matched adhered ridge waveguide - OSA Publishing

High-gain, wide-dynamic-range parametric interaction in Mg-doped LiNbO3 quasi-phasematched adhered ridge waveguide Rai Kou,1,2,4 Sunao Kurimura,1,2,* Kiyofumi Kikuchi,1,2 Akihiro Terasaki,1,2 Hirochika Nakajima,2 Katsutoshi Kondou,3 and Junichiro Ichikawa3 1 National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan Graduate School of Advanced Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan 3 New Technology Research Laboratories, Sumitomo Osaka Cement Co., LTD., 585 Toyotomi-cho, Funabashi, Chiba 274-8601, Japan 4 Currently with NTT Microsystem Integration Laboratories, 3-1, Morinosato Wakamiya, Atsugi, Kanagawa, Japan 243-0198 *[email protected] http://www.nims.go.jp/fcg/kurimura/ 2

Abstract: With recent developments and optimizations for quasi-phasematched adhered ridge waveguide (QPM-ARW), outstanding performances containing efficient amplification were demonstrated by difference frequency generation (DFG) and optical parametric amplification (OPA). A maximum channel conversion efficiency of +7.6 dB (570%) was achieved in a telecommunication band using a 50 mm-long device, when coupling with 160 mW pump. Simultaneously, the input signal was amplified up to +9.5 dB (890%). ©2011 Optical Society of America OCIS codes: (160.3730) Lithium niobate; (190.4410) Nonlinear optics, parametric processes; (230.7405) Wavelength conversion devices.

References and links K. Mizuuchi, A. Morikawa, T. Sugita, and K. Yamamoto, “Efficient second-harmonic generation of 340-nm light in a 1.4-μm periodically poled bulk MgO:LiNbO3,” Jpn. J. Appl. Phys. 42(Part 2, No. 2A), L90–L91 (2003). 2. S. V. Tovstonog, S. Kurimura, and K. Kitamura, “High power continuous-wave green light generation by quasiphase matching in Mg stoichiometric lithium tantalate,” Appl. Phys. Lett. 90(5), 051115 (2007). 3. M. Maruyama, H. Nakajima, S. Kurimura, N. E. Yu, and K. Kitamura, “70-mm-long periodically poled Mgdoped stoichiometric LiNbO3 devices for nanosecond optical parametric generation,” Appl. Phys. Lett. 89(1), 011101 (2006). 4. T. Tanemura, C. S. Goh, K. Kikuchi, and S. Y. Set, “Highly efficient arbitrary wavelength conversion within entire C-band based on nondegenerate fiber four-wave mixing,” IEEE Photon. Technol. Lett. 16(2), 551–553 (2004). 5. V. G. Ta’eed, N. J. Baker, L. B. Fu, K. Finsterbusch, M. R. E. Lamont, D. J. Moss, H. C. Nguyen, B. J. Eggleton, D. Y. Choi, S. Madden, and B. Luther-Davies, “Ultrafast all-optical chalcogenide glass photonic circuits,” Opt. Express 15(15), 9205–9221 (2007). 6. V. G. Ta’eed, M. D. Pelusi, B. J. Eggleton, D. Y. Choi, S. Madden, D. Bulla, and B. Luther-Davies, “Broadband wavelength conversion at 40 Gb/s using long serpentine As2S3 planar waveguides,” Opt. Express 15(23), 15047– 15052 (2007). 7. K. E. Stubkjaer, “Semiconductor optical amplifier-based all-optical gates for high-speed optical processing,” IEEE J. Sel. Top. Quantum Electron. 6(6), 1428–1435 (2000). 8. C. Joergensen, S. L. Danielsen, K. E. Stubkjaer, M. Schilling, K. Daub, P. Doussiere, F. Pommerau, P. B. Hansen, H. N. Poulsen, A. Kloch, M. Vaa, B. Mikkelsen, E. Lach, G. Laube, W. Idler, and K. Wunstel, “Alloptical wavelength conversion at bit rates above 10 Gb/s using semiconductor optical amplifiers,” IEEE J. Sel. Top. Quantum Electron. 3(5), 1168–1180 (1997). 9. S. Kurimura, Y. Kato, M. Maruyama, Y. Usui, and H. Nakajima, “Quasi-phase-matched adhered ridge waveguide in LiNbO3,” Appl. Phys. Lett. 89(19), 191123 (2006). 10. C. Q. Xu, H. Okayama, K. Shinozaki, K. Watanabe, and M. Kawahara, “Wavelength conversions 1.5-μm by difference-frequency-generation in periodically domain-inverted LiNbO3 channel waveguides,” Appl. Phys. Lett. 63(9), 1170–1172 (1993). 1.

