U.S. patent number RE42,497 [Application Number 12/205,248] was granted by the patent office on 2011-06-28 for fourier domain low-coherence interferometry for light scattering spectroscopy apparatus and method.
This patent grant is currently assigned to Duke University. Invention is credited to Adam Wax.
United States Patent |
RE42,497 |
Wax |
June 28, 2011 |
**Please see images for:
( Certificate of Correction ) ** |
Fourier domain low-coherence interferometry for light scattering
spectroscopy apparatus and method
Abstract
An apparatus and method for obtaining depth-resolved spectra for
the purpose of determining the size of scatterers by measuring
their elastic scattering properties. Depth resolution is achieved
by using a white light source in a Michelson interferometer and
dispersing a mixed signal and reference fields. The measured
spectrum is Fourier transformed to obtain an axial spatial
cross-correlation between the signal and reference fields with near
1 .mu.m depth-resolution. The spectral dependence of scattering by
the sample is determined by windowing the spectrum to measure the
scattering amplitude as a function of wavenumber.
Inventors: |
Wax; Adam (Chapel Hill,
NC) |
Assignee: |
Duke University (Durham,
NC)
|
Family
ID: |
33416114 |
Appl.
No.: |
12/205,248 |
Filed: |
September 5, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
10429756 |
May 6, 2003 |
7102758 |
Sep 5, 2006 |
|
|
Current U.S.
Class: |
356/497 |
Current CPC
Class: |
G01N
15/0211 (20130101) |
Current International
Class: |
G01B
9/02 (20060101) |
Field of
Search: |
;356/479,497 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0243005 |
|
Oct 1987 |
|
EP |
|
1021126 |
|
Jul 2004 |
|
EP |
|
99/18845 |
|
Apr 1999 |
|
WO |
|
00/42912 |
|
Jul 2000 |
|
WO |
|
2007/133684 |
|
Nov 2007 |
|
WO |
|
Other References
Xie, Tuqiang et al., "Fiber-Optic-Bundle-Based Optical Coherehence
Tomography," Optic Letters, vol. 30, No. 14, Jul. 15, 2005. cited
by other .
Pyhtila, John W. et al., "Fourier-Domain Angle-Resolved Low
Coherence Interferometry Through an Endoscopic Fiber Bundle for
Light-Scattering Spectroscopy," Optic Letters, vol. 31, No. 6, Mar.
15, 2006. cited by other .
Hausler, G. et al., "Coherence Radar and Spectral Radar--New Tools
for Dermatological Diagnosis," Journal of Biomedical Optics, vol.
3, Jan. 1998. cited by other .
Pyhtila, John W. et al., "Rapid, Depth-Resolved Light Scattering
Measurements using Fourier Domain, Angle-Resolved Low Coherence
Interferometry," Optics Express, vol. 12, No. 25, Dec. 13, 2004.
cited by other .
Pyhtila, John W. et al., "Determining Nuclear Morphology Using an
Improved Angle-Resolved Low Coherence. Interferometry System,"
Optics Express, vol. 11, No. 25, Dec. 15, 2003. cited by other
.
Wax, Adam et al., "Cellular Organization and Substructure Measured
Using Angle-Resolved Low-Coherence Interferometry," Biophysical
Journal, Apr. 2002, pp. 2256-2264, vol. 82. cited by other .
Wax, Adam et al., "Measurement of Angular Distributions by Use of
Low-Coherence Interferometry for Light-Scattering Spectroscopy,"
Optics Letters, Mar. 15, 2001, pp. 322-324, vol. 26, No. 6. cited
by other .
Wax, Adam et al., "Determination of Particle Size Using the Angular
Distribtion of Backscattered Light as Measured with Low-Coherence
Interferometry," Journal of the Optical Society of America, Apr.
2002, pp. 737-744, vol. 19, No. 4. cited by other .
Wax, Adam et al., "In Situ Detection of Neoplastic Transformation
and Chemopreventive Effects in Rat Esophagus Epithelium Using
Angle-Resolved Low-Coherence Interferometry," Cancer Research, Jul.
1, 2003, pp. 3556-3559, vol. 63, No. 13. cited by other .
Leitgeb, R. et al., "Performance of Fourier Domain vs. Time Domain
Optical Coherence Tomography," Optics Express, vol. 11, No. 8, Apr.
21, 2003, pp. 889-894. cited by other .
de Boer, Johannes F. et al., "Improved Signal-To-Noise Ratio in
Spectral-Domain Compared with Time-Domain Optical Coherence
Tomography," Optics Letters, vol. 28, No. 21, Nov. 1, 2003, pp.
2067-2069, http://oa.osa.org/abstract.cfm?id=86605. cited by other
.
Choma, Michael A. et al., "Sensitivity Advantage of Swept Source
and Fourier Domain Optical Coherence Tomography," Optics Express,
vol. 11, No. 18, Sep. 8, 2003, pp. 2183-2189. cited by other .
Kim, Y.L. et al., "Simultaneous Measurement of Angular and Spectral
Properties of Light Scattering for Characterization of Tissue
Microarchitecture and its Alteration in Early Precancer," IEEE
Journal of Selected Topics in Quantum Electronics, vol. 9, Issue 2,
Mar./Apr. 2003, pp. 243-256,
http://ieeexploreieee.sub.--org/xpl/freeabs.sub.--all.jsp?tp=&arnumber=12-
38988&isnumber=27791. cited by other .
Roy, Hemant K. et al., "Four-Dimensional Elastic Light-Scattering
Fingerprints as Preneoplastic Markers in the Rat Model of Colon
Carcinogenesis," Gastroenterology, vol. 126, Issue 4, Apr. 2004,
pp. 1071-1081,
http://www.gastrojoumal.org/article/PIIS0016508501000290/abstract.
cited by other .
Wax, Adam et al., "Prospective Grading of Neoplastic Change in Rat
Esophagus Epithelium Using Angle-Resolved Low-Coherence
Interferometry," Journal of Biomedical Optics, vol. 10(5),
Sep./Oct. 2005, pp. 051604-1 through 051604-10. cited by other
.
Brown, William J. et al., "Review and Recent Development of
Angle-Resolved Low-Coherence Interferometry for Detection of
Precancerous Cells in Human Esophageal Epithelium," IEEE Journal of
Selected Topics in Quantum Electronics, vol. 14, No. 1, Jan./Feb.
2008, pp. 88-97. cited by other .
Wax, Adam et al., "Fourier-Domain Low-Coherence Interferometry for
Light-Scaterring Spectroscopy," Optic Letters, vol. 28, No. 14,
Jul. 15, 2003, pp. 1230-1232. cited by other .
Backman, V. et al., "Measuring Cellular Structure at Submicrometer
Scale with Light Scattering Spectroscopy," IEEE J. Sel. Top.
Quantum Electron, vol. 7, Issue 6, Nov./Dec. 2001, pp. 887-893.
cited by other .
Backman, V. et al., "Detection of Preinvasive Cancer Cells," Nature
406, Jul. 6, 2000, pp. 35-36. cited by other .
Wojtkowski, M. et al., "Full Range Complex Spectral Optical
Coherence Tomography Technique in Eye Imaging ," Optics Letters,
vol. 27, Issue 16, Aug. 15, 2002, pp. 1415-1417. cited by other
.
Wojtkowski, M. et al., "In Vivo Human Retinal Imaging by Fourier
Domain Optical Coherence Tomography," J. Biomed. Opt., vol. 7, No.
3, Jul. 1, 2002, pp. 457-463. cited by other .
