WO2004111929A2

WO2004111929A2 - Improved system for fourier domain optical coherence tomography

Info

Publication number: WO2004111929A2
Application number: PCT/US2004/016745
Authority: WO
Inventors: Joseph A. Izatt
Original assignee: Duke University
Priority date: 2003-05-28
Filing date: 2004-05-27
Publication date: 2004-12-23
Also published as: WO2004111929A3; US20040239938A1; US20100265511A1; US9448056B2; US7697145B2

Abstract

Optical coherence tomography (OCT) is an imaging method which can image with micrometer-scale resolution up to a few millimeters deep into, for example, living biological tissues and preserved tissue samples. An improved apparatus and image reconstruction algorithm for parallel Fourier Domain OCT which greatly eases requirements for interferometer stability and also allows for more efficient parallel image acquisition is provided. The apparatuses and algorithms reconstruct images from interfered, low-coherence, multiwave length signals having a π radian phase difference relative to one another. Other numbers of signals and other phase differences may be alternatively used, with some combinations resulting in highe resolution and image stability. The apparatus also eliminates a need for bulk optics to modulate a phase delay in a reference arm (48 of the optical path. Images may be reconstructed using two spectrometers (66a, 71a), where each is coupled to a detector array (66b 71b) such as a photodiode array.

Description

IMPROVED SYSTEM FOR FOURIER DOMAIN OPTICAL COHERENCE TOMOGRAPHY

GOVERNMENT RIGHTS

This invention was made with United States Government support under Federal Grant No. BES 0134707 awarded by the National Science Foundation. The government has certain rights to this invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This apphcation claims priority to U.S. Provisional Patent Apphcation Serial No.

60/473,457 filed May 28, 2003, which is incorporated herein by reference.

Field of the Invention

The invention relates to imaging systems and more particularly to tomographic and

interferometric imaging systems with high resolution.

Background of the Related Art

Optical coherence tomography (OCT) is a method for noncontact optical imaging

taking advantage of sequential or scanned distance measurements developed primarily in the

1990's. In biological and biomedical imaging applications, OCT allows for micrometer-scale

imaging noninvasively in transparent and translucent biological tissues. The longitudinal

ranging capability of OCT is based on low-coherence interferometry, in which hght from a

broadband source is split between illuminating the sample of interest and a reference path.

The interference pattern of Hght reflected or backscattered from the sample and Hght from the reference delay contains information about the location and scattering amphtude of the

scatterers in the sample. In conventional (time-domain) OCT, this information is extracted

by scanning the reference path delay and detecting the resulting interferogram pattern as a

function of that delay.

The envelope of the interferogram pattern thus detected represents a map of the

reflectivity of the sample versus depth, called an "A-scan", with depth resolution given by

the coherence length of the source. In conventional OCT systems, multiple A-scans are

acquired while the sample beam is scanned laterally across the tissue surface, making a

continuous series of distance measurements and building up a two-dimensional map of

reflectivity versus depth and lateral extent caUed a "B-scan." The lateral resolution of the B-

scan is given by the confocal resolving power of the sample arm optical system, which is

usuaUy given by the size of the focused optical spot in the tissue.

Time-domain OCT systems have been designed to operate at moderate (~1

image/sec) and high speeds (up to video rate), and have been apphed for imaging in

biological appHcations such as imaging of embryonic development, as well as in medical

diagnostic appHcations such as imaging the structures of the anterior and posterior segments

of the eye, the skin, the gastrointestinal tract, and other tissues. SpeciaHzed probes,

endoscopes, catheters, and biomicroscope attachments have been designed to allow for

OCT imaging in these appHcations.

The time-domain approach in conventional OCT has been by far the most successful

to date in supporting biological and medical appHcations, and aU in-vivo human cknical trials

of OCT to date have utilized this approach. However, the time-domain approach in OCT suffers from some Hmitations. First, the requirement for mechanical scanning, such as with

bulk optics, of the reference delay in conventional OCT introduces complexity, expense, and

reduced reHabiHty, especially those which image at high speed and acquire A-scans at

kilohertz rates. The mechanical scanning reference delay Hne is typicaUy the most complex

optical apparatus in high-speed conventional OCT systems, and can be quite bulky as weU.

Second, since conventional OCT images are built up serially using a single , detector and

coUecting one pixel of image information at a time, no advantage is taken of modern ID and

2D array detection technologies which dominate other forms of optical imaging.

The serial coUection or scanning approach of time-domain OCT is also very wasteful

of sample arm Hght, in that an entire column of pixels is illuminated by that Hght while

reflected Hght is only coUected from one pixel at a time. This wastefulness of sample arm

Hght is costly because sources of broadband Hght suitable for use in OCT systems are

typicaUy expensive and limited in their output power capabiHty, and also because optical

damage to tissue structures often limits the maximum power which may be used in OCT

imaging, particularly in the retina. Where there is a Hmit on the amount of Hght which may

be used to illuminate the sample, the wastefulness of sample arm Hght translates directly into

increased image acquisition time. Further, the serial scanning approach in conventional OCT

requires that the sample under investigation remains stationary during the acquisition of each

A-scan, otherwise motion artifacts may appear in the image. FinaUy, primarily because of the

requirement for a mechanical delay scan, conventional high-speed OCT systems are typicaUy

expensive, bulky, and require frequent optical aHgnment. A potential solution to this need for a new approach has been variously termed

spectral radar, Fourier-domain OCT (FDOCT), complex Fourier OCT, Optical Frequency-

domain imaging, and swept-source OCT. In FDOCT, a different form of low-coherence

interferometry is used in which the reference delay is fixed (except for potential wavelength-

scale delay modulation in some implementations), and information about the location and

ampHtude of scatterers in the sample is derived from the optical spectrum of the Hght

returning from the sample and mixing with the reference. This spectral information is

typicaUy acquired by spectraUy dispersing the detector arm Hght using a spectrometer and

detecting it with an array detector such as a charge-coupled device (CCD), or else by using a

single detector and sweeping the source frequency as a function of time

The A-scan data coUected using FDOCT can be shown to be related (see below) to

the inverse Fourier transform of the spectral data thus acquired. Initial implementations of

FDOCT suffered from image artifacts resulting from: 1) large direct-current (DC) signals

appearing on the detector array arising from non-interfering Hght returning from the

reference delay and d e sample, thus dwarfing the much smaUer interferometric signals; and

2) autocorrelation of Hght signals between different reflections within the sample. As a result,

initial results of FDOCT imaging were fiUed with artifacts and were not comparable to

images obtained with time-domain OCT.

Recendy, newer implementations of FDOCT have appeared which take advantage of

techniques weU known from phase-shifting interferometry (PSI) to eliminate the sources of

both of the artifacts mentioned above. Since both artifacts resulted from Hght appearing on

the detector array which does not arise from interference between sample and reference arm Hght, the recently introduced technique of complex FDOCT eliminates these artifacts by

acquiring multiple spectra with different phase shifts introduced into the reference delay

path.

