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Polarization-sensitive OFDI using polarizationmultiplexed wavelength-swept laser Han Saem Cho and Wang-Yuhl Oh* Department of Mechanical Engineering, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, South Korea *Corresponding author: [email protected] Received March 28, 2014; revised June 4, 2014; accepted June 4, 2014; posted June 4, 2014 (Doc. ID 209161); published July 2, 2014 We demonstrate a novel polarization-sensitive optical frequency domain imaging system employing passive polarization multiplexing. Simple modification of a fiber delay line in the wavelength-swept light source enables illumination with two perpendicular polarizations that are required for determination of the Stokes vector components of the light reflected from each depth of the tissue. This simple all-passive approach provides a robust and low-cost solution for PS imaging replacing relatively complex conventional schemes such as polarization modulation or frequency-encoded polarization multiplexing. © 2014 Optical Society of America OCIS codes: (170.4500) Optical coherence tomography; (170.3880) Medical and biological imaging; (110.5405) Polarimetric imaging. http://dx.doi.org/10.1364/OL.39.004065
Polarization-sensitive optical coherence tomography (PS-OCT) [1,2] maps birefringence properties of the sample that is complementary to the structural intensity images. For PS-OCT systems where the birefringence of the sample arm does not vary over time, circularly polarized light can be used to illuminate the sample either by free-space optics [1,3] or polarization maintained fiber (PMF) [4] and waveplates in the interferometer ensuring that the incident polarization state did not coincide with the presumed linear birefringence axis of the sample. However, it is impractical to maintain a predetermined polarization state at the tissue surface when the sample arm of the system is not stationary, such as in an endoscopic or catheter-based system [5,6]. Illumination with a pair of polarization states, which are perpendicular to each other on the Poincaré sphere, either by polarization modulation [2,7], frequency multiplexing [8,9], passive polarization multiplexing [10,11], or passive polarization switching in the light source [12] had been demonstrated and suggested to overcome this hurdle, removing the requirement to know the precise polarization state incident on the sample. Since PS imaging is performed as long as the two incident polarizations stay perpendicular to each other on the Poincaré sphere, the birefringence information of the sample, especially local birefringence information on each depth position of the sample, is acquired even with unknown birefringence variation in the sample arm optics. In this Letter, we demonstrate a polarization-sensitive optical frequency domain imaging (PS-OFDI), one form of the second-generation OCT techniques, using a new wavelength-swept light source with passive polarization multiplexing. The novel scheme multiplexes two perpendicular polarization states on the Poincaré sphere on successive A-lines without using any active devices, thereby providing a robust and potentially low-cost solution for PS-OFDI. Figure 1(a) depicts an example of wavelength-swept light sources employed by many of the recent high-speed OFDI systems [13–16]. Following the laser cavity, a fiber delay line (FDL) was used to increase the sweep repetition rate with a wavelength scanning filter of a large free 0146-9592/14/144065-03$15.00/0
spectral range (FSR) and/or to output a unidirectional and close to 100% duty cycle wavelength sweep with a bidirectional wavelength scanning filter such as the tunable Fabry–Perot filter (TFPF). Before entering into the OFDI system, the output from the fiber delay line was amplified by a booster optical amplifier (BOA). For polarization-sensitive imaging, a polarization modulation scheme by active modulator [2,7] or a frequency-encoded polarization multiplexing scheme by acousto-optic frequency shifters [8] has been used to illuminate the sample with a pair of perpendicular polarization states. In the proposed scheme, the temporal multiplexing of
Fig. 1. Schematic diagrams of (a) a conventional wavelengthswept light source with a fiber delay line and a polarization modulator and (b) a proposed wavelength-swept light source with a passively polarization-multiplexing fiber delay line for PS-OFDI. Configurations of wavelength-swept light sources of (c) conventional and (d) newly proposed schemes. WSL, wavelength-swept laser; FDL, fiber delay line; BOA, booster optical amplifier; PM, polarization modulator; Coll, collimator; BS, cube beam splitter; PI-BOA, polarization-insensitive BOA; SOA, semiconductor optical amplifier; TFPF, tunable Fabry–Perot filter; MCU, microcontroller unit; and PC, polarization controller. © 2014 Optical Society of America