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Received 11 Apr 2011; revised 21 May 2011; accepted 22 May 2011; published 3 Jun 2011

6 June 2011 / Vol. 19, No. 12 / OPTICS EXPRESS 11867

11. M. H. Chou, I. Brener, M. M. Fejer, E. E. Chaban, and S. B. Christman, “1.5-μm-band wavelength conversion based on cascaded second-order nonlinearity in LiNbO3 waveguides,” IEEE Photon. Technol. Lett. 11(6), 653– 655 (1999). 12. K. R. Parameswaran, R. K. Route, J. R. Kurz, R. V. Roussev, M. M. Fejer, and M. Fujimura, “Highly efficient second-harmonic generation in buried waveguides formed by annealed and reverse proton exchange in periodically poled lithium niobate,” Opt. Lett. 27(3), 179–181 (2002). 13. Y. L. Lee, H. Suche, Y. H. Min, J. H. Lee, W. Grundkotter, V. Quiring, and W. Sohler, “Wavelength- and timeselective all-optical, channel dropping in periodically poled Ti:LiNbO3 channel waveguides,” IEEE Photon. Technol. Lett. 15(7), 978–980 (2003). 14. T. Suhara and M. Fujimura, Waveguide Nonlinear-Optic Devices (Springer-Verlag, Berlin, 2003). 15. C. Ware, L. K. Oxenløwe, F. Gómez Agis, H. C. H. Mulvad, M. Galili, S. Kurimura, H. Nakajima, J. Ichikawa, D. Erasme, A. T. Clausen, and P. Jeppesen, “320 Gbps to 10 GHz sub-clock recovery using a PPLN-based optoelectronic phase-locked loop,” Opt. Express 16(7), 5007–5012 (2008). 16. L. K. Oxenløwe, F. Gomez Agis, C. Ware, S. Kurimura, H. C. H. Mulvad, M. Galili, K. Kitamura, H. Nakajima, J. Ichikawa, D. Erasme, A. T. Clausen, and P. Jeppesen, “640 Gbit/s dock recovery using periodically poled lithium niobate,” Electron. Lett. 44(5), 370–372 (2008). 17. M. Nakazawa, S. Okamoto, T. Omiya, K. Kasai, and M. Yoshida, “256-QAM (64 Gb/s) Coherent optical transmission over 160 km with an optical bandwidth of 5.4 GHz,” IEEE Photon. Technol. Lett. 22(3), 185–187 (2010). 18. T. Kawanishi, T. Sakamoto, and M. Izutsu, “High-speed control of lightwave amplitude, phase, and frequency by use of electrooptic effect,” IEEE J. Sel. Top. Quantum Electron. 13(1), 79–91 (2007).

