U.S. patent application number 09/880120 was filed with the patent office on 2001-11-01 for grating based phase control optical delay line.
Invention is credited to Bouma, Brett E., Fujimoto, James G., Tearney, Guillermo.
Application Number | 20010036002 09/880120 |
Document ID | / |
Family ID | 27567815 |
Filed Date | 2001-11-01 |
United States Patent
Application |
20010036002 |
Kind Code |
A1 |
Tearney, Guillermo ; et
al. |
November 1, 2001 |
Grating based phase control optical delay line
Abstract
An apparatus for performing high speed scanning of an optical
delay and its application for performing optical interferometry,
ranging, and imaging, including cross sectional imaging using
optical coherence tomography, is disclosed. The apparatus achieves
optical delay scanning by using diffractive optical elements in
conjunction with imaging optics. In one embodiment a diffraction
grating disperses an optical beam into different spectral frequency
or wavelength components which are collimated by a lens. A mirror
is placed one focal length away from the lens and the alteration of
the grating groove density, the grating input angle, the grating
output angle, and/or the mirror tilt produce a change in optical
group and phase delay. This apparatus permits the optical group and
phase delay to be scanned by scanning the angle of the mirror. In
other embodiments, this device permits optical delay scanning
without the use of moving parts.
Inventors: |
Tearney, Guillermo;
(Cambridge, MA) ; Bouma, Brett E.; (Quincy,
MA) ; Fujimoto, James G.; (Cambridge, MA) |
Correspondence
Address: |
TESTA, HURWITZ & THIBEAULT, LLP
HIGH STREET TOWER
125 HIGH STREET
BOSTON
MA
02110
US
|
Family ID: |
27567815 |
Appl. No.: |
09/880120 |
Filed: |
June 13, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09880120 |
Jun 13, 2001 |
|
|
|
09603806 |
Jun 26, 2000 |
|
|
|
6282011 |
|
|
|
|
09603806 |
Jun 26, 2000 |
|
|
|
09079687 |
May 15, 1998 |
|
|
|
6111645 |
|
|
|
|
09603806 |
Jun 26, 2000 |
|
|
|
08607787 |
Feb 27, 1996 |
|
|
|
6134003 |
|
|
|
|
08607787 |
Feb 27, 1996 |
|
|
|
08577366 |
Dec 22, 1995 |
|
|
|
5748598 |
|
|
|
|
08607787 |
Feb 27, 1996 |
|
|
|
08252940 |
Jun 2, 1994 |
|
|
|
08607787 |
Feb 27, 1996 |
|
|
|
08916759 |
Aug 19, 1997 |
|
|
|
5784352 |
|
|
|
|
08916759 |
Aug 19, 1997 |
|
|
|
08492738 |
Jun 21, 1995 |
|
|
|
08492738 |
Jun 21, 1995 |
|
|
|
08033194 |
Mar 16, 1993 |
|
|
|
5459570 |
|
|
|
|
08033194 |
Mar 16, 1993 |
|
|
|
07692877 |
Apr 29, 1991 |
|
|
|
60046739 |
May 16, 1997 |
|
|
|
Current U.S.
Class: |
359/287 ;
359/278; 359/279; G9B/7.018; G9B/7.053; G9B/7.103; G9B/7.113 |
Current CPC
Class: |
A61B 3/1225 20130101;
G11B 7/005 20130101; G01B 2290/35 20130101; A61B 2562/0242
20130101; G02B 27/4255 20130101; B82Y 15/00 20130101; A61B 5/0084
20130101; G11B 7/1353 20130101; A61B 3/1005 20130101; G02B 27/4233
20130101; G11B 7/24 20130101; A61B 5/0066 20130101; G01B 11/2441
20130101; H01S 5/141 20130101; A61B 1/00096 20130101; G02B 5/1828
20130101; G01B 11/00 20130101; G11B 2007/0013 20130101; G02B 26/06
20130101; G11B 7/08564 20130101; A61B 1/00183 20130101; A61B 5/0064
20130101; G01N 21/4795 20130101; G01B 11/002 20130101; G01J 1/00
20130101; G01B 9/02002 20130101; G01B 2290/20 20130101; A61B 5/441
20130101; G11B 7/127 20130101; A61B 1/00172 20130101; G01B 9/02091
20130101; G01B 11/12 20130101; A61B 3/102 20130101; G01B 11/026
20130101; G02B 27/4244 20130101; A61B 5/6852 20130101 |
Class at
Publication: |
359/287 ;
359/278; 359/279 |
International
Class: |
G02F 001/23; G02F
001/01; G02F 001/11 |
Goverment Interests
[0002] This invention was made with government support under
Contract. No. NIH-RO1-EY11289-10 awarded by the National Institutes
of Health, Contract No. N00014-94-1-0717 awarded by the U.S. Office
of Naval Research, and Contract No. F49620-95-1-0221 awarded by the
U.S. Air Force Office of Scientific Research. The government has
certain rights in the invention.
Claims
What is claimed is:
1. An optical delay line apparatus, comprising: an optical input;
an optical output; and a plurality of optical elements in optical
communication with each other, wherein said plurality of optical
elements are used to guide an optical signal having an optical
spectrum from said optical input to said optical output, wherein at
least one of said plurality of optical elements is used to
spatially disperse the optical spectrum of the optical signal, and
wherein at least one of said plurality of optical elements is
adjustable to affect the phase delay and the group delay of the
optical signal between said optical input and said optical
output.
2. The optical delay line apparatus of claim 1 wherein said optical
input and said optical output are the same.
3. The optical delay line apparatus of claim 2 wherein said
plurality of optical elements further comprises: a spectrally
dispersive element in optical communication with said optical
input, said dispersive element spatially dispersing the optical
signal in response to the optical spectrum of the optical signal;
an optical imaging module in optical communication with said
dispersive element, said optical imaging module receiving a
dispersed optical signal from said dispersive element; and a
reflective element in optical communication with said optical
imaging module.
4. The optical delay line apparatus of claim 3 wherein said
dispersive element angularly disperses the optical spectrum of the
optical signal into spectral components, said optical imaging
module produces an image of the angularly dispersed optical signal
at said reflective element, said reflective element rotates about
an axis which is displaceable from a central wavelength of the
image to adjust the phase delay, and the angle of said reflective
element is alterable to adjust the group delay.
5. The optical delay line apparatus of claim 3 wherein said
dispersive element is a diffraction grating and wherein the group
delay is adjusted by altering the angle of said grating.
6. The optical delay line apparatus of claim 3 wherein said
dispersive element is a diffraction grating and the optical signal
impinges on said diffraction grating at an angle of incidence, and
wherein the group delay of the optical signal is adjusted by
altering the angle of incidence.
7. The optical delay line apparatus of claim 3 wherein said
dispersive element has a spatially periodic structure which
spatially disperses wavelength components of the optical spectrum
of the optical signal and wherein the group delay is adjusted by
altering the spatially periodic structure of said dispersive
element.
