Crosstalk in double-pumped fiber optic parametric amplifiers for wavelength division multiplexing systems J.M. Chavez Boggio, J.D. Marconi * , H.L. Fragnito Optics and Photonics Research Center, Instituto de Fı ´sica ÔGleb WataghinÕ Unicamp, 13083-970, CP 6165 Campinas, SP Brazil Received 18 March 2005; received in revised form 4 June 2005; accepted 8 August 2005 Abstract We study experimentally inter-channel crosstalk in double-pumped fiber optic parametric amplifiers constructed with conventional dispersion shifted fibers (DSFs) having different lengths (L A = 13.8, L B = 6.8, L C = 4.3, and L D = 0.8 km). For long fibers (L A and L B ), eye diagram measurements in a 5-channel (100 GHz spacing) system show that in order to have negligible crosstalk, the output sig- nal power per channel, P s , should be limited to P s < 0 dBm. By decreasing the fiber length (to L C ) it is possible to increase the output signal power and/or the number of signals while keeping the crosstalk on negligible levels. This trend was further confirmed by using a very short DSF (L D = 0.8 km). Finally, we experimentally demonstrate that a general trend in 2P-FOPAs is that spurious FWM increases with the number of signal channels up to a given number of channels when a saturation regime is reached. This saturation of the generation of spurious tones occurs when the bandwidth occupied by the signals exceeds 4–5 nm. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Optical parametric amplifier; Crosstalk; Wavelength division multiplexing 1. Introduction The fiber optic parametric amplifier (FOPA) is a prom- ising device for all-optical signal processing in future opti- cal networks [1–26]. FOPAs have been demonstrated having high gain and low noise figure at arbitrary wave- lengths by using a proper fiber design and tuning ade- quately one or two pump lasers. One important issue in FOPAs for wavelength division multiplexing (WDM) applications is inter-channel crosstalk, which arises princi- pally from two fonts: cross-gain saturation (XGS) and spu- rious four wave mixing (FWM). XGS is due to the instantaneous gain saturation experi- enced by a signal due to pump depletion, which is propor- tional to the number of signals transmitting simultaneously the bit one. The deleterious effects of XGS on FOPA per- formance have been investigated in detail in [27] and also in [28,29]. The other limiting effect arises from spurious FWM processes involving signals, idlers, and pumps. These processes can be very efficient in FOPAs due to the low dis- persion fiber used as nonlinear medium, i.e., the proximity of the signals and idlers to the zero dispersion wavelength (k 0 ). The first study on inter-channel crosstalk in FOPAs was reported by Krastev and Rothman in [29]. They stud- ied a two-channel WDM system being amplified by a sin- gle-pumped FOPA (1P-FOPA) and showed that XGS and spurious FWM involving two signals (and two idlers) and the pump could lead to signal eye closure. In [30] it was predicted, through numerical simulations with double- pumped FOPAs (2P-FOPAs), that crosstalk from spurious FWM and XGS can be alleviated if the 2P-FOPA is con- structed with shorter fibers and pumped at higher powers. In [31,32] it was presented a systematic numerical study of crosstalk by spurious FWM in 1P-FOPAs in terms of the output signal power, fiber length, nonlinear coefficient, 0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.08.015 * Corresponding author. Present address: Quantum Electronics, UNI- CAMP, Cidade Universitaria Zeferino Vaz, 13083970 Campina, Brazil. Tel.: +55 19 3788 5451; fax: +55 19 3788 5428. E-mail addresses: jmchavez@ifi.unicamp.br (J.M. Chavez Boggio), dmarconi@ifi.unicamp.br (J.D. Marconi). www.elsevier.com/locate/optcom Optics Communications 259 (2006) 94–103