Polarization-sensitive optical coherence tomography using self-phase modulation in fiber for broadband source generation

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PolarizationSensltlve Optical Coherence Tomography Using Self-phase Modulation in Fiber for Broadband Source Generation I-Jen Hsu, Shin-Ming Lin, Chih-Wei Lu, Chih- Ren Deng, Jui-Che Tsai, Yean-Woei Kiang, C.C. Yang, Chih-Yu Wang,‘ Chii-Wann Lin,**

Department of Electrical Engineering, Graduate Institute of Electro-Optical Engineering, and Graduate Institute of Communication Engineering, National Taiwan University, I ,

Roosevelt Road, Sec. 4, Taipei, Taiwan, R.O.C.; Email: [email protected]; *Department of Bioengineering, I-Shou University, Kaohsiung, Taiwan, R.O.C.; **Graduate Institute of Bioengineering, National Taiwan University, Taipei, Taiwan, R.O.C.

Optical coherence tomography (OCT) has been widely studied for high-resolution subsurface tis- sue scanning. Because the longitudinal resolu- tion in OCT scanning is inversely proportional to the light source bandwidth, pursuing means for generating broadband light source, either coher-

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GRIN lens. Fundamental characteristics (e.g., spatial resolution and dynamic range) were dis- cussed and OCT images of test sample and onion were shown. This compact OCT system may be utilized with imaging optics for endoscopic ap- plications.

J.M. Schmitt, “Optical coherence tomogra- phy (OCT),” IEEE J. Sel. Top. Quantum Elec- tron 5, 1205-1215 (1999).

A.M. Rollins, R. Ung-arunyawee, A. Chak, R.C.K. Wong, K. Kobayashi, M.V. Sivak, and J.A. Izatt, “Real-time in vivo imaging of hu- man gastrointestinal ultrastructure by use of endoscopic optical coherence tomography with a novel efficient interferometer design,” Opt. Lett. 24, 1358-1360 (1999).

T. Ikari, M. Sat0 and N. Tanno, “Optical fun- damental characteristics of partially delayed type Fizeau interferomter:’ Proceeding of Optical Society of Japan Symposium on Bio- medical Optics 1,30-31 (2000).

M. Sato, K. Seino, K. Onodera and N. Tanno, “Phase-drift suppression using harmonics in heterodyne detection and its application to optical coherence tomography,” Opt. Com- mun. 184,95-104 (2000). CTuY6 6:15 pm

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CTuY6 Fig. 1. Fiber input and output spectra when the power coupled into the fiber was 200 mW and the input central wavelength was 810

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ent or incoherent, is crucial for OCT develop- ment. In this paper, we report the use of self- phase modulation in a single-mode fiber to gen- erate broadband source for building a polarization OCT system.’ Due to the Kerr effect in fiber, self-phase modulation of a short pulse can induce a nonlinear phase distribution, which is equivalent to the generation of chirp or spec- trum broadening?

In experiments, about 100 fsec pulses from a mode-locked Tisapphire laser were end-coupled into a single-mode fiber of about 5 m. When the coupled power was between 200 to 400 mW, the output spectral width ranged from 50 to 70 nm. Figure 1 shows the spectra of the fiber input and output when the power coupled into the fiber was 200 mW and the input central wavelength was 810 nm. The original 10 nm wide spectrum has been broadened to around 50 nm. This pulsed light source of broad spectrum was used for a typical free-space OCT system (Michelson interferometer) with polarization control. In the system, a constant-speed stage was used in the reference arm for phase modulation and depth scanning. A lens with 3 mm in focal length was used for focusing light beam onto samples. The beam size at the focal point was estimated to be around 5 pm, which corresponded to the resolu- tion of the lateral scan. The high lateral resolu- tion was chosen at the expense of a short depth of focus or depth range of scanning, which was esti- mated to be around 100 Fm. The power incident upon samples was around 50 mW. In polariza- tion tomography operation, circularly polarized signals were applied onto samples. A polarizer was then used before the photo-detector for po- larization-gated scanning. The depth resolution was tested to be from 5 to 10 pm, depending on

the input power to the fiber. The sensitivity was estimated to be higher than 100 dB.

We have used this system to scan various tis- sue samples. Figures 2 and 3 show the results of polarization OCT scanning of chicken leg tissue. The lateral scan was along the direction of tissue filaments. Figures 2 and 3 correspond to the cas- es of the summation and difference of the two polarization components, respectively. The dif- ference image can show clearer features of such

CTuY6 Fig. 2. OCT scanning image of chick- en leg tissue with the summation of two polar- ization components.

CTuY6 Fig. 3. OCT scanning image of chick- en leg tissue with the difference of two polariza- tion components.

skeletal tissue, compared with the summation image. The light source bandwidth for these scans was 50 nm, corresponding to a depth reso- lution of about 10 pm. The circular features on the right could probably originate from certain cavity structures. This OCT system was used for tissue burn study. The results will also be present- ed. It is expected that with suitable control of group-velocity dispersion in fiber through taper- ing, the bandwidth of the OCT light source can be further increased.

References

1. J.F. de Boer, S.M. Srinivas, B.H. Park, T.H. Pham, 2. Chen, T.E. Milner, and J.S. Nelson, “Polarization effects in optical coherence to- mography of various biological tissues,” IEEE J. Sel. Top. Quantum Electron. 5, pp. e.g., G.P. Agrawal, Nonlinear Fiber Optics,

Academic Press (1989). 1200-1204 (1999). 2.

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Fredrik Laurell, Royal Inst. of Tech., Sweden, Presider

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Influence of crystallinity on bulk laser damage threshold of CsLiB6010 crystals Masashi Yoshimura, Tomosumi Kamimura, Richi Ono, Yoke Khin Yap, Yusuke Mori, and Takatomo Sasaki, Department ofEIectricaC Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan; Email: [email protected]

Tetsuo Kojima, Susumu Konno, Shuichi Fujikawa, and Koji Yasui, Advanced Technology R

& D Center, Mitsubishi Electric Corporation, 8-1 -

1 Tsukaguchi, Amagasaki, Hyogo 661 -8661, Japan

Mitsuhiro Tanaka and Yukikatsu Okada,

KogakuGiken Co., Ltd., 135 Nurumizu, Atsugi, Kanagawa 243, Japan

Because of cost-performance and convenient op-

eration, solid-state ultraviolet (W) lasers with nonlinear optical (NLO) crystals are in high de- mand for various industrial and medical applica- tions. CsLiB6010(CLBO) is suitable for fourth- and fifth-harmonics generation of pulsed NdYAG lasers.’.’ Generally, laser-induced dam- ages and W light absorption of NLO crystals limit the power scaling of UV lasers. Improve- ment of bulk quality can effectively reduce the absorption and enhance the laser damage thresh- old. Recently, we developed a strirring-solution technique to grow high-quality CLBO crystals.’ In this paper, we discuss influence of the crys- tallinity on the bulk laser damage threshold. The high-quality CLBO have approximately 2.5-fold higher damage threshold then fused quartz and conventional CLBO. As preliminary results, we also found the newly developed CLBO has lower absorption and can generate high-power U V