8. The optical delay line apparatus of claim 3 wherein said
dispersive element comprises an acousto-optic modulator having an
adjustable spatially periodic structure which varies in response to
a radio frequency drive waveform received by said acousto-optic
modulator.
9. The optical delay line apparatus of claim 8 wherein said radio
frequency drive waveform is repetitively altered to produce a
repetitive and substantially constant rate of change of the group
delay.
10. The optical delay line apparatus of claim 8 wherein said radio
frequency drive waveform is repetitively altered to produce a
repetitive change in group delay with a substantially constant
optical throughput efficiency.
11. The optical delay line apparatus of claim 3 wherein said
reflective element is a rotating polygon mirror.
12. An optical interferometric imaging system, comprising: an
optical source producing an optical signal having a broad bandwidth
optical spectrum, said optical signal in optical communication with
an interferometer; an optical delay line apparatus; a sample; and a
detector, wherein said interferometer has an input in optical
communication with said optical source, wherein said interferometer
is in optical communication with said optical delay line apparatus,
said optical delay line apparatus comprising: an optical input; an
optical output; and a plurality of optical elements in optical
communication with each other, wherein said plurality of optical
elements are used to guide the optical signal from said optical
input to said optical output; wherein at least one of said
plurality of optical elements is used to spatially disperse the
optical spectrum of the optical signal, and wherein at least one of
said plurality of optical elements is adjustable to affect the
phase delay and the group delay of the optical signal between said
optical input and said optical output, wherein said interferometer
is also in optical communication with a sample, and wherein said
interferometer has an optical output that is coupled to a detector,
said detector being in electrical communication with a processor,
said processor producing images of optical microstructural
properties of said sample.
13. The imaging system of claim 12 wherein at least one adjustable
optical element repeatedly scans to produce a substantially uniform
rate of change of optical group delay and a time varying optical
phase delay, and wherein said processor compensates for the time
varying optical phase delay.
14. An optical delay line apparatus comprising: an optical input;
an optical output; and a plurality of optical elements in optical
communication with each other, wherein said plurality of optical
elements guide an optical signal having an optical spectrum from
said optical input to said optical output, wherein at least one of
said plurality of optical elements spatially disperses the optical
spectrum of the optical signal, wherein at lest one of said
plurality of optical elements is adjustable to adjust the group
delay of the optical signal between said optical input and said
optical output, and wherein said delay line has the property that
upon interferometrically combining the optical signal transmitted
through said optical delay line with a portion of the optical
signal not transmitted through said optical delay line and
photodetection of the interferometrically combined optical signal,
a non-zero frequency heterodyne signal is achieved.
15. The apparatus of claim 14 used in conjunction with an
interferometric imaging system that requires the scanning of
optical group delay in order to perform imaging of optical
properties of a sample.
16. The apparatus of claim 15 wherein said adjustable optical
element is repetitively scanned and said imaging system has a
signal processing unit that compensates any non-uniform
rate-of-change of phase delay.
17. The apparatus and system of claim 15 wherein said
interferometric imaging system comprises a reference arm, a sample
arm, and an output, wherein said reference arm is coupled to said
optical delay line apparatus, and wherein said plurality of optical
elements further comprises: a spatially dispersive element in
optical communication with said optical input, said dispersive
element spatially dispersing the optical signal in response to the
optical spectrum of the optical signal; an optical imaging module
in optical communication with said dispersive element, said optical
imaging module receiving a dispersed optical signal from said
dispersive element; and a reflective element in optical
communication with said optical imaging module, said reflective
optical element placed away from the Fourier plane of the dispersed
optical spectrum so as to balance first order group velocity
dispersion between said sample and reference arms.
18. The apparatus of claim 17 wherein said spatially dispersive
element comprises a diffractive element.
19. The apparatus of claim 18 wherein said reflective element
comprises a scanning mirror in optical communication with said
diffractive element.
20. The apparatus of claim 18 wherein said diffractive element
comprises an angularly adjustable grating.
21. The apparatus of claim 14 further comprising an acousto-optic
modulator in optical communication with said optical delay
line.
22. The apparatus of claim 14 further comprising an electro-optic
beam deflector in optical communication with said optical delay
line.
23. The apparatus of claim 14 wherein said adjustable optical
element comprises a polygon scanning mirror.
24. The apparatus of claim 14 wherein said spatially dispersive
element has a periodic optical structure with an adjustable period,
said spatially dispersive element angularly dispersing the optical
signal.
25. The apparatus of claim 14 wherein said spatially dispersive
element comprises a holographic optical element.
26. An apparatus for performing scanning of an optical delay of an
optical signal, wherein the apparatus permits control of optical
group delay using adjustable optical elements that spatially
disperse the various wavelength components of the optical spectrum
of the optical signal such that upon interferometrically combining
the delayed optical signal with a portion of the optical signal not
transmitted though said apparatus and photodetection of the
interferometrically combined signal, a zero frequency heterodyne or
homodyne signal is achieved, wherein said optical delay line
apparatus is used in conjunction with an OCT imaging system, said
OCT imaging system comprising an optical source coupled to an
interferometer, said interferometer coupled to said optical delay
line apparatus input and output ports, said interferometer also
coupled to a sample via a probe module, said interferometer also
coupled to at least one photodetector, said photodetector coupled
to a signal processing and control unit, said signal processing and
control unit producing images of microstructural properties of the
sample.
27. An optical delay line comprising: a spatially dispersive
element receiving a beam of incident light having an optical
spectrum and dispersing the beam of incident light into spectral
components; an optical imaging module in optical communication with
said spatially dispersive element, said optical imaging module
receiving the dispersed beam of incident light; and a reflective
element in optical communication with said optical imaging module,
said reflective element receiving light imaged by said optical
imaging module and being adjustable to affect phase delay and group
delay of the beam of incident light.
28. The optical delay line of claim 27 wherein the light received
by said reflective element is reflected back through said optical
imaging module to said spatially dispersive element.
29. The optical delay line apparatus of claim 1 wherein said
optical delay line apparatus is used in conjunction with an OCT
imaging system, said OCT imaging system comprising an optical
source coupled to an interferometer, said interferometer coupled to
said optical delay line apparatus input and output, said
interferometer also coupled to a sample via a probe module, said
interferometer also coupled to at least one photodetector, said
photodetector coupled to a signal processing and control unit, said
signal processing and control unit producing images of said samples
optical microstructural properties.
