666 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 14, NO. 3, MAY/JUNE 2008 Impact of Pump-Phase Modulation on FWM-Based Wavelength Conversion of D(Q)PSK Signals Robert Elschner, Student Member, IEEE, Christian-Alexander Bunge, Member, IEEE, Bernd H ¨ uttl, Alexandre Gual i Coca, Carsten Schmidt-Langhorst, Reinhold Ludwig, Colja Schubert, and Klaus Petermann, Senior Member, IEEE Abstract—We investigate both theoretically and experimentally the impact of pump-phase modulation on differential phase shift keying (DPSK) and differential quadrature phase shift keying (DQPSK) signals in fiber-based parametric wavelength converters. It will be shown that the pump-phase modulation used to suppress stimulated Brillouin scattering introduces critical signal degrada- tions and optical SNR penalties on DPSK signals, and more severe on DQPSK signals. Different modulation schemes will be theo- retically investigated and the quantitative results are compared to system experiments. Finally, the theoretical results for a single conversion will be extended to multiple conversions to study the cascadibility of the wavelength converter. Index Terms—Differential phase shift keying (DPSK), differen- tial quadrature phase shift keying (DQPSK), stimulated Brillouin scattering (SBS), wavelength conversion. I. INTRODUCTION W AVELENGTH converters are commonly accepted as key components for flexible all-optical network ap- proaches like optical burst and packet switching [1], [2]. The wavelength converters are envisioned to fulfill multiple tasks inside the switching nodes and supposed to be involved in rout- ing, contention resolution, and signal regeneration [3]–[5]. Due to this demand from the system side, components suitable to match all practical requirements are rare, although a large num- ber of concepts have already been proposed based on different materials and nonlinear effects [6]–[10]. Among others, tun- ability, cascadibility, and transparency to bitrate and modulation formats are important characteristics of all-optical wavelength conversion to compete with optoelectronic solutions. Fiber-based components relying on four-wave mixing (FWM) seem very promising to match, in particular, the two latter requirements. Wavelength conversion of 80 Gb/s differential quadrature phase shift keying (DQPSK) [11] and multiwave- length conversion of up to 40 Gb/s per channel differential phase shift keying (DPSK) signals have been shown [12], [13]. We recently presented all-optical wavelength conversion of 320 Gb/s differentially phase-modulated signals based on FWM Manuscript received November 1, 2007; revised February 18, 2008. This work was supported by the Deutsche Forschungsgemeinschaft (DFG). R. Elschner, C.-A. Bunge, and K. Petermann are with the Fachgebiet Hochfrequenztechnik/Photonics, Technische Universit¨ at Berlin, Berlin 10587, Germany (e-mail: elschner@ieee.org; christian-alexander.bunge@tu-berlin.de; petermann@tu-berlin.de). B. H¨ uttl, A. Gual i Coca, C. Schmidt-Langhorst, R. Ludwig, and C. Schubert are with the Fraunhofer Institut f¨ ur Nachrichtentechnik, Heinrich- Hertz-Institut, Berlin 10587, Germany (e-mail: bernd.huettl@hhi.fhg.de; otdm@hhi.fraunhofer.de; carsten.schmidt-langhorst@hhi.fhg.de; ludwig@hhi. fhg.de; colja.schubert@hhi.de). Digital Object Identifier 10.1109/JSTQE.2008.920290 Fig. 1. (a) Principle setup of the FWM-based AOWC. (b) Schematical output spectrum. in a highly nonlinear fiber (HNLF) [14], [15] where we used continuous-wave (CW) pumping to provide modulation format transparency. In addition to the well-known fact that stimulated Brillouin scattering (SBS) limits the conversion efficiency of CW pumped FWM processes leading to low cascadibility [16], also the impact of the pump-phase modulation in order to sup- press SBS has been extensively investigated in the past years. Firstly, pump-phase modulation can generate signal gain dis- tortions that depend on the rise/fall time of the optical modula- tor [17]–[21]. Secondly, it leads to detrimental spectral broad- ening and phase distortions of the wavelength-converted idler wave when using a single pump [22]. Although the latter ob- stacle can be overcome for a specific idler using two pump sig- nals [23]–[26], applications using more than one idler [27], [28] or higher order idlers [29] are still affected. In this paper, we will theoretically describe the impact of the induced phase distortions on the performance of DPSK and DQPSK signals in terms of the bit error rate (BER). Quantita- tive results for a single conversion are compared to our recent experiments and extended to multiple conversions to study the cascadibility. Our results give a lower bound of the achiev- able optical SNR (OSNR) penalty and will identify the induced phase distortions as a major degradation source when dealing with differentially phase-modulated signals. This paper is organized as follows. In the second section, we briefly describe the limitations SBS sets on the efficiency of FWM. In the third section, we will discuss the phase distortions introduced by the pump-phase modulation. In the fourth section, we will theoretically derive the resulting OSNR penalty and compare the results with the recent experiments. Finally, in the fifth section, we will present results showing the cascadibility potential of the converter. II. FOUR-W AVE MIXING IN HIGHLY NONLINEAR FIBERS One of the most important features of the all-optical wavelength converter (AOWC) is low loss, which increases the cascadibility of the converter depicted in Fig. 1. Thus, the 1077-260X/$25.00 © 2008 IEEE Authorized licensed use limited to: Technische Universitaet Berlin. Downloaded on April 27, 2009 at 08:29 from IEEE Xplore. Restrictions apply.