476 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 5, MARCH 1, 2013 Multichannel Wavelength Conversion Using Four-Wave Mixing in Semiconductor Ring Lasers Antonio Pérez-Serrano, Julien Javaloyes, Member, IEEE, and Salvador Balle, Member, IEEE Abstract— We theoretically study all-optical simultaneous wavelength conversion of multiple channels by four-wave mixing in semiconductor ring lasers. Locking the semiconductor ring laser to a holding beam allows us to achieve large conversion efficiencies with good signal-to-noise ratio in several channels at multi-Gb/s bit rates. Cross-talk between signals, arising from the peculiar four-wave mixing cascade of modes in semiconductor ring lasers and their cross-gain saturation, is studied in detail. We show that it can be controlled by adjusting the intensity of the holding beam, the bias current of the laser, and the number, intensity, and wavelength of signals that one wants to convert. Index Terms—All-optical wavelength conversion, four-wave mixing (FWM), semiconductor lasers, semiconductor ring lasers (SRLs), traveling wave model (TWM). I. I NTRODUCTION A LL-OPTICAL multichannel wavelength converters allow to enhance the capacity and flexibility of future all- optical networks based on Wavelength-Division-Multiplexing (WDM). The possible candidates to perform this task should have the following properties: (a) transparency to the modula- tion format and speed of the incoming signal and the capacity to reuse the signal for further processing; (b) high integrability with other components such as laser sources and filters in Photonic Integrated Circuits (PICs); and (c) simultaneous and asynchronous conversion of more than one signal. Numerous approaches have been demonstrated mainly based on the use of non-linear effects such as Cross-Gain Modulation (XGM), Cross-Phase Modulation (XPM) or Four-Wave Mixing (FWM) in Semiconductor Optical Amplifiers (SOAs) [1]. Here, the incoming data signal on one particular channel is replicated onto one or several (multicast) channels by coupling it to one or several Continuous Wave (CW) sources. Recently, the concept of FWM-SOA-based converters has been extended Manuscript received August 27, 2012; revised December 27, 2012; accepted January 7, 2013. Date of publication January 16, 2013; date of current version February 8, 2013. The work of J. Javaloyes was supported in part by the Ramón y Cajal Program and in part by the Direcció General de Recerca, Desenvolupament Tecnológic i Innovació de la Conselleria d’Innovació, Interior i Justícia del Govern de les Illes Balears co-funded by the European Union FEDER funds. The work of S. Balle was supported by Project ALAS TEC2009-14581-C02-01. A. Pérez-Serrano is with the Weierstrass Institute for Applied Analysis and Stochastics, Berlin 10117, Germany (e-mail: perez@wias-berlin.de). J. Javaloyes is with the Departament de Física, Universitat de les Illes Balears, Palma de Mallorca E-07122, Spain (e-mail: julien.javaloyes@uib.es). S. Balle is with the Institut Mediterrani d’Estudis Avançats, IMEDEA, Esporles E-07190, Spain (e-mail: salvador@imedea.uib-csic.es). 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.2013.2240447 to perform simultaneous conversion of different incoming signals reaching rates of 10 Gb/s [2] and even higher bit rates (50 Gb/s) have been achieved by using quantum-dot SOAs [3]. However it should be noticed that SOA-based approaches have a high power consumption due to the high bias current needed in the SOA and the bias currents needed to generate the CW beams. Semiconductor Ring Lasers (SRLs) are highly integrable devices that have shown their potential for performing all- optical processing while having a low power consumption. Applications such as all-optical memory [4] and data process- ing [5] have been demonstrated exploiting the directional emission bistability exhibited by SRLs [6]. Furthermore, SRLs have shown cavity-enhanced FWM [7] that provides rich opportunities for implementing THz radiation generators, log- ical gates [8] and wavelength converters [9]. As compared with Fabry-Pérot (FP) resonators, SRLs exhibit a slightly superior FWM efficiency due to the absence of the (weak) Spatial Hole Burning (SHB) present in FP cavities. In addition, SRLs possesses the advantage of avoiding spurious reflections upon light injection: all the FWM signal remains in the same direction of propagation which contrasts with the FP case where it is divided in two counter-propagating parts. In this letter, we propose exploiting the FWM properties of SRLs for simultaneous wavelength conversion of multiple channels. We investigate numerically the dynamics of the SRL via a semi-classical Travelling Wave Model (TWM) [10] that allows us to naturally describe spatial effects and multimode operation in time-scales longer than 1 ps. The model is tested against the experimental results reported in [7] and [9], and then applied to analyze simultaneous multi-channel wavelength conversion. We discuss the impact of cross-talk effects and possible mitigation strategies. II. MODEL We summarize here the TWM developed in [10] for the slowly varying amplitudes of the clockwise and counter- clockwise fields, E ± (z , t ). We assume a single-transverse mode waveguide that supports a single TE mode of optical frequency ω 0 . We take the effect of the pres- ence of counter-propagating fields explicitly into account expressing the carrier density as N (z , t ) = N 0 (z , t ) + N +2 (z , t )e 2iq 0 z + N -2 (z , t )e -2iq 0 z , where q 0 = n g ω 0 /c, n g is the group refractive index, N 0 is the local average of the carrier density and N +2 (z , t ) = N ∗ -2 (z , t ) describes the amplitude of the carrier spatial modulation at half the optical wavelength, it describes the SHB. Scaling space and time to 1041–1135/$31.00 © 2013 IEEE