In a simple implementation of FDOCT, the reference delay consists of a mirror

mounted on a piezoelectric actuator (PZT). One spectrum is acquired at a given position of

the mirror, and then another is acquired with a path-length delay of π/2 (resulting in a

round-trip phase shift of π) introduced into the reference arm by the PZT. It is

straightforward to show that this π phase shift reverses the sign of the interferometric Hght

components but has no effect on the DC components of the detector arm Hght, so

subtracting the spectra obtained at 0 and π phase shifts results in a spectrum free of DC

artifacts. This spectrum can be considered the real part of the complex Fourier transform of

the A-scan. Thus, taking the inverse Fourier transform reconstructs the original A-scan.

However, since only the real part of the complex Fourier spectrum is acquired, the A-scan

data reconstructed is restricted to be symmetric. SpecificaUy, f(-z)=f*(z), and thus only A-

scan data for positive displacements (i.e., z>0) can be reconstructed.

As a further refinement of this phase- shifting technique, an additional spectrum may

be acquired for a path-length delay of π/4 (corresponding to a round-trip phase shift of

π/2). This spectrum (also optionally corrected for DC components by division by one of the

other spectra or by subtraction with a spectrum acquired with a path-length delay of 3π/4)

may be considered the imaginary part of the complex Fourier transform of the A-scan.

Thus, taldng the inverse Fourier transform of the complete complex spectrum (resulting from aU two, three, or four phase measurements) allows for unambiguous reconstruction of

aU depths in the sample limited only by spatial sampHng considerations. Additional

refinements to this approach may be apphed which are commonplace in phase-shifting

interferometry, such as the use of additional phase delays for increased accuracy in

measuring the complex spectrum.

Complex FDOCT thus addresses several of the needs for OCT systems with

decreased complexity and cost and increased reHabihty, having a mostiy fixed reference delay

and utilizing an array detector. However, serious Hmitations to these prior art complex

FDOCT implementations include: 1) a means is still required for displacing the reference

delay by distances on the scale of a wavelength; aU prior systems perform this function by

using bulk optical devices outside of the reference arm optical fiber; and 2) the spectra

obtained at different reference phases are obtained sequentially, thus the sample and

reference arms must be maintained interferometricaUy motionless during the entire A-scan

spectrum acquisition.

SUMMARY OF THE INVENTION

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.

Another object of the invention is to provide an approach to OCT which eliminates

the need for a mechanicaUy scanned reference delay and makes use of array detection

technologies to acquire signals from aU iUuminated axial pixels of an A-scan simultaneously. Another object of the invention is to enable the construction of OCT systems which

are inexpensive, compact, and are mechanicaUy stable such that they rarely require optical

reaHgnment.

Another object of the invention is to provide an improvement to FDOCT which

does not require any means for modulation of the reference arm path length, or may

accompHsh such modulation within the existing reference arm optical fiber.

Another object of the invention is to provide an FDOCT system which obtains the

multiple phase delays required for elimination of image artifacts and/or removal of

constraints on A-scan asymmetry simultaneously, thus relaxing constraints on sample and

reference motion during A-scan acquisition .

To achieve the aforementioned objects, an improved system for FDOCT is provided

which implements readout of multiple reference phases in two or more detector channels

simultaneously.

To further achieve the aforementioned objects, an improved system for FDOCT is

provided which ehminates the need for a mechanicaUy scanned reference delay and makes

use of array detection technologies or wavenumber swept sources to acquire signals from aU

iUuminated axial pixels of an A-scan simultaneously.

To further achieve the aforementioned objects, a method is provided which takes

advantage of inherent π phase differences between different ports of interferometers, and

which also utiHzes orthogonal polarization channels within the reference delay to encode

arbitrary phase delays is provided. To further achieve the aforementioned objects, a system is provided that utiHzes

photodiode arrays for optimal S/N ratio in FDOCT.

To further achieve the aforementioned objects, a system is provided that utiHzes

silicon-based photodiode arrays for FDOCT in the 830nm OCT window and InGaAs arrays

for FDOCT in the 1310 nm and 1550 nm spectral regions. Further advantages of the use of

dual-stripe and two-dimensional CCD and photodiode arrays are also disclosed.

Additional advantages, objects, and features of the invention will be set forth in part

in the description which foUows and in part will become apparent to those having ordinary

skiU in the art upon examination of the foUowing or may be learned from practice of the

invention. The objects and advantages of the invention may be realized and attained as

particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention wiU be described in detaU with reference to the following drawings in

which Hke reference numerals refer to Hke elements wherein:

Figure 1A is a schematic iUustration of a first embodiment of a FDOCT system, in

accordance with the present invention;

Figure IB is a schematic iUustration of a second embodiment of a FDOCT system,

simUar to the embodiment of Figure 1, that utiHzes a support frequency source, in

accordance with the present invention;

Figure 2 is a schematic Ulustration of a third embodiment of a FDOCT, system in

accordance with the present invention; Figure 3 is a schematic illustration of a fourth embodiment of a FDOCT system, in

accordance with the present invention;

Figure 4 is a schematic iUustration of a fifth embodiment of a FDOCT system, in

accordance with the present invention;

Figure 5 is a schematic Ulustration of a sixth embodiment of a FDOCT system, in

accordance with the present invention;

Figure 6 is a schematic iUustration of a seventh embodiment of a FDOCT system, in

accordance with the present invention;

Figure 7 is a generahzed schematic illustration of a eighth embodiment of a FDOCT

system, in accordance with the present invention; and

Figure 8 is a schematic iUustration of an imaging spectrometer, in accordance with the

present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figure 1, a first FDOCT 10 in accordance with one embodiment of the

present invention, is shown. The FDOCT 10 of Fig. 1 includes an optical circulator 34 with

three ports 34a, 34b and 34c, and a first fiber coupler 36 having four Michelson

interferometer ports 37a, 37b, 37c and 37d. The optical circulator 34 and the first fiber

coupler 36 together make up an optical manipulator 1. The first fiber coupler 36 is coupled

to a first polarization controller 42 by a single mode (SM) optical fiber 41. The first

polarization controUer 42 is coupled to a first phase modulator 44 by a SM optical fiber 48.

The first phase modulator 44 is coupled to a SM optical fiber 51 which terminates in a reflector 46. Together the first polarization controUer 42, first phase modulator 44 and

reflector 46, plus SM optical fibers 48 and 46, form a reference arm.

A second polarization controUer 54 is coupled by a SM optical fiber 52 to the first

fiber coupler 36. The second polarization controUer 54 is opticaUy coupled to a first lens 56

by a SM optical fiber 65. The first lens 56 is configured to capture a signal exiting the SM

optical fiber 65, and direct the signal to scanning optics 58, preferably a moveable mirror.