Leitgeb, R. et al., "Spectral Measurement of Absorption by
Spectroscopic Frequency-Domain Optical Coherence Tomography," Optic
Letters, vol. 25, Issue 11, Jun. 1, 2000, pp. 820-822. cited by
other .
Morgner, U. et al., "Spectroscopic Optical Coherence Tomography,"
Optic Letters, vol. 25, Issue 2, Jan. 15, 2000, pp. 111-113. cited
by other .
Tuchin, V. et al., Tissue Optics: Light Scattering Methods and
Instruments for Medical Diagnosis, SPIE, May 2000. cited by other
.
Amoozegar, Cyrus et a., "Experimental Verification of
T-matrix-based Inverse Light Scattering Analysis for Assessing
Structure of Spheroids as Models of Cell Nuclei," Applied Optics,
vol. 48, No. 10, to be published Apr. 1, 2009, 7 pages. cited by
other .
Graf, R. N. et al., "Parallel Frequency-Domain Optical Coherence
Tomography Scatter-Mode Imaging of the Hamster Cheek Pouch Using a
Thermal Light Source," Optics Letters, vol. 33, No. 12, Jun. 15,
2008, pp. 1285-1287. cited by other .
Robles, Francisco et al., "Dual Window Method for Processing
Spectroscopic OCT Signals with Simultaneous High Spectral and
Temporal Resolution," Optical Society of America, 2008, 12 pages.
cited by other .
Keener, Justin D. et al., "Application of Mie Theory to Determine
the Structure of Spheroidal Scatterers in Biological Materials,"
Optics Letters, vol. 32, No. 10, May 15, 2007, pp. 1326-1328. cited
by other .
Chalut, Kevin J. et al., "Application of Mie Theory to Assess
Structure of Spheroidal Scattering in Backscattering Geometries,"
J. Opt. Soc. Am. A, vol. 25, No. 8, Aug. 2008, pp. 1866-1874. cited
by other .
Chalut, Kevin J., et al., "Label-Free, High-Throughput Measurements
of Dynamic Changes in Cell Nuclei Using Angle-Resolved Low
Coherence Interferometry," Biophysical Journal, vol. 94, Jun. 2008,
pp. 4948-4956. cited by other .
Giacomelli, Michael G. et al., "Application of the T-matrix Method
to Determine the Structure of Spheroidal Cell Nuclei with
Angle-resolved Light Scattering," Optics Letters, vol. 33, No. 21,
Nov. 1, 2008, pp. 2452-2454. cited by other .
Wax, Adam, "Studying the Living Cell Using Light Scattering and
Low-Coherence Interferometry," Laser Biomedical Research Center,
MIT Spectroscopy Laboratory, presented at Case Western Reserve
University 2002, Feb. 1, 2002. cited by other .
Pyhtila, John W. et al., "Polarization Effects on Scatterer Sizing
Accuracy Analyzed with Frequency-Domain Angle-Resolved
Low-Coherence Interferometry," Applied Optics, vol. 46, No. 10,
Apr. 1, 2007. cited by other .
Pyhtila, John W. et al., "Coherent Light Scattering by In Vitro
Cell Arrays Observed with Angle-Resolved Low Coherence
Interferometry," SPIE, vol. 5690, 2005. cited by other .
Wax, Adam et al., "Angular Light Scattering Studies Using
Low-Coherence Interferometry," SPIE, vol. 4251, 2001. cited by
other.
|
Primary Examiner: Lee; Hwa S. A
Attorney, Agent or Firm: Withrow & Terranova PLLC
Claims
What is claimed is:
1. A method of obtaining depth-resolved spectra of a sample for
determining size and depth characteristics of scatterers within the
sample, comprising the steps of: emitting a beam onto a splitter,
wherein the splitter .[.is fixed with respect to the sample, and
wherein the splitter.]. splits light from the .[.bean.]. .Iadd.beam
.Iaddend.to produce a reference beam, .[.which is reflected to
produce a reflected reference beam,.]. and an input beam to the
sample .[.comprised of a substrate having a first surface and a
second surface.].; cross-correlating the .[.reflected.]. reference
beam with a .[.reflected.]. sample beam scattered from the sample
as a result of the input beam by mixing the .[.reflected.].
reference beam and the .[.reflected.]. .Iadd.scattered
.Iaddend.sample beam; spectrally dispersing the mixed
.[.reflected.]. reference beam and the .[.reflected.].
.Iadd.scattered .Iaddend.sample beam to yield a single
.[.spectrally resolved,.]. .Iadd.spectrally-resolved
.Iaddend.cross-correlated reflection profile having depth-resolved
information about the .[.reflected.]. .Iadd.scattered
.Iaddend.sample beam; and generating a spectroscopic depth-resolved
reflection profile.[., by processing.]. .Iadd.that includes
characteristics of scatterers within the sample by:.Iaddend.
.Iadd.providing one or more spectral windows of .Iaddend.the single
spectrally-resolved cross-correlated reflection profile .[.by: at a
plurality of different center wavelengths, applying a window to the
single spectrally-resolved cross-correlated reflection profile
at.]. .Iadd., each of the one or more spectral windows having
.Iaddend.a given center wavelength to obtain spectral information
.[.at the given center wavelength.]. .Iadd.about the sample for
each of the one or more spectral windows.Iaddend.; and
.[.converting the windowed.]. .Iadd.applying a Fourier transform to
the .Iaddend.spectral information .[.via a Fourier transform.]. to
recover depth-resolved information about the sample at .[.all.].
.Iadd.each given .Iaddend.center .[.wavelengths.]. .Iadd.wavelength
.Iaddend.simultaneously.
2. The method of claim 1, further comprising recovering size
information about the scatterers from the spectroscopic
depth-resolved reflection profile.
3. The method of claim 2, wherein recovering the size information
is obtained by measuring a frequency of a spectral modulation in
the spectroscopic depth-resolved reflection profile.
4. The method of claim 2, wherein recovering the size information
is obtained by comparing the spectroscopic depth-resolved
reflection profile to a predicted analytical or numerical
scattering distribution of the sample.
5. The method of claim 1, wherein .[.applying a processing
algorithm.]. .Iadd.providing one or more spectral windows
.Iaddend.is comprised of .[.applying a.]. .Iadd.providing one or
more .Iaddend.Gaussian .[.window.]. .Iadd.windows.Iaddend.,
.Iadd.one or more .Iaddend.multiple simultaneous windows, or
.Iadd.one or more .Iaddend.other window.
6. The method of claim 1, wherein the splitter is comprised from
the group consisting of a beam splitter and an optical fiber
splitter.
7. The method of claim 1, wherein emitting a beam onto the splitter
comprises emitting a collimated beam.
8. The method of claim 7, wherein the input beam comprises a
collimated beam.
9. The method of claim 7, wherein the .[.reflected.]. reference
beam comprises a collimated beam.
10. The method of claim 1, wherein the beam is comprised of a light
comprised of white light from an arc lamp or thermal source.
11. The method of claim 1, wherein cross-correlating the
.[.reflected.]. reference beam with the .[.reflected.].
.Iadd.scattered .Iaddend.sample beam comprises determining an
interference term by measuring the intensity of the .[.reflected.].
.Iadd.scattered .Iaddend.sample beam and the .[.reflected.].
reference beam independently and subtracting them from the total
intensity of the .[.reflected.]. .Iadd.scattered .Iaddend.sample
beam.
12. The method of claim 1, wherein the .[.reflected.]. reference
beam is .[.created by reflecting the reference beam.].
.Iadd.reflected .Iaddend.off of a reference mirror.
13. The method of claim 1, wherein the length of the path of the
reference beam is fixed.
14. The method of claim 1, wherein the splitter is attached to a
fixed reference arm.
