15th OptoElectronics and Communications Conference (OECC2010) Technical Digest, July 2010, Sapporo Convention Center, Japan
9C3-2
Optical Coherence Tomography Based on Optical Frequency Comb Generator with Single-sideband Modulator Zuyuan He, Quang Nam Ho, Weiwen Zou*, Koji Kajiwara, Hiroshi Takahashi, and Kazuo Hotate Department of Electrical Engineering and Information Systems, The University of Tokyo, Tokyo 113-8656, Japan *Currently with Dept. of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Tel: +81 3 5841 6746, Fax: +81 3 5841 8563, E-mail:
[email protected]
Abstract — A novel optical coherence tomography (OCT) scheme is proposed based on an optical frequency comb (OFC) generator with single-sideband modulator. The OFC generator has a flat spectrum of 7.46-nm full width at half magnitude at 1550-nm band. By changing the comb interval around 10 GHz slightly, the detection depth is scanned through the imaging sample at high speed. Elimination of the coherence side-lobes of the generated optical comb is also studied by setting the OFC’s seed laser at optimized wavelength. Tomography images of a glass cell with air or milk filled are successfully achieved with a spatial resolution of ~90 Pm, which can be further improved by using a gainflattening filter in the OFC generator.
I. INTRODUCTION
Optical Frequency Comb Generator isolator Comb seed laser 10%
30:70 coupler 90% 10:90 coupler EDF
Figure 1 shows the experimental setup of the OFCOCT. The left part denotes an OFC generator [7] and the right corresponds to the OCT interferometer [5]. A
coupler
Scan fm
isolator AOM
SSBM
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circulator 40MHz
collimator
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Computer isolator 980-nm laser
II. EXPERIMENTAL SETUP isolator
Interferometer
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isolator
Optical coherence tomography (OCT) has become a promising technique for biomedical as well as industrial applications [1-6]. Compared to the time-domain OCT [1-2] and the spectrum-domain OCT [3-4], OCT based on the synthesis of optical coherence function technique (SOCF-OCT) [5-6] can realize tomography imaging in scattering objects with higher dynamic range because of using highly coherent laser source. The SOCF-OCT uses a stepwise frequency-modulated laser diode as the light source to synthesize its power spectrum into a comb shape; then, its coherence function becomes a sharp peak. By modifying the waveform of a stepwise phase modulation [5], the coherence peak is scanned over the imaging object to get the reflection/scattering distribution. The modification of the waveform, however, takes quite long time. In this paper, we propose a novel scheme to overcome above problem by using an optical frequency comb (OFC) generator as the light source. The OFC generator is based on a ring cavity with a single-sideband (SSB) modulator. With a seed laser at 1550 nm, the OFC generator has a flat spectral span with a full width at half magnitude (FMHM) of ~7.46 nm. By changing the wavelength of the OFC’s seed laser, the side-lobes in the coherence function are suppressed. We succeeded in the tomography imaging of a glass cell with air or milk filled by simply tuning the OFC’s comb interval around 10 GHz and 7 GHz, respectively. The spatial resolution of the OFC-OCT is about 90 Pm currently.
tunable laser at 1550 nm is used as the OFC seed laser, whose output is launched into a ring cavity through a 30/70 coupler. An optical SSB modulator driven by a microwave synthesizer at around 10 GHz or 7 GHz is used to down-shift the frequency of the light wave in the ring cavity. By proper bias control, ~30-dB suppression of the carrier and the unwanted sideband was achieved. The erbium-doped fiber (EDF) pumped with s 980-nm laser is used to compensate the optical loss in the ring cavity. Lightwave from the OFC is output by a 10/90 coupler isolated by an isolator. In the ring cavity, the optical frequencies with a unique frequency interval are continuously shifted by the SSB modulator. As a result, a large number of spectrally separated elements are generated. The phase relation among the frequency elements in the OFC generator changes unsteadily due to variation in the length of the ring cavity. However, phase fluctuation does not deteriorate the stability of the frequency interval because it is precisely equal to the microwave frequency. Therefore, the OFC generator used here does not need strict control of the phase fluctuation and the stabilization of the ring length. Comparing to the OFC generator based on a mode-locked laser and the Fabry-Perot phase modulator [7], which requires a stabilization technique to strictly control the phase fluctuation and the stabilization of the optical length, the novel method proposed in this study is more cost-effective. A half of the output of the OFC generator is launched into the imaging object through a 50/50 coupler, an optical circulator, and an inline collimator. The other half is used as a local optical oscillator, whose frequency is shifted by 40 MHz with an acousto-optic modulator (AOM). The reflected or backscattered wave from the imaging object under test interferes with the local optical
sample (mirror or glass)
isolator BPD
40MHz
ESA
Fig. 1. Experimental setup of OFC-based OCT. The left part is the OFC generator and the right part is the interferometer.
