Abstract

We recently demonstrated strong coupling of light and mechanical vibrations in a "dual-nanoweb" fibre (Fig. 1(a)), comprising two very thin, closely spaced silica membranes supported inside a capillary. The optomechanical nonlinearity is strongest when the structure is driven at its mechanical resonant frequency Ω₀/2π ~ 6 MHz [1], and is further enhanced by operating in vacuum [2]. We observed the generation of an optical frequency comb, with multiple sidebands spaced by Ω₀, when the structure was pumped with only a few mW of CW laser light [3]. We explained this by ultra-high-gain stimulated Raman-like scattering (SRLS), initiated by a strong population of thermally-driven phonons. Although under these circumstances theory predicts thresholdless oscillation, the experiments showed a clear threshold at a power of a few mW. We attributed this to competition between several slightly different mechanical resonances, each localized at a different position along the structurally inhomogeneous fibre. At threshold, the resonance with the highest SRLS gain switches into coherent oscillation, creating a frequency comb.

© 2015 IEEE