Abstract

We present numerical solutions of a model which describes a nested fiber Raman cavity using Bragg reflectors by taking into account all interactions between forward and backward traveling waves [1]. The numerical algorithm uses the COLSYS package. The model allows to calculate, for a fixed pump power and a given number n of Stokes orders growing inside the nested cavity, the optimum fiber length and the optimum output coupling for the (n+1)th Stokes line to be extracted from the cavity. We have performed measurements of both Raman gain and linear losses for a few optical fibers in order to insert into the calculations realistic fiber parameters. We apply our model to the cases in which a laser source at 1117 nm pumps a two-step Raman cascade leading to a 1240 nm output or a five-step cascade leading to a 1480-nm output. The latter case corresponds to the experimental situation discussed by Grubb et al. [2]. By using our model we can reproduce with good accuracy the data of Ref.². We find that the efficiency of the Raman laser described in [2] could be significantly improved by reducing the fiber length. In order to provide a second, more complete test of the model a new experiment was performed at the General Physics Institute of the Russian Academy of Sciences in Moskow. The pump is an Ytterbium fiber laser emitting a maximum power of 3.5 W at the wavelength 1090 nm. The laser is working on the first Stokes line at 1150 nm. The output mirror has a reflectivity of 20%. Measurements of the output power as a function of the pump power are reported in Fig.1 for two distinct values of the fiber length L. By using measured values of the Raman gain and linear losses, a fit of the model to the experimental data was performed. The agreement is rather good, as shown in Fig.l. This confirms that numerical simulation of Raman fiber lasers can be used with some confidence to predict laser behaviour and to optimise laser performance.

© 2001 EPS