486 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 2, FEBRUARY 2005 Compact Optical 3R Regeneration Using a Traveling-Wave Electroabsorption Modulator Hsu-Feng Chou, Zhaoyang Hu, John E. Bowers, Fellow, IEEE, and Daniel J. Blumenthal, Fellow, IEEE Abstract—We propose and demonstrate a novel reamplification, retiming, and reshaping regeneration approach that utilizes the photocurrent signal and the nonlinear electrooptical transfer function of a traveling-wave electroabsorption modulator in an electrical ring oscillator. All the required functionalities such as clock recovery, pulse generation, and nonlinear decision are implemented with the same device, leading to a very compact con- figuration. In addition, wavelength conversion and electrical signal monitoring can be realized at the same time. With a degraded 10-Gb/s return-to-zero input, 1.0 dB of negative power penalty and 50% timing jitter reduction are measured after regeneration. Index Terms—Clock recovery, electroabsorption modulator (EAM), regeneration, traveling-wave (TW), wavelength conversion. I. INTRODUCTION T O EXTEND the reach of ultralong-haul transmission and to increase the scalability of all-optical networks, reampli- fication, retiming, and reshaping (3R) regeneration techniques are required to restore accumulated impairments in both the amplitude and the time domains. Architectures based on non- linearities in fibers, semiconductor optical amplifiers (SOAs), and electroabsorption modulators (EAMs) have been demon- strated recently [1]–[5]. Reamplification of optical signals using the erbium-doped fiber amplifier (EDFA) is a well-known suc- cess in various transmission systems. An alternative is the SOA, which offers very compact reamplification due to its small size and integration potential. However, the pattern dependence of SOA originating from the gain recovery dynamics is a major challenge in practical application. On the other hand, in order to retime and reshape return-to-zero (RZ) signals, three func- tionalities must be implemented. First, clock recovery from the degraded signal is necessary to acquire synchronization with a reduced jitter, which is often realized with the aid of elec- tronics. The recovered electrical clock then drives an optical pulse source to generate a clean pulse train, which provides re- timing and lateral reshaping. Finally, the generated pulse train is modulated by a nonlinear decision gate controlled by the de- graded signal, where vertical reshaping is obtained. By imple- menting the three functionalities individually, effective 3R re- generations can be realized [1], [2] but many components are required. Incorporating some of these functionalities together Manuscript received July 12, 2004; revised October 6, 2004. This work was funded by Defense Advanced Research Projects Agency (DARPA)/MTO under CS-WDM Grant N66001-02-C-8026 The authors are with the Department of Electrical and Computer Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106-9560 USA (e-mail: Hsu-Feng.Chou@ieee.org). Digital Object Identifier 10.1109/LPT.2004.839453 Fig. 1. (a) Configuration of the compact PAW-Regeneration. (b) E-O transformation in the TW-EAM without an electrical clock and (c) with an electrical clock. with fewer components would be advantageous. For example, an EAM can work both as a phase comparator for clock re- covery and a nonlinear decision gate [4]. A self-pulsating laser can recover an optical sinusoidal pulse train, combining clock recovery and an optical pulse source [5]. In this letter, we propose a compact 3R approach that in- corporates all three functions for retiming and reshaping with a traveling-wave EAM (TW-EAM) [6]. This 3R regeneration is based on the photocurrent-assisted wavelength conversion (PAW-Conversion) [7], [8] and is called PAW-Regeneration. In addition, simultaneous clock recovery is realized by setting the TW-EAM in an electrical ring oscillator [9]. 3R regeneration of 10-Gb/s RZ data is demonstrated. II. PRINCIPLE OF PAW-REGENERATION The configuration of PAW-Regeneration is schematically shown in Fig. 1(a), where the core is PAW-Conversion inside the TW-EAM [7]. Conventionally, absorption saturation due to a strong input signal is used to achieve wavelength conversion and is the enabling mechanism for many 3R regenerators [2], [4]. However, it is not absolutely necessary for the proposed 1041-1135/$20.00 © 2005 IEEE