Narrowband wavelength selective detector applicable SD-OCT based on Fabry-Perot tunable filter and balanced photoreceiver

Tae Joong Eom1, Vitali. A. Tougbaev1, Bong-Ahn Yu1, Woojin Shin1,

Yeung Lak Lee1, and Do-Kyeong Ko1

1Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju, 500-712, Korea Tel: +82-62-970-3344, Fax: +82-62-970-3419, E-mail: eomtj@gist.ac.kr

Chang-Seok Kim2, Eun Joo Jung2, Jae Seok Park2, and Myung Yung Jeong2

2Departmen of Nanosystem Engineering, Pusan National University, Pusan, 609-735, Korea

Eun Seo Choi3

3Department of Physics, Chosun University, Gwangju, 501-759, Korea

ABSTRACT

We demonstrate a novel implementation of spectral domain OCT by using a proposed sweeping detector at 1320 nm wavelength range. A fiber pigtailed Fabry-Perot tunable filter is newly adapted to receive spectral interferometer information using a photo-receiver instead of using charged couple detector arrays. In order to show a possibility of the scheme in other view point, we have changed the position of the Fabry-Perot tunable filter of the interferometer. The combination of a super luminescent LED and a semiconductor optical amplifier was used as an optical source. Its output power is about 10 mW and the spectral bandwidth is about 60 nm. The filtered light after passing thorough the Fabry-Perot tunable filter has 0.15 nm instantaneous spectral linewidth with 1.3 mW average output power. The system with an axial resolution of 12 µm performed OCT imaging of a cornea of a rat eye proving potential about the application of the proposed sweeping detector OCT.

Keywords: swept optical source, swept optical detector, tunable filter, optical coherence tomography, biological imaging

1. INTRODUCTION An optical coherence tomography (OCT) as an optical imaging method has provided depth-resolved morphological

information of biological samples without invasive process1, 2. In the time-domain (TD) implementation of OCT, the position of the reference mirror in the interferometer is rapidly scanned in order to obtain a depth profile (A-line) within a sample3. To overcome the limitations in scanning speed, frequency domain OCT (FD-OCT) utilizing a swept laser source is being developed4, 5. Compared to a spectral-domain OCT (SD-OCT) by using a charge-coupled device (CCD) or area CCD camera6, FD-OCT can offer various advantages. Among them, an enhanced imaging speed based on swept source is presented by using a fast scanning device with high-repetition rate. More recently, novel OCT system with a wavelength range of 800 nm was reported to reduce the effect of water absorption of a sample7. Since the resolution in FD-OCT is also given by spectral properties of a source used, an ultra-wide band source is considered as a key-technology to achieve ultra-high resolution imaging. Recently, a semiconductor optical amplifier (SOA) having about 100-nm spectral bandwidth is employed for optical source of FD-OCT8. To achieve higher scanning speed of FD-OCT system, Electro-optic wavelength-tunable fiber laser based on cascaded composite Sagnac loop filters was also reported9. In order to increase the axial resolution of OCT imaging, a femto-second(fs) laser system was applied to make ultra-wide

Coherence Domain Optical Methods and Optical Coherence Tomography in Biomedicine XII,edited by Joseph A. Izatt, James G. Fujimoto, Valery V. Tuchin, Proc. of SPIE Vol. 6847, 68470R, (2008)

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band spectrum10, 11. Recently, Kerr-lens mode-locked Cr3+:LiCAF was reported for OCT imaging in order to achieve a smooth, Gaussian-like output spectrum, while preserving as much bandwidth and output power as possible12.

One way to high-speed imaging is offered by spectral-domain (SD) OCT, in which the interference signal is detected separately by use of a line-scan charge-CCD) or area CCD camera. Until now, however, the CCD system has been limited by the depth of range, the hard removal of DC noise, the narrow dynamic range and the limited imaging resolution of CCD. Especially, though many broadband light sources have been developed in the range of 1300 nm, it is hard to find a large number (> 1000) pixel InGaAs CCD to utilize the light source effectively around 1300 nm. The wavelength range of 1300 nm is attractive range for real application of OCT, because that the component of the wavelength range has been already developed in order to use optical communication systems.

In this paper, we demonstrate a novel SD-OCT in the center wavelength of 1300 nm based on a fiber pigtailed Fabry-Perot tunable filter (FP-TF) and a balanced photo receiver, instead of using a CCD camera. In order to show a possibility of a proposed method, we have experimentally showed two different systems depending on a position of the FP-TF. Also, this method will reduce the inter symbol interference from side-lobes after a Fourier transform process to get OCT image. We have previously reported similar effect by using a Ti:Sapphire fs laser and an external tunable filter13. The dithering phenomena of the longitudinal lasing mode can be occurred from unstable lasing mode. Because traditional FD-OCT systems use a swept fiber laser that has a possibility of the internal multi modal lasing from a wavelength tunable filter within the laser cavity. There is no optical source dithering as changing output wavelength because the tunable filter exits at outside of the laser cavity.