30. An apparatus for performing optical coherence tomography,
comprising: an optical source producing an optical signal having an
optical spectrum; and an interferometer having an input in optical
communication with said optical source, an optical combiner, a
sample arm for transmitting a first portion of the optical signal
to a sample and receiving the scattered signal from the sample, a
reference arm and an output, wherein said reference arm comprises
an optical delay line, said optical delay line comprising: an
optical input port adapted to receive an optical signal having an
optical spectrum; a plurality of optical elements in optical
communication with each other, one of said plurality of optical
elements being in optical communication with said optical input
port and receiving the optical signal from said optical input port,
and at least one of said plurality of optical elements spatially
dispersing the optical spectrum of the optical signal; and an
optical output port receiving the spatially dispersed optical
signal from said plurality of optical elements; wherein at least
one of said plurality of optical elements is adjustable to
independently affect phase delay and group delay of the optical
signal; a detector in optical communication with said output of
said interferometer; and a processor in electrical communication
with said detector; wherein the second portion of the optical
signal from said reference arm is combined at said combiner with
the first portion of the optical signal from said sample arm, the
combined signal generating a homodyne signal at said detector, and
said processor produces structural images of the sample.
31. An optical delay line comprising: an optical input; an optical
output; and a plurality of optical elements in optical
communication with each other, wherein at least one of said
plurality of optical elements is in optical communication with said
optical input and receives an optical signal from said optical
input, wherein at least one of said plurality of optical elements
spatially disperses the optical spectrum of the received optical
signal, wherein at least one of said plurality of optical elements
is in optical communication with said spatially dispersed optical
spectrum, wherein at least one of said plurality of optical
elements is in optical communication with said optical output and
delivers the optical signal to said optical output, and wherein at
least one of said plurality of optical elements is adjustable to
affect phase delay and group delay of the optical signal between
said optical input and said optical output.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to co-pending to
provisional patent application Ser. No. 60/046,739, filed May 16,
1997, the entirety of which is incorporated herein by
reference.
FIELD OF THE INVENTION
[0003] The invention relates to the field of optical measurement
using a rapid scanning optical delay line and more specifically to
the field of optical coherence tomography.
BACKGROUND
[0004] For many applications in optical ranging and optical imaging
using interferometric based techniques, it is necessary to use a
scanning optical delay line as a component of the measurement
apparatus. A conventional scanning optical delay line produces a
delay by propagating the optical beam through a variable path
length. Such a conventional delay line produces a change in phase
delay and group delay which is determined by the geometric path
length divided, respectively, by the phase velocity and group
velocity of light in the medium of propagation.
[0005] Previous optical delay scanning devices have largely relied
on scanning of the optical path length in order to achieve delay
scanning. Devices using linear actuators, spinning mirrors or
cam-driven linear slides have been demonstrated. Most current
mechanical scanning optical delay lines are not rapid enough to
allow in vivo imaging owing to the presence of motion artifacts.
Piezoelectric optical fiber stretchers that allow rapid scanning
have been demonstrated but they suffer from high power
requirements, nonlinear fringe modulation due to hysteresis and
drift, uncompensated dispersion mismatches, and poor mechanical and
temperature stability. In addition the concept of using a system of
diffraction gratings and lenses has been demonstrated for
stretching and compressing short optical pulses, pulse shaping and
phase control. A combination grating and lens device has been
demonstrated for scanning delay in a short pulse autocorrelator.
The device produces a change in group delay by angular adjustment
of a mirror, however, it does not permit the phase delay to be
adjusted independently of the group delay.
[0006] Such delay lines are useful in performing Optical Coherence
Tomography (OCT). OCT is a relatively new optical imaging technique
that uses low coherence interferometry to perform high resolution
ranging and cross sectional imaging by illuminating the object to
be imaged with low coherence light and measuring the back reflected
or back scattered light as a function of time delay or range.
Optical ranging and imaging in tissue is frequently performed using
a modified Michelson or other type interferometer. Precision
measurement of optical range is possible since interference is only
observed when the optical path length to the scattering features
within the specimen and the reference path optical path length
match to within the coherence length of the light.
[0007] The axial reflectance of structures within the specimen is
typically obtained by varying the reference arm length using a
mechanical scanning linear galvanometer translator and digitizing
the magnitude of the demodulated interference envelope or direct
digitization of the fringes. A cross-sectional image is produced by
recording axial reflectance profiles while the position of the
optical beam on the sample to be imaged is scanned. Such imaging
can be performed through various optical delivery systems such as a
microscope, hand-held probe, catheter, endoscope, or
laparoscope.
SUMMARY OF THE INVENTION
[0008] Unlike conventional scanning optical delay lines, the change
in phase delay using a grating based phase controlled delay line is
more independently adjustable from the change in group delay, so
that when the delay line is used in conjunction with an
interferometer, the modulation of interference fringes produced by
delay line scanning may be more precisely controlled. In one
embodiment a diffraction grating disperses an optical beam into
different spectral frequency or wavelength components which are
collimated by a lens. A mirror is placed one focal length away from
the lens and the alteration of the grating groove density, the
grating input angle, the grating output angle, or the mirror tilt
produces a change in optical group and phase delay. Specifically,
if the mirror tilt produces a change in group delay, the offset of
the beam with respect to the center axis of tilt controls the phase
delay and the resultant modulation frequency at the interferometer.
Moreover, if the grating-lens pair is incident on the center axis
of the tilting mirror, group delay is produced without changing the
phase delay. Then other external modulation techniques may be
applied to control the frequency of modulation of the interference
fringes, or OCT detection can be performed directly at baseband
using a phase diversity homodyne detection technique.
[0009] In the preferred embodiment, the device permits optical
delays to be scanned by scanning an angle of a mirror, thus
providing higher speed optical delay scanning than conventional
optical delay lines which typically require longitudinal or range
scanning of mirrors or other optical retroreflecting elements. In
other embodiments, the device permits high speed scanning by
varying the periodicity of an acousto-optically generated
diffraction grating or other device parameters. In addition, since
interferometric optical ranging and imaging techniques depend upon
the frequency of modulation of the interference fringes produced by
the interferometer, this device permits the design of higher
performance interferometric ranging and imaging systems.
[0010] The optical delay line apparatus is designed so that it may
be used with Low Coherence Interferometry (LCI), Optical Coherence
Tomography (OCT), or other interferometric based optical ranging
and imaging techniques. This apparatus is especially useful for the
implementation of OCT in applications which require high speed
imaging because these applications require high speed scanning of
optical delay. In medical imaging or in vivo imaging applications,
the apparatus permits high speed imaging by reducing or eliminating
blurring from motion artifacts and permitting real time
visualization. The medical applications of this device in OCT
imaging include but are not limited to in vivo medical diagnostic
imaging of the vascular system; gastrointestinal tract; urinary
tract; respiratory tract; nervous system; embryonic tissue; OB/GYN
tissue; and any other internal human organ systems. Other medical
applications include a rapid scanning OCT system for performing
guiding surgical intervention. This device may be also used in OCT
imaging for non-medical applications including imaging in
biological specimens, materials, composite materials,
semiconductors, semiconductor devices and packages, and other
applications requiring high speed imaging.
[0011] The optical delay lines of the invention presented here are
an improvement over existing mechanical delay lines because the
sweep speed of the scan can be increased and the phase delay and
group delay of the scanning can be more independently controlled.