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the two perpendicular polarizations was implemented by the FDL in the light source, allowing PS imaging without either polarization modulation or frequency-encoded polarization multiplexing scheme. The even and odd wavelength sweeps from the FDL are separated and their polarizations are aligned to two linear states that are perpendicular to each other on the Poincaré sphere, respectively, as shown in Fig. 1(b). These two perpendicularly polarized wavelength sweeps were recombined at a cube beam splitter and then amplified by a polarization-insensitive BOA. To experimentally demonstrate the passive polarization multiplexing approach, we built a short cavity length wavelength-swept laser (WSL) with an intracavity TFPF (Lambdaquest X1300-160-1000) driven at its first resonant frequency [14]. The intracavity SOA (Thorlabs BOA1130U2) with high polarization dependent gain, which ensures single polarization laser output, was modulated so that the laser was turned on only during the forward wavelength sweep of the filter. After splitting the laser output by 50∕50 coupler, one of them is delayed by half of the filter scan period. Unlike the conventional wavelength-swept light source using an FDL [Fig. 1(c)], a pair of linear polarizers aligned 45 deg to each other was placed in front of the cube beam splitter and the polarizations of the original wavelength sweep and the delayed copy are adjusted to match the orientations of each linear polarizer, before being combined at a cube beam splitter as shown in Fig. 1(d). These interleaved wavelength sweeps were amplified by a polarization-insensitive BOA (PI-BOA, InPhenix IPSAD1304C) before entering into the system interferometer. The average output power of the light source was 40 mW. It provides forward wavelength sweeps with alternating polarization states at a rate of 117.2 kHz with 100% duty cycle as shown in Fig. 2(a). The wavelength tuning range was 110 nm centered at 1290 nm [Fig. 2(b)] and the instantaneous linewidth was measured to be 0.14 nm, which corresponds to a full axial ranging depth of 5.2 mm (the range over which sensitivity remains within 6 dB of that at zero delay). The axial resolution and the sensitivity were measured to be 9.7 μm in air and at 106 dB, respectively. Figure 3 shows a schematic of the OFDI system that we built for PS imaging. Ninety percent of the light source output was directed to the sample arm, and the remaining 10% was directed to the reference arm of the system interferometer. Half of the reference arm light was coupled to the reference mirror, and another half was directed to a fiber Bragg grating (FBG) for trigger signal generation. An acousto-optic frequency shifter (AOTF, Brimrose Inc. AMF-85-1300) running at 85 MHz was used
Fig. 2.
Laser output (a) trace and (b) spectra.
Fig. 3. PS-OFDI setup with the passively polarizationmultiplexed wavelength-swept light source. Circ, Circulator; GM, galvanometric mirror scanner; RM, reference mirror; FBG, fiber Bragg grating; FS, frequency shifter; AMTIR, AMTIR dummy glass block; and PBS, polarization beam splitter.
in the reference arm for full-range imaging [17]. The light backscattered from the sample was combined with the reference arm light at the cube beam splitter of the detection part. A polarization-diverse, balanced detection scheme was implemented with a pair of balanced receivers (Thorlabs PDB460C, 200 MHz) for PS imaging, minimization of polarization dependent signal fading, speckle reduction, and reduction of source RIN (relative intensity noise) and auto-correlation noise [5,7]. A linear polarizer with a rotation angle of 45 deg relative to the orientation of the polarization beam splitter (PBS) was placed at the reference arm light input port of the detection setup to ensure the same reference arm power for each orthogonal polarization channel. We also placed a dummy AMTIR glass block dimensionally matching that used in the frequency shifter in the reference arm for dispersion compensation at the sample arm light input port of the detection setup. A signal from each balanced receiver corresponding to each orthogonal polarization channel was acquired by a two-channel, high-speed, and high-resolution digitizer (Signatec PX14400, 14 bits) at a rate of 340 MS∕s. We first imaged a partial reflector sample to check the polarization state of the imaging light on the sample. Stokes vector components of the light reflected from each depth of the sample were determined by measuring projections onto a pair of orthogonal polarization states through the polarization-diverse detection. Figure 4(a) shows the normalized Stokes vector components acquired from 512 consecutively acquired A-lines. The states of polarizations (SOPs) of even and odd A-lines are plotted as red and blue dots on the Poincaré sphere, respectively. While the Stokes vectors corresponding each A-line set are well localized of the Poincaré sphere, average angle difference between the two sets of vectors were measured to be 86 deg, verifying that the passive polarization multiplexing scheme provides illumination
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muscle are clearly distinguished because of the strong contrast of local retardation from the PS image. In conclusion, we have demonstrated a novel PS-OFDI scheme based on the wavelength-swept light source with passive polarization multiplexing. This approach allows illumination of the sample with a pair of alternating polarization states that are perpendicular to each other on the Poincaré sphere by simple addition of a pair of linear polarizers in the fiber delay line of the light source. Since the proposed scheme is simple and all passive, it provides a robust and low-cost solution for PS imaging, replacing a relatively complex approach such as an active polarization modulation scheme or a frequencyencoded polarization multiplexing scheme that has been used in conventional PS-OFDI systems. This research was supported in part by the NRF of Korea, grant 2010-0017465, by the MSIP of Korea, grant GFP/(CISS-2012M3A6A6054200) and grant NIPA-2013H0401-13-1007.