2. EXPERIMENTAL FOR SWEEPING DETECTOR The proposed OCT system configuration is shown schematically in Fig. 1. The system consists of a broadband light source around 1.3 um, fiber couplers and optical circulators for the Michelson interferometer, the FP-TF for a variable filter into the photo receiver. Briefly, a broadband light source was combined a super luminescent laser diode and a semiconductor amplifier (Inphenix Inc.) and it has a center wavelength at ~1320 nm, a full width at half maximum (FWHM) of 60 nm and a maximum output power of 10 mW. The axial resolution was 12 µm in air based on the FWHM of the coherence envelope.

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Figure 1. Configuration of the proposed spectral domain OCT based on a sweeping detector.

The optical beams are employed a sample arm (70 % port) and a reference arm (30 % port) through a circulator. The splitting ratio was easily controlled by a tunable directional coupler (Fiberpro Inc.). Two arms have each

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polarization controllers in order to maximize the interference between sample and reference light at the detector. The reflected power from the reference mirror was attenuated by using a variable neutral density filter before returning to the reference arm. The reference mirror was kept stationary throughout the experiments. Reflected beams from the respective arms combine at a 50/50 coupler after passing through the circulators again. The imaging of sample was controlled by a scanning galvanometer.

Resulting interference signal is multiplexed by the division of wavelength through FP-TF to propagate to the dual-balanced photo-receiver. The detector signal is synchronously digitized with data acquisition board. Data acquisition software was designed to ensure saving of one captured frame onto a memory of computer. A data buffers were held in memory until by a spread thread. The Fast Fourier Transform (FFT) thread has run independently, grabbing data for the acquired frame and data processing for OCT image display. To prove the imaging performance of the system, we imaged a transparent cover glasses and air-gap between them by scanning the sample beam. In Fig. 2, the interference detector signal at the photo receiver is shown for the sample of two slide glasses. The well balanced interference signal removed background DC signal. Figure 2 shows the complex depth encoded signal using the Discrete (D-FFT) algorithm. Thickness of each cover glass and air-gap were 250 µm (n = 1.5) and 10 µm (n = 1.0), respectively. Dispersion imbalance between two arms was managed by placing a glass plate and the neutral density filter in the reference arm. The OCT image of the stacked slide glasses was also shown in Fig. 2.

Figure 2. Complex depth encoded signal using discrete FFT, interference signal through the photoreceiver, and OCT image of two cover glasses. The image was cropped to remove the DC peak from the broadband source envelope.

The second experiment of OCT imaging is shown with an IR detection card and a rat eye ex vivo in Fig. 3. The image size was 512 x 1024 pixels and the images were cropped to remove the DC peak from the broadband source envelope. The maximum intensity of the image was taken as the relative maximum intensity in the frame of the OCT image. Figure

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3(a) shows a surface and depth image of the IR detection card. The OCT image nearby a cornea and an iris of a rat eye ex vivo is shown in Fig. 3(b). The part of iris and cornea are clearly identifiable.

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Figure 3. OCT images of (a) IR detection card and (b) nearby a cornea and a iris of a rat eye ex vivo.

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3. SWEEPING SOURCE WITHOUT DITHERING In additionally, we have changed the position of the FP-TF of the interferometer to prove the imaging performance

without dithering of a wavelength swept source. Figure 5 shows the experimental setup with changing position of the FP-TF front of the optical fiber interferometer. We have used same experimental setup for the previous experimental of the sweeping detector. As shown in Fig. 5, variable couplers and circulators are used to build a Michelson interferometer. The optical beams are separated into a sample arm (70% port) and a reference arm (30% port) through a coupler. Reflected beams from both arms are combined at a 50/50 coupler after passing through the circulators. Figure 5 shows the interference detector signal at the photo receiver for the sample of a cover glass.

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Figure 5. Interference signal after passing through the FP-TF and the fiber interferometer for a cover glass.

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The experiment of OCT imaging for reducing sidelobe effect from dithering of a wavelength swept source is shown with an IR detection card and a rat eye ex vivo in Fig. 6. The image size was reduced 512 x 300 pixels from 512 x 1024 pixels and the images were cropped to remove the autocorrelation peak. Figure 6 shows an OCT image nearby a cornea and an iris of a rat eye ex vivo. The part of cornea, iris, and lens of the rat eye are clearly identifiable.

