Optical Power Limiting By SHG In The Čerenkov Regime

Abstract

The interest in χ(2)-materials and related nonlinear wave-propagation effects has been driven for a long time by potential applications in efficient blue-light generation by Second Harmonic Generation (SHG) and electro-optical switching [1]. From a practical point of view the interest in SHG has been dominated by guided mode - guided mode (here referred to as type A) interactions rather than by guided mode - radiation mode (type B) interactions (also called Čerenkov radiation) since in the latter case analytical relations are difficult to obtain (due to the problems associated with normalisation of radiation modes) and because the geometrical shape of the emerging SH light is not favourable. Numerical studies on type B interactions [2] have merely focused on the SHG efficiency of structures in which the index of the the core at 2 ω ( n c 2 ω ) is larger than the index of the cladding at 2 ω ( n cl 2 ω ) and in which both are larger than the effective index of the guided modes at ω ( N eff ω ) . In this paper we report on numerical investigations on type B SHG of structures with n cl 2 ω > n c 2 ω > N eff ω . We find that in the Čerenkov regime the depletion of the fundamental can be almost complete when there is phase-matching to a leaky mode at 2ω due to the high propagation losses (10-1000 dB/cm) of these modes [3], The increasingly more complete conversion with increasing input power makes this type of interaction very interesting for optical power limiting of the input beam at co. Benefits of this scheme are a large dynamic range, the potential absence of heating (because it does not require intrinsic absorption), adjustable limiting levels and a wide wavelength bandwidth.