The scanning optics 58, together with second lens 61, directs the signal onto a sample 62 and

receive a reflected signal therefrom. The second polarization controUer 54, first lens 56,

scanning optics 58, and second lens 60, a SM optical fiber 52 and SM optical fiber 65 form a

sample arm. AppHcant notes that the terms "signal," "beam," and "Hght" are used

synonymously to include aU forms of electromagnetic radiation suitable for use in imaging

systems.

Also connected by a SM optical fiber 64 to the first fiber coupler 36 is a first detector

66 having a first spectrometer 66a and first array detector 66b. A second detector 71 is

coupled to the optical circulator 34 by a SM optical fiber 68. The optical circulator 34 may

additionaUy be configured to receive a signal from a source 72 through a SM optical fiber 74.

The first and second detectors, 66 and 71, and SM optical fibers 64 and 68 together form a

detector portion of the FDOCT 10.

The FDOCT system 10 is preferably implemented using a SM fiber Michelson

interferometer iUuminated by a broadband short-coherence length Hght source 72. Light

from the source 72 may be evenly spht between sample and reference arms by d e first fiber

coupler 36. The sample arm can optionaUy include a second polarization controller 54 for controlHng the polarization state of the optical signal and scanning mirror 58 and lens 61 for

scanning and focusing the sample arm signal onto the sample 62.

The reference arm may have a fixed path length, preferably obtained by placing a

reflector 46 on the tip of the reference arm fiber 51 (thus eliminating bulk optics entirely in

the reference arm and the substantial losses incurred in couphng out of and back into the

SM fiber 51).

The reference arm may also optionaUy include a first polarization controUer 42 for

matching the polarization state in the reference arm to that in the sample arm, and may also

optionaUy include a first phase modulator 44 which is capable of selectively causing

wavelength-scale variations in the reference delay under user control. The first phase

modulator 44 can be placed in the sample arm in this and aU subsequent implementations

without any loss of functionaUty. Light from the third port 34c of the first circulator 34 and

Hght from the third Michelson interferometer port 37c of the fiber coupler 36, having 180°

phase difference between them may be coupled into a pair of detectors, 71 and 66

respectively. Detectors 66 and 71 each preferably include spectrometers 66a and 71a, and

array detector, 66b and 71b, respectively. This configuration is designed to place copies of

the phase-shifted optical spectrum onto a matched pair of array detectors.

The FDOCT 10 of Fig. 1 acquires a pair of FDOCT spectra, with π radians phase

difference between them, on a pair of array detectors to eliminate most motion artifacts

associated with conventional phase-shift interferometry. Spectrometers 66a and 71a, and

array detectors 66b and 71b are preferably matched as closely as possible in their optical and electronic characteristics. This can be accomphshed by using spectrometers and detector

arrays of matching design.

In operation, the FDCOT 10 acquires spectra having a relative phase delay of 180°

between them from interferometer ports 37c and 3 of the interferometer, and differences the

spectra in order to eliminate sample and reference arm DC and sample arm autocorrelation

terms. The resulting difference spectrum is inverse Fourier transformed to acquire a one¬

sided A-scan. Care must be taken to assure that the length of the reference arm is adjusted

so that no reflections are observed for z<0.

In an alternative mode of operation, the FDOCT 10 acquires a 180° relative phase

delay between them and differences them in order to eliminate most sample and reference

arm DC and sample arm autocorrelation terms, as designated above. The first phase

modulator 44 in the reference arm is then adjusted for 90° of additional reference delay.

Simultaneous spectra having 90° and 270° phase delay between them are then acquired and

differenced in order to eliminate most sample and reference arm DC and sample arm

autocorrelation terms. The first and second difference spectra may then be taken as the real

and imaginary parts, respectively, of the complex Fourier transform of the two-sided A-scan.

An inverse Fourier transform may be performed on the complex data to obtain the A-scan

free of symmetry considerations. Additional phase delays of the first phase modulator 44

may also be selected, and orthogonal pairs of spectra obtained, according to estabhshed

algorithms for phase-shift interferometry.

Figure IB shows a second FDOCT 15, in accordance with a second embodiment of

the present invention. The FDOCT 15 fifteen of Figure IB is similar to the FDOCT system 10 of Figure 1A, except that a swept-frequency source 720 is used in place of the broadband

short-coherence length Hght source 72 of Figure 1A. In addition, single-channel detectors

800A and 800B are used in place of the spectrometer/array detector combinations of Figure

1A.

The swept-frequency source 720 is preferably a narrowband Hght source whose

frequency can be swept as a function of time. In the embodiment of Figure IB, the

spectrum of the interferometer output(s) is obtained by monitoring the output of the

detectors 800A, 800B as a function of time while the frequency of the swept-frequency

source 720 is swept.

The additional embodiments discussed below wiU be shown with a broadband Hght

source, and with spectrometer/array detector(s) that aree used to resolve the spectrum of the

interferometer output. However, it should be appreciated that all of the embodiments

described below can also be implemented in a swept-source configuration, such as the

configuration shown in Figure IB, by replacing the broadband source with a swept-

frequency narrowband source, and by replacing each detector with a single-channel time-

resolved detector. When Fourier Domain OCT is performed using a swept-source

implementation, then all of the same advantages conferred by obtaining multiple

simultaneous phase differences, either from multiple output ports of the various

interferometer topologies, from polarization encoding of phase in the interferometer arms,

or a combination of both approaches, wiU apply.

Figure 2 shows a second FDOCT embodiment 20, in accordance with die present

invention. Si Uar to the FDOCT embodiment 10 shown in Figure 1, the FDOCT embodhnent 20 of Fig. 2 has a fiber coupler 36 connected to a first polarization controUer

42 by a SM optical fiber 41. The first polarization coupler 42 is connected to a phase

modulator 44 by a SM optical fiber 48. The phase modulator 44 has a fiber 51 extending

tiierefrom terminating in a reflector 46. AdditionaUy, the first fiber coupler 36 is connected

to a second polarization controUer 54 by a SM optical fiber 52. The second polarization

controUer 54 is optically coupled to a first lens 56 by a SM optical fiber 64. The first lens 56

directs an optical output signal from fiber 64 to scanning optics 58. Scanning optics 58 and

lens 61 direct the optical signal to sample 62. Reflected optical signals from the sample 62

are coupled back into fiber 64 via lens 61 scanning optics 58 and lens 56.

The first fiber coupler 36 is also connected to a first detector 66 by a SM optical fiber

64. In a variation from the first FDOCT embodiment 10, the first fiber coupler 36 is

connected to a second fiber coupler 76 through a SM optical fiber 38. The second fiber

coupler 76 is connected to a second detector 71 through a SM optical fiber 68, and includes

a 0° port 75 and a 180° port 77. The second fiber coupler 76 also includes a SM optical fiber

stub 78 connected to the 180° port 77. A low coherence source 72 may also be connected to

the second fiber coupler 76 through a SM optical fiber 74.

