IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 6, MARCH 15, 2007 357 All-Optical Conversion From RZ to NRZ Using Gain-Clamped SOA T. G. Silveira, A. Teixeira, G. Tosi Beleffi, D. Forin, P. Monteiro, H. Furukawa, and N. Wada Abstract—An all-optical converter from return-to-zero (RZ) pulses to the nonreturn-to-zero (NRZ) format is presented. The converter operates in two stages: the laser generated in a gain-clamped semiconductor optical amplifier (SOA) is modulated with the data signal; afterwards this signal is wavelength-converted by cross-gain modulation in a common SOA. The setup is non- inverting and can feature wavelength conversion. Experimental error-free conversion from 5- and 40-ps RZ pulses to NRZ format is presented at 10 Gb/s using a bit sequence. Index Terms—Format conversion, gain-clamped semiconductor optical amplifier (GC-SOA), reshaping, wavelength conversion (WC). I. INTRODUCTION W AVELENGTH-DIVISION multiplexing (WDM) and optical time-division multiplexing (OTDM) are key techniques for the future optical networks [1]. These networks should be able to support several pulse formats, from which return-to-zero (RZ) and nonreturn-to-zero (NRZ) are the most common [2]; therefore, it is essential to develop all-optical converters between these formats. One example of the format conversion need is the interface between the ultrafast OTDM networks and the access networks, operating at lower bit rates [2]. In the former, RZ short optical pulses are usually preferred, while in the latter, NRZ pulse shape is generally favored due to its lower spectral occupancy and increased jitter tolerance. Several schemes have been proposed to achieve conversion between these two formats, such as the following: a structure using cross-gain modulation (XGM) and WDM-to-TDM con- version [1]; a semiconductor optical amplifier (SOA)-based nonlinear optical loop mirror [2]; a Fabry–Pérot laser with an injection-locking scheme [3]; interferometer structures with SOA [4]; optical filtering [5]; and four-wave mixing in SOA [6]. In this letter, we propose and experimentally demonstrate at 10 Gb/s a scheme to convert RZ pulses to NRZ. The setup con- sists in two stages. In the first, the input RZ signal modulates Manuscript received September 8, 2006; revised January 5, 2007. The work of T. G. Silveira was supported by FCT under the BDE/15543/2005 Scholarship. T. G. Silveira and P. Monteiro are with Siemens Networks S.A., 2720-093 Amadora, Portugal, and also with Instituto de Telecomunicações, 3810-193 Aveiro, Portugal (e-mail: silveira@av.it.pt; teixeira@ua.pt). A. Teixeira is with Instituto de Telecomunicações, 3810-193 Aveiro, Portugal. G. Tosi Beleffi and D. Forin are with the Instituto Superiore della Commu- nicazioni e delle Tecnologie dell’Informazione, 00144 Rome, Italy (e-mail: davide.forin@comunicazioni.it; giorgio.tosibeleffi@comunicazioni.it). H. Furukawa and N. Wada are with the Research Department, Photonic Network Group, National Institute of Information and Communications Technology, Tokyo 184-8795, 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.2007.891643 Fig. 1. Schematic representation of the variations in the GC-SOA carrier den- sity and in the internal laser, when low- and high-power pulses are injected. (a) Low-power pulses. (b) High-power pulses. the internal laser of a gain-clamped SOA (GC-SOA) [7]; in the second, this signal is wavelength-converted using XGM in a common SOA. The advantages of this method are simplicity, since it does not require interferometric schemes or nonlinear loops, a noninverted converted signal is retrieved, and the output signal wavelength can be matched to the input one featuring eventual wavelength conversion (WC). II. OPERATION PRINCIPLE The GC-SOA [7] is similar to a common SOA, where lasing is induced by a distributed Bragg reflector (DBR). When a data signal is injected into the GC-SOA, the internal laser power will vary inversely to the input signal power in order to obtain a con- stant cavity gain. However, if the input power is high enough, the carriers are depleted bellow the lasing threshold, the laser turns OFF and the GC-SOA acts as a common saturated SOA. Once the input power level decreases (so that the carrier density at the excited state becomes sufficient to provide the necessary gain) the internal laser turns ON again. In Fig. 1, the GC-SOA carrier density and the internal laser power temporal evolutions are represented when two consecu- tive optical pulses are injected, considering a pulse spacing close to the maximum modulation rate of the GC-SOA internal laser. When a “low power” pulse train is injected [Fig. 1(a)], the carrier density recovery time to the lasing threshold level is lower than ; therefore, all the transitions of the input signal are tracked by the laser. For a “high-power” pulse train [Fig. 1(b)], the GC-SOA is driven into deeper saturation and is higher than , hence, the laser remains OFF during two consecutive pulses. In the “high-power” mode, the internal laser is NRZ-modulated with the inverted equivalent of the input logical information. 1041-1135/$25.00 © 2007 IEEE