Dispersion compensation by a tunable all-optical signal regenerator Er’el Granot a, * , Shalva Ben-Ezra a , Reuven Zaibel a , Sagie Tsadka a , Paul R. Prucnal b a KaiLight Photonics, 2b Bergman, Rehovot 76124, Israel b Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA Received 12 June 2006; received in revised form 20 October 2006; accepted 19 December 2006 Abstract The effect of dispersion on an NRZ signal in 120 km of SMF28 fiber is partially compensated by a tunable all-optical signal regen- erator (TASR). The TASR is comprised of a semiconductor optical amplifier incorporated in an asymmetric Sagnac loop. It is shown theoretically that the primary mechanism underlying dispersion compensation in the TASR relies on its ability to control the optical phase of the carrier. Our results are supported by numerical simulations and experimental results. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction One of the main factors limiting the performance of fiber optic transmission systems is dispersion. The stan- dard fiber which is used in most long-distance transmis- sion systems, SMF28, has a small but finite chromatic dispersion coefficient of D = 17 ps/nm km in the C-band (1530–1560 nm). For short fiber spans, chromatic disper- sion has a negligible effect on the shape of the received optical data, but for long distances it can cause severe dis- tortion. On the other hand, dispersion can be helpful in reducing nonlinear effects such as four wave mixing. Therefore, though dispersion is a useful tool in mitigating nonlinear effects, it must be compensated after long dis- tances to permit data recovery. In this letter, we propose and demonstrate an integrated dispersion compensation device that is suitable for use in long-haul fiber optic transmission systems. A number of methods have previously been developed for deleting or compensating dispersion effects [1–11]. The most ubiquitous of these is dispersion compensating fiber (DCF) [12–15], which has a very large negative disper- sion coefficient, allowing the compensation of dispersion in a long fiber using a relatively short length of DCF. How- ever, DCF typically introduces a substantial amount of loss into the system. In some cases it may be advantageous to avoid using DCF by closely spacing the regenerators and transponders. In this way, instead of eliminating dispersion completely, the signal is regenerated before the dispersion effects accu- mulate to the point that the signal cannot be recovered. Recently, it has been demonstrated that a tunable-all- optical signal regenerator (TASR), which is based on a semiconductor optical amplifier (SOA) incorporated in an asymmetric Sagnac loop (ASL) [16], can partially compen- sate for the effects of dispersion while also regenerating the signal [17]. Clearly, such a device has a considerable advan- tage over electronic regenerators, which cannot compen- sate for dispersion. Originally the TASR was designed to regenerate a signal and convert its carrier wavelength (if necessary). However, it was shown that not only does it improve the bit-error rate and its signal-to-noise ratio (SNR) [16] but it also com- pensates some of the dispersion effects [17], when the TASR was placed before the dispersive line. On the face of it, it is surprising that an interferometer can compensate for dispersion effects, and it is not clear how, by adjusting an interferometer, one can compensate for different fiber lengths. In this paper, we show theoretically (verified by simula- tion) and experimentally that the main mechanism of the 0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.12.040 * Corresponding author. E-mail address: erel@yosh.ac.il (E. Granot). www.elsevier.com/locate/optcom Optics Communications 273 (2007) 121–126