The FDOCT embodiment 20 of Figure 2 is similar in many respects to the FDOCT

embodiment 10 of Figure 1, except that a second fiber coupler 76 is used in the source arm

to provide one of the orthogonal phase components, in place of the first circulator 34 of

Figure 1. Use of a second fiber coupler 76 may be preferable for decreasing system cost or if

circulators are not available to meet the specified wavelength or bandwidth requirements.

The penalty for use of the second fiber coupler 76 in place of a circulator wiU result in a higher insertion loss (3dB for a fiber coupler versus ~0.7 dB for a circulator) in the forward

direction, plus a 3dB loss in the reverse direction of the 0° port 75, which wiU need to be

matched by an equal amount of attenuation of the 180° port 64 in order to match DC levels

on the detectors 66 and 71.

Thus, the FDOCT embodiment 20 may experience a total loss of source Hght of

approximately 6dB loss (not counting circulator insertion losses) as compared to the

FDOCT embodiment 10. As in the FDOCT embodiment 10 of Fig. 1, the separate

spectrometers 66a and 71a, and array detectors 66b and 71b of the first and second detectors

66 and 71 could be replaced by an imaging spectrometer and a dual-row or three-color

detector array. Also, any of the three modes of operation discussed in connection with the

FDOCT embodiment 10 of Fig. 1 may also be used in the FDOCT of Fig. 2.

Figure 3 iUustrates a third FDOCT embodiment 30, in accordance with the present

invention. SimUar to the FDOCT embodiments 10 and 20 discussed above, the FDOCT

embodiment 30 includes a fiber coupler 36 coupled to a second polarization controller 54

through a SM optical fiber 52. The first fiber coupler 36 includes interferometer ports 37a,

37b, 37c and 37d. The second polarization controUer 54 is opticaUy coupled to a first lens

56 through a SM optical fiber 56. The first lens 56 directs and receives signals to and from a

sample 62 through scanning optics 58 and second lens 61.

Also attached to the first fiber coupler 36 is a reference arm including a second phase

modulator 84 connected to the first fiber coupler 36 with a SM optical fiber 82. The second

phase modulator 84 is connected to a third polarization controUer 88 with a SM optical fiber

86. The third polarization controller 88 is connected to a tiαird fiber coupler 94 through a SM optical fiber 92. The tiiird fiber coupler 94 is also connected to the first fiber coupler 36

with a SM optical fiber 96. The third fiber coupler 94 is connected to a first detector 66 and

a second detector 71 through SM optical fibers 64 and 68, respectively. The FDOCT

embodiment 20 of Figure 2 may also include a source 72 coupled to the first fiber coupler 36

through a SM optical fiber 75.

Figure 3 iUustrates a FDOCT embodiment 30, in accordance with the present

invention, which takes advantage of the intrinsic phase difference between interferometer

ports 37a and 37b of the first fiber coupler 36, but has a transmissive reference delay rather

than a reflective one. Other aspects of the FDOCT embodiment 30 are similar to the

previously discussed FDOCT embodiments 10 and 20. The FDOCT embodiment 30 of

Fig. 3 may experience a loss of 3dB of the sample arm reflected Hght (which is returned into

the source). However, this 3dB loss is less than the corresponding configurations in other

FDOCT embodiments, and is approximately the same loss experienced by time-domain

OCT in a conventional Michelson interferometer. Thus, the FDOCT embodiment 30 of

Figure 3 is the preferred implementation when a circulator is unavaUable or undesirable. As

in the previously discussed FDOCT embodiments 10 and 20, optimally the first and second

spectrometers 66a and 71a and first and second array detectors 66b and 71b could be

replaced by an imaging spectrometer and a dual-row or three-color detector array. Also, any

of the three modes of operation discussed above in connection with FDOCT embodiments

10 and 20, may also be used.

Figure 4 iUustrates a fourth FDOCT embodiment 40, in accordance with the present

invention. The FDOCT embodiment 40 includes a first fiber coupler 36 which is coupled to a first polarization controller 42 by a SM optical fiber 41. The first fiber coupler 36 has four

interferometer ports 37a, 37b. 37c, and 37d. One interferometer port 37a is coupled to a

first polarization controller 42 by a SM fiber 41, and the second interferometer port 37b is

coupled to a second polarization controUer 54 by a SM fiber 52. The first polarization

controUer 42 is coupled to a first phase modulator 44 by a SM optical fiber 48. The second

polarization controUer 54 is coupled to the first fiber coupler 36 by a SM optical fiber 52,

and to a second circulator 102 by a SM optical fiber 64. The second circulator 102 has a SM

optical fiber 110 opticaUy coupled to a first lens 56, which directs and receives a signal to and

from a sample 62 through scanning optics 58 and second lens 61. The second circulator 102

is also coupled to a fourth fiber coupler 98 through a SM optical fiber 104. The first phase

modulator 44 is also coupled to the fourth fiber coupler 98 by a SM fiber 50.

The fourth fiber coupler 98 is coupled to first and second detector 66 and 71 through

SM optical fibers 64 and 68, respectively. The FDOCT embodiment 40 may also include a

first source 72 coupled to the first fiber coupler 36 through a SM optical fiber 74 and a

second source 106 coupled to the fiber coupler 36 through a SM optical fiber 108.

The FDOCT embodiment 40 of Fig. 4 takes advantage of the intrinsic phase

difference between interferometer ports 37a and 37b, and also has a transmissive delay.

However, the FDOCT embodiment 40 uses a second circulator 102 to direct Hght onto the

sample 62. The FDOCT embodiment 40 also places the second circulator 102 within one of

the arms of the interferometer, where chromatic and polarization mode dispersion effects

within the second circulator 102 may be problematic. However, the embodiment 40 makes highly efficient use of source Hght (except for

insertion losses in the circulator 102 itself), and also aUows for the introduction of a second

source 106. This may be preferable in order to increase the power of low-coherence Hght on

the sample 62 from avaUable Hght sources 72 and 106, and also may be used to increase the

bandwidth of illumination by using sources with displaced center wavelengths. As in the

previously discussed FDOCT embodiments 10, 20 and 30, the separate spectrometers 66a

and 71a and array detectors 66b and 71b could be replaced by an imaging spectrometer and

a dual-row or three-color detector array. Also, any of the three modes of operation discussed

above in connection with the previously discussed FDOCT embodiments 10, 20 and 30 may

be used in the FDOCT embodiment 40 of Fig. 4.

AU the FDOCT embodiments discussed above may take advantage of the intrinsic π

phase delay which is found between output ports of Michelson and Mach-Zehnder

interferometers. As described above, this phase delay may be used to sknultaneously obtain

pairs of spectra which may be differenced to remove non-interferometric noise from the

spectral data. However, it also may be desirable to obtain pairs of spectra with π/2 phase

delay simultaneously, to aUow for both removal of non-interferometric noise and also for

unambiguous calculation of sample reflectivity without symmetry artifacts. The

embodiments described below take advantage of polarization to encode arbitrary phase

delays into spectra which may be measured simultaneously.

