Ultra-low threshold optical nonlinear oscillator using a high-Q microsphere

Summary form only given. High-Q micro-resonators are attractive for nonlinear light generation due to their long photon storage time and small mode volumes, which lead to high circulating intensities and hence strong reductions of the necessary power for stimulated and parametric optical processes. We use silica microspheres coupled to a tapered fiber which allows stable and long-term study of cavity-enhanced nonlinear processes. The weak intrinsic nonlinearity of silica is overcome by the ultrahigh-Q of the optical modes, since the threshold for nonlinear oscillation in a resonator is reduced by Q/sup 2//V, where V is the modal volume. The quadratic dependence on quality factor is due to the simultaneous resonance of both pump and nonlinear wave, which leads to extremely low thresholds even for weak processes like stimulated Raman scattering in silica. In our work, we observed a Raman threshold of only 60 /spl mu/W in a microsphere of 40 /spl mu/m diameter having a Q of 10/sup 8/. This is the lowest threshold achieved in silica to date and represents a 1000/spl times/ reduction in threshold over all micro-droplet work.