154 OPTICS LETTERS / Vol. 35, No. 2 / January 15, 2010

Polarization-maintaining fiber basedpolarization-sensitive optical coherence

tomography in spectral domain

Hui Wang, Muhammad K. Al-Qaisi, and Taner Akkin*Department of Biomedical Engineering, University of Minnesota, 312 Church Street Southeast, Minneapolis,

Minnesota 55455, USA*Corresponding author: akkin@umn.edu

Received October 6, 2009; revised December 2, 2009; accepted December 7, 2009;posted December 16, 2009 (Doc. ID 118137); published January 13, 2010

We demonstrate a polarization-sensitive spectral-domain optical coherence tomography system based onpolarization-maintaining fiber technology. Using a single-line-scan camera, the system produces reflectivityand retardance information along a depth profile with a single measurement. The relative axis orientationis available as well. System design and characterization and images of a biological tissue are presented.© 2010 Optical Society of America

OCIS codes: 170.4500, 230.5440, 170.3880.

Optical coherence tomography (OCT) noninvasivelyimages depth-resolved tissue microstructure with ahigh resolution [1]. By taking advantage of polariza-tion information of light, polarization-sensitive opti-cal coherence tomography (PS-OCT) is able to pro-vide an additional contrast based on birefringence[2], which can be useful in applications such as burndepth determination, cancer diagnosis, and glaucomaassessment. In recent years, the development ofhigh-speed spectral domain OCT technology madethe technique more suitable in clinical settings.Then, spectral domain polarization-sensitive opticalcoherence tomography (SDPS-OCT) interferometersin bulk have been reported using dual [3] or single [4]camera(s) in the detection. The free-space setups aresimple but inconvenient in alignment and inappro-priate for endoscopic applications. A fiber-basedSDPS-OCT with a single camera has also been re-ported by using a polarization modulator to inducedifferent states during successive measurements(A-lines) [5].

Polarization-maintaining fibers (PMFs) have twoaxes (fast and slow channels) that support the propa-gation of two orthogonal polarization modes in the fi-ber with a high isolation. A PMF-based low-coherenceinterferometer has been demonstrated for birefrin-gence measurement [6]. Then a time-domain PS-OCTsystem based on the PMF technology and frequencymultiplexing has been reported for imaging tissue bi-refringence with a single detector and a single mea-surement [7]. Since the polarization state of light ismaintained, the PMF-based systems combine the ad-vantages of fiber technology with straightforwardanalyses of bulk setups. Moreover, the operation issimpler with no need of polarization controllers, andexternal disturbances such as motion artifacts do notcause polarization transformations which can beproblematic for conventional fiber (non-PMF)-basedOCT.

In this Letter, we report a PMF-based SDPS-OCT.This system is capable of generating depth-resolved

reflectivity and birefringence information along a

0146-9592/10/020154-3/$15.00 ©

depth profile (A-line) with a single measurement.The relative axis orientation is also available. As thepolarization state of light incident on the sample (cir-cular) is fixed, the retardance image does not requirecompensation. A custom-built spectrometer simulta-neously acquires spectral interferences on the or-thogonal channels of the PMF. The temporal reso-lution is determined by the integration time of a linescan camera in the spectrometer. The results suggestaccurate and sensitive imaging for biomedical appli-cations.

Figure 1(a) shows the schematic setup of theSDPS-OCT. The light source is a 25 mW superlumi-nescent diode with a FWHM bandwidth of 50 nm cen-tered at 840 nm. The light from the source is linearlypolarized in a fiber bench and coupled into one of theorthogonal channels of the PMF. A 2�2 polarization-maintaining (PM) coupler splits the light into the ref-erence (70%) and sample (30%) arms. In the refer-ence arm, the collimated light travels through aquarter-wave plate (QWP) oriented at 22.5° with re-spect to the incoming polarization state. The light re-turning from the reference arm is linearly polarizedat 45°; thus, it couples back into the orthogonal chan-nels of the PMF with equal power. In the samplearm, light passes through a QWP oriented at 45°,which ensures circularly polarized light incident on asample. Since tissue birefringence alters the polar-ization state, the backscattered light couples backinto both channels of the PMF.