Fig. 4. (a) Normalized Stokes vectors of a partial reflector plotted on Poincaré sphere. Intensity and local birefringence images of (b) and (c) rubber phantoms, and (d) chicken tendon and muscle tissue. NS-RP, non-strained rubber phantom; S-RP, strained rubber phantom. Scale bar: 500 μm.
of the sample with a pair of polarization states perpendicular to each other on the Poincaré sphere on each successive pair of A-lines. To demonstrate PS-OFDI imaging, we first imaged rubber phantoms [18,19]. Local retardation at each depth position of the sample was extracted across six depth pixels (21 μm in tissue) [18–20] through geometrical reasoning on the Poincaré sphere [7] following in-plane median filtering and out-of-plane frame averaging of the Stokes vector components. Hence, the local retardation image shows the strength of local birefringence across a given depth range (six axial pixels here) at each location in the imaging sample. Figures 4(b) and 4(c) show that a rubber phantom strained by stretching exhibited strong birefringence and almost no birefringence was observed in the non-strained rubber phantom as expected, while strained and non-strained rubber phantoms are not distinguishable in the intensity images. For biological tissue imaging, we imaged a chicken muscle and tendon tissue ex vivo as shown in Fig. 4(d). While the muscle and tendon show slightly different levels of scattering in the intensity image, a very strong signal was observed from the tendon region in the local retardation image, showing the existence of strong birefringence in the tissue as expected. Regions of tendon and
References 1. M. R. Hee, D. Huang, E. A. Swanson, and J. G. Fujimoto, J. Opt. Soc. Am. B 9, 903 (1992). 2. C. E. Saxer, J. F. de Boer, B. H. Park, Y. Zhao, Z. Chen, and J. S. Nelson, Opt. Lett. 25, 1355 (2000). 3. J. F. de Boer, T. E. Milner, M. J. C. van Gemert, and J. S. Nelson, Opt. Lett. 22, 934 (1997). 4. E. Götzinger, B. Baumann, M. Pircher, and C. K. Hitzenberger, Opt. Express 17, 22704 (2009). 5. S. H. Yun, G. J. Tearney, B. J. Vakoc, M. Shishkov, W. Y. Oh, A. E. Desjardins, M. J. Suter, R. C. Chan, J. A. Evans, I. K. Jang, N. S. Nishioka, J. F. de Boer, and B. E. Bouma, Nat. Med. 12, 1429 (2006). 6. B. J. Vakoc, M. Shishkov, S. H. Yun, W. Y. Oh, M. J. Suter, A. E. Desjardins, J. A. Evans, N. S. Nishioka, G. J. Tearney, and B. E. Bouma, Gastrointest. Endosc. 65, 898 (2007). 7. B. H. Park, M. C. Pierce, B. Cense, and J. F. de Boer, Opt. Express 11, 782 (2003). 8. W. Y. Oh, S. H. Yun, B. J. Vakoc, M. Shishkov, A. E. Desjardins, B. H. Park, J. F. de Boer, G. J. Tearney, and B. E. Bouma, Opt. Express 16, 1096 (2008). 9. W. Y. Oh, B. J. Vakoc, S. H. Yun, G. J. Tearney, and B. E. Bouma, Opt. Lett. 33, 1330 (2008). 10. B. Baumann, W. Choi, B. Potsaid, D. Huang, J. S. Duker, and J. G. Fujimoto, Opt. Express 20, 10229 (2012). 11. Y. Lim, Y. Hong, L. Duan, M. Yamanari, and Y. Yasuno, Opt. Lett. 37, 1958 (2012). 12. W. Wieser, G. Palte, C. M. Eigenwillig, B. R. Biedermann, T. Pfeiffer, and R. Huber, Opt. Express 20, 9819 (2012). 13. W. Y. Oh, B. J. Vakoc, M. Shishkov, G. J. Tearney, and B. E. Bouma, Opt. Lett. 35, 2919 (2010). 14. H. S. Cho, S.-J. Jang, K. Kim, A. V. Dan-Chin-Yu, M. Shishkov, B. E. Bouma, and W. Y. Oh, Biomed. Opt. Express 5, 223 (2014). 15. W. Wieser, B. R. Biedermann, T. Klein, C. M. Eigenwillig, and R. Huber, Opt. Express 18, 14685 (2010). 16. T. Klein, W. Wieser, C. M. Eigenwillig, B. R. Biedermann, and R. Huber, Opt. Express 19, 3044 (2011). 17. S. H. Yun, G. J. Tearney, J. F. de Boer, and B. E. Bouma, Opt. Express 12, 4822 (2004). 18. M. Villiger, E. Z. Zhang, S. K. Nadkarni, W. Y. Oh, B. E. Bouma, and B. J. Vakoc, Opt. Lett. 38, 923 (2013). 19. M. Villiger, E. Z. Zhang, S. K. Nadkarni, W. Y. Oh, B. J. Vakoc, and B. E. Bouma, Opt. Express 21, 16353 (2013). 20. E. Z. Zhang, W. Y. Oh, M. L. Villiger, L. Chen, B. E. Bouma, and B. J. Vakoc, Opt. Express 21, 1163 (2013).
