each second optical path, and wherein said means for processing
includes means for processing the received images from the plurality of
paths to effect parallel scanning of the sample.
33. A system as claimed in claim 1 including a balanced receiver for
cancelling intensity noise.
34. A system for performing optical imaging on a sample comprising:
an optical radiation source;
an optical path leading to said sample, said optical path terminating in a
probe module, said probe module including means for controlling the
transverse position on said sample at which imaging is being performed,
said sample position being selectively changed by said means for
controlling to scan the sample in at least one transverse direction;
means for applying optical radiation through the optical path including the
probe module to the sample;
means for controlling an optical characteristic of the radiation source
output to control the longitudinal range within the sample from which
imaging information is being obtained;
means for receiving optical reflections from said sample and responsive at
least in part to said received reflections for generating an output having
a frequency proportional to the length of the optical path to the
longitudinal range in the sample being imaged;
means for detecting said output; and
means for processing the detected output to obtain a selected image of the
sample.
35. A system as claimed in claim 34 wherein said optical radiation source
is a frequency modulatable source, wherein said optical characteristic
controlling means controls an input to the source to modulate its output
frequency, and including a reference optical reflector, and an additional
optical path leading to said reflector, and wherein said means for
receiving receives reflections from said sample and said reflector and
includes means for combining said reflections to generate interference
fringes and an output having a frequency which is a function of the
difference in such path lengths.
36. A system for performing optical imaging and measurements on a sample
comprising:
a short coherence length optical radiation source;
a reference optical reflector;
a first optical path leading to said reflector;
a second optical path leading to said sample;
means for applying optical radiation from said source through the first
optical path to said reflector and through the second optical path to the
sample;
means for altering the length of the second optical path to alter in the
relative lengths of said optical paths in accordance with a predetermined
velocity profile, having an instantaneous velocity V at each point on the
profile;
means for combining reflections from the reflector received through the
first optical path and reflections from the sample received through the
second optical path, the resulting combined optical output having
interference fringes at length matched points on the two paths, the means
for altering causing periodic changes in the longitudinal range position
in the sample for the length matched points of the second optical path;
a probe module terminating said second optical path, said probe module
including means for controlling the longitudinal range focus for the
module in the sample so that the longitudinal range focus is maintained
substantially at said length-matched point as said point is periodically
changed;
means for detecting said output; and
means for processing the detected output to obtain a selected image of the
sample.
37. A system as claimed in claim 36 wherein said probe module includes at
least one focusing lens for radiation received from said second optical
path, and wherein said longitudinal range controlling means includes means
for moving a focusing lens in the direction of the radiation passing
therethrough to control longitudinal range.
38. A system for performing optical imaging on a sample comprising:
at least one optical radiation source;
at least one optical reflector;
a plurality of first optical paths leading from said at least one source to
said at least one reflector;
a plurality of second optical paths leading from said at least one source
to selected transverse points on said sample;
means for combining reflections from the at least one reflector received
through each first optical path with reflections from the sample received
through a corresponding second optical path, each resulting combined
optical output having interference fringes at matched points on the two
paths; and
means for processing the combined optical outputs to obtain a plurality of
images of the sample in parallel, said means including means for detecting
each of said outputs and means for processing the detected outputs.
39. A system as claimed in claim 38 including an optical radiation source
for each first optical path/second optical path pair.
40. A method for performing optical imaging on a sample comprising the
steps of:
(a) causing short coherence length optical radiation to impinge on a
reference reflector and on the sample through first and second optical
paths, respectively;
(b) altering the relative lengths of said paths in accordance with a
predetermined profile;
(c) selectively changing the transverse position on the sample at which
scanning is being performed;
(d) combining reflections from the reflector received through the first
optical path and reflections from the sample received through the second
optical path, the resulting combined optical output having interference
fringes at length matched points on the two paths;
detecting said output; and
processing the detected output to obtain a selected image of the sample.
41. A method as claimed in claim 40 wherein the relative rates at which
altering step (b) and changing step (c) are performed are such that points
at all longitudinal ranges of interest are scanned for a given transverse
sample position before the transverse position is changed to initiate
scanning at a new transverse position.
