Dual-Wavelength Pumped Dispersion-Compensating Fibre Raman Amplifiers

Abstract

Fibre Raman amplifiers (FRAs) use optical pumping to provide low-noise gain in fibre waveguides by means of stimulated Raman scattering (SRS). They can be operated over a range of telecommunications windows, from below 1300 nm to beyond 1650 nm, often with broader spectra than those of erbium doped fibre amplifiers (EDFAs). The gain medium can be transmission fibre or dispersion compensating fibre (DCF). DCF-based Raman amplifiers simultaneously boost the propagating signals and compensate for accumulated chromatic dispersion, thereby fulfilling a dual role (Bromage, 2004, Urquhart et al., 2007). Dispersion compensating Raman amplifiers (DCRAs) normally consist of modules incorporating several kilometres of DCF plus up to around twelve pumps at different wavelengths (Islam, 2004; Namiki et al., 2005), usually launched contra-directionally with respect to the signals, as illustrated in Fig. 1. The Raman gain is often several decibels above the transparency condition of the DCF medium to mitigate the loss of associated passive components. A single pump excites a gain profile with a full width at half height of ~7 GHz but it is far from spectrally uniform, rendering it unsuitable for wavelength division multiplexed (WDM) communications. Gain flattening is thus required and it is normally achieved by the multiple pumps. Complicated optical interactions occur within the fibre, in which power is coupled from the pumps to the signals, from one pump to another and from one signal to another. Additionally, there are the noise processes of amplified spontaneous Raman scattering and amplified distributed Rayleigh backscattering, which can be sufficiently powerful to contribute to the gain saturation. Nevertheless, by carefully optimising the launched powers, the desired spectral equalisation can be achieved. Multi-wavelength pumped DCF modules have been used to provide gain bandwidths that exceed 100 nm with uniformities of better than 0.3 nm but they are complicated sub-systems (Giltrelli and Santagiustina, 2004; Namiki et al., 2004; Neto et al, 2009). Wavelengthstabilised pump lasers are expensive and the resulting gain spectra are sensitive to the precise values of the launched powers. Sophisticated simulation software with advanced