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Polarization-sensitive OFDI using polarizationmultiplexed wavelength-swept laser Han Saem Cho and Wang-Yuhl Oh* Department of Mechanical Engineering, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, South Korea *Corresponding author: [email protected] Received March 28, 2014; revised June 4, 2014; accepted June 4, 2014; posted June 4, 2014 (Doc. ID 209161); published July 2, 2014 We demonstrate a novel polarization-sensitive optical frequency domain imaging system employing passive polarization multiplexing. Simple modification of a fiber delay line in the wavelength-swept light source enables illumination with two perpendicular polarizations that are required for determination of the Stokes vector components of the light reflected from each depth of the tissue. This simple all-passive approach provides a robust and low-cost solution for PS imaging replacing relatively complex conventional schemes such as polarization modulation or frequency-encoded polarization multiplexing. © 2014 Optical Society of America OCIS codes: (170.4500) Optical coherence tomography; (170.3880) Medical and biological imaging; (110.5405) Polarimetric imaging. http://dx.doi.org/10.1364/OL.39.004065
Polarization-sensitive optical coherence tomography (PS-OCT) [1,2] maps birefringence properties of the sample that is complementary to the structural intensity images. For PS-OCT systems where the birefringence of the sample arm does not vary over time, circularly polarized light can be used to illuminate the sample either by free-space optics [1,3] or polarization maintained fiber (PMF) [4] and waveplates in the interferometer ensuring that the incident polarization state did not coincide with the presumed linear birefringence axis of the sample. However, it is impractical to maintain a predetermined polarization state at the tissue surface when the sample arm of the system is not stationary, such as in an endoscopic or catheter-based system [5,6]. Illumination with a pair of polarization states, which are perpendicular to each other on the Poincaré sphere, either by polarization modulation [2,7], frequency multiplexing [8,9], passive polarization multiplexing [10,11], or passive polarization switching in the light source [12] had been demonstrated and suggested to overcome this hurdle, removing the requirement to know the precise polarization state incident on the sample. Since PS imaging is performed as long as the two incident polarizations stay perpendicular to each other on the Poincaré sphere, the birefringence information of the sample, especially local birefringence information on each depth position of the sample, is acquired even with unknown birefringence variation in the sample arm optics. In this Letter, we demonstrate a polarization-sensitive optical frequency domain imaging (PS-OFDI), one form of the second-generation OCT techniques, using a new wavelength-swept light source with passive polarization multiplexing. The novel scheme multiplexes two perpendicular polarization states on the Poincaré sphere on successive A-lines without using any active devices, thereby providing a robust and potentially low-cost solution for PS-OFDI. Figure 1(a) depicts an example of wavelength-swept light sources employed by many of the recent high-speed OFDI systems [13–16]. Following the laser cavity, a fiber delay line (FDL) was used to increase the sweep repetition rate with a wavelength scanning filter of a large free 0146-9592/14/144065-03$15.00/0
