Abstract

In all-solid state lasers with additive-pulse modelocking (APM), to reach the threshold of selfstarting the nonlinear phase of approximately π/10, acquired in the external cavity, is needed.[l] Since the nonlinearity is weak this demands an initiating kick and leads to a phase distortion of the output. Alternative way of approaching desirable values of the phase shift may be based on a cascaded second- order nonlinearity. This causes a pulse of fundamental frequency to experience a nonlinear phase shift while passing through a phase-mismatched frequency doubler.[2,3] The value of such an effective refractive index is of the order of 10^-13 cm²/ W for poled silica fibres,[4] what is three orders of magnitude larger than n₂ produced by a third-order nonlinearity. This implies that such third-order phenomenon as an APM laser can be implemented with mWatt power level in a fibre geometry on the basis of cascading.

© 1996 Optical Society of America