Figure 5 Ulustrates a fifth FDOCT embodiment 50, in accordance with the present

invention. The FDOCT embodiment 50 includes a non-polarizing beam spHtter 114

opticaUy coupled to a λ/n waveplate 116 and a fixed reference mirror 118. The non- polarizing beam spHtter 114 is also opticaUy coupled to a sample 62 through scanning optics

58 and second lens 61. The non-polarizing beam spHtter 114 is additionaUy opticaUy

coupled to a polarizing beam spHtter 120. Polarizing beam spHtter 120 is opticaUy coupled

to a first detector 66 through lens 122, and a second detector 121 through lens 124. The

FDOCT embodiment 50 may also include a source 72 optically coupled to the non-

polarizing beam spHtter 114 through a polarizer 112.

The FDOCT embodiment 50 is similar to a bulk-optic Michelson interferometer

which may encode a 90° phase shift into two polarization channels, which are separated

outside of the interferometer by the polarizing beam spHtter 120. The two polarization

channels may be directed into a matched pair of spectrometers 66a and 71a and array

detectors, 66b and 71b. Light emitted from the preferably low-coherence source 72 may be

Hnearly polarized at 45° from the vertical by a polarizer 112 placed in the source arm. The

non-polarizing beamspHtter 114 sphts this Hght evenly between sample and reference arms.

A λ/n waveplate 116 (for n=2, 4, 8, etc.) may be placed in the reference arm, with its fast

axis oriented verticaUy (i.e., at 0° to the vertical).

Thus, the horizontal component of the Hght in the reference arm may experience a

phase delay of 4π/n radians with respect to the vertical component after double-passing the

λ/n waveplate 116. These two components may be separated by the polarizing beamspHtter

120 in the detector arm, which sends the phase-delayed components of the reference arm

Hght, along with an equal division of the Hght reflected from the sample 62, into a matched

pair of spectrometers 66a and 71a, and array detectors, 66b and 71b. For example, for n=8, i.e. an eighth-wave plate in the reference arm, there wiU be a

λ/4 or 90° phase difference between the spectra obtained from the reference and sample

arms, which is sufficient for unambiguous reconstruction of the sample reflectivity from the

complex spectrum thus obtained. For other values of n, i.e., n=4 (quarter-wave plate), n=2

(half-wave plate), other phase delays between the coUected spectra may also be obtained as

needed for various phase-shift interferometry reconstruction algorithms. Although only 2

phase delays may be encoded into polarization, the polarization-based approach of the

FDOCT embodiment 50 may be combined with the intrinsic interferometer port phase

difference methods of the other FDOCT embodiments discussed above to obtain at least 4

simultaneous spectra with different phase delays.

As another example, the addition of a circulator into the source arm of the FDOCT

embodiment 50, which may direct Hght into another polarizing beamspHtter and two more

spectrometers, would allow for the simultaneous acquisition of spectra having 0°, 90°, 180°,

and 270° phase differences. It will be clear to one of ordinary skiU in the art that numerous

other implementations of the inventive concept of polarization encoding of phase may be

used as extensions of the FDOCT embodiment 50, such as the rotation of all polarization-

sensitive elements in the embodiment 50 by a fixed angle, or numerous alternative

placements of the polarization-sensitive elements (including placing of the λ/n plate 116 in

the sample arm instead of the reference arm), whUe stiU faUing within the scope of the

present invention.

Modes of operation of the FDOCT embodiment 50 of Figure 5 includes acquiring

simultaneous spectra having 0° and 180° phase delay by use of a λ/4 waveplate in the reference arm. The spectra may be differenced in order to ehminate sample and reference

arm DC and sample arm autocorrelation terms. The resulting difference spectrum may be

inverse Fourier transformed to acquire a one-sided A-scan. The length of the reference arm

is preferably adjusted so that no reflections are observed for z<0.

Another mode of operation for use with the FDOCT embodiment 50 of Figure 5

includes acquiring simultaneous spectra having 0° and 90° phase delay between them and

differencing them in order to ehminate sample and reference arm DC and sample arm

autocorrelation terms. These spectra may be taken as the real and imaginary parts,

respectively, of the complex Fourier transform of the two-sided A-scan. The inverse Fourier

transform may then be performed on the complex data to obtain the A-scan free of

symmetry considerations.

Another mode of operation for use with the FDOCT embodiment 50 of Figure 5

includes acquiring simultaneous spectra having 0°, 90°, 180°, and 270° phase difference

between them, by use of a combination of polarization encoding of phase and intrinsic

phase delay between interferometer ports, as described above. Pairs of spectra having 180°

phase delay between them may be differenced in order to ehminate sample and reference

arm DC and sample arm autocorrelation terms. The differenced pairs of spectra may then be

taken as the real and imaginary parts, respectively, of the complex Fourier transform of the

two-sided A-scan. The inverse Fourier transform may be performed on the complex data to

obtain the A-scan free of symmetry considerations.

Figure 6 iUustrates a sixth FDOCT embodiment 60, in accordance with the present

invention. The FDOCT embodiment 60 includes a first fiber coupler 36 coupled to a first polarization controUer 42 through a SM optical fiber 41. The first polarization controUer 42

is coupled to a SM optical fiber 51, which terminates in a reflector 46. The first fiber coupler

36 is also coupled to a second polarization controUer 54 through a SM optical fiber 52. The

second polarization controUer 54 is opticaUy coupled to a first lens 56 through a SM optical

fiber 64. The first lens 56 is opticaUy coupled to a sample 62 through a scanning optics 58

and second lens 61. The first fiber coupler 36 is also coupled to a tiiird lens 126 through a

coupler 64. The third lens 126 is optically coupled to a polarizing beam spHtter 120. The

polarizing beam spHtter 120 is opticaUy coupled to a lens 122 and a first detector 66. The

polarizing beam spHtter 120 is also opticaUy coupled to a lens 124 and a second detector 71.

It is preferable to have the capabiHty for arbitrary simultaneous dual phase delays

between acquired spectra in a fiber interferometer, since most practical OCT systems to date

make use of the flexibility of fiber optic systems for medical and biological appHc-ations. The

FDOCT embodiment 60 iUustrates one possible implementation of a fiber-optic

interferometer for imaging, which uses polarization for phase encoding. In this embodiment,

the Hght source 72 is either polarized or a polarization element 112 (such as a fiber polarizer)

is used in the source arm. Preferably, the interferometer is constructed from polarization-

maintaining fiber (PMF), although previous work in polarization-sensitive OCT has shown

that non-PMF fiber is also capable of maintaining phase relationships between orthogonal

polarization states propagating through the fiber. In the FDOCT embodiment 60 of Figure

6, a fiber polarization controUer 42 in the reference arm may be used to simulate the λ/n

waveplate 116 in the FDOCT embodiment 60, and the second polarization controUer 54 in the sample arm may be used to correct for stress-induced birefringence in the sample arm

fiber assembly.