spectral range (FSR) and/or to output a unidirectional and close to 100% duty cycle wavelength sweep with a bidirectional wavelength scanning filter such as the tunable Fabry–Perot filter (TFPF). Before entering into the OFDI system, the output from the fiber delay line was amplified by a booster optical amplifier (BOA). For polarization-sensitive imaging, a polarization modulation scheme by active modulator [2,7] or a frequency-encoded polarization multiplexing scheme by acousto-optic frequency shifters [8] has been used to illuminate the sample with a pair of perpendicular polarization states. In the proposed scheme, the temporal multiplexing of
Fig. 1. Schematic diagrams of (a) a conventional wavelengthswept light source with a fiber delay line and a polarization modulator and (b) a proposed wavelength-swept light source with a passively polarization-multiplexing fiber delay line for PS-OFDI. Configurations of wavelength-swept light sources of (c) conventional and (d) newly proposed schemes. WSL, wavelength-swept laser; FDL, fiber delay line; BOA, booster optical amplifier; PM, polarization modulator; Coll, collimator; BS, cube beam splitter; PI-BOA, polarization-insensitive BOA; SOA, semiconductor optical amplifier; TFPF, tunable Fabry–Perot filter; MCU, microcontroller unit; and PC, polarization controller. © 2014 Optical Society of America
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the two perpendicular polarizations was implemented by the FDL in the light source, allowing PS imaging without either polarization modulation or frequency-encoded polarization multiplexing scheme. The even and odd wavelength sweeps from the FDL are separated and their polarizations are aligned to two linear states that are perpendicular to each other on the Poincaré sphere, respectively, as shown in Fig. 1(b). These two perpendicularly polarized wavelength sweeps were recombined at a cube beam splitter and then amplified by a polarization-insensitive BOA. To experimentally demonstrate the passive polarization multiplexing approach, we built a short cavity length wavelength-swept laser (WSL) with an intracavity TFPF (Lambdaquest X1300-160-1000) driven at its first resonant frequency [14]. The intracavity SOA (Thorlabs BOA1130U2) with high polarization dependent gain, which ensures single polarization laser output, was modulated so that the laser was turned on only during the forward wavelength sweep of the filter. After splitting the laser output by 50∕50 coupler, one of them is delayed by half of the filter scan period. Unlike the conventional wavelength-swept light source using an FDL [Fig. 1(c)], a pair of linear polarizers aligned 45 deg to each other was placed in front of the cube beam splitter and the polarizations of the original wavelength sweep and the delayed copy are adjusted to match the orientations of each linear polarizer, before being combined at a cube beam splitter as shown in Fig. 1(d). These interleaved wavelength sweeps were amplified by a polarization-insensitive BOA (PI-BOA, InPhenix IPSAD1304C) before entering into the system interferometer. The average output power of the light source was 40 mW. It provides forward wavelength sweeps with alternating polarization states at a rate of 117.2 kHz with 100% duty cycle as shown in Fig. 2(a). The wavelength tuning range was 110 nm centered at 1290 nm [Fig. 2(b)] and the instantaneous linewidth was measured to be 0.14 nm, which corresponds to a full axial ranging depth of 5.2 mm (the range over which sensitivity remains within 6 dB of that at zero delay). The axial resolution and the sensitivity were measured to be 9.7 μm in air and at 106 dB, respectively. Figure 3 shows a schematic of the OFDI system that we built for PS imaging. Ninety percent of the light source output was directed to the sample arm, and the remaining 10% was directed to the reference arm of the system interferometer. Half of the reference arm light was coupled to the reference mirror, and another half was directed to a fiber Bragg grating (FBG) for trigger signal generation. An acousto-optic frequency shifter (AOTF, Brimrose Inc. AMF-85-1300) running at 85 MHz was used
Fig. 2.
