Fibre-length dependence of critical power for stimulated Raman scattering

The fibre-length dependence of critical powers, at which Stokes lights begin to build up, is investigated on stimulated Raman scattering in single-mode fibres. A mode-locked and Q-switched Nd:YAG laser is used as a pumping laser. The critical powers for the first five Stokes decrease with an increase in the length of fibre and become constant above 200 m long. The critical powers of 8 W, 20 W, 30 W, 53 W and 73 W for 1st, 2nd, 3rd, 4th and 5th Stokes, respectively, are obtained for fibres longer than 200 m.