IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 19, OCTOBER 1, 2008 1597 40-Gb/s Multichannel NRZ to CSRZ Format Conversion Using an SOA Tiago Silveira, Ana Ferreira, António Teixeira, and Paulo Monteiro Abstract—An all-optical converter from nonreturn-to-zero (NRZ) to carrier-suppressed return-to-zero modulation format is proposed and experimentally demonstrated. The converter is based on cross gain and phase modulation in a semiconductor optical amplifier. Single- and multichannel operation is exper- imentally assessed at 40 Gb/s. In single-channel operation, the required optical signal-to-noise ratio for a bit-error rate of is improved by 3 dB, in comparison to the input NRZ. Considering multichannel operation with two and four channels, this improve- ment decreases by only 0.6 and 1.5 dB, respectively. Index Terms—Carrier-suppressed return-to-zero (CSRZ), format conversion, semiconductor optical amplifier (SOA). I. INTRODUCTION O PTICAL networks are facing growing bandwidth de- mands, motivated by the increase of data exchange. Advanced modulation formats and line coding offer high spectral density and enhanced tolerance to transmission impairments, enabling cost-efficient networks [1], [2]. Car- rier-suppressed return-to-zero (CSRZ) modulation format is a candidate for such networks due to promising characteristics. These include high tolerance to nonlinear transmission effects and polarization-mode dispersion; moreover, when combined with tight optical filtering, CSRZ allows very high spectral efficiency without significant degradation of the return-to-zero (RZ) pulse shape [1]–[3]. Simple and robust format converters from/to CSRZ are required at the interface between optical networks using different modulation formats, from which nonreturn-to-zero (NRZ) and RZ are the most common. Conversion from NRZ and RZ to CSRZ is commonly executed by modulating an incoming NRZ/RZ signal in a Mach–Zehnder modulator (MZM) driven with a clock signal at half the signal bit rate [1]. Due to the MZM, this method presents high insertion loss, polarization dependence, and poor cost effectiveness. In [4], all-optical conversion to CSRZ in a semiconductor optical amplifier (SOA)-based op- tical loop mirror has been proposed. However, only 10-Gb/s single-channel operation has been demonstrated; moreover, this setup relies on a nonlinear loop, which is inherently complex. Manuscript received February 17, 2008; revised June 10, 2008. Current ver- sion published September 5, 2008. This work was supported by Fundação para a Ciência e a Tecnologia under scholarships BDE/15543/2005 and SFRH/BD/ 24249/2005. T. Silveira and P. Monteiro are with Nokia Siemens Networks SA, 2720-093 Amadora, Portugal (e-mail: tiago.silveira@nsn.com). A. Ferreira and A. Teixeira are with Instituto de Telecomunicações, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal. 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.2008.928535 Fig. 1. 40-Gb/s CSRZ signal obtained from an NRZ signal by the common technique [1]. (a) Optical spectrum. (b) Temporal intensity and phase. In this work, we propose and experimentally demonstrate an all-optical converter from NRZ to CSRZ based on cross- gain modulation (XGM) and cross-phase modulation (XPM) in SOA. This method is simple, low cost, presents negligible po- larization dependence, and high spectral bandwidth. Moreover, SOA provide amplification. Single- and multichannel operation is experimentally demonstrated at 40 Gb/s with characterization through bit-error-rate (BER) measurements. A similar setup was used in [5] to obtain conversion from NRZ to RZ; however, a CSRZ signal was not sought and only simulation results with single wavelengths were presented. The required operation con- ditions (such as optical power values, optical clock temporal alignment, and characteristics of the optical filter after SOA) were not appropriate to enable conversion to the carrier-sup- pressed format. II. OPERATION PRINCIPLE To assist the analysis of the converter proposed in this work, Fig. 1 characterizes a 40-Gb/s CSRZ signal, generated from an optical NRZ signal by the common technique of [1]. The output CSRZ signal optical spectrum is presented in Fig. 1(a). The central frequency of the CSRZ signal spectrum [frequency 0 in Fig. 1(a)] coincides with the optical carrier frequency of the input NRZ signal. In Fig. 1(b), the temporal intensity and phase of the CSRZ signal are depicted. The CSRZ signal inten- sity is similar to that of a common RZ signal. The optical signal phase is analyzed considering a low pass equivalent of the op- tical signal, where a reference frequency is considered. When corresponds to the central frequency of the CSRZ signal [0 GHz in Fig. 1(a)], constant phase is observed within the bit period, and phase shifts occur between adjacent bits. To facilitate the comprehension of the proposed converter, the signal phase is also analyzed considering detuned from the CSRZ central frequency towards negative frequencies by half the signal bit rate [ GHz in Fig. 1(a)]. In this case, the signal phase can be obtained by adding to the signal time domain phase obtained with ( repre- sents time and is a frequency equal to the signal bit rate). Considering GHz, linear phase with slope equal to 1041-1135/$25.00 © 2008 IEEE