15. The method of claim 1, wherein the sample is attached to a
fixed sample arm.
16. The method of claim 1, wherein dispersing the mixed reflected
reference beam and .[.reflected.]. .Iadd.scattered .Iaddend.sample
beam is performed using a spectrograph.
17. A method of obtaining depth-resolved spectra of a sample
comprised of a substrate having a first surface and a second
surface for determining size and depth characteristics of
scatterers within the sample, comprising the steps of: emitting a
beam onto a splitter .[.wherein the splitter is fixed with respect
to the sample.]., wherein the splitter splits light from the beam
to produce a .[.reference beam, which is reflected to produce a
reflected.]. reference beam, and an input beam to the sample
comprised of a substrate having a first surface and a second
surface; cross-correlating the .[.reflected.]. reference beam with
a first .[.reflected.]. .Iadd.scattered .Iaddend.sample beam
comprised of a first portion of light scattered from the first
surface, by mixing the .[.reflected.]. reference beam and the first
portion of light; cross-correlating the .[.reflected.]. reference
beam with a second .[.reflected.]. .Iadd.scattered .Iaddend.sample
beam comprised of a second portion of light scattered from the
second surface, by mixing the .[.reflected.]. reference beam and
the second portion of light; spectrally dispersing the mixed
.[.reflected.]. reference beam and the first .[.reflected.].
.Iadd.scattered .Iaddend.sample beam to yield a .Iadd.first
.Iaddend.single .[.first spectrally dispersed,.].
.Iadd.spectrally-resolved .Iaddend.cross-correlated reflection
profile having depth-resolved information about the first surface
of the substrate; spectrally dispersing the mixed .[.reflected.].
reference beam and the second .[.reflected.]. .Iadd.scattered
.Iaddend.sample beam to yield a .Iadd.second .Iaddend.single
.[.second spectrally dispersed,.]. .Iadd.spectrally-resolved
.Iaddend.cross-correlated reflection profile having depth-resolved
information about the second surface of the substrate; generating a
first spectroscopic depth-resolved reflection profile .[.by
processing the single spectrally dispersed, first cross-correlated
reflection profile by:.]. .Iadd.that includes characteristics of
scatterers within the sample by:.Iaddend. .[.at a plurality.].
.Iadd.providing one or more first spectral windows .Iaddend.of
.[.different center wavelengths, applying a window to.]. the
.Iadd.first .Iaddend.single .[.first spectrally
dispersed,.]..Iadd.spectrally-resolved .Iaddend.cross-correlated
reflection profile.Iadd., each of the one or more first spectral
windows .Iaddend.at a given center wavelength to obtain .[.spectral
information at the given center wavelength; and converting the
windowed spectral information via a Fourier transform to recover
depth-resolved.]. .Iadd.first .Iaddend.spectral information about
the first surface of the substrate .[.at all center wavelengths
simultaneously.]. .Iadd.for each of the one or more first spectral
windows.Iaddend.; and .Iadd.applying a Fourier transform to the
first spectral information to recover depth information about the
first surface at each given center wavelength simultaneously;
and.Iaddend.generating a second spectroscopic depth-resolved
reflection profile .[.by processing the single.]. .Iadd.that
includes characteristics of scatterers within the sample by:
providing one or more .Iaddend.second .[.spectrally dispersed,
cross-correlated reflection profile by: at a plurality of different
center wavelengths, applying a window to the single.].
.Iadd.spectral windows of the .Iaddend.first .[.spectrally
dispersed,.]. .Iadd.single spectrally.Iaddend.-.Iadd.resolved
.Iaddend.cross-correlated reflection profile.Iadd., each of the one
or more second spectral windows .Iaddend.at a given center
wavelength to obtain .[.spectral information at the given center
wavelength; and.]. .[.converting the windowed spectral information
via a Fourier transform to recover depth-resolved.]. .Iadd.second
.Iaddend.spectral information about the second surface of the
substrate .[.at all center wavelength simultaneously.]. .Iadd.for
each of the one or more second spectral windows; and applying a
Fourier transform to the second spectral information to recover
depth information about the second surface at each given center
wavelength simultaneously.Iaddend..
18. The method of claim 17, wherein recovering size information
about the sample is comprised of determining a ratio of the first
spectroscopic depth-resolved reflection profile and the second
spectroscopic depth-resolved reflection profile.
19. The method of claim 17, wherein the first surface is the front
of the substrate and the second surface is the back of the
substrate or a sample attached to or near the back of the
substrate.
20. An apparatus for obtaining depth-resolved spectra of a sample
in order to determine the size and depth characteristics of
scatterers within the sample, comprising: .[.a sample that receives
a sample beam and reflects a reflected sample beam in response,
wherein the reflected sample beam contains light scattered from the
sample;.]. a receiver .[.that is fixed with respect.].
.Iadd.adapted .Iaddend.to .[.the sample, that receives.].
.Iadd.receive .Iaddend.a .[.reflected.]. reference beam and .[.the
reflected.]. .Iadd.a scattered .Iaddend.sample beam .[.and
cross-correlates.]. .Iadd.containing light scattered from a sample
in response to the sample receiving a sample beam, wherein the
receiver is further adapted to cross-correlate .Iaddend.the
.[.reflected.]. reference beam with the .[.reflected.].
.Iadd.scattered .Iaddend.sample beam; a detector .[.that.].
.Iadd.adapted to .Iaddend.spectrally .[.disperses.]. .Iadd.disperse
.Iaddend.the cross-correlated .[.reflected.]. reference beam and
.[.reflected.]. .Iadd.scattered .Iaddend.sample beam to yield a
single .[.spectrally dispersed,.]. .Iadd.spectrally-resolved
.Iaddend.cross-correlated reflection profile having depth-resolved
information about the .[.reflected.]. .Iadd.scattered
.Iaddend.sample beam; and a processor unit adapted to.[.:.].
generate a spectroscopic depth-resolved reflection profile.[., by
processing the single spectrally-resolved cross-correlated
reflection profile by: at a plurality.]. .Iadd.that includes
characteristics .Iaddend.of .[.different center wavelengths,
applying a window to.]. .Iadd.scatterers within the sample by:
providing one or more spectral windows of .Iaddend.the single
spectrally-resolved cross-correlated reflection profile.Iadd., each
of the one or more spectral windows .Iaddend.at a given center
wavelength to obtain spectral information .[.at the given center
wavelength.]. .Iadd.about the sample for each of the one or more
spectral windows.Iaddend.; and .[.converting.]. .Iadd.applying a
Fourier transform to .Iaddend.the spectral information .[.via
Fourier transform.]. to recover .[.depth-resolved spectral.].
.Iadd.depth .Iaddend.information about the sample at .[.all.].
.Iadd.each given .Iaddend.center .[.wavelengths.]. .Iadd.wavelength
.Iaddend.simultaneously.
21. The apparatus of claim 20, wherein the processor unit is
further adapted to recover size information about the sample from
the spectroscopic depth-resolved reflection profile.
22. The apparatus of claim 20, wherein the processor unit is
further adapted to recover .[.the.]. size information by measuring
a frequency of a spectral modulation in the spectroscopic
depth-resolved reflection profile.
23. The apparatus of claim 20, .Iadd.wherein .Iaddend.the processor
unit is further adapted to recover the size information by
.[.comparing the spectroscopic depth-resolved reflection profile to
a predicted analytical or numerical scattering distribution of the
sample.]. .Iadd.measuring a frequency of a spectral modulation in
the spectroscopic depth-resolved reflection profile.Iaddend..
