An Optical Differential 8-PSK Modulator Using Cascaded QPSK Modulators Yanfu Yang (1) , Linghao Cheng (1) , Zhaohui Li (1) , Chao Lu (1) , Qianjin Xiong (2) , Xiaogeng Xu (2) , Lei Liu (2) , H Y Tam (3) , P K A Wai (1) (1) Photonics Research Centre, Dept. of Electronic & Information Eng, The Hong Kong Polytechnic Univ., Hung Hom, Kowloon, Hong Kong SAR Phone:+852 2766 4094 Fax: +852 2362 8439 yyf02@eie.polyu.edu.hk . (2) Huawei Technologies Co., Ltd., Shenzhen, P. R. China. (3) Photonics Research Centre, Dept. of Electrical Eng, The Hong Kong Polytechnic Univ., Hong Kong SAR Abstract A new optical D8PSK modulator implementation is proposed. The proposed scheme removes the need for precise driving signal voltage control. Simulation results show the scheme enables modulation signal generation with more relaxed EO bandwidth requirements. Introduction Various optical multilevel modulation formats have been studied extensively to meet the increasing demand for capacity in optical communication systems. In particular differential quadrature phase- shift keying (DQPSK) scheme has been investigated in long-haul transmission experiments 1,2 due to its advantage of high nonlinear tolerance and better OSNR performance comparing with other alternative formats. Recently, optical differential 8-level phase- shift keying (D8PSK) system has been proposed and verified experimentally 3 . It offers higher spectral efficiency than DQPSK scheme and can be used in high spectral efficiency direct detection or coherent detection optical communications systems. The implementation of D8PSK modulator typically uses a cascade structure including a QPSK modulator and a phase modulator (PM). However, the generated constellation diagram is a strong function of the driving voltage and the frequency response of the PM. The voltage amplitude of the driving signal for PM has to be precisely tuned to obtain the desired phase deviation of ʌ/4. Currently there is no reported technique for achieving automatic tuning of the driving voltage to meet this requirement. Furthermore, any bandwidth limitation or non-flat amplitude spectral response of the driving amplifier or the modulator will translate directly to the deviation of the resulted constellation diagram from the ideal one and thus will impact on the system performance. Therefore PM modulator with high electrical-optical (EO) bandwidth and broadband driver amplifier is necessary in this D8PSK optical modulator implementation. In this paper, a new optical D8PSK modulator implementation is proposed for eliminating the problems above. The scheme uses two cascaded QPSK modulators with an interferometer in between. The simulation results further show that the modulator has more relaxed requirements for EO bandwidth of the DQPSK modulators used and can achieve performance close to that of an ideal D8PSK modulator. Working Principle Fig.1a and Fig.1b show the configurations of two possible implementations of D8PSK modulators. In the conventional modulator implementation, a QPSK modulator is followed by a phase modulator 3 .The proposed new D8PSK modulator in Figure 1b can be constructed with a delay-line Interferometer (DLI) placed between two QPSK modulators. The first DQPSK modulator together with the DLI serve the same function as the phase modulator used in a conventional D8PSK modulator implementation by selection between two subsets of DQPSK constellations with ʌ/4 phase shift between them. While the second DQPSK modulator serves the same function as the DQPSK modulator in conventional implementation. The DQPSK constellation {Ɏi=(i-1)ʌ/2+ʌ/4, i=1..4} after the first QPSK modulator is shown in the left diagram of Fig.1c. The driving signals {I’ Q’} are selected such that the differential phase between two successive output symbols are either zero or ʌ/2. As a result there are eight possible combinations for neighbouring driving signals, as seen in Table1. In the DLI one symbol will interfere with the next one with ʌ/4 phase offset. For the differential phase of zero or ʌ/2 (illustrated in Diagram I and II of Fig.1c) the ĭI’(1) ĭII’(1) After the First QPSK modulator After the second QPSK modulator ĭ1 [0 0] ĭ2[1 0] ĭ3[1 1] ĭ4[0 1] After the DLI (II) ĭ1 ʌ/4 n n+1 ĭ1 ĭI(1) ĭII(1) ĭI’(2) ĭI’(3) ĭI’(4) ĭII’(2) ĭII’(3) ĭII’(4) ʌ/4 n&n+1 ĭ2 The First QPSK modulator DLI I’ Q’ ʌ/2 ʌ/4 T I Q ʌ/2 The Second QPSK modulator QPSK modulator PM(ʌ/4) PM modulator I Q ʌ/2 D LD LD a b c (I) Fig. 1: (a) conventional modulator (b) new proposed modulator (c) the working principle of the new modulator ECOC 2009, 20-24 September, 2009, Vienna, Austria Paper P3.19 978-3-8007-3173-2 © VDE VERLAG GMBH