Although the FDOCT embodiment 60 is just one example of polarization phase

encoding in a fiber interferometer, any of the fiber interferometers shown in the FDOCT

embodiments described above could be altered to use polarization phase encoding by the

addition of a PBS and matched pair of spectrometers to each output port 37a and 37b.

Modifications of the FDOCT embodiments described above in this way would result in the

simultaneous coUection of spectra with 4 phase delays (pairs of which are separated by 180°),

since each of those implementations have dual interferometer outputs.

Figure 7 shows a generahzed embodiment 100 of a FDOCT system, in accordance

with the present invention. The FDOCT 100 includes a signal manipulator 12. The signal

manipulator 12 is coupled to a reference portion 14 by a coupler 20, and a sample portion 16

by a coupler 22. The signal manipulator 12 is also coupled to a detector 18 by a coupler 24.

The signal manipulator 12 may also be coupled to a source 26 by a coupler 28. The sample

portion 16 is coupled 31 to a sample 32.

Examples of signal manipulator 12 of Figure 7 can include, but are not limited to, the

combination of the first optical circulator 34 and the first fiber coupler 36 of Figure 1, the

second fiber coupler 76 and first fiber coupler 36 of Figure 2, and the first fiber coupler 36

of Figures 3 and 4. AdditionaUy, the signal manipulator 12 of Figure 7 may also correspond

to the polarizer 110 and nonpolarizing beam spHtter 114 of Figure 5, and the polarizing

element 112 with the fiber coupler 36 of Figure 6. The reference portion 14 of Figure 7 may include, but is not Hmited to, the combined

fkst polarization controUer 42, fkst phase modulator 44, and reflector 46 of Figures 1 and 2.

The reference portion 14 of Figure 7 may also include, but is not Hmited to, the second

phase modulator 84 and thkd polarization controUer 88 of Figure 3, and the fkst

polarization controUer 42 and fkst phase modulator 44 of Figure 4. The reference portion

14 of Figure 7 may also include, but is not Hmited to, the combination of the λ/n plate 116

and reference mkror 118 of Figure 5, and the fkst polarization controUer 42 and reflector 46

of Figure 6.

The sample portion 16 of Figure 7 may include, but is not Hmited to, the second

polarization controUer 54, fkst lens 56, scanning optics 58 and second lens 61 of Figures 1,

2, 3 and 6. The sample portion of Figure 7 may also include, but is not Hmited to, the

combination of the second polarization controUer 54, second optical ckculator 102, first lens

56, scanning optics 58 and second lens 61 of Figure 4, and the scanning optics 58 and

second lens 61 of Figure 5.

The detector 18 of Figure 7 may include, but is not limited to, the first detector 66

and second detector 71 of Figures 1 and 2. The detector 18 of Figure 7 may also include,

but is not Hmited to, the combination of the first and second detectors 66 and 71, and the

thkd fiber coupler 94 of Figure 3, and the first and second detectors 66 and 71, and the

fourth fiber coupler 98 of Figure 4. Finally, the detector 18 of Figure 7 may include, but is

not Hmited to, the fkst and second detectors 66 and 71, the lens 124 and the lens 122, and

the polarization beam spHtter 120 of Figures 5 and 6, as weU as the thkd lens 126 of

Figure 6. It should also be noted that the source 26 shown in Figure 7 may include, but is not

Hmited to, the fkst source 72 of Figures 1, 2, 3, 5 and 6. The source 26 of Figure 7 may also

include, but is not Hmited to, the fkst source 72 and second source 106 of Figure 4.

Referring to Figure 8, a detector 66 with imaging capabiHties suitable for use with

FDOCT embodiments 10, 20, 30, 40, 50, 60 and 100 is shown. The detector 66 has dual

input fibers 128 and 129 coupled to an input sHt 136 of the detector 66. The detector 66

also includes an output area 130. The output area includes a fkst array 132 and a second

array 134 which may receive two spectra, 138 and 140.

In operation, the detector 66 receives two multi-frequency signals carried in the dual

input fibers 128 and 129, at the input sHt 136. The input signals preferably have a phase

difference between them, and the phase difference is preferably 90 degrees. Each input

signal is then dispersed according to frequency and the resulting spectra 138 and 140, are

dkected onto the arrays 132 and 134. The arrays 132 and 134, may then measure power as a

function of frequency for each spectrum 138 and 140.

The previously described FDOCT embodiments 10, 20, 30, 40, 50, 60 and 100 aU

preferably take advantage of the detection of multiple simultaneous optical spectra in the

detector arm of the interferometer, corresponding to multiple phase-delayed components of

a complex FDOCT signal. Figures 1-7 iUustrate simple cases of dual-channel detection of

spectra separated by orthogonal (90°) or opposite (180°) phase obtained through the use of

intrinsic phase delays associate with interferometer ports, or through polarization

multiplexing. A combination of intrinsic and polarization-derived phase delays could also be

used to obtain at least four simultaneous phase delays, which would preferably be detected in an equal number of spectral channels. Although four or more simultaneous phase delays

may be deskable to accommodate some phase-shift interferometry reconstruction

algorithms, coUection of two sknultaneous phases (optimaUy separated by 90°) would be one

preferred embodiment of the invention, as that would aUow for almost complete removal of

autocorrelation noise and calculation of complex double-sided spectra with the least

complexity and expense.

The Hght source 72 in the aforementioned embodiments is preferably a low-

coherence source. Multiple Hght sources 72 and 106, may also be used. The spectrometers

66 and 71 used in the FDOCT embodiments should preferably be selected for maximum

optical throughput and optimal matching of thek dispersion to the spectral content of the

low-coherence source, to avoid artifacts associated with the spatial frequency response of the

array (i.e., the dispersion should be chosen so that the spectrum nearly fiUs the detector

array). Grating spectrometers currently exhibit the optimal combination of characteristics to

satisfy these constraints, however other spectrometer types may be used. If space utiHzation

is not a serious constraint, prism-based spectrometers may give better throughput at the cost

of increased requked path length.

Preferably, the detector arrays utiHzed are photodiode arrays with the maximum weU

depth available, and optimized for response in the wavelength range of the source. Using

current detector array technology, this corresponds to the use of siHcon photodiode arrays

for the popular 830 nm OCT window and for any other desked OCT spectral windows

below approximately 1000 nm, and for InGaAs photodiode arrays for the popular 1310 nm

OCT window and for any other desked OCT spectral windows in the near-infrared beyond 1000 nm. Charge-coupled device (CCD) arrays may also be used, however current-

generation CCDs utiHze siHcon substrates and may thus be unsuitable for imaging at the

popular OCT wavelengths above 1000 nm.

