1 August 1999 Ž . Optics Communications 166 1999 85–88 www.elsevier.comrlocateroptcom Fibre Raman amplifiers for broadband operation at 1.3 m m D.V. Gapontsev ) , S.V. Chernikov, J.R. Taylor Physics Department, Imperial College, Prince Consort Road, London, SW7 2BZ, UK Received 10 March 1999; accepted 5 May 1999 Abstract A high-power fibre laser operating at 1.24 mm was used to provide broad band Raman gain around 1.3 mm in compact, efficient, silica fibre-amplifier configurations. The gain and noise performances of these broad band amplifiers are described, as well as the mechanism for further increasing and flattening the operational bandwidth. q 1999 Elsevier Science B.V. All rights reserved. PACS: 4265C; 4281 Keywords: Raman amplifier; Circulators; Telecommunications; Raman 1. Introduction Recent progress in high-power fibre lasers operat- ing in the range of 1.03–1.55 mm which are already commercially available give the possibility of devel- oping efficient fibre Raman amplifiers for telecom- munications. Raman amplifiers can operate in the entire window of transparency of silica single mode fibres and particularly in the second telecommunica- Ž . tion window 1.3 mm where rare earth silica-based amplifiers have so far been notably unsuccessful. Several cascaded, resonant Raman amplifiers based on Bragg gratings and WDM couplers have been w x reported 1–3 . In this paper, we demonstrate the design of a broadband Raman amplifier in the spec- tral range around 1.3 mm. The amplifier is based on ) Corresponding author. Fax: q44-171-594-7782; e-mail: d.gapontsev@ic.ac.uk optical circulators to increase the bandwidth of the amplifier and has an optimised configuration to achieve a high gain with a low noise figure. 2. Amplifier configurations and performance The amplifier configuration is shown in Fig. 1. A dual-stage configuration was used for high-gain op- eration with no significant increase of the noise wx figure determined by double Rayleigh scattering 4 . Various configurations and several different fibres were tested during the optimisation procedure. The dual stage configuration comprising 6 km of disper- Ž . sion shifted fibre DSF and 2 km of small core, high Germanium concentration fibre was found to per- form best. It was also found that the lowest noise figure was achieved when a counter-propagating pump and signal based on DSF were launched at the first stage. Optical circulators were used to multiplex and demultiplex the pump and signal lasers, which 0030-4018r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0030-4018 99 00244-8