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Right Side Lobe/Main Peak [dB]
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3dB comb span : 7.46nm
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Fig. 2. Optical spectrum of the OFC generator. The full width at half magnitude is ~7.46 nm.
Reflectivity [dB]
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Fig. 4. Suppression of sidelobes. (a) experiment and (b) simulation.
Experiment
-3 0
Resolution ~97䃛m
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L e f t S d /M a in P e a k
0
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Right Side Lobe/Main Peak[dB]
W a vR ei g lhet Snd /gM ta hi n P[ enam k ]
-6 0 1552
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䢯䢴䢲 䢯䢵䢲
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(b)
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9 .6
P o s itio n [m m ]
9 .8
1 0 .0
Resolution ~80䃛m
䢢
Simulation
䢯䢶䢲
Fig. 5. Tomogram of a glass cell with air (a) or milk (b) filled.
䢯䢷䢲 䢳䢷䢷䢵䢰䢲䢢䣰䣯 䢳䢷䢷䢴䢰䢺䢢䣰䣯 䢳䢷䢷䢴䢰䢸䢢䣰䣯
䢯䢸䢲 䢯䢹䢲 䢢 䢻
䢻䢰䢴
䢻䢰䢶
䢻䢰䢸
Position [mm]
䢻䢰䢺
coherence peaks. To eliminate them, we tuned the wavelength of the seed laser to apodize the spectrum of the comb. As shown in Fig. 4(a) and (b), there is an optimal wavelength (1552.6 nm) of the seed laser to efficiently suppress the coherence sidelobes. At the above optimal wavelength, we carried out the tomography imaging of a glass cell with air or liquid milk filled. The cell comprises of two walls (front and rear) and each wall includes two glass faces. The distance between the front and the rear walls is 10 mm. The crosssectional position (x-axis) is scanned by a transit stage. First, we set the fm = 10 GHz to measure the tomography image of the cell sample with air filled, which is shown in Fig. 5(a). The tomography image of the sample with liquid milk filled [see Fig. 5(b)] is also successfully detected by setting fm = 7 GHz. Because of the different refractive index between air and liquid milk, the optical path with milk filled is longer than that with air filled. Comparing Fig. 5(a) and (b), the random scattering nature and the attenuation of liquid milk can be observed clearly.
䢳䢲
Fig. 3. (a) Measured and (b) simulated reflection along the optical path of a mirror at different seed laser’s wavelength.
oscillator at the second 50/50 coupler, and the interference signal at 40 MHz is detected with a balanced photo detector (BPD), and sampled by an electrical spectral analyzer (ESA). The OFC frequency interval fm is scanned by changing the microwave frequency to the SSB modulator; thus the coherence peak is scanned over the object so that the reflection/scattering at different longitudinal position of the imaging object is sampled continuously. All operations are controlled by a LabVIEW program. The measurement range zc and the spatial resolution 'z of the OFC-based OCT are given by [5], c D uc zc , and ' z , 2 nf m 2 nf s where c is the light speed in vacuum, fs the frequency span of the OFC generator, n the refractive index of the imaging object, and Įӌ㸯 an interferometric coefficient.
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, and J. G. Fujimoto, Science 254, 1178 (1991). [2] J. A. Izatt, M. R. Hee, G. M. Owen, E. A. Swanson, and J. G. Fujimoto, Opt. Lett. 19, 590 (1994). [3] S. R. Chinn, E. A. Swanson, and J. G. Fujimoto, Opt. Lett. 22, 340 (1997). [4] S. H. Yun, G. J. Tearney, B. E. Bouma, B. H. Park, and J. F. de Boer, Optics Express Vol. 11, No.26, Dec. (2003). [5] Z. He and K. Hotate, Optics Letters, Vol. 24, No. 21, pp. 1502-1504, Nov. (1999). [6] S. Choi, T. Shioda, Y. Tanaka, and T. Kurokawa, Opt. Lett. 31, 1976 (2006). [7] T. Kawanishi, T. Sakamoto, S. Shinada, and M. Izutsu, IEICE Electronics Express 1, 217 (2004).
III. EXPERIMENTAL RESULTS Figure 2 shows a typical example of the OFC’s spectrum with a FMHW of ~7.46 nm. Figure 3(a) shows the reflection from a mirror as the function of the optical path, i.e., the coherence function. The 3-dB width of the peak (the spatial resolution) is estimated to be ~97 Pm, which can be further improved by using a gain-flattening filter in the ring cavity to expand the bandwidth of the OFC. Simulation was also carried out as shown in Fig. 3(b) based on the measured spectrum. The simulation matches well with the experimental result. As shown in Fig. 3(a) and (b), there are quite high sidelobes of the
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