24. The apparatus of claim 20, wherein .[.applying a processing
algorithm.]. .Iadd.providing one or more spectral windows
.Iaddend.is comprised of .[.applying a.]. .Iadd.providing one or
more .Iaddend.Gaussian .[.window.]. .Iadd.windows.Iaddend.,
.Iadd.one or more .Iaddend.multiple simultaneous windows, or
.Iadd.one or more .Iaddend.other window.
25. The apparatus of claim 20, wherein the receiver is comprised of
a splitter.
26. The apparatus of claim 25, wherein the splitter is comprised
from the group consisting of a beam splitter and an optical fiber
splitter.
27. The apparatus of claim 20, wherein the sample beam comprises a
collimated beam.
28. The apparatus of claim 20, wherein the .[.reflected.].
reference beam comprises a collimated beam.
29. The apparatus of claim 20, wherein the received beam is
comprised of a light comprised from the group consisting of a white
light generated by an arc lamp or thermal source.
30. The apparatus of claim 20, wherein the length of the path of
the reference beam is fixed.
31. The apparatus of claim 20, wherein the receiver is attached to
a fixed reference arm.
32. The apparatus of claim 20, wherein the sample is attached to a
fixed sample arm.
33. The apparatus of claim 20, wherein the detector is comprised of
a dispersive element.
34. The apparatus of claim 33, wherein the dispersive element is a
spectrograph.
35. An apparatus for obtaining depth-resolved spectra of a sample
comprised of a substrate having a first surface and a second
surface in order to determine the size and depth characteristics of
scatterers within the sample, comprising: .[.a sample that receives
a sample beam and reflects a first and second reflected sample beam
in response, wherein the first reflected sample beam is comprised
of a first portion of light scattered from the first surface of the
sample, and where the second reflected sample beam is comprised of
a second portion of light scattered from the second surface of the
sample;.]. a receiver .[.that is fixed with respect to the sample,
that receives.]. .Iadd.adapted to receive .Iaddend.a
.[.reflected.]. reference beam .[.and the.]. .Iadd., a
.Iaddend.first .Iadd.scattered sample beam containing light
scattered from a first surface in response to the first surface
receiving a sample beam, .Iaddend.and .Iadd.a .Iaddend.second
.[.reflected.]. .Iadd.scattered .Iaddend.sample .[.beams.].
.Iadd.beam containing light scattered from a second surface in
response to the first surface receiving a sample beam, .Iaddend.and
.[.cross-correlates.]. .Iadd.cross-correlate .Iaddend.the
.[.reflected.]. reference beam with the first .[.reflected.].
.Iadd.scattered .Iaddend.sample beam, and the .[.reflected.].
reference beam with the second .[.reflected.]. .Iadd.scattered
.Iaddend.sample beam; a detector .[.that.]. .Iadd.adapted to
.Iaddend.spectrally .[.disperses.]. .Iadd.disperse .Iaddend.the
cross-correlated .[.reflected.]. reference beam and .Iadd.the
.Iaddend.first .[.reflected.]. .Iadd.scattered .Iaddend.sample beam
to yield a .Iadd.first .Iaddend.single .[.first spectrally
dispersed,.]. .Iadd.spectrally-resolved .Iaddend.cross-correlated
reflection profile having depth-resolved information about the
first surface, and spectrally .[.disperses.]. .Iadd.disperse
.Iaddend.the cross-correlated .[.reflected.]. reference beam and
.Iadd.the .Iaddend.second .[.reflected.]. .Iadd.scattered
.Iaddend.sample beam to yield a .Iadd.second .Iaddend.single
.[.second spectrally dispersed,.]. .Iadd.spectrally-resolved
.Iaddend.cross-correlated reflection profile having depth-resolved
information about the second surface; and a processor unit adapted
to: generate a first spectroscopic depth-resolved reflection
profile.[., by processing the single first cross-correlated
reflection profile by, at a plurality of different center
wavelengths:.]. .Iadd.that includes characteristics of scatterers
within the sample by:.Iaddend. .[.applying a window to the.].
.Iadd.providing one or more first spectral windows of the first
.Iaddend.single .[.first.]. .Iadd.spectrally-resolved
.Iaddend.cross-correlated reflection profile.Iadd., each of the one
or more first spectral windows .Iaddend.at a given center
wavelength to obtain .[.spectral information at the given center
wavelength; and converting the spectral information via Fourier
transform to recover depth-resolved.]. .Iadd.first
.Iaddend.spectral information about the first surface of the
.[.sample at all center wavelengths simultaneously.].
.Iadd.substrate for each of the one or more first spectral
windows.Iaddend.; and .Iadd.applying a Fourier transform to the
first spectral information to recover depth information about the
first surface at each given center wavelength simultaneously;
and.Iaddend. generate a second spectroscopic depth-resolved
reflection profile.[.,.]. .Iadd.that includes characteristics of
scatterers within the sample as a function wavelength and depth
.Iaddend.by .[.processing the single.]..Iadd.:.Iaddend.
.Iadd.providing one or more .Iaddend.second .[.cross-correlated
reflection profile by: at a plurality of different center
wavelengths, applying a window to the single.]. .Iadd.spectral
windows of the .Iaddend.second .Iadd.single spectrally-resolved
.Iaddend.cross-correlated reflection profile.Iadd., each of the one
or more second spectral windows .Iaddend.at a given center
wavelength to obtain .Iadd.second .Iaddend.spectral information
.[.at the given center wavelength.]. .Iadd.about the second surface
of the substrate for each of the one or more second spectral
windows.Iaddend.; and .[.converting the spectral information via.].
.Iadd.applying a .Iaddend.Fourier transform to .Iadd.the second
spectral information to .Iaddend.recover .[.depth-resolved
spectral.]. .Iadd.depth .Iaddend.information about the second
surface .[.of the sample.]. at .[.all.]. .Iadd.each given
.Iaddend.center .[.wavelengths.]. .Iadd.wavelength
.Iaddend.simultaneously.
36. The apparatus of claim 35, wherein the processor unit is
further adapted to recover size information about the sample by
determining a ratio of the first spectroscopic depth-resolved
reflection profile and the second spectroscopic depth-resolved
reflection profile.
37. The apparatus of claim 35, wherein the first surface is the
front of the substrate and the second surface is the back of the
substrate or a sample attached to or near the back of the
substrate.
.Iadd.38. The method of claim 1, comprising: providing the one or
more spectral windows to the single spectrally-resolved
cross-correlated reflection profile as a plurality of spectral
windows at a plurality of different center wavelengths to obtain
the spectral information for each of the one or more spectral
windows; and applying the Fourier transform to the spectral
information to recover the depth-resolved information about the
sample at all of the plurality of different center wavelengths
simultaneously..Iaddend.
.Iadd.39. The method of claim 17, comprising: providing the one or
more first spectral windows to the first single spectrally-resolved
cross-correlated reflection profile as a plurality of spectral
windows at a plurality of different center wavelengths to obtain
the first spectral information for each of the plurality of
spectral windows; and applying the Fourier transform to the first
spectral information to recover the depth-resolved information
about the first surface of the substrate at all of the plurality of
different center wavelengths simultaneously..Iaddend.
.Iadd.40. The method of claim 17, comprising: providing the one or
more second spectral windows to the second single
spectrally-resolved cross-correlated reflection profile as a
plurality of spectral windows at a plurality of different center
wavelengths to obtain the second spectral information for each of
the plurality of spectral windows; and applying the Fourier
transform to the second spectral information to recover the
depth-resolved information about the second surface of the
substrate at all of the plurality of different center wavelengths
simultaneously..Iaddend.
