2ND-HARMONIC GENERATION IN GLASS-FIBERS

Experiments on frequency doubling of 1064 nm and 1053 nm radiation with non-phase-matched and (close to) phase-matched germaniadoped silica fibres are reported. The greatest conversion efficiencies are obtained with non-phase-matched fibres that are single-mode at the second-harmonic wavelength, in which phase-matching is thought to arise from a self-written grating. Competition between grating self-writing and self-erasure in these fibres in the absence of an external seed is also reported. Phase-matched fibres allow the formation of the nonlinearity to be studied independently of the phase matching. The nonlinearity is optically induced by exposing the fibre to short-wavelength visible radiation. An enhancement of 104 to 105 in conversion efficiency between the fundamental and the phase-matched LP31 second-harmonic mode over the intrinsic background second harmonic has been observed. We also report on the occurrence of phase-matched second-harmonic generation (SHG) and nonphase-matched SHG in the same fibre. © 1990 Taylor & Francis Ltd.