This decoupling of group delay and phase delay permits the control
of fringe modulation in a manner not previously possible by other
optical delay scanning methods. Additionally, the disclosed delay
scheme can be embodied with no moving parts. Finally, this optical
delay line apparatus can be incorporated into OCT systems to enable
high speed reference arm optical path length scanning using
heterodyne or homodyne detection. This scanning technology is
necessary for high speed OCT imaging to for a variety of
applications (e.g., in vivo medical imaging in human tissue). It
has been shown that OCT has ten times greater resolution than
intravascular ultrasound (IVUS) and endoscopic ultrasound (EUS) in
the application of diagnosing tissue pathology. Similar findings
have shown that OCT may be clinically useful for performing high
resolution imaging of other organ systems, including the skin and
gastrointestinal tract.
[0012] The delay line includes common optical components, has
modest power requirements, generates repeatable and controllable
optical delays, and is temperature stable. Moreover, since the
phase delay and group delay are adjustable, the modulation
frequency which is produced in interferometric imaging techniques
can be controlled thus simplifying the detection electronics. This
is especially important for detection scenarios which involve
direct electronic digitization (A/D conversion) of the detected
optical interference signal.
[0013] The grating based phase control optical delay line produces
optical group and phase delay by dispersing the spectrum with a
grating, and applying a temporally modulated linear wavelength
dependent phase. The linear wavelength dependent phase can be
achieved by reflecting the spread spectrum from a tilted mirror. If
the angle of the mirror is rapidly scanned, a time dependent
optical group delay line is produced. The optical delay line can
then be inserted into the reference arm of an interferometer for
performing high speed OCT.
[0014] The phase control delay line is powerful because it allows
group delay to be produced by scanning the angle of a beam, instead
of employing mechanical linear translation to vary optical path
length. The phase control delay line also allows flexibility in the
heterodyne or IF beat frequency. Commercially available mechanical
beam scanners such as the galvanometer, resonant scanner, rotating
polygon mirror, and scanning holographic optical elements are one
to two orders of magnitude faster than mechanical linear
translators. In addition, rapid optical beam scanning can be
performed by devices such as acousto-optic modulators which contain
no moving parts. These components are used in a variety of
applications such as bar code readers, laser printers, and real
time video scanning subsystems.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] This invention is pointed out with particularity in the
appended claims. The above and further advantages of this invention
may be better understood by referring to the following description
taken in conjunction with the accompanying drawings, in which:
[0016] FIGS. 1A and 1B are block diagrams of a grating based phase
control optical delay line in a single pass configuration and a
double pass configuration, respectively.
[0017] FIG. 2 is a block diagram of a grating based phase control
optical delay line using an acousto-optic modulator and a
reflection grating to scan an input beam.
[0018] FIG. 3 is a block diagram of a grating based phase control
optical delay line using an acousto-optic modulator and a
diffraction grating to scan an input beam.
[0019] FIG. 4 is a block diagram of a grating based phase control
optical delay line using a scanning mirror to change the grating
input angle.
[0020] FIG. 5 is a block diagram of a grating based phase control
optical delay line using a steerable grating.
[0021] FIG. 6 is a block diagram of a grating based phase control
optical delay line using a radially scanned circular holographic
optical element.
[0022] FIG. 7 is a block diagram of a grating based phase control
optical delay line having a scanning mirror.
[0023] FIG. 8 is a block diagram of a machined polygon with
reflecting facets which can be used as a scanning mirror.
[0024] FIGS. 9A and 9B show a circular holographic optical element
which can be used as a diffraction grating in a grating based phase
control optical delay line.
[0025] FIG. 10 is a block diagram of a generic pulse shaping
apparatus for delay line scanning in OCT systems.
[0026] FIG. 11 is a block diagram of a tilted mirror configuration
for pulse shaping.
[0027] FIG. 12 is a block diagram of a grating based phase control
optical delay line in an OCT system.
[0028] FIG. 13 shows a block diagram of a grating based phase
control optical delay line in a double pass configuration.
[0029] FIG. 14 shows a grating based phase control optical delay
line with elements that can be modified to change the scanning
group delay.
[0030] FIG. 15 shows a plot of the path length delay as a function
of the grating input angle, .theta..sub.i.
[0031] FIG. 16 is a block diagram of a grating based phase control
optical delay line using a scanning grating.
[0032] FIG. 17 shows a block diagram of a scanning optical delay
line apparatus using a rotating circular holographic optical
element to produce a scanning group delay.
[0033] FIG. 18 shows a block diagram of a scanning optical delay
line apparatus using an acousto-optic modulator and a diffraction
grating.
[0034] FIG. 19 shows a plot of the path length delay produced by
the apparatus of FIG. 18 as a function of the RF driving
frequency.
[0035] FIG. 20 is a block diagram of a grating based phase control
optical delay line using a scanning mirror with its axis of
rotation offset from the optical axis.
[0036] FIG. 21 is a block diagram of a grating based phase control
optical delay line using a polygon mirror with its axis of rotation
offset from the optical axis.
DETAILED DESCRIPTION
[0037] The optical group delay scanning can be accomplished by
scanning the components or parameters of the system in a variety of
ways. In one embodiment (FIG. 1A), the grating based phase control
optical delay line includes a diffraction grating 10 having a
grating spacing (d) and lens 14. A mirror 18 is placed
approximately one focal length (f) away from the lens 14. The
grating 10 disperses the spectrum of the incident optical beam 22.
The Fourier transform of the dispersed optical beam 24 is present
at the mirror 18. If the mirror 18 is tilted (angle .gamma.), a
phase ramp or linear phase shift of the optical spectrum across the
mirror 18 is applied. The inverse Fourier transformation of the
spectrum is achieved as the light propagates back through the lens
14 toward the grating 10. The inverse Fourier transform of a linear
phase shift is a time delay, therefore, as the light is reflected
back from the mirror 18, it results in a temporal group delay of
the incident beam 22. Changes in group delay can also be made to
occur by changing the grating groove density (d), the grating input
angle (.theta..sub.i), the grating output angle (.theta.(d)) or
mirror tilt (.gamma.). A double passed configuration (i.e., the
reflected light 23 approximately follows, in reverse direction, the
incident light path 22) as shown in FIG. 1b can be used to assure
that the incident optical path 22 is coaxial with the reflected
optical path 23. The double passed configuration thus improves
coupling of the reflected beam 23 back into the optics used to
launch the incident beam 22.
[0038] Referring to FIG. 12, an OCT system using a scanning optical
delay line 11 includes an interferometer with a reference path 13,
a sample path 15, a splitter/combiner 19, a broadband source 31, a
detector 21, and a processor 25. The scanning optical delay line 11
is located at the end of the reference arm 13 of the
interferometer. The sample arm 15 includes a probe module 27 to
direct light to the sample 29 and collect light scattered from the
sample 29.