The detection arm consists of a custom-builtpolarization-sensitive spectrometer. A 50 mm achro-matic collimating lens directs the light to a 1200lines/mm transmission grating. A Wollaston prismwith a splitting angle of 6° separates the orthogonalpolarization channels. Then, a 100 mm achromaticlens focuses the spectra onto a single-line-scan CCDcamera, which contains 2048 pixels (1024 pixels foreach channel). The spectrometer was calibrated us-ing a tunable Ti:sapphire laser and a commercialspectrometer. The procedure was to identify CCD

pixel numbers for 14 wavelengths and to apply

2010 Optical Society of America

January 15, 2010 / Vol. 35, No. 2 / OPTICS LETTERS 155

second-order interpolation to find wavelengths of allpixels for both channels.

When the path difference between the referenceand sample arms is within the imaging depth, spec-tral modulations related to depth information occur[8]. After applying an inverse Fourier transform algo-rithm, the reflectivity R�z� and the phase retardance��z� information are extracted from the magnitudes,A1�z� and A2�z�, of the complex signals on each chan-nel [3] as

R�z� � A1�z�2 + A2�z�2, �1�

��z� = arctan�A1�z�/A2�z��. �2�

The optical axis orientation can be obtained by usingthe phase information, �1�z� and �2�z�, of the com-plex depth profiles [3]. Because of an arbitrary phasedelay between the PMF channels, our system yields arelative optical axis orientation ��z�,

��z� = ��1�z� − �2�z��/2. �3�

To test the system sensitivity for reflectivity mea-surement as a function of the ranging depth, a mirrorand an attenuator (neutral density filter) were usedto mimic a sample. With a power of 2 mW coupled tothe sample arm, the reflected power from the sample�Ps� was 110 nW, measured at the fiber tip of the de-

Fig. 1. (Color online) (a) SDPS-OCT system: SLD, super-luminscent diode; FB, fiber bench; P, polarizer; C, collima-tor; QWP, quarter-wave plate; L, lens; M: mirror; GM, galvomirror; G, grating; W, Wollaston prism. (b) Dynamic rangeand depth-dependent decay in reflectivity.

tection arm. The spectra were acquired at a rate of 25

kHz �40 �s/A-line� for eight depth locations sepa-rated from each other by 250 �m. Dispersion imbal-ance between the interferometer arms was compen-sated in software [9]. Figure 1(b) shows thereflectivity profiles �10 log�R�z���. The peak at zero isdue to the autocorrelation of the source spectrum.The ratio of the peak at 250 �m and the averagenoise level between 0.4 and 2.25 mm results in a dy-namic range of 59.8 dB. Without attenuation, themeasured power returning from the sample to the de-tection arm �Ps�� was 827 �W. Therefore, the sum ofthe dynamic range and the attenuation contribution�10 log�Ps� /Ps�=38.8 dB� yields a sensitivity of 98.6dB. The peak drops by 16.1 dB at a depth of 2 mmbecause of the finite size of the CCD pixels. Smallpeaks between 1.3 and 1.5 mm indicate a fixed noisepattern caused by the QWP in the reference arm andcan be removed by the subtraction of the referencearm signal. Cross coupling in the PM coupler and im-perfect splicing of the PMF segments if axes are notaligned carefully may cause ghost lines, which aredisplaced from the imaging depth by using longerPMF segments ��15 m� in the reference and samplearms. The leakage between the PMF channels, on theother hand, may contribute to the noise floor.

To characterize the retardance measurement, avoltage-controlled liquid crystal variable retarder

Fig. 2. (Color online) (a) Retardance measurement of avoltage driven variable retarder (solid line) and the manu-facturer’s test data (dashed line); (b) relative optical axisorientation (circles: left y-axis) and retardance (squares:right y-axis) measurements of a known retarder. Solid lineindicates the expected slope.