42. A method as claimed in claim 40 wherein the relative rates at which
altering step (b) and changing step (c) are performed are such that points
at all transverse positions to be scanned in at least one dimension are
scanned at a given longitudinal range in the sample before the
longitudinal range is altered to cause sampling at a new longitudinal
range to be performed.
43. A method as claimed in claim 40 wherein the changing step (c) includes
the step of performing a two-dimensional transverse scan at a longitudinal
range in the sample determined by said altering step.
44. A method as claimed in claim 40 wherein said imaging involves
non-invasive cross-sectional imaging in biological specimens.
45. A method as claimed in claim 44 wherein said cross-sectional imaging is
performed on various eye sections.
46. A method as claimed in claim 40 wherein measurements taken during a
given scan of the sample through all longitudinal ranges and transverse
positions may have spurious intensity variations; and
including the steps of performing a plurality of scans on said sample, and
averaging said scans to compensate for said intensity variations. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention related to optical imaging, including utilizing such images
to perform precision measurements on biological and other samples.
BACKGROUND OF THE INVENTION
There are may industrial, medical, and other applications where high
resolution (generally less than 10 micrometer) images of, and measurements
of distances, thicknesses, and optical properties of, a biological or
other sample are required.
Copending application Ser. No. 07/692,877 mentioned above describes an
optical coherence domain reflectometer (OCDR) technique for performing
such measurements generally with a single scan in the longitudinal
direction. However, there are many applications, including medical
applications, where a need exists for such scans to be conducted in two or
three dimensions rather than in a single dimension, including transverse
directions at a given longitudinal depth, thereby providing
multidimensional imaging and measurements. Therefore, a need exists for a
means to perform such scanning in at least one transverse direction at a
selected longitudinal depth, with the capability of also scanning in the
longitudinal direction.
Further, particularly in medical application, it is frequently desirable to
provide such scans inside of tubular or other structures such as blood
vessels, the bronchial tree of the lungs, the gastrointestinal tract, the
genital tract or the urinary tract, using an angioscope or endoscope. In
order for such scanning to be performed, a probe must be provided which is
capable of being mounted in an endoscope or angioscope for performing
internal scans.
While typically a scan would be completed through the full depth range at a
given lateral and/or transverse position before repositioning to the next
position, this may require scanning of the mirror or other element used
for performing longitudinal range or depth scans at a rate higher than the
capacity of existing equipment. This is particularly true where the
longitudinal scan produces a Doppler shift frequency which affects the
interferometric signal frequency and hence the system sensitivity. It is,
therefore, desired that such scanning be performed at a constant velocity.
However, since very high speed longitudinal scanning at a constant
velocity is difficult to achieve, where two or three dimensional scanning
is being performed, other scan patterns may be required. Further, in some
applications, it may be desirable to perform transverse scanning in one or
two dimensions at a selected longitudinal position or depth.
Another problem which becomes particularly serious when transverse scanning
is being performed is that the bandwidth of the received signals increases
beyond the inherent Doppler frequency shift of the system. In such cases,
aliasing (i.e. variations in image intensity) may occur. It is, therefore,
desirable that a technique be provided to enhance resolution by
eliminating or averaging out such intensity variations.
Another problem with the prior system is that, if scanning is to be
conducted over an extended depth range, a smaller numerical aperture must
be used so as to extend the depth of focus. However, this reduces lateral
resolution and the received optical signal power throughout the range. A
need, therefore, exists for a technique which permits the use of a large
numerical aperture over an extended depth range within a sample.
Further, some of the problems described which result from performing
longitudinal scanning by mechanically moving a mirror or other element may
be overcome by performing this scan electronically, for example by varying
the optical frequency or amplitude of the light incident from the light
source. However, for certain applications, for example imaging a dynamic
biological sample such as the eye, the scanning speed required to do
three-dimensional scanning may be such that a parallel scanning technique
may be preferable or may be required.
SUMMARY OF THE INVENTION
Thus, a need exists for improved optical coherence domain reflectometer
(OCDR) optical imaging and measurement systems or for other imaging and
measurement systems, particularly electronically scanned systems, which
are capable of performing two and three dimensional scans at a selected
and/or over an extended longitudinal or depth range for either internal or
external samples with sharp focus and high resolution and sensitivity over
the range.
