IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 20, OCTOBER 15, 2006 2081 Cascadability of Optical 3R Regeneration for NRZ Format Investigated in Recirculating Loop Transmission Over Field Fibers Masaki Funabashi, Zuqing Zhu, Zhong Pan, Bo Xiang, Loukas Paraschis, David L. Harris, and S. J. B. Yoo, Senior Member, IEEE Abstract—This letter demonstrates optical 3R regeneration in 10-Gb/s nonreturn-to-zero field transmission. The 3R regenerator utilizes semiconductor-optical-amplifier-based Mach–Zehnder interferometer wavelength converters, a synchronous modulator, and a Fabry–Pérot filter to realize optical 3R regeneration including all-optical clock recovery. The cascadablity of the 3R regenerator is investigated in recirculating loop transmission experiments with various regeneration spacings up to 462 km (corresponding to an input optical signal-to-noise ratio (OSNR) of 22 dB). Trans- mission with the 3R regenerator shows significant performance improvement over that without 3R regeneration. A 66-km-spaced 3R regeneration with a 33-dB input OSNR achieves 1000-hop cascaded error-free transmission (66 000 km in distance) with no hop-to-hop power penalties. A 264-km-spaced 3R regeneration with a 25-dB input OSNR also achieves 100-hop cascaded trans- mission (26 400 km in distance) with a bit-error-rate error floor at . Index Terms—All-optical clock recovery, Fabry–Pérot filter (FPF), field fiber, nonreturn-to-zero (NRZ), optical regeneration. I. INTRODUCTION O PTICAL signals after fiber transmission undergo various distortions due to the impairments caused by noise, at- tenuation, and fiber dispersion and nonlinearity. Optical signal regeneration is a promising technology for advanced optical networking in overcoming these impairments by restoring the signal quality [1]. While numerous publications reported on optical 3R regeneration in well-controlled laboratory environ- ments [2]–[5], only a few have demonstrated a field trial of optical 2R [6] or 3R regeneration [7]. Moreover, the majority of previous studies on 3R regeneration techniques employ a return-to-zero (RZ) format [2]–[5], [7], and only a few have discussed 3R regeneration for the most commonly used non- return-to-zero (NRZ) format [8], [9]. The main advantage of optical regeneration is to enhance transmission distance by cas- cading regenerators, and recirculating loop setups are typically Manuscript received May 9, 2006; revised July 25, 2006. This work was sup- ported in part by Furukawa Electric, Cisco System URP Program, and in part by the National Science Foundation under NSF 0335301 and NSF 9986665. M. Funabashi is with the Department of Electrical and Computer En- gineering, University of California, Davis, CA 95616 USA, and also with Furukawa Electric Co., Ltd., Yokohama 220-0073, Japan. Z. Zhu, Z. Pan, B. Xiang, and S. J. B. Yoo are with the Department of Elec- trical and Computer Engineering, University of California, Davis, CA 95616 USA (e-mail: yoo@ece.ucdavis.edu). L. Paraschis is with Advanced Technology, Core Routing, Cisco Systems, San Jose, CA 95134 USA. D. L. Harris is with Advanced Technology Labs, Next Gen Transport, Sprint, Burlingame, CA 94010 USA. Color versions of Figs. 1–4 are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2006.883222 Fig. 1. (a) Experimental setup. (b) Field fiber map. (c) Eye diagrams at the locations (A)–(E) when km and the lap number is Lap2. used to investigate the cascadability [2]–[5]. Reference [5] also investigated the regeneration spacing dependence of the cascadability for the RZ format in a laboratory environment. This letter demonstrates 3R regeneration for a 10-Gb/s NRZ signal in a recirculating loop transmission over field fiber links. The utilization of semiconductor-optical-amplifier-based Mach–Zehnder interferometer (SOA-MZI) wavelength con- verters, a Fabry–Pérot filter (FPF), and a synchronous modulator realizes 3R regeneration, as well as all-optical clock recovery from the NRZ signal [10]–[12]. The cascadability of the 3R regenerator is investigated with various regeneration spacings and optical signal-to-noise ratios (OSNRs). II. EXPERIMENTAL SETUP Fig. 1(a) and (b) shows the experimental setup and the field fiber map. The recirculating loop transmission setup enables cascadability evaluation of an inline 3R regenerator. As a trans- mission line, up to seven sets of field fibers were used. The field fibers are standard single-mode fibers in a loop-back con- figuration between Burlingame and Palo Alto. Each field fiber span has a round-trip length of 66 km, followed by a two-stage erbium-doped fiber amplifier with a dispersion-compensating fiber (DCF) in its middle stage. The 3R regenerator was located after a predetermined number ( ) of the fiber spans ( or ; corresponding to a regeneration spacing ( ) of 66, 264, or 462 km). After the first, fourth, and seventh span, residual dispersion values were within 30 ps/nm, and OSNR degraded to 33, 25, and 22 dB, respectively, from the initial value of 45 dB 1041-1135/$20.00 © 2006 IEEE