.Iadd.41. The method of claim 17, comprising: providing the one or
more first spectral windows to the first single spectrally-resolved
cross-correlated reflection profile as a plurality of first
spectral windows at a plurality of different center wavelengths to
obtain the first spectral information for each of the plurality of
first spectral windows; providing the one or more second spectral
windows to the second single spectrally-resolved cross-correlated
profile as a plurality of second spectral windows at the plurality
of different center wavelengths to obtain the second spectral
information for each of the plurality of second spectral windows;
applying the Fourier transform to the first spectral information to
recover the depth-resolved information about the first surface of
the substrate at all of the plurality of different center
wavelengths simultaneously; and applying the Fourier transform to
the second spectral information to recover the depth-resolved
information about the second surface of the substrate at all of the
plurality of different center wavelengths
simultaneously..Iaddend.
.Iadd.42. The apparatus of claim 20, wherein the processor unit is
adapted to: provide the one or more spectral windows to the single
spectrally-resolved cross-correlated reflection profile as a
plurality of spectral windows at a plurality of different center
wavelengths to obtain the spectral information for each of the
plurality of spectral windows; and apply the Fourier transform to
the spectral information to recover the depth-resolved information
about the sample at all of the plurality of different center
wavelengths simultaneously..Iaddend.
.Iadd.43. The apparatus of claim 35, wherein the processor unit is
adapted to: provide the one or more first spectral windows to the
first single spectrally-resolved cross-correlated reflection
profile as a plurality of spectral windows at a plurality of
different center wavelengths to obtain the first spectral
information for each of the plurality of spectral windows; and
apply the Fourier transform to the first spectral information to
recover the depth-resolved information about the first surface of
the substrate at all of the plurality of different center
wavelengths simultaneously..Iaddend.
.Iadd.44. The apparatus of claim 35, wherein the processor unit is
further adapted to: provide the one or more second spectral windows
to the second single spectrally-resolved cross-correlated
reflection profile as a plurality of spectral windows at a
plurality of different center wavelengths to obtain the second
spectral information for each of the plurality of spectral windows;
and apply the Fourier transform to the second spectral information
to recover the depth-resolved information about the second surface
of the substrate at all of the plurality of different center
wavelengths simultaneously..Iaddend.
.Iadd.45. The method of claim 2, in which the scatterers are cell
nuclei..Iaddend.
.Iadd.46. The method of claim 3, in which the scatterers are cell
nuclei..Iaddend.
.Iadd.47. The method of claim 4, in which the scatterers are cell
nuclei..Iaddend.
.Iadd.48. The method of claim 21, in which the scatterers are cell
nuclei..Iaddend.
.Iadd.49. The method of claim 22, in which the scatterers are cell
nuclei..Iaddend.
.Iadd.50. The method of claim 23, in which the scatterers are cell
nuclei..Iaddend.
.Iadd.51. The method of claim 1, wherein the bandwidth of at least
one of the one or more spectral windows is between approximately
4.4 nm and 21.0 nm..Iaddend.
.Iadd.52. The method of claim 1, wherein the bandwidth of each of
the one or more spectral windows is between approximately 4.4 nm
and 21.0 nm..Iaddend.
.Iadd.53. The method of claim 17, wherein the bandwidth of at least
one of the one or more spectral windows of the first single
spectrally-resolved cross-correlated reflection profile is between
approximately 4.4 nm and 21.0 nm..Iaddend.
.Iadd.54. The method of claim 17, wherein the bandwidth of at least
one of the one or more spectral windows of the second single
spectrally-resolved cross-correlated reflection profile is between
approximately 4.4 nm and 21.0 nm..Iaddend.
.Iadd.55. The method of claim 48, wherein the bandwidth of at least
one of the one or more spectral windows of the first single
spectrally-resolved cross-correlated reflection profile is between
approximately 4.4 nm and 21.0 nm..Iaddend.
.Iadd.56. The method of claim 17, wherein the bandwidth of each of
the one or more spectral windows of the first single
spectrally-resolved cross-correlated reflection profile is between
approximately 4.4 nm and 21.0 nm..Iaddend.
.Iadd.57. The method of claim 17, wherein the bandwidth of each of
the one or more spectral windows of the second single
spectrally-resolved cross-correlated reflection profile is between
approximately 4.4 nm and 21.0 nm..Iaddend.
.Iadd.58. The apparatus of claim 20, wherein the bandwidth of at
least one of the one or more spectral windows is between
approximately 4.4 nm and 21.0 nm..Iaddend.
.Iadd.59. The apparatus of claim 20, wherein the bandwidth of each
of the one or more spectral windows is between approximately 4.4 nm
and 21.0 nm..Iaddend.
.Iadd.60. The apparatus of claim 35, wherein the bandwidth of at
least one of the one or more spectral windows of the first single
spectrally-resolved cross-correlated reflection profile is between
approximately 4.4 nm and 21.0 nm..Iaddend.
.Iadd.61. The apparatus of claim 35, wherein the bandwidth of at
least one of the one or more spectral windows of the second single
spectrally-resolved cross-correlated reflection profile is between
approximately 4.4 nm and 21.0 nm..Iaddend.
.Iadd.62. The apparatus of claim 61, wherein the bandwidth of at
least one of the one or more spectral windows of the first single
spectrally-resolved cross-correlated reflection profile is between
approximately 4.4 nm and 21.0 nm..Iaddend.
.Iadd.63. The apparatus of claim 35, wherein the bandwidth of each
of the one or more spectral windows of the first single
spectrally-resolved cross-correlated reflection profile is between
approximately 4.4 nm and 21.0 nm..Iaddend.
.Iadd.64. The apparatus of claim 35, wherein the bandwidth of each
of the one or more spectral windows of the second single
spectrally-resolved cross-correlated reflection profile is between
approximately 4.4 nm and 21.0 nm..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for
obtaining depth-resolved spectra for the purpose of determining
structure by measuring elastic scattering properties. More
particularly, Fourier domain, low-coherence interferometry
techniques are applied to light scattering spectroscopy. This
approach permits the viewing and recovery of depth-resolved
structures, as well as obtaining spectroscopic information about
scattered light as a function of depth.
2. Background of the Related Art
Accurately measuring small objects or other physical phenomena is a
goal that is pursued in many diverse fields of scientific endeavor.
For example, in the study of cellular biology and cellular
structures, light scattering spectroscopy (LSS) has received much
attention recently as a means for probing cellular morphology and
the diagnosing of dysplasia. The disclosures of the following
references are incorporated by reference in their entirety:
Backman, V., V. Gopal, M. Kalashnikov, K. Badizadegan, R. Gurjar,
A. Wax, I. Georgakoudi, M. Mueller, C. W. Boone, R. R. Dasari, and
M. S. Feld, IEEE J. Sel. Top. Quantum Electron., 7(6): p. 887-893
(2001); Mourant, J. R., M. Canpolat, C. Brocker, O. Esponda-Ramos,
T. M. Johnson, A. Matanock, K. Stetter, and J. P. Freyer, J.
Biomed. Opt., 5(2): p. 131-137 (2000); Wax, A., C. Yang, V.
Backman, K. Badizadegan, C. W. Boone, R. R. Dasari, and M. S. Feld,
Biophysical Journal, 82: p. 2256-2264 (2002); Georgakoudi, I., E.