In accordance with the above, this invention provides a method and
apparatus for performing optical imaging on a sample by applying optical
radiation, which radiation has a short coherence length for preferred
embodiments, to a reference optical reflector and to the sample through
first and second optical paths respectively. The optical paths are
preferably fiber optic paths. The longitudinal range within the sample
from which imaging information is obtained is controlled by, for example,
altering the relative lengths of the paths or by varying the frequency or
intensity of the source in accordance with a predetermined profile. The
lateral or transverse position on the sample at which imaging or
measurements are being performed is also selectively changed. This results
in imaging being performed on the sample in at least one transverse
dimension. Where the profile for longitudinal scanning is a steppable
profile, transverse scanning in one or two dimensions may be performed at
any selected longitudinal range. Reflections from the reflector through
the first optical path and reflections from the sample received through
the second optical path are combined, the resulting combined optical
output having interference fringed at matched points, for example, length
matched points, on the two paths and having an instantaneous modulating
frequency which may include a Doppler shift frequency at a frequency
f.sub.D .about.NV/.lambda. for embodiments where relative path lengths are
being altered with a velocity profile having an instantaneous velocity V
at each point on the profile. The combined output is detected and the
detected output is processed to obtain a selected image of the sample.
The second optical path is preferably terminated in a probe module which
includes a means for controlling the transverse position on the sample at
which imaging is being performed, and a means for selectively changing
this position in at least one dimension to scan the sample. The velocity V
may be sufficiently high so that the Doppler shift frequency is
sufficiently high to meet the bandwidth requirements to overcome the
predominate low frequency noise for the system and for signal aliasing.
Where this is not the case, a means is provided for causing a vibratory or
other change in a modulating frequency f.sub.M, resulting in a modulating
frequency which is a selected combination of f.sub.D and f.sub.M. This
change may be effected by at least one acousto-optic modulator (AOM) in at
least one of the optical paths. For one embodiment, there are two AOM's in
one of the optical paths, with f.sub.M being the difference frequency
shift caused by the two AOM's. The probe position controller may include
means for moving a probe at the end of the second optical path or the
distal end of a fiber optic element forming the second optical path in at
least one dimension substantially perpendicular to the direction in which
optical radiation is applied to the sample to provide two-dimensional or
three-dimensional scanning of the sample.
For other embodiments, the probe module may include mirrors or other means
for steering the optical radiation to a position on the sample and for
optically changing the transverse position in at least one dimension,
generally perpendicular to the direction in which the optical radiation is
applied to the sample. Where three dimensional scanning is desired, the
transverse position is changed in two directions. The means for optically
changing transverse position may include at least one movable mirror in
the optical path of the radiation for angularly translating the radiation
at an angle dependent on mirror position. One mirror may be movable in two
orthogonal directions to angularly translate the radiation in a direction
which varies in two dimensions, whereby three dimensional scanning is
achieved, or this objective may be achieved utilizing two mirrors
successively spaced along the optical path, with the mirrors being movable
in different, generally orthogonal, directions.
For other embodiments, the probe module is a mechanism for scanning
internal channels such as an angioscope or endoscope. For such
applications, the probe module may include an outer sheath. For one
embodiment, the probe module also includes an inner sheath rotatably
mounted within the outer sheath, an optical means for directing radiation
from the second optical path through the inner sheath, and a means movable
with the inner sheath for directing the radiation at a selected position
on the internal channel, the selected position varying as the inner sheath
is rotated. This embodiment preferably uses a mirror mounted to rotate
with the inner sheath to reflect radiation passing through the inner
sheath in a selected direction beyond the end of the outer sheath.
For another embodiment, a bundle of optical fibers is mounted in the outer
sheath. A first end of a selected one or more of such optical fibers are
optically connected to the second optical path, a means being provided for
controlling the optical fiber(s) to which the second optical path is
connected. There is also a means for establishing a selected transverse
position on the sample for each of the optical fibers and for optically
connecting a second end of each optical fiber to the corresponding
selected transverse position.
