Bi-material crystalline whispering gallery mode microcavity structure for thermo-opto-mechanical stabilization

Abstract

We fabricated a calcium fluoride (CaF2) whispering gallery mode (WGM) microcavity with a computer controlled ultra-precision cutting process. We observed a thermo-opto-mechanical (TOM) oscillation in the CaF2 WGM microcavity, which may influence the stability of the optical output when the cavity is employed for Kerr comb generation. We studied experimentally and numerically the mechanism of the TOM oscillation and showed that it is strongly dependent on cavity diameter. In addition, our numerical study suggests that a microcavity structure fabricated with a hybrid material (i.e. CaF2 and silicon), which is compatible with an ultra-high Q and high thermal conductivity, will allow us to reduce the TOM oscillation and stabilize the optical output.

Development of CaF₂-brass hybrid WGM microcavity by using ultra-precision machining

“…As mentioned in Section 2, TO and TE effects cause an unstable cavity performance, and the transmittance spectra of the conventional cavity form a distorted dip. The detailed explanation about the unstable mechanism was provided elsewhere (Itobe et al, 2016). Due to that, a lot of distorted resonance peaks were seen in the range of quite short region for the conventional cavity.…”

“…In order to suppress these phenomena, the cavity structure, which enables heat to easily diffuse from the resonance body, is of high importance. We proposed a hybrid cavity that consists of a cavity and a heatsink using different materials (Itobe et al, 2016). Fig.…”

“…The wavelength was continuously changed during measurement to find the resonance wavelength in the normal sweep direction (from shorter to longer wavelength). The measurement procedure has been described in detail elsewhere (Itobe et al, 2016;Mizumoto et al, 2017). The measurement setup is illustrated in Fig.…”

“…Finally, the cavity system must be robust against thermal changes due to trapped light that can be converted into heat. In particular, thermo-optic (TO) and thermal expansion (TE) effects will lead to an unstable light output (Itobe et al, 2016).…”

“…This behavior is a result of the competition between different TO and TE effects occurring at different timescales (Wang et al, 2013). To avoid the phenomena, we proposed a new microcavity shape (Itobe et al, 2016) that allows heat to diffuse quickly and suppresses TO effects at a given optical input power. In this study, the hybrid microcavity was designed based on a finite element method (FEM) simulation of heat transfer.…”