Low Threshold Optical Oscillations in a Whispering Gallery ModeCaF2Resonator

Abstract

We have observed low-threshold optical hyperparametric oscillations in a high-Q fluorite whispering gallery mode resonator. The oscillations result from the resonantly enhanced four-wave mixing occurring due to Kerr nonlinearity of the material. We demonstrate that, because of the narrow bandwidth of the resonator modes as well as the high efficiency of the resonant frequency conversion, the oscillations produce stable narrow-band beat-note of the pump, signal, and idler waves. A theoretical model for this process is described.

Full Stabilization of a Microresonator-Based Optical Frequency Comb

“…One mode of a toroidal microresonator is pumped with a tunable external cavity diode laser (ECDL) amplified by an erbium doped fiber amplifier (EDFA), leading to the generation of a frequency comb. In contrast to earlier work [12,21] reporting only phase modulation in the four-wave mixing process, we were able to directly measure the 86-GHz mode spacing beat by sending the comb to a fast photodiode (3 dB cutoff at 50 GHz). To measure the 86-GHz beat note, it is mixed down to a 30-MHz radio frequency (rf) signal using a harmonic mixer and the 6th harmonic of a local oscillator around 14.3 GHz.…”

“…This approach is based on continuously pumped fused silica microresonators on a chip, in which frequency combs are generated via parametric frequency conversion through four-wave mixing [10], mediated by the Kerr nonlinearity [11][12][13][14][15]. In this energy-conserving process, two pump photons are converted into a symmetric pair of sidebands with a spacing given by the free spectral range of the microcavity.…”

Optical frequency comb generation from a monolithic microresonator

“…This is a priori not expected to be satisfied, since the distance between adjacent modes ν FSR = |ν m − ν m+1 | (the free spectral range, FSR) can vary due to both material and intrinsic cavity dispersion which impact n eff and thereby render optical modes (having frequencies ν m = m · c 2π·R·n eff , where c is the speed of light in vacuo) non-equidistant. Indeed, it has only recently been possible to observe these processes in microcavities (made of crystalline [15] CaF 2 and silica [14,23]). Importantly, this mechanism could also be employed to generate optical frequency combs: the initially generated signal and idler sidebands can interact among each other and produce higher order sidebands (cf.…”

“…Early work generated frequency combs by intra-cavity phase modulation [10,11], while to date frequency combs are generated utilizing the comb-like mode structure of mode-locked lasers, whose repetition rate and carrier envelope phase can be stabilized [12]. Here, we report an entirely novel approach in which equally spaced frequency markers are generated from a continuous wave (CW) pump laser of a known frequency interacting with the modes of a monolithic high-Q microresonator [13] via the Kerr nonlinearity [14,15]. The intrinsically broadband nature of parametric gain enables the generation of discrete comb modes over a 500 nm wide span (≈ 70 THz) around 1550 nm without relying on any external spectral broadening.…”

“…Furthermore, we note that parametric interactions do also occur in other types of microcavities -e.g. CaF 2 [15] provided the material exhibits a third order nonlinearity and sufficiently long photon lifetimes. As such the cavity geometry is not conceptually central to the work and the reported phenomena should become equally observable in other types of high-Q microresonators, such as silicon, SOI or crystalline based WGM-resonators.…”