For still another embodiment, wherein the first and second optical paths
are in the form of first and second optical fibers respectively, the probe
includes a means for securing a distal end of the second optical fiber to
an inner wall of the sheath. This means includes means for moving the
distal end toward and away from the wall. Means are also provided for
optically connecting the distal end of the fiber to the sample, this means
establishing a selected focal position on the sample for each position of
the distal end relative to the wall.
For some embodiments, the probe module includes a means for controlling the
focus for the module in the sample so that this depth focus is maintained
substantially at a point in the sample from which imaging information is
being obtained as this point is periodically changed during a longitudinal
scan of the sample. Such focal plane may be accomplished by moving a
focusing lens of the probe module in the direction of the radiation
passing therethrough to control focus depth.
Multi-dimensional scanning may be accomplished utilizing at least three
different scan patterns. For one scan pattern, the rates at which the
relative lengths of the optical paths are altered and at which the
transverse position on the sample is changed are such that points at all
longitudinal ranges of interest are scanned for a given sample transverse
position before the scan beam is moved to initiate imaging at a new
transverse position. Alternatively, the relative rates at which the
longitudinal range altering and sample transverse position changing occur
may be such that all imaging positions in at least one transverse
dimension are scanned at a given longitudinal range in the sample before
the longitudinal range is altered to cause scanning at a new range to be
performed. The latter scanning procedure may be desirable where very high
speed, uniform velocity longitudinal scanning would be required if the
first scanning pattern were utilized. A third scanning pattern is to step
the longitudinal position control to a selected longitudinal position and
to then perform scanning in one or two dimensions ar such longitudinal
positions.
For some embodiments, a plurality of optical paths may be provided to
permit parallel scanning on the sample. In addition, for some embodiments
a characteristic of the optical source, such as its frequency or intensity
is controlled or varied to control the longitudinal point in the sample
being imaged, the received reflections resulting in an output having a
frequency proportional to the optical length of the path to the
longitudinal point or plane in the sample being imaged at the time. This
output is detected and processed to obtain the image.
Because of aliasing or other problems, spurious intensity variations may
occur in an image produced as a result of a two or three dimensional scan.
To overcome this problem, AOM's may be used as previously indicated or a
plurality of scans may be performed on a sample, with the scans being
averaged to compensate for intensity variations.
For preferred embodiments, the measurements involve non-invasive
cross-sectional imaging in biological specimens. One particular useful
application for the invention is in producing cross-sectional images of
various eye sections.
The foregoing and other objects, features and advantages of the invention
will be apparent from the following more particular description of
preferred embodiments of the invention as illustrated in the accompanying
drawings.
IN THE DRAWINGS
FIG. 1A is a schematic block diagram of an optical coherence domain
reflectometer in accordance with a preferred embodiment of the invention.
FIG. 1B is a schematic block diagram of an alternative embodiment of the
invention utilizing a frequency modulated optical source.
FIG. 2 is a block diagram illustrating One embodiment of a probe module to
achieve multi-dimensional scanning.
FIG. 3A is a diagram to an alternative probe module for performing two or
three dimensional scanning.
FIG. 3B is a diagram of an alternative probe module for achieving three
dimensional scanning.
FIG. 3C is a diagram of an alternative probe module for performing circular
scanning.
FIGS. 4A and 4B are diagrams of two additional probe module embodiments for
performing multidimensional scanning.
FIG. 5 is a side sectional side diagrammatic view of one embodiment of an
endoscopic probe module.
FIG. 6 is a side sectional diagrammatic view of a second embodiment of
endoscopic probe module.
FIG. 7 is a side sectional diagrammatic view of a third embodiment of
endoscopic probe module.
FIG. 8A is a diagram illustrating a first scan pattern for two dimensional
scanning of a sample in accordance with the teachings of this invention.
FIG. 8B is a diagram illustrating a second scan pattern for two dimensional
scanning of a sample in accordance with the teachings of this invention.
FIG. 8C is a diagram illustrating a third scan pattern for two dimensional
scanning of a sample in accordance with the teachings of this invention.
FIG. 9 is a schematic block diagram of a parallel scanned embodiment.
FIG. 10 is a schematic block diagram of a balanced receiver embodiment.
