2610 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 12, DECEMBER 2005 Athermal Optical Demodulator for OC-768 DPSK and RZ-DPSK Signals Xiang Liu, Senior Member, IEEE, Alan H. Gnauck, Senior Member, IEEE, Xing Wei, Member, IEEE, Jay (Y. C.) Hsieh, Chiayu Ai, and Vincent Chien Abstract—We report an athermal optical delay-interfer- ometer capable of demodulating any OC-768 differential phase-shift keying (DPSK) signal on the ITU 50-GHz grid over the -band. The demodulator is based on a free-space Michelson interferometer. Experiments are performed with a 42.7-Gb/s nonreturn-to-zero DPSK signal and a 42.7-Gb/s return-to-zero DPSK signal with 67% duty cycle. Receiver sensitivity (at a bit error rate of 10 ) of better than 35.5 dBm is achieved for both signals. Negligible temperature-induced penalty is observed over an operational temperature range between 0 C and 70 C. The penalty associated with laser frequency offset (from the ITU grid) is also investigated. Index Terms—Differential phase-shift keying (DPSK), Michelson interferometer, optical delay interferometer (DI). I. INTRODUCTION O PTICAL differential phase-shift keying (DPSK) has recently attracted attention as a promising modulation format [1] that offers high receiver sensitivity, high toler- ance to major nonlinear effects in high-speed transmissions [2], and high tolerance to coherent crosstalk [3]. In DPSK, data information is carried by the optical phase difference between adjacent bits. For direct detection of DPSK signal (by conventional intensity detectors), a demodulator is needed to convert the phase-coded signal into an intensity-coded signal. Conventionally, the demodulator is an optical 1-bit delay interferometer (1-bit DI) based on an all-fiber design or a planar lightwave circuit design. These designs are intrinsi- cally temperature-sensitive. Since precise control of the phase difference between the two optical paths of the DI is required [4], accurate temperature control and stabilization of the DI are required. Recently, we demonstrated an athermal DI, based on a free-space Michelson interferometer, for demodulating a 42.7-Gb/s nonreturn-to-zero (NRZ) DPSK signal [5]. Here, we report the use of the athermal DI for demodulating both NRZ-DPSK and return-to-zero (RZ) DPSK signals on the ITU 50-GHz grid over the entire -band in the temperature range of 0 C 70 C. In addition, the penalty associated with laser frequency offset (from the ITU grid) is measured. Manuscript received July 13, 2005; revised September 2, 2005. X. Liu and A. H. Gnauck are with Lucent Technologies, Bell Laboratories, Holmdel, NJ 07733 USA (e-mail: xliu20@lucent.com). X. Wei is with Lucent Technologies, Bell Laboratories, Murray Hill, NJ 07974 USA. J. Hsieh, C. Ai, and V. Chien are with Optoplex Corporation, Fremont, CA 94538 USA (e-mail: jayhsieh@optoplex.com). Digital Object Identifier 10.1109/LPT.2005.859410 Fig. 1. Schematic of an athermal DI based on a free-space optical Michelson interferometer. is the speed of the light, is the round-trip length of one path, and is the round-trip time delay between the two paths of the DI. II. DESIGN AND CHARACTERISTICS OF THE ITU-COMPLIANT ATHERMAL DI The schematic of the athermal DI is shown in Fig. 1. This de- vice is based on a free-space optical Michelson interferometer with a free spectral range of 50 GHz, consisting of an optical beam splitter (BS) and two reflection mirrors. The incident beam from the left-hand side of a BS splits into two beams, which are reflected by the two mirrors before interfering with each other at a slightly different location (than that of the input beam) on the BS. The round-trip differential time delay between the two optical paths of the DI satisfies: ps , where THz (the reference frequency of the ITU grid), and is a small integer. In order to obtain good extinc- tion ratio and to minimize the polarization-dependent frequency shift, the power splitting ratio of the BS is very close to 50/50 and the phase of the BS is insensitive to the state of polarization. In addition, to achieve the athermal property, the length differ- ence between the two paths varies less than 10 nm over the op- erational temperature range between 0 C and 70 C. This is accomplished by connecting the mirrors with the BS through zerodur, which has an extremely low thermal expansion coeffi- cient. An air gap of 3 mm in one optical path is used for a final adjustment that locks the passband of the athermal DI onto the ITU grid. The device is hermetically sealed. The insertion loss from the input port to either of the output ports is less than 1.5 dB. Worth mentioning is its small form factor: mm. The measured polarization-dependent frequency shift is 0.3 GHz. With the athermal design, the fre- quency drift is less than 0.75 GHz from 0 C to 70 C. This corresponds to a temperature dependence of 0.02 GHz C, which is 50 times smaller than conventional fiber-based DIs. Fig. 2 shows the transmission curves at the constructive port of the athermal DI at temperatures of 0 C, 30 C, and 70 C. The extinction ratio is greater than 25 dB. The maximum frequency 1041-1135/$20.00 © 2005 IEEE