1. Introduction Nonlinear optical devices using ferroelectric materials such as LiNbO3 and LiTaO3 have made great technological breakthrough and achieved broad public acceptance in the past decade. The reason is simple: every required frequency can be generated from IR through UV light region [1–3] as if a made-to-order system by quasi phase matching (QPM). The technology can be applied to the field of optical wavelength switching for future wavelength division multiplexing (WDM) network in telecommunication bands. Besides the QPM, highly nonlinear fiber (HNLF) [4] is a well-known wavelength converter by four wave mixing (FWM), however its weak third order nonlinearity χ (3) caused low conversion efficiency, even putting a few tens- or hundreds-meters-long HNLF. Recently, although a new material chalcogenide glass (As2S3) [5,6] is also known, the narrow waveguides result in high coupling/propagation loss and its conversion efficiency reduction. On the other hand, a gain medium, semiconductor optical amplifier (SOA) is adequate for placing in WDM system using cross-phase-modulation (XPM), cross-gain-modulation (XGM) and FWM effects [7,8]. Nevertheless, the semiconductor device causes several serious problems with XPM and XGM effects: frequency chirping, signal-to-noise ratio degradation, narrow input dynamic range and limited channel number. Thus, in order to achieve the compatibility between high efficiency and high quality in the wavelength conversion, periodically poled magnesium doped LiNbO 3 (Mg:LN) material is very attractive for future network vision, due to advantages of its large second order nonlinearity χ(2) (d33 = 25 pm/V) and ultrafast parametric conversion process. An adheredridge-waveguide (ARW) structure [9] that we have presented is also a key role to enhance the conversion performance. When comparing with the previously known waveguide fabrication techniques such as proton exchange [10–12] and Ti diffusion [13], ARW structure allow to achieve a tighter confinement and higher overlap for interacting modes by the step index profile (Δn~1), and suppress unnecessary effects such as degradations for nonlinear coefficient and low durability for long term operation. In our previous report [9], second harmonic generation (SHG) normalized conversion efficiency of 370%/Wcm2 in 11.4 mm (480%/W) was demonstrated with a combination of QPM and ARW by dicing approach for waveguides, but the method yielded moderate propagation loss due to sidewall roughness that limits interaction length of the device. This work, with our recent developments and optimizations in fabrication processes, it has become possible to attain low-insertion-loss, high-design-flexibility and efficient channel conversion that containing optical parametric amplification (OPA) in difference frequency generation (DFG) using a 50 mm-long QPMARW device.

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2. Device fabrication All fabrication processes can be separated as wafer-bonding and -polishing, periodical-poling, and waveguide-dry-etching steps. First of all, a Z-cut 500 µm-thick 5.0 mol% Mg:LN with a sputtered SiO2 layer was adhered to a 500 µm-thick non-doped congruent LN (CLN) substrate by epoxy, and precisely lapped to 4 µm thickness by mechano-chemical polishing (CMP). Each of SiO2 and epoxy adhesive layers is approximately 0.5 µm thick. All adhered and polished wafers underwent measurements by interferometer to certain that the thickness distribution of Mg:LN layer is no greater than 5%. Following the typical wafer cleaning processes, periodic photoresist patterns of 16.5, 16.7 and 16.9 µm were formed on the +Z face by photolithography, and cut the wafer to chip size by a dicing saw. After evaporation of a metal film on the both sides as electrodes, we proceeded with the electric field periodical poling step. For the reason of coercive field difference between Mg:LN and CLN materials, just the polarization of Mg:LN layer was reversed when applying 9.0 kV/mm electric field and 1200 pulses in a vacuum chamber at 100 °C. A long application time corresponds to the charge transfer time to stabilize polarization reversal. Finally, we obtained a uniform periodically poled structure over 50 mm length. As discussed already in our previous report [9], the poling process after adhesion and polishing enables to avoid excess loss on the top surface in comparison with poling process before adhesion. For dry etching process, we firstly cleansed the layers of metal and photoresist on the both sides, then formed a new mask pattern on the +Z face as an etching mask, and fabricated ridge waveguides by electron cyclotron resonance-reactive ion etching (ECR-RIE) which provides waveguides design with more flexibility like taper couplers and folding type integrations. 3. Results and discussions An overview photomicrograph of ARWs is shown in Fig. 1. The ridge height, width and length of the ARWs were approximately 3 µm, 3.5-12.0 µm and 50 mm, respectively. Figure 2 shows a scanning electron microscope (SEM) cross-sectional photograph of 9, 8 and 7 µmwide waveguides: each waveguide width corresponds to a dimension of photomask design. With the improvement of a fabrication approach for waveguides from dicing to ECR-RIE, we were placing our hopes for reducing the propagation loss, and actually confirmed the sidewall roughness was fairly reduced in the observation by SEM. In optical characteristics, we firstly measured the insertion loss for different device length to determine the propagation loss using a CW tunable diode laser at 1550 nm and a CW Ti:sapphire laser at 780 nm. The insertion loss of the 8 µm-wide waveguide was 4.4 dB at

Fig. 1. Overview photomicrograph from the top face of ARWs (26 waveguides per each section: total 52 waveguides in the above photograph).

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Fig. 2. SEM cross-sectional photograph of 9, 8 and 7 µm-wide waveguides. An insulatorsandwich structure is fabricated by adhesion and CMP processes.

1550 nm and 3.2 dB at 780 nm, in the 50 mm-long device, for the TM-mode without antireflection coating. Under the same conditions, the propagation loss was approximately