Abstract

Phase-matched second-harmonic generation (SHG) in optical fibres has been studied by several workers ^{[1] [2] [3]}. The origin of the second-order non-linearity in a material which should be centro-symmetric is unclear. A model proposed recently^[3] speculates that a third order interaction creates a spatially alternating static electric-field which orients defects and removes material symmetry. The periodic variation in the effective second-order nonlinearity χ⁽²⁾ then allows phase-matching. Oullette et al ^[4] W have demonstrated the erasure of a phase-matching seeded grating ^[3] using blue or green Argon laser wavelengths. Recently, Fermann et al ^[5] showed alignment of defects by a strong external static electric-field applied to an optical fibre transmitting Ar⁺ radiation at 488 nm wavelength. Static-field alone ^[6] does not seem to ‘prepare’ the fibre, while irradiation with Argon light alone ^[5] seems to erase the grating. Unlike previously reported work, we show that radiation from an Ar⁺ laser operating at 514.5 nm on a single longitudinal mode (SLM) is sufficient on its own to enhance SHG of 1064 nm wavelength light from a Q-switched Nd³⁺:YAG laser in an elliptical cored fibre which is close to the phase-matched condition for one set of fundamental and second-harmonic modes.

© 1989 Optical Society of America