DETAILED DESCRIPTION
Referring first to FIG. 1A, an optical coherence domain reflectometer
(OCDR) 10 is shown which incorporates the teachings of this invention. In
particular, the output from a short coherence length (broad spectral
bandwidth) optical source 12 is coupled as one input to an optical coupler
14. Such coupling may be through a suitable optical path, which for the
preferred embodiment is a fiber optic path 16. Source 12 may, for example,
be a light emitting diode, super luminescent diode or other white light
source of suitable wavelength, or may be a short-pulse laser. Such sources
preferably having a coherence length of less than 10 micrometers for
preferred embodiments. As will be discussed later, it is desirable that
the coherence length of source 12 be minimized to enhance the resolution
of the system.
The other input to coupler 14 is from a laser 18 generating an optically
visible output which is applied to the coupler through a fiber optic path
20. As is discussed in greater detail in the copending application, laser
18 does not contribute to the normal operation of the system and is
utilized only to provide a source of visible light for proper alignment
with a sample, when the light from diode 12 is in the infrared region and
thus not visible.
The output from coupler 14 is applied as an input to coupler 22 through
fiber optic path 24. The light or optical energy received at coupler 22 is
split between a first fiber optic path 26 leading to scanning/sample
assembly 28 and a second fiber optic path 30 leading to a reference
assembly 32. Assembly 28 may include a lens assembly formed of one or more
lenses for focusing light received from optical path 26 on a sample to be
scanned and various mechanisms for causing lateral, transverse or
longitudinal motion of the light relative to the sample. In particular,
while for the preferred embodiment longitudinal scanning is performed by
movement at the reference assembly, it is also possible for the sample or
probe to be moved longitudinally, or for longitudinal scanning to
otherwise be performed at assembly 28. The assembly may also include a
mechanism for controlling the longitudinal or depth position of the focus
in conjunction with the longitudinal scan position. A probe module portion
of assembly 28 may be designed for positioning adjacent to an outer
surface of the sample, for example adjacent to a patient's eye for
scanning and imaging or taking measurements on the patient's eye, or it
may be adapted to be positioned inside the sample, being, for example,
part of an angioscope or endoscope for scanning internal body or other
channels. For purposes of FIG. 1A, the sample being scanned and/or imaged
is included in the assembly 28. Various mechanisms which may function as
the assembly 28 in accordance with various embodiments of the invention
are shown in FIGS. 2-7.
For all embodiments, light transmitted by the probe to the sample is
reflected by the sample back through the probe module to fiber 26. The
optical fiber of path 26 may be wrapped around a piezoelectric crystal
transducer or actuator 34 which vibrates (i.e. expands and contracts) in
response to an applied electrical signal to cause slight expansion and
contraction of the optical fiber and to thus modulate the optical signal
passing through the fiber. As will be discussed later, this added
modulation may facilitate detection.
Reference assembly 32 may include a collimating lens 36, first and second
acousto-optic modulators 38 and 40 (AOM 1 and AOM 2), a corner-cube
retro-reflector 42 and an end mirror 44. For the preferred embodiment,
corner cube 46 is mounted to a mechanism 46 which reciprocates the corner
cube toward and away from both optical path 30 and end mirror 44 in a
particular pattern to effect longitudinal scanning of the sample. As
discussed in greater detail in the beforementioned copending application,
the corner-cube is preferably moved at a uniform, relatively high velocity
(for example greater than 1 CM/SEC), causing Doppler shift modulation used
to perform heterodyne detection. The length or extent of movement of cube
42 by mechanism 46 is at least slightly greater than half the desired
scanned depth range in the sample. The scanning pattern for mechanism 46
preferably has a uniform velocity V, at least during the portions thereof
during which scanning occurs, and may, for example, be a ramp pattern, or
a sawtooth pattern. Further, as discussed in the copending application, a
sine wave or other scan pattern can be utilized with suitable compensation
in other elements of the circuit.
Alternatively, scanning in the longitudinal or depth dimension may be
accomplished by reciprocating end mirror 44 with a suitable mechanism such
as mechanism 46 rather than corner cube 42. However, if this is done, the
effective stroke is reduced by 50% so that the end mirror 44 must be moved
through a path which is slightly greater than the desired scan depth range
rather than through a path equal to half such range. The greater travel
stroke required for the mechanism 44 in this instance may adversely affect
the scan rate achievable and may also limit the modulating Doppler shift
frequency, requiring the use of additional modulating elements. If corner
cube 46 is eliminated completely, the system becomes more susceptible to
errors resulting from wobble of the end mirror as it is reciprocated.