[0039] In some embodiments, the grating groove density (d) and
grating input angle (.theta..sub.i) are varied using an
acousto-optic modulator 28 (AOM) (FIGS. 2 and 3). An AOM 28 can
scan a beam without using any moving parts. The AOM 28 forms a high
frequency sound wave in a crystal which interacts with the crystal
to form a variable refractive index grating. Acoustic energy is
transferred to the crystal by means of a small piezoelectric
transducer (PZT) or other transducer attached to one end. A radio
frequency (RF) signal is applied to the PZT to create an acoustic
wave in the crystal. This acoustic wave varies the refractive index
of the crystal to produce a Bragg grating. The light diffracted by
the grating is transmitted through the crystal at an angle
determined by the grating spacing. If the RF frequency is scanned,
the grating spacing changes, altering the angle. In FIG. 2, the AOM
scans the incident angle .theta..sub.i. In FIG. 3, the AOM
dispersion of the incident beam 22 is augmented by directing the
light transmitted through the AOM 28 through a diffraction grating
32. The second grating 10, 32 is used to increase the dispersion
produced by the AOM 28 and increase the group delay. In another
embodiment, a telescope is used in between the AOM 28 and the
grating 10 as shown in FIG. 2.
[0040] A configuration using an AOM 28 has the additional advantage
of no moving parts in the rapid scanning optical delay line. In
addition, this configuration can achieve higher scanning speeds
than many existing methods. Moreover, changes in coupling and phase
fringe modulation frequency can be compensated for by applying a
customized AOM RF input signal. For example, if the amplitude of
the diffracted light decreases with the AOM scan angle then the RF
drive amplitude can be increased to compensate. More importantly,
if the output angle is not linear with RF drive frequency, then the
RF drive waveform can be adjusted from a single sawtooth or
triangle waveform to compensate for a linear output angle or other
desirable output angle (e.g., a sinusoidal output angle) as a
function of time.
[0041] In yet another embodiment (FIG. 4) the mechanical optical
delay scanning apparatus functions by changing the grating input
angle (.theta..sub.i), using a polygon scanning mirror 34, a
galvanometer, resonant scanner, or a piezoelectric mirror tilter in
the path of the incident beam 22. A telescope 36 is placed between
the scanning mirror 34 and the grating 10 to avoid beam walkoff at
the grating 10. If .gamma. is non-zero, the delay is scanned as the
angle of the mirror 34 is scanned.
[0042] In still another embodiment (FIG. 5), the grating 10 can
also be physically or mechanically scanned in angle using a
galvanometer, a resonant scanner, or a piezoelectric mirror tilter.
For example, a small light weight grating can be placed on the
rotating shaft of a galvanometer to achieve a steerable grating
10.
[0043] In an embodiment in which the grating 10 is a circular
holographic optical element 40 (HOE), the grating 40 may be
radially scanned (FIG. 6). The HOE 40 changes the transmitted
diffraction angle (.DELTA..theta.) of a beam as it is rotated. One
simple configuration consists of a circular element 42 with wedge
subsections 44 (FIG. 9a). Each wedge consists of a diffraction
grating with grating spacing (d) that varies as a function of angle
(.theta.) (FIG. 9b). As the HOE 42 is rotated, d and
.DELTA..theta.(.lambda.) change, producing a varying group delay.
If the HOE 42 is rotated using a high speed motor, the change in
grating spacing d diffracts the beam at a different angle .theta..
Usually the holographic scanner 42 is only used with monochromatic
light. The grating 10 can spatially disperse a broad bandwidth
source. This property is advantageous for phase control because the
rotating HOE grating 42 can replace both the grating 10 and the
angular scanner 18, 34 (FIGS. 1 and 4, respectively).
[0044] In still yet another embodiment (FIG. 7), in either a single
pass or a double pass configuration, the angle (.gamma.) of the
mirror 18 following the grating 10 and lens 14 can be scanned using
a polygon scanning mirror, a galvanometer, resonant scanner, or a
piezoelectric mirror tilter.
[0045] A polygonal scanning mirror 46 (FIG. 8) consists of a
machined polygon 47 with highly reflecting facets 48. A high speed
motor (not shown) is used to rotate the polygon 47. As the polygon
47 rotates, the input beam 22 is reflected off of one of the facets
48, producing an angular scan. Since air bearing motors are
available that can scan at up to 40,000 rpm, a polygonal scanning
mirror 46 with 24 facets 48 can produce 16,000 angular scans per
second. This technology is well-suited for generating linear
angular scans at high speeds.
[0046] A galvanometer used for linear mechanical scanning of
optical delay includes a retroreflector or corner cube mounted on a
lever arm. Due to mechanical resonances and the large force
required to drive the relatively high moment of inertia associated
with a retroreflector mounted to a lever arm, the maximum frequency
of galvanometer-based linear translators is typically only
approximately 100 Hz. The galvanometer is similar in structure to a
torque motor, consisting of a mirror mounted to a moving magnet
rotor positioned between stator coils. The stator coils can provide
a variable magnetic field which causes the rotor to turn. Without
the large mass of a lever arm, this device is capable of angular
scanning with high linearity and frequencies up to a few kHz.
Scanning frequencies are maximized by reducing the mass of the
rotor and attached mirror. Thus, for high scan frequencies, the
mirror must be small in size, limiting the maximum beam size on the
mirror. A linear angular scan is possible because the galvanometer
is heavily damped to prevent coupling into its natural mechanical
resonances.
[0047] A resonant scanner can also be used. The resonant scanner
only oscillates at or near its mechanical resonance frequency.
Thus, resonant scanners produce a sinusoidal change in angle as a
function of time. If the near linear portions of the rising and
falling edges of the sinusoidal angular scan are used, a 66% duty
cycle can be achieved with a 2:1 slope change. Thus, for
applications which require a linear angular scan (e.g., OCT imaging
in which the interference output of the interferometer is detected
and demodulated using a fixed band pass filter), the resonant
scanner can provide a 66% duty cycle with a signal-to-noise (SNR)
loss that is dependent on the noise equivalent bandwidth (NEB). The
resonant scanner, however, can oscillate at speeds up to 20 kHz,
permitting its use for real time OCT imaging if the decreased SNR
is acceptable. By way of example, if each scan of the optical delay
is used to acquire an axial set of image pixels, then images of 500
pixels at 15 to 30 image frames per second correspond to 7.5 to 15
kHz scan frequencies.
[0048] Alternatively, a resonant galvanometer can be used with
resulting nonlinear phase and group delays as a function of time.
This nonlinear behavior can be compensated using post-detection
electronic processing as is known in the art (e.g., a Doppler
tracking receiver). For many delay line applications such as OCT,
it is sometimes desirable to have a non-zero IF frequency or
heterodyne frequency that results when the output of the delay line
is interferometrically combined with some of the original light not
transmitted through the delay line and photodetected. By offsetting
the center of rotation of the tilt mirror 18 relative to the chief
ray passing through the lens 14, the heterodyne frequency can be
adjusted. Since the phase and group delay are decoupled in this
process, the heterodyne frequency can be adjusted without affecting
the group delay. Moreover, if the grating 10 and lens 14 are
located on an axis which intercepts the axis of the tilting mirror
18, group delay is produced without changing the phase delay. This
configuration can be used to apply an external modulation to the
local oscillator for optimal matching of the optical heterodyne
frequency to the system demodulation electronics or for performing
homodyne detection in an OCT imaging system.
