IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 10, MAY 15, 2008 773 Polarization-Insensitive Operation of Lithium Niobate Mach–Zehnder Interferometer With Silica PLC-Based Polarization Diversity Circuit Kenya Suzuki, Member, IEEE, Takashi Yamada, Osamu Moriwaki, Member, IEEE, Hiroshi Takahashi, Member, IEEE, and Masayuki Okuno Abstract—We demonstrate the polarization-insensitive oper- ation of a Mach–Zehnder interferometer (MZI)-based lithium niobate switch with a silica waveguide polarization beam splitter. The splitter is composed of an MZI with a half wave plate installed in one arm, and exhibits a polarization extinction ratio of more than 15 dB. The switch functions for both polarizations with an extinction ratio of more than 20 dB, a polarization-dependent loss of 0.1 dB and a switching speed of 40 ps. Index Terms—Integrated optics, lithium niobate (LN) phase shifter, optical switch, polarization beam splitter, silica-based planar lightwave circuit (PLC). I. INTRODUCTION T HE continuing rapid growth of internet traffic has mo- tivated the extensive progress made on optical network systems. Optical networks, such as optical packet and burst networks, need fast optical switches. In such networks, the frame gaps for optical packets are of nanosecond order [1]. Various approaches have been proposed to meet the demand for fast switches, including a semiconductor-based switch that operates by total internal reflection [1], lead–lanthanum–zir- conate–titanate-based switches [2], [3], a SiGe–Si multimode interference switch [4], and lithium niobate (LN)-based switches [5]–[8]. Of these, LN-based switches appear to be the most attractive, because they are capable of high switching speeds with low loss characteristics. One of the drawbacks of LN-based switches is their polarization-dependent operation. The polarization-insensitive operation of LN switches has been investigated with a view to overcoming this problem. However, one of the methods for achieving polarization insensitivity requires a high switching voltage of more than 70 V [7], which is difficult to generate at the gigahertz level. An alternative method has been proposed that utilizes a polarization-mode converter in an LN waveguide [8]. However, this method requires a long polarization conversion waveguide ( 1 cm) for each Mach–Zehnder interferometer (MZI), resulting in a large switch, which makes it difficult to realize large port count switches. Manuscript received December 7, 2007; revised January 14, 2008. This work was supported in part by CREST, JST. K. Suzuki, T. Yamada, O. Moriwaki, and H. Takahashi are with NTT Photonics Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan (e-mail: kenya@ aecl.ntt.co.jp). M. Okuno is with NTT Electronics Corporation, Naka, Ibaraki 311-0122, Japan. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2008.921109 Fig. 1. Schematic configuration of a polarization-insensitive MZI consisting of an LN phase shifter array and silica-based PLCs with integrated polarization beam splitter/combiners. In this letter, we propose and demonstrate an alternative method for realizing an LN-based MZI switch by using a polarization diversity configuration [9]. Polarization diversity is achieved by employing integrated beam splitter/combiners in silica-based planar lightwave circuits (PLCs). They are directly attached to the LN waveguide chip with hybrid integration technology [9]–[11]. The advantage of hybrid integration is that we can benefit from the design flexibility offered by silica PLCs and the high-speed operation of LN phase shifters. This configuration is promising for realizing more complicated, highly functional switches by increasing the number of LN waveguide arrays. II. POLARIZATION-INDEPENDENT OPERATION WITH POLARIZATION DIVERSITY As is well known, an LN phase shifter has a polarization-de- pendent modulation property. The transverse-magnetic (TM) mode exhibits extraordinary polarization for -cut LN and cou- ples to the -component of the electric field through the co- efficient. In contrast, the transverse-electric (TE) mode exhibits ordinary polarization and couples to the field through the co- efficient, which is almost one third of the coefficient. There- fore, the phase modulation index for the TM mode is about three times that of the TE mode. This results in the polarization de- pendence of the LN phase shifter. We utilized a polarization diversity configuration for each LN phase shifter to eliminate the polarization dependence. Fig. 1 shows the schematic configuration of a 2 2 switch we made as a first demonstration. It consists of a -cut traveling-wave Ti : LN phase shifter array and input and output 1041-1135/$25.00 © 2008 IEEE