For simpHcity, Figures 1-7 above illustrate the phase-delayed spectral channels as

being dispersed and detected in separate detectors. As shown in Figure 8, however, the

spectrometers and arrays in the detectors shown in each implementation can be replaced by

a single imaging spectrometer having a multiple-stripe or two-dimensional detector array,

with the input fibers arranged in close vertical piOximity to one another so as to have thek

spectra imaged onto the separate rows (stripes) of the detector array. Such an arrangement

would have the significant advantages of allowing for optimal matching of the spectra placed

on aU channels (since aU channels would use the same grating and other spectrometer

optics), as weU as the cost and space savings achievable by using a single spectrometer. Dual-

stripe photodiode arrays have been commerciaUy avaUable in the past, and also three-row

CCD arrays designed for 3-color Hne scanning are currentiy commerciaUy available. Two of

the three rows of a 3-color Hne scanner array could also be used in place of a dual-row array.

For a preferred embodiment of two FDOCT channels separated by 90° or 180°, a

single dual-stripe photodiode array mounted onto an imaging spectrometer would be the

preferred detector. Alternatively, a two-dimensional CCD or photodiode array could be used

to either a) simulate a dual-stripe array by using the binning capabilities of such an array to

coUect dual simultaneous spectra, or b) coUect more than two sknultaneous spectra through

an appropriate alternative binning algorithm. However, two-dimensional CCDs still have significant weU-depth Hmitations, and large two-dimensional photodiode arrays are not yet

commerciaUy avaUable.

It should be noted that the term "optical ckculator" is used herein to mean any type

of device capable of dkectional coupHng of electromagnetic radiation incident on port 1 to

port 2, while simultaneously coupHng electromagnetic radiation incident on port 2 to port 3

Also, as used herein, a "fiber coupler" is used to mean any device which receives an input

signal of electromagnetic radiation and divides that signal between two output ports. It

should be noted that as used herein, a fiber coupler may have multiple ports wherein each

port can serve as an input port for a selected pak of output ports as well as function as an

output port for a selected input port. The fiber coupler spHtting ratio for all embodiments is

ideally 50/50, however this spHtting ratio may be modified to account for nonideal

performance of other components, for example to compensate for the insertion loss of

ckculators or other elements. "Polarization controUer" is used herein to mean any

semiconductor or bulk optical device used to selectively manipulate d e polarization of an

input signal and output the manipulated signal. "Optical fiber" is used to mean any device

or set of devices used to dkect electromagnetic radiation along a prescribed patii. Thus,

"optical fiber" can mean a signal strand of opticaUy transparent material bounded by a region

of contrasting index of refraction, as weU as mkrors or lenses used to dkect electromagnetic

radiation along a prescribed path.

As used herein, "phase modulator" means any semiconductor or bulk device used to

modulate or otherwise alter the phase of an input electromagnetic signal and output the

manipulated electromagnetic signal. "Reflector" is used herein to mean any device capable of reflecting an electromagnetic signal. Thus, "reflector" can be used to mean a mkror, an

abrupt transmission in an index of refraction as weU as a periodicaUy spaced array structure

such as a Bragg reflector. "Scanning optics" means any system configured to sweep an

electromagnetic signal across a chosen area.

"Detector" is used herein to mean any device capable of measuring energy in an

electromagnetic signal as a function of wavelength. Additionally, "source" is used to mean

any source of electromagnetic radiation, and preferably means a low coherence source of

electromagnetic radiation.

The foregoing embodiments and advantages are merely exemplary and are not to be

construed as Hmiting the present invention. The present teaching can be readily apphed to

other types of apparatuses. The description of the present invention is intended to be

Ulustrative, and not to limit the scope of the claims. Many alternatives, modifications, and

variations wiU be apparent to those skiUed 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.

Claims

WHAT IS CLAIMED IS:

1. A multi- frequency interferometric imager, comprising:

a signal manipulator configured to receive a signal from a source and produce

a reference signal and a sample signal;

a reference portion configured to receive the reference signal from the signal

manipulator and to send the reference signal back to the signal manipulator;

a sample portion configured to receive the sample signal from the signal

manipulator and to send the sample signal back to the signal manipulator, wherein the signal

manipulator is further configured to combine the reference signal and the sample signal and

to produce a fkst and a second measurement signal with a 180 degree phase difference

therebetween; and

a detector portion configured to receive the fkst and second measurement

signals.

2. The multi- frequency interferometric imager of claim 1, wherein the reference

portion comprises:

a fkst polarization controUer;

a first phase modulator apphcably coupled to the fkst polarization controUer;

and

a reference delay unit optically coupled to the first phase modulator.

3. The multi- frequency interferometric imager of claim 1, wherein the reference

portion comprises a fkst phase modulator opticaUy coupled to a fkst polarization controUer.

4. The multi- frequency interferometric imager of claim 1, wherein the signal

manipulator comprises a fkst fiber coupler opticaUy coupled to a second fiber coupler, and

wherein the reference portion is configured to receive a signal from the fkst fiber coupler

and output a signal to the second fiber coupler.

5. The multi-frequency interferometric imager of claim 4, wherein the detector

portion is configured to receive a signal from the second fiber coupler.

6. The multi- frequency imager of claim 6, wherein the detector portion is

configured to receive a fkst signal and a second signal from the second fiber coupler,

wherein the fkst signal and the second signal comprise different phases.

7. The multi- frequency interferometric imager of claim 1, wherein the sample

portion comprises a second polarization controller opticaUy coupled to scanning optics, and

wherein the sample portion is further configured to be opticaUy coupled to a sample.

8. The multi- frequency interferometric imager of claim 1, wherein the signal

manipulator comprises an optical ckculator opticaUy coupled to a fkst fiber coupler.

9. The multi- frequency interferometric imager of claim 8, wherein the reference

portion and the sample portion are each configured to receive a signal from the first fiber

coupler.

10. The multi- frequency interferometric knager of claim 9, wherein the detector

portion is configured to receive a signal from the first fiber coupler and a signal from the

second fiber coupler.

11. The multi- frequency interferometric imager of claim 10, wherein the detector

portion comprises a fkst detector configured to receive a signal from the first fiber coupler

and a second detector configured to receive a signal from the optical ckculator.

12. The multi- frequency interferometric imager of claim 1, wherein the signal

manipulator comprises a first fiber coupler optically coupled to a second fiber coupler.

13. The multi-frequency interferometric imager of claim 12, wherein the reference

portion and the sample portion are opticaUy coupled to the first fiber coupler.

14. The multi-frequency interferometric imager of claim 13, wherein the detector

portion is configured to receive a signal from the fkst fiber coupler and the second fiber

coupler.