[0049] In addition, a double passed configuration can be used in
all of the scanning methods described above to ensure that the
incident optical path 22 is coaxial with the reflected optical
path. Thus, the double passed configuration eliminates the lateral
offset of the reflected beam 23 (shown in FIG. 1) and thus improves
the coupling efficiency back into the light path 22. All of the
above methods provide a way to change both optical group delay and
phase delay, thus allowing control over the optical fringe
modulation frequency.
[0050] When combined with angular beam scanning, the phase control
optical delay can be a versatile method for producing a scanning
optical group delay. Phase control is a technique that uses a
lens-grating pair 10, 14 to alter the temporal properties of
ultrafast pulses by manipulating the spectrum. This technique has
been used for the temporal shaping of ultrafast pulses. A schematic
of the generic pulse shaping apparatus is shown in FIG. 10. The
pulse shaping apparatus consists of two identical reflection
grating-lens pairs 10, 14 and an amplitude, A(x), and/or phase,
.phi.(x) mask 50 placed midway between and one focal length,
.function., away from both lenses 14. The grating disperses the
spectrum of the incident optical beam. If the separation between
the lens 14a and grating 10a is equal to the focal length of the
lens (i.e., L=.function.), the Fourier transform of the dispersed
optical beam 24 occurs at the mask 50. The mask 50 modifies the
spectrum either by phase or amplitude modulation. The modified
spectrum is inverse Fourier transformed by the second lens 14b,
causing an alteration of the temporal profile of the pulse. This
transmission system can be used for delay line scanning in OCT
systems.
[0051] In another embodiment, the pulse shaping apparatus employs a
folded geometry configuration (FIG. 11). This configuration has two
advantages. First, only one grating-lens pair 10, 14 is used. In
addition, the folded geometry enables coupling back into the
reference arm collimating lens 15 and optical fiber 17 without
additional optical components.
[0052] Phase manipulation can provide optical group delay by
dispersing the spectrum with a grating and then applying a
temporally modulated linear wavelength dependent phase. The
wavelength dependent angular diffraction of the incident collimated
beam is given by the grating equation, 1 ( ) = arcsin ( m d - sin (
1 ) ) , ( 1 )
[0053] where m is the diffracted order of the reflected beam 24, d
is the ruling spacing of the grating 10, and .theta..sub.i is the
incident angle on the grating 10. If L=.function., each wavelength
is distributed along the x axis after the lens, at the
position,
x(.lambda.)=.function. tan(.theta..sub.0-.theta.(.lambda.)) (2)
[0054] where .theta..sub.0 is the diffracted angle the center
wavelength of the source, .lambda..sub.0. The Fourier transform of
the input beam now resides at the plane of the mirror 18. Since the
Fourier transform of a linear phase ramp in the spectral domain
corresponds to a delay in the time domain, a temporal group delay
is obtained by placing a phase mask at the mirror, and is described
by:
.phi.(x(.lambda.))=-x(.lambda.).tau. (3)
[0055] The modified spectrum is then inverse Fourier transformed by
propagating back through the folded phase control apparatus,
creating a temporal delay of the input beam 22. The magnitude of
the optical delay is proportional to the spectral dispersion of the
grating 10, the focal length of the lens 14, and the slope of the
phase ramp, .tau.. Note that as described later by offsetting the
center of rotation of the mirror 18 (FIG. 1a) with respect to the
chief ray at angle .theta..sub.0, the phase control device can be
used to independently adjust the phase delay and group delay.
[0056] Arbitrary phase masks, such as a liquid crystal arrays, have
been proposed for pulse shaping, however, a complicated phase mask
is not necessary for producing an optical group delay only.
Instead, the phase-mask-mirror combination can be replaced with a
single tilted mirror (FIG. 11). If the mirror 18 is tilted with an
angle (.gamma.), a linear wavelength dependent phase is applied to
the incident beam 24. A 100 Hz linear scanning group delay line
using a piezoelectric mirror tilter has been previously presented
for construction of a high speed autocorrelator to measure pulse
durations.
[0057] One difficulty with using a tilted mirror 18 to produce the
group delay is that the light 26 reflected from the tilted mirror
18 is no longer collinear with the incident beam 24. Beam walkoff
due to deflection by the tilted mirror 18 limits coupling of the
reflected beam 26 back into the reference arm collimating lens 15
and single mode fiber 17 (FIG. 12). One solution is to use a double
pass configuration (FIG. 13). In this configuration, the beam
emerging from the collimating 15 is decentered on grating 10 so the
diffracted beam 24 is decentered on the lens 14. The beam 24 is
refracted by the lens 14, which is corrected for spherical
aberration, onto the tilted mirror 18. The tilted mirror 18
reflects the beam 26 through the lower portion of the lens 14. The
light is then diffracted off the grating 10 and onto the double
pass mirror 19. The double pass mirror 19 is aligned to allow the
beam 26 to retrace its path back to the collimator. This
configuration allows the folded configuration to be used with a
tilted mirror 18 while avoiding beam walkoff and resultant coupling
losses into the optical fiber 17 or equivalent source. In addition,
since the phase control apparatus is double passed, the delay
produced for a given mirror tilt is also doubled. All of the
devices described can use some form of this double pass
geometry.
[0058] In addition to enabling high speed group delay scanning,
another advantage of the phase control apparatus for OCT is the
capability to compensate dispersion mismatch between the reference
and sample arms. An analysis performed to determine the group
velocity dispersion (GVD) for a grating compressor describes the
dispersion in the double passed configuration to be, 2 2 2 0 = - 0
3 ( L - f ) c 2 2 [ cos ( 0 ) ] - 3 2 . ( 4 )
[0059] When the lens 14 is not one focal length away from the
grating 10, an additional wavelength dependent phase delay is added
to the pulse, creating positive dispersion for L<.function. or
negative dispersion for L>.function.. This property of the phase
control apparatus enables compensation of the dispersion imbalance
between the reference and sample arms in the OCT system by simply
changing the lens-grating separation.
[0060] One powerful aspect of the phase control paradigm is its
versatility. Analysis has revealed that altering any one of several
optical comments in the phase control apparatus can produce a
change in group delay (FIG. 14). Specifically, a scanning group
delay can be obtained by tilting the mirror 18, changing the
incident angle .theta..sub.i on the grating 10, tilting the grating
10, or changing the grating spacing d.
[0061] A simple ray trace analysis can be used to determine an
approximate analytical expression for the group delay produced by
changing the Fourier plane mirror tilt 10 by an angle, .gamma.. The
wavelength dependent phase shift produced by the tilted mirror 18
can be easily determined from the geometry of FIG. 13 or FIG.