Laser output (a) trace and (b) spectra.
Fig. 3. PS-OFDI setup with the passively polarizationmultiplexed wavelength-swept light source. Circ, Circulator; GM, galvanometric mirror scanner; RM, reference mirror; FBG, fiber Bragg grating; FS, frequency shifter; AMTIR, AMTIR dummy glass block; and PBS, polarization beam splitter.
in the reference arm for full-range imaging [17]. The light backscattered from the sample was combined with the reference arm light at the cube beam splitter of the detection part. A polarization-diverse, balanced detection scheme was implemented with a pair of balanced receivers (Thorlabs PDB460C, 200 MHz) for PS imaging, minimization of polarization dependent signal fading, speckle reduction, and reduction of source RIN (relative intensity noise) and auto-correlation noise [5,7]. A linear polarizer with a rotation angle of 45 deg relative to the orientation of the polarization beam splitter (PBS) was placed at the reference arm light input port of the detection setup to ensure the same reference arm power for each orthogonal polarization channel. We also placed a dummy AMTIR glass block dimensionally matching that used in the frequency shifter in the reference arm for dispersion compensation at the sample arm light input port of the detection setup. A signal from each balanced receiver corresponding to each orthogonal polarization channel was acquired by a two-channel, high-speed, and high-resolution digitizer (Signatec PX14400, 14 bits) at a rate of 340 MS∕s. We first imaged a partial reflector sample to check the polarization state of the imaging light on the sample. Stokes vector components of the light reflected from each depth of the sample were determined by measuring projections onto a pair of orthogonal polarization states through the polarization-diverse detection. Figure 4(a) shows the normalized Stokes vector components acquired from 512 consecutively acquired A-lines. The states of polarizations (SOPs) of even and odd A-lines are plotted as red and blue dots on the Poincaré sphere, respectively. While the Stokes vectors corresponding each A-line set are well localized of the Poincaré sphere, average angle difference between the two sets of vectors were measured to be 86 deg, verifying that the passive polarization multiplexing scheme provides illumination
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muscle are clearly distinguished because of the strong contrast of local retardation from the PS image. In conclusion, we have demonstrated a novel PS-OFDI scheme based on the wavelength-swept light source with passive polarization multiplexing. This approach allows illumination of the sample with a pair of alternating polarization states that are perpendicular to each other on the Poincaré sphere by simple addition of a pair of linear polarizers in the fiber delay line of the light source. Since the proposed scheme is simple and all passive, it provides a robust and low-cost solution for PS imaging, replacing a relatively complex approach such as an active polarization modulation scheme or a frequencyencoded polarization multiplexing scheme that has been used in conventional PS-OFDI systems. This research was supported in part by the NRF of Korea, grant 2010-0017465, by the MSIP of Korea, grant GFP/(CISS-2012M3A6A6054200) and grant NIPA-2013H0401-13-1007.
Fig. 4. (a) Normalized Stokes vectors of a partial reflector plotted on Poincaré sphere. Intensity and local birefringence images of (b) and (c) rubber phantoms, and (d) chicken tendon and muscle tissue. NS-RP, non-strained rubber phantom; S-RP, strained rubber phantom. Scale bar: 500 μm.