E. Sheets, M. G. Muller, V. Backman, C. P. Crum, K. Badizadegan, R.
R. Dasari, and M. S. Feld, Am J Obstet Gynecol, 186: p. 374-382
(2002); Backman, V., M. B. Wallace, L. T. Perelman, J. T. Arendt,
R. Gurjar, M. G. Muller, Q. Zhang, G. Zonios, E. Kline, T.
McGillican, S. Shapshay, T. Valdez, K. Badizadegan, J. M. Crawford,
M. Fitzmaurice, S. Kabani, H. S. Levin, M. Seiler, R. R. Dasari, I.
Itzkan, J. Van Dam, and M. S. Feld, Nature, 406(6791): p. 35-36
(2000); Wax, A., C. Yang, M. Mueller, R. Nines, C. W. Boone, V. E.
Steele, G. D. Stoner, R. R. Dasari, and M. S. Feld, Cancer Res,
(accepted for publication).
The LSS technique examines variations in the elastic scattering
properties of cell organelles to infer their sizes and other
dimensional information. In order to measure cellular features in
tissues and other cellular structures, it is necessary to
distinguish the singly scattered light from diffuse light, which
has been multiply scattered and no longer carries easily accessible
information about the scattering objects. This distinction or
differentiation can be accomplished in several ways, such as the
application of a polarization grating, by restricting or limiting
studies and analysis to weakly scattering samples, or by using
modeling to remove the diffuse component (s).
As an alternative approach for selectively detecting singly
scattered light from sub-surface sites, low-coherence
interferometry (LCI) has also been explored as a method of LSS.
Experimental results have shown that using a broadband light source
and its second harmonic allows the recovery of information about
elastic scattering using LCI [7].
More recently, angle-resolved LCI (a/LCI) has demonstrated the
capability of obtaining structural information by examining the
angular distribution of scattered light from the sample or object
under examination. The a/LCI technique has been successfully
applied to measuring cellular morphology and to diagnosing
intraepithelial neoplasia in an animal model of carcinogenesis.
The above references are incorporated by reference herein where
appropriate for appropriate teachings of additional or alternative
details, features and/or technical background.
SUMMARY OF THE INVENTION
The claimed exemplary embodiments of the present invention address
some of the issues presented above.
An object of the invention is to solve at least the above problems
and/or disadvantages and to provide at least the advantages
described hereinafter.
In one exemplary embodiment of the present invention, an apparatus
comprises a first receiver that receives a first reference light
and outputs a second reference light. A second receiver that
receives a first sample light and outputs a second sample light and
wherein the second sample light contains light scattered from a
sample when at least a portion of the first sample light is
scattered from a sample. A cross-correlator that receives and
cross-correlates the second reference light with the second sample
light. The cross-correlator may be a spatial cross-correlator.
In another exemplary embodiment of the present invention, a
reference arm receives a first reference light and outputs a second
reference light. A sample receives a first sample light and outputs
a second sample light and wherein the second sample light contains
light scattered from the sample when at least a portion of said
first sample light is scattered from the sample. A spatial
cross-correlator receives and cross correlates the second reference
light with the second sample light. The spatial cross-correlator
comprises a detector and a processing unit. The detector outputs an
interference term to the processing unit. The processing unit
processes the interference term to yield depth resolved
cross-correlation reflection profiles of the sample. The processing
unit first applies a Gaussian window and then a Fourier transform
transforms the interference term to yield depth resolved
cross-correlation reflection profiles of the sample. The Fourier
transform obtains an axial spatial cross-correlation between a
signal field(s) and a reference field(s). A light source outputs
light, which contains the first sample light and the first
reference light.
In another exemplary embodiment of the present invention, a method
comprises receiving a first reference light and outputting a second
reference light. A first sample light is received and a second
sample light is output. The second sample light contains light
scattered from a sample when at least a portion of the first sample
light is scattered from a sample along with the reception and cross
correlation of the second reference light with the second sample
light.
In another exemplary embodiment, a method comprises receiving light
and splitting at least a portion of the light into reference light
and sample light. At least a portion of said reference light is
reflected from a reference surface to yield reflected reference
light. At least a portion of the sample light is scattered from a
sample to yield scattered sample light, and the scattered sample
and the reflected reference light are mixed. Information is
recovered about the scattered sample light. The mixing comprises
detecting an intensity of the scattered sample light and the
reflected reference light. Recovering information comprises
extracting an interference term from a total intensity. Recovering
information can further comprise applying a mathematical operator
to the interference term to recover the spectral information about
the scattered sample light at a particular depth to yield depth
resolved cross-correlation reflection points of the sample. The
mathematical operator used is preferably a Gaussian window.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in detail with reference to the
following drawings in which like reference numerals refer to like
elements wherein:
FIG. 1A is a diagram of an exemplary embodiment of an fLCI
system;
FIG. 1B is a diagram of another exemplary embodiment of an fLCI
system using fiber optic coupling;
FIG. 2 is a diagram illustrating exemplary properties of a white
light source;
FIG. 3 is a diagram of an exemplary axial spatial cross-correlation
function for a coverslip sample;
FIG. 4 is a diagram of exemplary spectra obtained for front and
back surfaces of a coverglass sample when no microspheres are
present;
FIG. 5 is a diagram of exemplary spectra obtained for front and
back surfaces of a coverglass sample when microspheres are
present;
FIG. 6 is a diagram of exemplary ratios of spectra in FIGS. 4 and 5
illustrating scattering efficiency of spheres for front and back
surface reflections;
FIG. 7 is a diagram of a generalized version of the system shown in
FIG. 1;
FIG. 8 is a block diagram of an exemplary embodiment of a method in
accordance with the present invention; and
FIG. 9 is a block diagram of another exemplary embodiment of a
method in accordance with the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In the following detailed description of the various exemplary
embodiments, reference is made to the accompanying drawings that
show, by way of illustration, specific embodiments in which the
invention may be practiced. In the drawings, like numerals describe
substantially similar components throughout the several views.
These embodiments are described in sufficient detail to enable
those skilled in the art to practice the invention. Other
embodiments may be utilized without departing from the scope of the
present invention. Moreover, it is to be understood that various
embodiments of the invention, although different, are not
necessarily mutually exclusive. For example, a particular feature,
structure, or characteristic described in one embodiment may be
included within other embodiments. Therefore, the following
detailed description is not to be taken in a limiting sense. The
scope of the present invention is delineated by the claims, along
with the full scope of equivalents to which such claims are
entitled.
The contents of the following references are incorporated by
reference in their entirety: Wojtkowski, M., A. Kowalczyk, R.
Leitgeb, and A. F. Fercher, Opt. Lett., 27(16): p. 1415-1417
(2002); Wojtkowski, M., R. Leitgeb, A. Kowalczyk, T. Bajraszewski,
and A. F. Fercher, J. Biomed. Opt., 7(3): p. 457-463 (2002);
Leitgeb, R., M. Wojtkowski, A. Kowalczyk, C. K. Hitzenberger, M.
Sticker, and A. F. Fercher, Opt. Lett., 25(11): p. 820-822
(2000).
In general, spectral radar makes use of techniques where
depth-resolved structural information is recovered by applying a
Fourier transform to the spectrum of two mixed fields. In fLCI, the
aforementioned approach used in spectral radar applications is
extended to recover not only depth-resolved structure, but also to
obtain spectroscopic information about scattered light as a
function of depth. The capabilities of fLCI enable extracting the
size of polystyrene beads in a sub-surface layer based on their
light scattering spectrum. The apparatus and method according to
exemplary embodiments of the invention can be applied to many
different areas. One such area of application is to recover nuclear
morphology of sub-surface cell layers.
One exemplary embodiment of the fLCI scheme is shown in FIG. 1A.
White light from a Tungsten light source 100 (e.g. 6.5 W, Ocean
Optics.TM.) is coupled into a multimode fiber 101 (e.g. 200 .mu.m
core diameter). The output of the fiber 101 is collimated by an
achromatic lens 102 to produce a beam 104 (e.g. a pencil beam 5 mm
in diameter). The beam 104 is then forwarded to an fLCI system
10.
