Abstract

Ever since Osterberg and Margulis¹ reported the growth of second-harmonic generation in optical fibres, and the observation by Stolen and Tom,² the question has remained: how is the initial frequency doubled light generated? In particular Stolen and Tom observed that pumping the fibre with 4.8 kW peak of 1064 nm radiation, the frequency doubled light grew in the LP^2ω₁₁ mode from the fundamental wavelength LP^2ω₀₁ mode. They noted that symmetry has to be broken by the nonlinearity for the even^ω mode to odd^2ω mode interaction to be observed. We have investigated the intrinsic nonlinearity in optical fibres by using phase-matched fibres and low optical powers. We postulated that the assymmetry of the nonlinearity which allows the observation of the LP^2ω₁₁ mode, will also allow the observation all the higher modes of the same order. It is practically impossible to fabricate a fibre which is phase-matched for the interaction, LP^ω₀₁ → LP^2ω₁₁. We therefore fabricated a fibre which was phase matched for the higher mode-interaction, LP^ω₀₁ →LP^2ω₁₂, and another for LP^ω₀₁ →LP^2ω₁₃. It has been demonstrated for the first time that the fibres frequency double 10's of milliwatts of CW 1064 nm radiation into the LP^2ω₁₂, and the LP^2ω₁₃ modes with no prior preparation of the fibre. These observations indicate that there is indeed an intrinsic χ⁽²⁾ nonlinearity in the fibre, theoretically studied by Terhune and Weinberger³ which must therefore act as the source of the initial sh radiation.

© 1990 Optical Society of America