Figure 6. OCT imaging of a rat eye ex vivo using the FP-TF and the fiber interferometer.

4. DISCUSSION AND CONCLUSION We introduce and demonstrate a new SD-OCT technology for 1300 nm region broadband light sources. This system uses FP-TP with a high wavelength resolution, instead of CCD array, to receive spectral interferometer information. We experimentally demonstrated the imaging comparison of spectral OCT using the FP-TF and the balanced photo receiver. This method will reduce the inter symbol interference from side-lobes after a Fourier transform process to get OCT image. The dithering phenomena of the longitudinal lasing mode can be occurred from unstable lasing mode. The uniform and stable multi-wavelength spectral distribution is helpful to obtain the higher sensitivity from the lower exposure intensity source to get a better quality spectral OCT image.

ACKNOWLEDGEMENTS This work was partially supported by Ministry of Commerce, Industry and Energy (MOCIE) of Republic of Korea through the research program for commercializing of new technologies (Grant No. 10023577) and the Ministry of Science and Technology (MOST) of Republic of Korea through APRI-Research Program of GIST.

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. Puliafito, J. G. Fujimoto, “Optical coherence tomography,” Science 254, 1178-1181 (1991).

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2 J. M. Schmitt, “Optical coherence tomography (OCT): a review,” IEEE J. Sel. Top. Quantum Electron., 5, 1205-1215, 1999. 3 A. F. Fercher, W. Drexler, C. K. Hitzenberger, and T. Lasser, “Optical Coherence Tomography - Principles and Applications,” Rep. Prog. Phys. 66, 239-303 (2003). 4 S. Yun, G. Tearney, B. Bouma, B. Park, and J. de Boer, “High-speed spectral-domain optical coherence tomography at 1.3 µm wavelength,” Opt. Express 11, 3598-3604, 2003. 5 S. H. Yun, C. Boudoux, M. C. Pierce, J. F. de Boer, G. J. Tearney and B. E. Bouma, “Extended-cavity semiconductor wavelength-swept laser for biomedical imaging,” IEEE Photon. Technology Lett., 16, pp. 293-295, 2004. 6 N. Nassif, B. Cense, B. Park, M. Pierce, S. Yun, B. Bouma, G. Tearney, T. Chen, and J. de Boer, “In vivo high-resolution video-rate spectral-domain optical coherence tomography of the human retina and optic nerve,” Opt. Express 12, 367-376 (2004) 7 H. Lim, J. F. de Boer, B. H. Park, E. C. Lee, R. Yelin, and S. H. Yun, “Optical frequency domain imaging with a rapidly swept laser in the 815-870 nm range,” Opt. Express 14, 5937-5944, 2006. 8 W. Y. Oh, S. H. Yun, G. T. Tearney and B. E. Bouma, “Wide tuning range wavelength-swept laser with two semiconductor optical amplifiers,” IEEE Photon. Technology Lett., 17, pp. 678-680, 2005. 9 C. S. Kim, F. N. Farokhrooz and J. U. Kang, “Electro-optic wavelength-tunable fiber laser based on cascaded composite Sagnac loop filters,” Opt. Lett., 29, pp. 1677-1679, 2004. 10 B. Bouma, G. J. Tearney, S. A. Boppart, M. R. Hee, M. E. Brezinski, and J. G. Fujimoto, “High-resolution optical coherence tomographic imaging using a mode-locked Ti:Al2O3 laser source,” Opt. Lett. 20, 1486- (1995). 11 S. Bourquin, A. Aguirre, I. Hartl, P. Hsiung, T. Ko, J. Fujimoto, T. Birks, W. Wadsworth, U. Bünting, and D. Kopf, “Ultrahigh resolution real time OCT imaging using a compact femtosecond Nd:Glass laser and nonlinear fiber,” Opt. Express 11, 3290-3297, 2003. 12 P. Wagenblast, T. Ko, J. Fujimoto, F. Kaertner, and U. Morgner, “Ultrahigh-resolution optical coherence tomography with a diode-pumped broadband Cr3+:LiCAF laser,” Opt. Express 12, 3257-3263, 2004. 13 T. J. Eom, H.-G. Choi, J. H. Sung, C.-S. Kee, D.-K. Ko, and J. Lee , E. S. Choi, “Wavelength tunable broadband source based on Ti:Sapphire femtosecond laser applicable to FD-OCT system,” Photonic West 2007, 2007-1-23, San Jose, CA, 2007.

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