This illumination scheme achieves Kohler illumination in that the
fiber acts as a field stop, resulting in the proper alignment of
incident or illuminating light and thereby achieving critical
illumination of the sample. In the fLCI system 10, the white light
beam is split by the beamsplitter 106 (BS) into a reference beam
105 and an input beam 107 to the sample 108. The light scattered by
the sample 108 is recombined at the BS 106 with light reflected by
the reference mirror 114 (M).
The reference beam 105 in conjunction with the reference mirror 114
forms a portion of a reference arm that receives a first reference
light and outputs a second reference light. The input beam 107 and
the sample 108 form a portion of a sample arm that receives a first
sample light and outputs a second sample light.
Those skilled in the art will appreciate that the light beam can be
split into a plurality of reference beams and input beams (e.g. N
reference beams and N input beams) without departing from the
spirit and scope of the present invention. Further, the splitting
of the beams may be accomplished with a beamsplitter or a fiber
splitter in the case of an optical fiber implementation of and
exemplary embodiment of the present invention.
In the exemplary embodiment of the present invention shown in FIG.
1A, the combined beam is coupled into a multimode fiber 113 by an
aspheric lens 110. Again, other coupling mechanisms or lens types
and configurations may be used without departing from the spirit
and scope of the present invention. The output of the fiber
coincides with the input slit of a miniature spectrograph 112 (e.g.
USB2000, Ocean Optics.TM.), where the light is spectrally dispersed
and detected.
The detected signal is linearly related to the intensity as a
function of wavelength I(.lamda.), which can be related to the
signal and reference fields (E.sub.s, E.sub.r) as:
<I(.lamda.)>=<|E.sub.s(.lamda.)|.sup.2>+<|E.sub.r(.lamda.)-
|.sup.2>+2Re<E.sub.s(.lamda.)E*.sub.r(.lamda.)>cos .phi.
(1) where .phi. is the phase difference between the two fields and
<. . .> denotes an ensemble average.
The interference term is extracted by measuring the intensity of
the signal and reference beams independently and subtracting them
from the total intensity.
The axial spatial cross-correlation function, .GAMMA..sub.SR(z)
between the sample and reference fields is obtained by resealing
the wavelength spectrum into a wavenumber (k=2.pi./.lamda.)
spectrum then Fourier transforming:
.GAMMA..sub.SR(z)=.intg.dke.sup.ikz<E.sub.s(k)E*.sub.r(k)>cos
.phi.. (2)
This term is labeled as an axial spatial cross-correlation as it is
related to the temporal or longitudinal coherence of the two
fields.
Another exemplary embodiment of an fLCI scheme is shown in FIG. 1B.
In this exemplary embodiment, fiber optic cable is used to connect
the various components. Those skilled in the art will appreciate
that other optical coupling mechanisms, or combinations thereof,
may be used to connect the components without departing from the
spirit and scope of the present invention.
In FIG. 1B, white light from a Tungsten light source 120 is coupled
into a multimode fiber 122 and the white light beam in the
multimode fiber is split by the fiber splitter (FS) 124 into a
reference fiber 125 and an sample fiber 127 to the sample 130. The
fiber splitter 124 is used to split light from one optical fiber
source into multiple sources.
The reference light in reference fiber 125, in conjunction with a
lens 126 (preferably an aspheric lens) and the reference mirror
128, forms a portion of a reference arm that receives a first
reference light and outputs a second reference light. Specifically,
reference light in reference fiber 125 is directed to the reference
mirror 128 by lens 126, and the reference light reflected by the
reference mirror 128 (second reference light) is coupled back into
the reference fiber 125 with lens 126. The sample light in sample
fiber 127 and the sample 130 form a portion of a sample arm that
receives a first sample light and outputs a second sample light.
Specifically, sample light in sample fiber 127 is directed to the
sample 130 by lens 131 (preferably as aspheric lens), and at least
a portion of the sample light scattered by the sample 130 is
coupled into the sample fiber 127 by lens 131. In the exemplary
embodiment shown in FIG. 1B, the sample 130 is preferably spaced
from lens 131 by a distance approximately equal to the focal length
of lens 131.
At least a portion of the reflected reference light in reference
fiber 125 and at least a portion of the scattered sample light on
sample fiber 127 are coupled into a detector fiber 133 by the FS
124.
The output of detector fiber 133 coincides with the input of a
miniature spectrograph 132, where the light is spectrally dispersed
and detected.
FIG. 2 illustrates some of the properties of a white light source.
FIG. 2(a) illustrates an autocorrelation function showing a
coherence length (l.sub.C=1.2 .mu.m). FIG. 2(a) shows the
cross-correlation between the signal and reference fields when the
sample is a mirror, and this mirror is identical to the reference
mirror (M). In this exemplary scenario, the fields are identical
and the autocorrelation is given by the transform of the incident
field spectrum, modeled as a Gaussian spectrum with center
wavenumber k.sub.o=10.3 .mu.m.sup.-1 and l/e width
.DELTA.k.sub.l/e=2.04 .mu.m.sup.-1 (FIG. 2(b)).
FIG. 2(b) shows an exemplary spectrum of light source that can be
used in accordance with the present invention.
From this autocorrelation, the coherence length of the field,
l.sub.c=1.21 .mu.m is determined. This is slightly larger than the
calculated width of l.sub.c=2/.DELTA.k.sub.l/c=0.98 .mu.m, with any
discrepancy most likely attributed to uncompensated dispersion
effects. Note that rescaling the field into wavenumber space is a
nonlinear process which can skew the spectrum if not properly
executed [13].
In data processing, a fitting algorithm is applied (e.g. a cubic
spline fit) to the rescaled wavenumber spectrum and then resampled
(e.g. resample with even spacing). The resampled spectrum is then
Fourier transformed to yield the spatial correlation of the sample.
Those skilled in the art will appreciate that other frequency based
algorithms or combinations of algorithms can be used in place of
the Fourier transform to yield spatial correlation. One example of
a software tool that can be used to accomplish this processing in
real time or near real time is to use LabView.TM. software.
In one exemplary embodiment of the present invention, the sample
consists of a glass coverslip (e.g., thickness, d.about.200 .mu.m)
with polystyrene beads which have been dried from suspension onto
the back surface (1.55 .mu.m mean diameter, 3% variance). Thus, the
field scattered by the sample can be expressed as:
E.sub.s(k)=E.sub.front(k)e.sup.ik.sup..delta..sup.z+E.sub.back(k)e.sup.ik-
(.sup..delta..sup.z+nd) (3)
In equation 3, E.sub.front and E.sub.back denote the field
scattered by the front and back surfaces of the coverslip, and
.delta.z is the difference between the path length of the reference
beam and that of the light reflected from the front surface and n
the index of refraction of the glass. The effect of the
microspheres will appear in the E.sub.back term as the beads are
small and attached closely to the back surface. Upon substituting
equation 3 into equation 2, a two peak distribution with the width
of the peaks given by the coherence length of the source is
obtained.
In order to obtain spectroscopic information, a Gaussian window is
applied to the interference term before performing the Fourier
transform operation. Those skilled in the art will appreciate that
other probabilistic windowing methodologies may be applied without
departing from the spirit and scope of the invention. This makes it
possible to recover spectral information about light scattered at a
particular depth.