of the sample with a pair of polarization states perpendicular to each other on the Poincaré sphere on each successive pair of A-lines. To demonstrate PS-OFDI imaging, we first imaged rubber phantoms [18,19]. Local retardation at each depth position of the sample was extracted across six depth pixels (21 μm in tissue) [18–20] through geometrical reasoning on the Poincaré sphere [7] following in-plane median filtering and out-of-plane frame averaging of the Stokes vector components. Hence, the local retardation image shows the strength of local birefringence across a given depth range (six axial pixels here) at each location in the imaging sample. Figures 4(b) and 4(c) show that a rubber phantom strained by stretching exhibited strong birefringence and almost no birefringence was observed in the non-strained rubber phantom as expected, while strained and non-strained rubber phantoms are not distinguishable in the intensity images. For biological tissue imaging, we imaged a chicken muscle and tendon tissue ex vivo as shown in Fig. 4(d). While the muscle and tendon show slightly different levels of scattering in the intensity image, a very strong signal was observed from the tendon region in the local retardation image, showing the existence of strong birefringence in the tissue as expected. Regions of tendon and
References 1. M. R. Hee, D. Huang, E. A. Swanson, and J. G. Fujimoto, J. Opt. Soc. Am. B 9, 903 (1992). 2. C. E. Saxer, J. F. de Boer, B. H. Park, Y. Zhao, Z. Chen, and J. S. Nelson, Opt. Lett. 25, 1355 (2000). 3. J. F. de Boer, T. E. Milner, M. J. C. van Gemert, and J. S. Nelson, Opt. Lett. 22, 934 (1997). 4. E. Götzinger, B. Baumann, M. Pircher, and C. K. Hitzenberger, Opt. Express 17, 22704 (2009). 5. S. H. Yun, G. J. Tearney, B. J. Vakoc, M. Shishkov, W. Y. Oh, A. E. Desjardins, M. J. Suter, R. C. Chan, J. A. Evans, I. K. Jang, N. S. Nishioka, J. F. de Boer, and B. E. Bouma, Nat. Med. 12, 1429 (2006). 6. B. J. Vakoc, M. Shishkov, S. H. Yun, W. Y. Oh, M. J. Suter, A. E. Desjardins, J. A. Evans, N. S. Nishioka, G. J. Tearney, and B. E. Bouma, Gastrointest. Endosc. 65, 898 (2007). 7. B. H. Park, M. C. Pierce, B. Cense, and J. F. de Boer, Opt. Express 11, 782 (2003). 8. W. Y. Oh, S. H. Yun, B. J. Vakoc, M. Shishkov, A. E. Desjardins, B. H. Park, J. F. de Boer, G. J. Tearney, and B. E. Bouma, Opt. Express 16, 1096 (2008). 9. W. Y. Oh, B. J. Vakoc, S. H. Yun, G. J. Tearney, and B. E. Bouma, Opt. Lett. 33, 1330 (2008). 10. B. Baumann, W. Choi, B. Potsaid, D. Huang, J. S. Duker, and J. G. Fujimoto, Opt. Express 20, 10229 (2012). 11. Y. Lim, Y. Hong, L. Duan, M. Yamanari, and Y. Yasuno, Opt. Lett. 37, 1958 (2012). 12. W. Wieser, G. Palte, C. M. Eigenwillig, B. R. Biedermann, T. Pfeiffer, and R. Huber, Opt. Express 20, 9819 (2012). 13. W. Y. Oh, B. J. Vakoc, M. Shishkov, G. J. Tearney, and B. E. Bouma, Opt. Lett. 35, 2919 (2010). 14. H. S. Cho, S.-J. Jang, K. Kim, A. V. Dan-Chin-Yu, M. Shishkov, B. E. Bouma, and W. Y. Oh, Biomed. Opt. Express 5, 223 (2014). 15. W. Wieser, B. R. Biedermann, T. Klein, C. M. Eigenwillig, and R. Huber, Opt. Express 18, 14685 (2010). 16. T. Klein, W. Wieser, C. M. Eigenwillig, B. R. Biedermann, and R. Huber, Opt. Express 19, 3044 (2011). 17. S. H. Yun, G. J. Tearney, J. F. de Boer, and B. E. Bouma, Opt. Express 12, 4822 (2004). 18. M. Villiger, E. Z. Zhang, S. K. Nadkarni, W. Y. Oh, B. E. Bouma, and B. J. Vakoc, Opt. Lett. 38, 923 (2013). 19. M. Villiger, E. Z. Zhang, S. K. Nadkarni, W. Y. Oh, B. J. Vakoc, and B. E. Bouma, Opt. Express 21, 16353 (2013). 20. E. Z. Zhang, W. Y. Oh, M. L. Villiger, L. Chen, B. E. Bouma, and B. J. Vakoc, Opt. Express 21, 1163 (2013).