15. The multi- frequency interferometric imager of claim 14, wherein the detector

portion comprises a fkst detector is configured to receive a signal from the fkst fiber coupler

and a second detector configured to receive a signal from the second fiber detector.

16. The multi- frequency interferometric imager of claim 1, wherein the signal

manipulator comprises a fkst fiber coupler.

17. The multi- frequency interferometric imager of claim 16, wherein the sample

portion comprises:

a first polarization controUer; and

an optical ckculator opticaUy coupled to the first polarization controUer.

18. The multi- frequency interferometric imager of claim 17, wherein the detector

portion comprises a second fiber coupler, and first and second detectors configured to

receive signals from the second fiber coupler.

19. The multi- frequency interferometric imager of claim 18, wherein the

reference portion and the sample portion are coupled to the second fiber coupler.

20. The multi- frequency interferometric imager of claim 1, wherein the signal

manipulator comprises:

a polarizer; and

a non-polarizing beam spHtter opticaUy coupled to the polarizer.

21. The multi- frequency interferometric imager of claim 20, wherein the reference

portion comprises:

a λ/n plate; and

a reference delay unit optically coupled to the λ/n plate.

22. The multi- frequency interferometric imager of claim 21, wherein the sample

portion comprises scanning optics opticaUy coupled to a sample.

23. The multi-frequency interferometric imager of claim 22, wherein the detector

portion comprises a polarizing beam spHtter optically coupled to a fkst and second lenses, a

first detector configured to be optically coupled to the first lens, and a second detector

configured to be optically coupled to the second lens.

24. The multi- frequency interferometric imager of claim 1, wherein the signal

manipulator comprises a polarizing element configured to be optically coupled to a fiber

coupler.

25. The multi-frequency interferometric imager of claim 24, wherein the reference

portion and the sample portion are opticaUy coupled to the fiber coupler.

26. The multi-frequency interferometric imager of claim 25, wherein the detector

portion comprises:

fkst and second lenses opticaUy coupled to a polarizing beam spHtter;

a first detector coupled to the first lens; and

a second detector coupled to the second lens.

27. The multi-frequency interferometric imager of claim 1, wherein the reference

arm is configured to selectively alter the polarization of a signal.

28. The multi- frequency interferometric imager of claim 1, wherein the reference

arm is configured to selectively alter the phases of a fkst and second polarization of a signal.

29. The multi- frequency interferometric imager of clakn 28, wherein a fkst

polarization of a signal forms a fkst signal and a second polarization of a signal forms a

second signal and, wherein the fkst signal and the second signal have an approximately π

radian phase difference.

30. The multi-frequency interferometric imager of claim 29 where the polarization

of the fkst signal is orthogonal to the polarization of the second signal.

31. The multi-frequency interferometric imager of claim 1, wherein the sample

portion is configured to selectively alter the phase of a signal.

32. The multi- frequency interferometric imager of claim 1, wherein the detector

portion comprises a spectrometer and an array detector, and wherein the spectrometer and

the array detector are configured to measure a power of a signal as a function of frequency

33. The multi-frequency interferometric imager of claim 30, wherein the array

detector comprises a photo-detector array.

34. The multi- frequency interferometric imager of claim 2, wherein the reference

delay unit comprises a reflector.

35. The multi- frequency interferometric imager of claim 21, wherein the reference

delay unit comprises a reflector.

36. A device, comprising:

a signal manipulator configured to receive a signal from a source;

a reference portion configured to opticaUy couple the signal manipulator to a

reference delay unit;

a sample portion configured to opticaUy couple the signal manipulator to a

sample; and a detector portion configured to receive a signal from the signal manipulator,

wherein the signal can be used to measure a distance to the sample without scanning.

3.7. The device of claim 36, wherein the reference delay unit comprises a reflector.

38. A method, comprising:

receiving a fkst spectra and a second spectra having a 180 degree phase

difference therebetween from a signal manipulator;

determining a difference between the fkst and second spectra; and

inverse Fourier transforming the difference between the fkst and second

spectra.

39. The method of claim 38, further comprising adjusting a portion of the first

and second spectra to reduce reflections therein for z<0.

40. A method, comprising:

receiving a fkst spectra and a second spectra having a 180 degree phase

difference therebetween from a signal manipulator;

determining a first difference spectra between the fkst and second spectra;

receiving a thkd and fourth spectra having a 180 degree phase difference

tiierebetween from a signal manipulator, wherein the fkst and second spectra are orthogonal

to the thkd and fourth spectra, respectively; determining a second difference spectra between the thkd and fourth spectra;

complex Fourier transforming the first and second difference spectra to

produce a complex Fourier transform, wherein the fkst difference spectra comprises the real

part of the complex Fourier transform and the second difference spectra comprises the

imaginary part of the complex Fourier transform; and

inverse Fourier transforming the complex Fourier transform.

41. The method of claim 40, wherein the fkst spectra has a 0 degree phase shift

relative to a reference spectra.

42. The method of claim 41, wherein the phase difference between the fkst

spectra and the thkd spectra is approximately 90 degrees.

43. A method, comprising:

creating a fkst and a second spectra;

shifting the phase of the fkst and second spectra relative to one another with a

retarder until the first and second spectra have a 180 degree phase difference;

determining a difference spectra between the fkst and second spectra; and

inverse Fourier transforming the difference spectra.

44. The method of claim 43, further comprising adjusting a portion of the first

and second spectra to reduce reflections therein for z<0.

45. The method of claim 43, wherein the retarder is a λ/4 waveplate.

46. The method of claim 43, wherein the fkst spectra has a 0 degree phase shift

relative to a reference spectra.

47. A method, comprising:

receiving a fkst spectra and a second spectra having a 0 degree phase

difference therebetween from a signal manipulator;

determining a fkst difference spectra between the fkst and second spectra;

receiving a thkd and fourth spectra having a 90 degree phase difference

therebetween from a signal manipulator, wherein the fkst and second spectra are orthogonal

to the thkd and fourth spectra, respectively;

determining a second difference spectra between the thkd and fourth spectra;

complex Fourier transforming the fkst and second difference spectra to

imaginary part of the complex Fourier transform; and

inverse Fourier transforming the complex Fourier transform.

48. A method, comprising:

creating a fkst, second, thkd and fourth spectra, wherein there is a 90 degree

phase shift between the fkst, second, thkd and fourth spectra, respectively, a 180 degree

phase shift between the fkst and thkd spectra, and a 180 degree phase shift the second and

fourth spectra;

determining a fkst difference spectra between the fkst and thkd spectra;

determining a second difference spectra between the second and fourth

spectra;

complex Fourier transforming the fkst and second difference spectra to

imaginary part of the complex Fourier transform; and

inverse Fourier transforming the complex Fourier transform.

49. The method of claim 48, wherein the fkst spectra comprises a fkst

polarization, the second spectra comprises a second polarization, the third spectra comprises

a thkd polarization, and the fourth spectra comprises a fourth polarization.