Th.P2.15 342 ICTON 2008 978-1-4244-2626-3/08/$25.00 ©2008 IEEE Four-Wave Mixing in Non-Zero Dispersion Shifted Fibers Michal P. Nikodem, Wojciech Żurawski, Krzysztof M. Abramski Laser and Fiber Electronics Group Institute of Telecommunication, Teleinformatics and Acoustics Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland Tel: (4871) 320 30 24, Fax: (4871) 320 31 89, e-mail: michal.p.nikodem@pwr.wroc.pl ABSTRACT In this paper we investigate Four-Wave Mixing (FWM) in Non-Zero Dispersion Shifted Fiber (NZ DSF). Theoretical background and experimental results are presented. We study the impact of the channel separation on FWM efficiency. We present a method of simultaneous chromatic dispersion and fiber nonlinear coefficient measurement based on FWM efficiency dependence on channel spacing. Keywords: optical fiber, four-wave mixing, chromatic dispersion, nonlinear coefficient. 1. INTRODUCTION One of the biggest challenges in optical transmission systems, with growing transmission rates and system capabilities, is to overcome the nonlinear effects in optical fibers. Four-wave mixing seems to be the most harmful in case of dense wavelength division multiplexing (DWDM) systems. FWM can be useful in some applications (e.g. wavelength conversion). Nevertheless, crosstalk that it causes in WDM or FDM transmission lines limits system capabilities [1,2]. Several methods for theoretical analysis of FWM in optical fiber has been shown. Very often split-step Fourier method (SSFM) is used [3,4]. Analytical model presented by Hill et. al. [5] is also common. It is simpler and faster than SSFM. Such model can be used to analyze optical transmission systems performance and limitations. On the other hand they can be used for fiber chromatic dispersion and fiber nonlinearity measurement [6]. It is commonly used for finding zero dispersion wavelength in dispersion shifted fibers (mainly due to relatively high FWM efficiency in such fibers) [7]. In this paper we explore four-wave mixing in non-zero dispersion shifted fiber. We introduce theoretical model that can be used in FWM analysis. We present experimental results showing the impact of the channel spacing on FWM efficiency in case of two different types of NZ DSF. Using described model and obtained results fiber chromatic dispersion and nonlinearity parameter is measured. 2. THEORETICAL ANALYSIS We consider the situation where two or three continuous-wave beams are launched into the fiber. Power of each channel is P 0 and the channel spacing is Δf. FWM signal power can be than written as [8,9] ( ) 2 3 0 2 2 exp 9 eff FWM L L P d P ⋅ - = α γ η , (1) where η is the FWM efficiency, d is the degeneracy factor (d = 3 and 6 in case of two and three channels respectively), γ is the nonlinearity parameter, α is the fiber attenuation, L and L eff are the length and the effective length of the fiber respectively. Nonlinearity parameter is defined as follows [3]: eff A n ⋅ = λ π γ 2 2 , (2) where n 2 is the nonlinear-index coefficient, A eff is the effective core area. Effective length of the fiber L eff takes into account power decreasing due to attenuation and is defined as ( ) α αL L eff - - = exp 1 . (3) FWM efficiency η is written as ( ) ( ) ( ) ( ) ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ - - ⋅ Δ ⋅ - ⋅ + ⋅ Δ + = 2 2 2 2 2 exp 1 2 / sin exp 4 1 L L L α β α β α α η (4) where Δβ is the phase mismatch