14,
.phi.(.lambda.)=-2kz(.lambda.) (5)
[0062] or
.phi.(.lambda.)=-2kx(.lambda.)tan(.gamma.). (6)
[0063] The diffracted angle for the center wavelength of the source
is 3 0 = ( 0 ) = arcsin ( 0 d - sin ( i ) ) . ( 7 )
[0064] If the phase delay is reformulated as a function of
frequency, the wavelength dependent phase shift induced by the
folded phase control apparatus is, 4 ( ) = - 2 c f tan ( ) tan ( 0
- arcsin [ 2 c d - sin ( i ) ] ) . ( 8 )
[0065] Since the group delay is defined as 5 g ( ) = 0 , ( 9 )
[0066] after differentiation and substitution of the center
wavelength, the group delay becomes 6 0 = 2 c 0 , ( 10 ) g ( ) = -
2 f 0 tan ( ) c d cos ( 0 ) . ( 11 )
[0067] The change in group delay is twice that of equation (11) for
the double passed phase control system. Given the double passed
group delay length, l.sub.g=2 .tau..sub.gc, for small angular
deviations of the mirror by .gamma., the total group delay length
is a linear function of the scan angle, For values at Littrow's
angle, 7 l g ( ) = 4 f 0 d cos ( 0 ) . ( 12 ) i = L = arcsin ( 2 d
) , ( 13 )
[0068] d=150 lines per mm, .DELTA..gamma.=10.degree., and
.function.=10 cm, the total group delay length calculated using
equation (12) is 14 mm.
[0069] Since rapid scanning requires a small mirror, vignetting of
the spectrum is a potential problem. The beam spread on the mirror
18,
.DELTA.x(.lambda..sub.max,.lambda..sub.min)=.function.(tan[.theta.(.lambda-
..sub.max)]-tan[.theta.(.lambda..sub.min)]), (14)
[0070] determines the maximum allowable mirror size for the rapid
scanning delay line. For the parameters given above with a
.lambda..sub.max-.lambd- a..sub.min bandwidth of 200 nm, the beam
spread is 3 mm. Thus, the mirror 18 must be at least 3 mm or
clipping of the spectrum will occur, resulting in the convolution
of the autocorrelation function with a sinc function. For this
reason, other configurations of the phase control apparatus that do
not require a moving mirror 18, can be utilized for high resolution
applications.
[0071] Optical group delay may also be produced by scanning the
grating incident beam angle, .theta..sub.i, using a scanning
component 58 such as a rotating polygon mirror, a galvanometer, or
a resonant scanner (FIG. 14). The configuration differs from the
previous method by the inclusion of a fixed angle, .gamma., a
device for scanning .theta..sub.i, and a telescope 56 between the
scanning mirror 58 and the grating 10. Since the tilted mirror 18
is fixed, it can be large enough to accommodate any bandwidth
source. The telescope 56 is inserted between the scanning component
58 and the grating 10 to prevent beam walkoff at the grating 10. To
accomplish this, the image and object planes of the telescope 56
must match the positions of the scanning mirror 58 and the grating
10.
[0072] An analytical expression for the optical group delay
produced by this configuration can be formulated in a similar
manner to the scanning mirror configuration, except the independent
variable is now .theta..sub.i. If .gamma. is small, after
differentiation of the wavelength dependent phase and evaluation at
the center wavelength, the double passed group delay length, 8 l g
( i ) = - 4 f tan [ { i0 - ( - arcsin ( 0 d - sin ( i ) ) ) } + 0 d
1 - ( 0 d - sin ( i ) ) 2 ] , ( 15 )
[0073] where .theta..sub.i0 is the angle of diffraction from the
grating 10 for .lambda..sub.0 for the central scan position. A plot
of the path length delay calculated from equation (15) is shown in
FIG. 15 for d=150 lines per mm, .function.=10 cm,
.gamma.=3.degree., and a variation of the grating incident angle
.theta..sub.i by 10.degree.. For these parameters, the delay
obtained using this method is also an approximately linear function
of the independent variable, .theta..sub.i.
[0074] Angular scanning of the grating 10 in the folded phase
control apparatus also creates a group delay (FIG. 16). The primary
advantage to this configuration is that a telescope 56 is not
necessary because beam walkoff at the grating 10 does not occur.
This approach requires placing a grating 10 on a galvanometer
mirror or polygon scanning mirror as shown.
[0075] Another interesting modification of the phase control
apparatus permits high speed group delay scanning. If the grating
10 is a transmission HOE, such as that shown in FIG. 9, the groove
density of the grating may be scanned in a rapid fashion. This may
be accomplished by using a rotating circular HOE 42 with grating
spacing that varies as a function of angle. As the HOE 42 is
rotated, the change in grating spacing alters the extent of the
spectral spreading (FIG. 17). Since the wavelength dependent phase
delay is proportional to ruling of the grating, rotating the HOE 42
also produces a scanning group delay.
[0076] A more elegant method for changing the grating groove
density is the use of an AOM 28 (FIG. 18). In this configuration,
the wavelength spread is augmented by directing the light 24
transmitted through the AOM 28 through a diffraction grating 32
(FIG. 18). The diffraction grating 32 is necessary because the
change in grating spacing (RF bandwidth) for commercially available
AOMs 28 is not sufficient to produce an adequate group delay scan
for OCT. A telescope (not shown) with a high magnification can be
placed between the AOM 28 and the grating 32 to enhance the change
in diffraction provided by the AOM 28.
[0077] A plot of the path length delay produced by an
AOM-diffraction grating pair 28, 32 as a function of the RF driving
frequency is presented in FIG. 19. To generate this data, an
analytical expression of the group delay for a changing grating
spacing, d, was formulated. The parameters used for generating the
data include the use of a slow shear wave TeO.sub.2 AOM 28
(c.sub.s=0.6 km/s, n=2.35 where c.sub.s is the velocity of sound
and n is the index of refraction), an RF center frequency 50 MHz,
.function.=5 cm, and .gamma.=4.degree.. The secondary diffraction
grating had a ruling of 1200 lines per mm. The group delay produced
by this configuration is nonlinear. This nonlinearity can be
corrected during a group delay scan by modifying the RF waveform
sweep frequency. This can be beneficial when the delay line
apparatus is used in OCT systems. In addition, changes in frequency
dependent diffraction efficiency can be compensated for by altering
the RF signal amplitude. Another difference between the AOM
scanning method is that a Doppler shift (2 .nu..sub.RF) is
transferred to the local oscillator signal. This modulation
frequency may be removed by using the AOM 28 in a double pass
configuration. The AOM configuration is preferable over the
mechanical angular scanning configurations because it allows real
time (15 kHz) path length scanning with no moving parts.
[0078] In order to use the scanning path length delay lines
presented in the previous section, the phase delay must be analyzed
to determine the heterodyne modulation frequency. Unlike other
rapid scanning optical delay lines, such as the linear mechanical
translator or the piezoelectric optical fiber stretcher, the change
in phase delay using the phase control method is not directly
related to the change in group delay. In FIG. 20, the center
wavelength is directed towards the tilting mirror 18 and is offset
from the axis of rotation by x.sub.0. If the mirror surface 60
approximately intersects the axis of rotation 62, the phase delay
can be written as, 9 ( , t ) = 4 f t [ 0 - arcsin ( d - sin ( i ) )
+ x 0 f ] , ( 16 )
[0079] which is a modification of equation. (8) that incorporates a
lateral offset of the galvanometer.