The windowed interference term takes the form:
<E.sub.s(k)E*.sub.r(k)>exp
[-((k-k.sub.w)/.DELTA.k.sub.w).sup.2]. (4)
The proper sizing of a windowed interference term can facilitate
the processing operation. For example, by selecting a relatively
narrow window (.DELTA.k.sub.w small) compared to the features of
E.sub.s and E.sub.k, we effectively obtain <Es(kw)E*r(kw) >.
In processing the data below, we use .DELTA.k.sub.w=0.12
.mu.m.sup.-1 which degrades the coherence length by a factor of
16.7. This exemplary window setting enables the scattering at 50
different wavenumbers over the 6 .mu.m.sup.-1 span of usable
spectrum.
In FIG. 3, an axial spatial cross-correlation function for a
coverslip sample is showed according to one embodiment of the
invention. FIGS. 3(a) and (b) shows the depth resolved
cross-correlation reflection profiles of the coverslip sample
before and after the processing operations. In FIG. 3(a), a high
resolution scan with arrows indicating a peak corresponding to each
glass surface is shown. In FIG. 3(b), a low resolution scan is
obtained from the scan in FIG. 3(a) is shown by using a Gaussian
window.
Note that the correlation function is symmetric about z=0,
resulting in a superposed mirror image of the scan. Since these are
represented as cross-correlation functions, the plots are symmetric
about z=0. Thus the front surface reflection for z>0 is paired
with the back surface reflection for z<0, and vice versa.
In FIG. 3(a), the reflection from the coverslip introduces
dispersion relative to the reflection from the reference arm,
generating multiple peaks in the reflection profile. When the
spectroscopic window is applied, only a single peak is seen for
each surface, however several dropouts appear due to aliasing of
the signal.
To obtain the spectrum of the scattered light, we repeatedly apply
the Gaussian window and increase the center wavenumber by 0.12
.mu.m.sup.-1 between successive applications. As mentioned above,
.DELTA.k.sub.w=0.12 .mu.m.sup.-1 is used to degrade the coherence
length by a factor of 16.7. This results in the generation of a
spectroscopic depth-resolved reflection profile.
FIGS. 4(a) and (b) show the spectrum obtained for light scattered
from the front (a) and back (b) surfaces of a coverglass sample
respectively, when no microspheres are present. The reflection from
the front surface appears as a slightly modulated version of the
source spectrum. The spectrum of the reflection from the rear
surface however has been significantly modified. Thus in equation
3, we now take E.sub.front(k)=E.sub.s(k) and
E.sub.back(k)=T(k)E.sub.s(k), where T(k) represents the
transmission through the coverslip.
In FIG. 5, the spectra for light scattering obtained for front (a)
and back (b) surfaces of a coverglass sample when microspheres are
present on the back surface of the coverslip are shown in FIGS.
5(a) and (b). It can be seen that the reflected spectrum from the
front surface has not changed significantly, as expected. However,
the spectrum for the back surface is now modulated. We can examine
the scattering properties S(k) of the microspheres by writing the
scattered field as E.sub.spheres(k)=S(k)T(k)E.sub.s(k) and taking
the ratio E.sub.spheres(k)/E.sub.back(k)=S(k), which is shown as a
solid line in FIG. 6(a). It can be seen from this ratio that the
microspheres induce a periodic modulation of the spectrum.
In FIG. 6(a), a ratio of the spectra found in FIG. 4 and FIG. 5 is
shown. This illustrates the scattering efficiency of spheres for
front (represented by the dashed line) and back (represented by the
solid line) surface reflections. In FIG. 6(b), a correlation
function obtained from ratio of back surface reflections is shown.
The peak occurs at the round trip optical path through individual
microspheres, permitting the size of the spheres to be determined
with sub-wavelength accuracy.
For comparison, the same ratio for the front surface reflections
(dashed line in FIG. 6(a)) shows only a small linear variation.
Taking the Fourier transform of S(k) yields a clear correlation
peak (FIG. 6(b)), at a physical distance of z=5.24 .mu.m. This can
be related to the optical path length through the sphere by z=2nl
with the index of the microspheres n=1.59. The diameter of the
microspheres to be l=1.65 .mu.m+/-0.33 .mu.m, with the uncertainty
given by the correlation pixel size. Thus with fLCI, we are able to
determine the size of the microspheres with sub-wavelength
accuracy, even exceeding the resolution achievable with this white
light source and related art LCI imaging.
There are many applications of the various exemplary embodiments of
the present invention. One exemplary application of fLCI is in
determining the size of cell organelles, in particular the cell
nucleus, in epithelial tissues. In biological media, for example,
the relative refractive indices are lower for organelles compared
to microspheres and thus, smaller scattering signals are expected.
The use of a higher power light source will permit the smaller
signals to be detected. Other examples include detection of
sub-surface defects in manufactured parts, including fabricated
integrated circuits, detection of airborne aerosols, such as nerve
agents or biotoxins, and detection of exposure to such aerosols by
examining epithelial tissues within the respiratory tract.
Additionally, the larger the size of the nucleus (compared to the
microspheres in this experiment), the higher the frequency
modulation of the spectrum. Those skilled in the art will
appreciate that higher frequency oscillations are detected at a
lower efficiency in Fourier transform spectroscopy techniques.
Therefore, in order to detect these higher frequency oscillations,
a higher resolution spectrograph is used.
FIG. 7 illustrates a generalized embodiment of the fLCI system
shown in FIG. 1 and discussed in greater detail above. In FIG. 7, a
light source 700 (e.g. a multi-wavelength light) is coupled into an
fLCI system 702. Within the fLCI system 702, a sample portion 704
and a reference portion 706 are located. The sample portion 704
includes a light beam and light scattered from a sample. For
example, the sample portion 704 may include a sample holder, a free
space optical arm, or an optical fiber. The reference portion 706
includes a light beam and light that is reflected from a reference.
For example, the reference portion 706 may include an optical
mirror. A cross-correlator 708 receives and cross-correlates light
from the sample with light from the reference.
FIG. 8 illustrates another exemplary embodiment of the present
invention. In FIG. 8, a method is disclosed where a first reference
light is received 800 and a second reference light is output 802. A
first sample light is received 804 and a second sample light is
output 806. The second sample light contains light scattered from a
sample when at least a portion of the first sample light is
scattered from a sample. The second reference light with the second
sample light are received and cross-correlated 808.
FIG. 9 illustrates another exemplary embodiment of the present
invention. In FIG. 9, a method is disclosed where light is received
900 from a sample that has been illuminated. At least a portion of
the light is split into reference light and sample light 902. At
least a portion of said reference light is reflected from a
reference surface to yield reflected reference light 904. At least
a portion of the sample light is scattered from a sample to yield
scattered sample light 906. The scattered sample light and the
reflected reference light are mixed 908. Spectral information is
recovered about the scattered sample light 910.
The foregoing example illustrates how the exemplary embodiments of
the present invention can be modified in various manners to improve
performance in accordance with the spirit and scope of the present
invention.
From the foregoing detailed description, it should be apparent that
fLCI can recover structural information with sub-wavelength
accuracy from sub-surface layers based on measuring elastic
scattering properties. The simplicity of the system makes it an
excellent candidate for probing cellular morphology in tissue
samples and may one day serve as the basis for a biomedical
diagnostic device.
The foregoing embodiments and advantages are merely exemplary and
are not to be construed as limiting the present invention. The
present teaching can be readily applied to other types of
apparatuses. The description of the present invention is intended
to be illustrative, and not to limit the scope of the claims. Many
alternatives, modifications, and variations will be apparent to
those skilled in the art. In the claims, means-plus-function
clauses are intended to cover the structures described herein as
performing the recited function and not only structural equivalents
but also equivalent structures.
* * * * *
References