Kerr Nonlinearity induced Optical Frequency Comb Generation in Microcavities

Owing to the ultra-high optical Q-factors and small mode volumes, fused silica toroidal microcavities are well suited for generation of light via nonlinear frequency conversion. Optical parametric oscillations (OPO) in microcavities have recently been reported in (Kippenberg et al., 2004) and theoretically explained in (Matsko et al., 2005). Furthermore, it has been shown that the parametric process can cascade. The paper shows a typical optical spectrum obtained by pumping an 80 micron diameter toroidal microcavity with a 1553 nm external cavity laser. Highly efficient evanescent coupling from a tapered optical fiber is used to excite the whispering-gallery modes of the microtoroids. The nonzero dispersion of the cavity renders the resonance modes of the cavity non-equidistant. Since the generated OPO sidebands are equidistant one would expect that they are not resonant in the cavity. However, owing to the effect of cavity enhanced cross-and self phase modulation the parametric oscillation and four wave mixing which give rise to the frequency cascade can pull cavity modes equidistant which is experimentally shown here. The measurement principle is based on the fact, that two superimposed equidistant combs can generate an equidistant comb of beat notes in the RF-domain. As reference comb a femtosecond fiber laser frequency comb is used.