It is also possible to eliminate the end mirror by arranging the corner
cube for a single pass configuration. In this configuration, incoming
light to the corner cube is aligned with the corner cube vertex. This also
results in a 50% decrease in the effective stroke. In addition, as
discussed above, mechanism 46 may be eliminated in the reference assembly
32, with longitudinal scanning being performed in assembly 28 by moving
either the probe or sample in the longitudinal direction. This will be
discussed later. If this is done, then corner cube 42 is not required and
light from path 30 may impinge directly on mirror 44.
Finally, while for preferred embodiments utilizing a Doppler shift
frequency, mechanism 46 moves a corner cube or end mirror at a velocity
which, as indicated above, is substantially constant in the scanning
range, for some embodiments to be discussed, Doppler shift modulation in
the longitudinal direction is not utilized and movement of the mirror is
effected primarily to control the desired scan depth. For such embodiments
and others, mechanism 46 may operate in step fashion to control the
desired scan depth.
The total length of path 26 between coupler 22 and a selected depth point
in a sample being scanned and the total length of path 30 between coupler
22 and end mirror 44, should be substantially equal for each depth point
of the sample during a scan of selected depth range. In addition, to
prevent group velocity dispersion which would decrease spatial resolution,
the lengths of the optical fibers in paths 26 and 30 should also be
substantially equal. Alternatively, the group velocity dispersion may be
equalized by placing optical materials of known group velocity dispersion
and thickness in the light paths to compensate for any inequality. For
example, where the fiber in the reference path may need to be shorter than
that in the sample probe, a length of high dispersion material may be
included in the reference path. It is also important that the termination
of the optical fibers utilized in this system be angle polished and/or
anti-reflection coated to minimize reflections and maximize throughput.
Mechanism 46 may be any one of a variety of devices adapted for performing
the translation function. For example, mechanism 46 could be a stepper
motor, the motion of which is applied to corner-cube 42 or mirror 44
through an averaging mechanism for embodiments where uniform velocity is
required. A DC servo-motor might also be utilized to obtain the desired
motion. Various electromagnetic actuators, for example a speaker coil, may
also be utilized for this function. With such electromagnetic actuators,
detection of mirror position and servo control thereof may be required in
order to achieve uniform motion where required. More specifically, in a
uniform motion system, a signal indicative of desired mirror position at
each point in the mirror travel path could be compared against a signal
from a detector of actual mirror position and any resulting error signals
utilized to control the actuator to maintain the mirror moving at the
desired constant velocity. It is also possible to use a servo control
galvanometer driven linear translator for the mechanism 46.
One potential problem in the reference mechanism 32 is wobble of the mirror
being translated which may adversely effect the accuracy of distance
determinations. Such wobble is partially compensated for in the embodiment
of FIG. 1A by corner-cube 42, such corner cubes generally having the
property that, regardless of the angle at which a beam is incident
thereon, the beam will always return in exactly the same direction at
which the beam was incident. Other techniques known in the art and/or
discussed in the before-mentioned copending application may also be
utilized to deal with the wobble problem.
Reflections received from assemblies 28 and 32 are applied through optical
paths 26 and 30, respectively to optical coupler 22. These signals are
combined in coupler 22, resulting in interference fringes for
length-matched reflections, (i.e. reflections for which the difference in
reflection path length is less than the source coherence length) and the
resulting combined output is coupled onto fiber optic path 50.
To maximize interference between light returning from the reference and
sample optical paths, their polarization should be substantially the same.
To accomplish this polarization matching, a polarization controller may be
placed in one of the optical paths 26 or 30. For purposes of illustration,
a polarization controller 51 is shown in optical path 30 in FIG. 1A. Such
a polarization controller compensates for changes in polarization in the
fiber optic paths. Alternatively, polarization maintaining fibers and
couplers may be utilized in the system to achieve the desired result.
Further, in applications where polarization is randomly varying, a
polarization diversity receiver can be utilized in th | | |
