Experimental studies of the WDM signal crosstalk in two-pump fiber optical parametric amplifiers Kenneth K.Y. Wong a, * , Guo-Wei Lu b , Lian-Kuan Chen b a Department of Electrical and Electronic Engineering, The University of Hong Kong, Room 516, Chow Yei Ching Building, Pokfulam Road, Hong Kong b Department of Information Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong Received 4 May 2006; received in revised form 21 September 2006; accepted 25 September 2006 Abstract We showed that cross-gain modulation is dominant over another kind of common signal quality degradation, namely four-wave mix- ing, in a two-pump fiber optical parametric amplifier (OPA) WDM system. We also demonstrated experimentally that the signal quality degradation can be improved significantly by a two-orthogonal-pump OPA (2OP-OPA) over the conventional two-parallel-pump OPA (2PP-OPA). Ó 2006 Elsevier B.V. All rights reserved. Keywords: Four-wave mixing (FWM); Cross-gain modulation (XGM); Optical amplifiers; Optical fibers; Optical transmission; Parametric amplification 1. Introduction Fiber optical parametric amplifiers (OPAs) have recently been demonstrated to be practical amplifiers with high gain [1], large bandwidth [2] and polarization-inde- pendence [3]. The quality of signals emerging from OPAs used as signal processors has been investigated by several groups recently, especially with regard to the pump-to-sig- nal RIN transfer [4], cross-phase modulation (XPM) amongst WDM channels [5], etc. However, four-wave mix- ing (FWM) and cross-gain modulation (XGM) seem to be the fundamental limits for using OPA as WDM amplifier. Previous work has shown that this kind of degradation is already severe with only three WDM channels in one- pump OPA (1P-OPA) system [6]. It was also shown that unequal channel spacing slightly improves the degradation. However, the XGM effect still provides a basic detrimental effect when using OPA in WDM systems, mediated through the depletion of the pump(s). Comparing with 1P-OPA, two-pump OPA (2P-OPA) provides an extra degree of freedom, such that a flattened gain spectrum can be achieved by trading with the gain bandwidth [7]. Previous efforts include the investigation of the relationship between the spurious FWM and signal power and length of nonlinear medium for a 2P-OPA [8]. It has also been shown that with low gain and output power, the crosstalk level and power penalty are acceptable [9–11], though the condi- tions may not be necessarily fulfilled for practical applica- tions, especially using fiber OPA as power booster amplifier. Therefore, we will focus on the large-signal behavior in 2P-OPA in this paper. Besides, with comple- mentary phase-dithering, one can obtain a narrow-line- width idler spectrum, as well as effective suppression of stimulated Brillouin scattering (SBS) [12]. Mid-span spec- tral inversion (MSSI) using two-orthogonal-pump OPA (2OP-OPA) in a 320-km transmission link has been demon- strated [13]. However, there has not yet been any quantita- tive study about the signal degradation of the WDM channels for such systems. In this paper, we will investigate experimentally how the 2OP-OPA improves performance in terms of XGM and FWM, compared to 2-parallel-pump OPA (2PP-OPA). 0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.09.067 * Corresponding author. Tel.: +852 28578483; fax: +852 25598738. E-mail address: kywong@eee.hku.hk (K.K.Y. Wong). www.elsevier.com/locate/optcom Optics Communications 270 (2007) 429–432