[0080] The heterodyne modulation frequency for a source with a
Gaussian spectral distribution, is determined by the phase shift at
the center wavelength,
.phi.(.lambda.,t).vertline..sub..lambda..sub..sub.0. (17)
[0081] The phase shift for the scanning mirror configuration with a
linear change in angle as a function of time, .gamma.t, is then
because for this case, 10 ( 0 , t ) = 4 f tx 0 0 , ( 18 ) 0 =
arcsin ( d - sin ( i ) ) . ( 19 )
[0082] Thus, the envelope of the autocorrelation function produced
by the scanning linear group delay is modulated by a sinusoid,
cos(2.pi..function..sub.pt), (20)
[0083] where the modulation frequency, 11 f p = 2 x 0 0 . ( 21
)
[0084] As can be seen by equation (18), if the center wavelength of
the spectrum is incident on the mirror axis of rotation
(x.sub.0=0), no modulation frequency is applied to the local
oscillator, even though a scanning linear group delay is produced.
Thus, the interferometric signal consists of the envelope of the
autocorrelation function without any modulation. This can be useful
for OCT imaging systems that perform homodyne detection. This
feature of the tilting mirror configuration can be advantageous. If
an independent phase modulation is applied to the local oscillator,
the system would be capable of scanning at different speeds without
changing the center frequency of the band pass filter before
demodulation. A phase diversity homodyne detection system would be
useful for OCT in this instance.
[0085] Furthermore, by translating the scanning mirror 18 so that
the center wavelength is offset from the axis of rotation
(x.sub.0.noteq.0), an arbitrary modulation frequency can be applied
to the local oscillator. This feature allows complete control over
the center frequency of the local oscillator. The modulation
frequency (i.e., phase delay) may be varied by simply translating
the tilting mirror 18 perpendicular to the optical axis 64 of the
beam. The range of center modulation frequencies that may be
achieved is only limited by spectral vignetting due to the finite
size of the scanning mirror 18.
[0086] To this simple approximation, the group-phase delay
independence of the phase control apparatus is an advantage for
scanning mirrors 18 with an axis of rotation 62 that intersects the
mirror surface 60. When the mirror surface 60 is separated from the
axis of rotation 62 by a distance, r, however, the group-delay and
phase-delay properties are more complex. To an approximation, the
group-delay is linear in angle but not in phase delay. For real
time OCT applications (>1 kHz), a polygon mirror 46 is the
optimal scanning device for rapidly changing the angle, .gamma.
(FIG. 21). In this case, to a first order approximation, x.sub.0
changes across a single scan,
x.sub.0(t).congruent.r tan[(.OMEGA.-.OMEGA..sub.0)t], (22)
[0087] where .OMEGA. is the rotation angle and .OMEGA..sub.0 is the
angle at which the center wavelength of the source is coincident
with the center of the polygon mirror facet. In the limit of small
.OMEGA.-.OMEGA..sub.0, x.sub.0 is a linear function of t. The
modulation frequency in this case becomes, 12 f p ( t ) = 2 r ( - 0
) t 0 . ( 23 )
[0088] While the change in group delay produced by the polygon
scanning mirror 46 is linear, the change in phase is quadratic as a
function of time. Since the modulation frequency shifts linearly
over the scan, the polygon scanning mirror 46 cannot be used in
conjunction with a demodulation method that incorporates a fixed
band pass filter. This is an unfortunate result because the polygon
scanning mirror 46 is the best mechanical means for obtaining high
speed (>1 kHz) linear group delays. The varying modulation
frequency can be overcome, however, by using an alternative
demodulation scheme, such as adaptive frequency mixing detection,
where the frequency at which the demodulation is performed is
varied to track the variation in the modulation frequency. This
scheme is particularly well suited for OCT imaging
applications.
[0089] Alternative phase control configurations, such as scanning
the grating angle of incidence .theta..sub.i or the grating ruling
density, also produce a nonlinear phase delay. By evaluating
equation. (8) at .lambda..sub.0 for these scanning methods, the
phase shift becomes 13 ( t ) = 4 f 0 [ i0 - arcsin ( 0 d ( t ) -
sin ( i ( t ) ) ) ] . ( 24 )
[0090] As with the polygon scanning mirror 46, the phase is a
nonlinear function of time and again, these methods can only be
used in conjunction with an adaptive frequency mixing demodulation
scheme for OCT imaging applications. Based on the phase produced by
the polygon scanner 46, scanning the grating angle of incidence
.theta..sub.i, and the grating ruling density, it is clear that a
change in the demodulation method is warranted to exploit the full
potential of the phase control paradigm. Moreover, a Doppler
tracking or adaptive frequency mixing detection method which tracks
the changing IF heterodyne frequency is useful, especially for OCT
imaging systems.
[0091] Typical operating parameters for one embodiment of the phase
control optical delay are given in terms of an experimental
example. Scanning the angle, .gamma., with a galvanometer produces
a linear optical group delay scan with a constant modulation
frequency. Because the demodulation electronics used in this study
required a constant modulation frequency, a folded double passed
scanning mirror configuration was used to perform coherence gating
in the high speed OCT system.
[0092] A self-phase modulated Kerr Lens Modelocked Cr4+:Forsterite
laser was used as the source of low coherence light for the high
speed OCT system. The laser was set to an output power of 30 mW.
After being transmitted by the fiber optic beamsplitter, the sample
arm power was 12 mW. The FWHM spectrum of the source was 75 nm,
corresponding to a Gaussian autocorrelation FWHM of 10 .mu.m. The
center of the beam was offset on the scanning mirror 18 to produce
a modulation frequency of 750 kHz. The FWHM bandwidth of the signal
was approximately 350 kHz. This modulation frequency was chosen to
enable band pass filtering of the interferometric signal without
accepting any contributions from low frequency noise. In order to
produce linear angular scans at 1 kHz, the mirror size was
minimized to a width of 6 mm. Because of this constraint, the full
bandwidth of the self-phase modulated source was not used. If the
entire spectrum was employed (200 nm), one side of the spectrum
would have been clipped by the edges of the scanning mirror.
[0093] The galvanometer was driven with a 1 kHz triangle waveform,
enabling 2000 scans per second, twice the speed of the PZT-based
high speed OCT system. This rapid scanning rate enabled image
acquisition at 4 frames per second for an image size of 512
(lateral).times.256 (axial) pixels or 8 frames per second for an
image size of 256 (lateral).times.256 (axial) pixels. The phase
control method produced axial scans that were not corrupted by
dropout artifacts due to hysteresis. The total galvanometer scan
angle of 3.degree. provided an optical path length delay of 3
mm.
* * * * *