(LCVR) was inserted into the sample arm between

156 OPTICS LETTERS / Vol. 35, No. 2 / January 15, 2010

the QWP and the back reflector. The data were re-corded when the voltage applied to the LCVR waschanged from 0 to 10 V with an incremental step sizeof 10 mV. Figure 2(a) demonstrates good agreementbetween the retardance (solid curve) obtained from��z� measurements and the manufacturer’s test data(dashed curve). Small deviations may result frommisalignments, mismatch between our wavelength(840 nm) and that of manufacturer’s test (848.7 nm),and temperature dependence of the LCVR.

Measurement of relative optical axis orientationwas examined by using a known retarder (QWP at633 nm) as the sample. The data were recorded in ro-tational steps of 10° of the retarder over a range of180°. Figure 2(b) shows the relative axis orientationmeasurements (circles) with the expected slope (solidline). The mean value of the error is 2.64°. Figure2(b) also shows the phase retardance measurements(square) with a mean value of 68.38° and a standarddeviation of 0.83°. Errors, which are negligible, may

Fig. 3. (Color online) SDPS-OCT’s (a) reflectivity (dynamicrange, 30 dB; dark, low; light, high), (b) phase retardance(dark, 0°; light, 90°), and (c) relative axis orientation (blue,�90°; red, 90°) images of the nail and skin of a humanfinger.

arise from misalignments and imperfect polarizationcomponents.

The imaging performance of our SDPS-OCT sys-tem was demonstrated by stacking multiple A-lineswhile a galvanometer-controlled mirror scanned thebeam over a human nail and skin. Cross-sectionalimages containing 1000 A-lines were acquired andstored at 20 fps �50 �s/A-line�. The real-time displayrate for the reflectivity and retardance images wasabout 1.5 fps. Figures 3(a)–3(c) show the reflectivity,phase retardance, and relative axis orientation im-ages, respectively. The measured axial resolution isabout 5.5 �m in tissue. The reflectivity image, whichhas a dynamic range of 30 dB, is relatively uniformacross the tissue. High-contrast banding patternscan be seen in the retardance image. The bandingpatterns indicate that the nail in this sample is morebirefringent than the skin. The relative axis orienta-tion image is fairly uniform across the lateral direc-tion of the nail; however, red, yellow, and light greenspots in the skin portion suggest different axis orien-tations.

In conclusion, we demonstrated a high-speedSDPS-OCT system for imaging tissue reflectivity, bi-refringence, and relative axis orientation. This PMF-based approach is unique in the fiber based PS-OCTimplementations because it is capable of generatingbirefringence information along a depth profile witha single measurement. Although PMF setups requirecareful implementation, simple design and operationof the PMF-based system make it attractive for real-time physiological studies and clinical applications.

This work was funded in part by a research grantfrom the National Institutes of Health (NIH)(EB006588; cofunded by the National Institute ofBiomedical Imaging and Bioengineering and the Na-tional Eye Institute), and McKnight Land-Grant Pro-fessorship.

References

1. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W.G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory,C. A. Pufialito, and J. G. Fujimoto, Science 254, 1178(1991).

2. J. F. de Boer, T. E. Milner, M. J. C. Van Gemert, and J.S. Nelson, Opt. Lett. 22, 934 (1997).

3. E. Götzinger, M. Pircher, and C. K. Hitzenberger, Opt.Express 13, 10217 (2005).

4. B. Baumann, E. Götzinger, M. Pircher, and C. K.Hitzenberger, Opt. Express 15, 1054 (2007).

5. B. Cense, M. Mujat, T. C. Chen, B. H. Park, and J. F. deBoer, Opt. Express 15, 2421 (2007).

6. D. P. Davé, T. Akkin, and T. E. Milner, Opt. Lett. 28,1775 (2003).

7. M. Al-Qaisi and T. Akkin, Opt. Express 16, 13032(2008).

8. A. F. Fercher, C. K. Hitzenberger, G. Kamp, and S. Y.El-Zaiat, Opt. Commun. 117, 43 (1995).

9. B. Cense, N. A. Nassif, T. C. Chen, M. C. Pierce, S. Yun,B. H. Park, B. E. Bouma, G. J. Tearney, and J. F. deBoer, Opt. Express 12, 2435 (2004).