Optical frequency comb generation in coupled nonlinear microcavities

Abstract

Microcavity based optical frequency combs with Kerr nonlinearity attract much attentions for their various applications and compact configurations for chip-scale integration. Kerr soliton combs are typically generated in nonlinear microcavities with anomalous dispersion pumped by red-detuned continuous wave lasers. Although soliton combs have been repeatedly demonstrated in the laboratories, there are several challenges limit the use of microcavity combs beyond the laboratories. The first challenge is that current soliton Kerr comb generation scheme requires tunable lasers for the access of the soliton states. Tunable lasers are usually large scale and high-power consumption making them unsuitable for chip-scale integration. The second challenge is that soliton Kerr combs can only be generated in ultralow loss microcavities. The low loss requirement limits the choice of cavity materials and size. The cost of fabricating low loss cavities also limits commercial applications of microcavity combs. In this thesis, we propose optical frequency comb generation by utilizing coupled nonlinear microcavities to overcome these challenges. By theoretical analysis, we find that the coupled microcavities support modulation instability generation in the blue-detuned side of the nonlinear cavity's resonance, whereas there is no blue-detuned modulation instability generation in a single nonlinear microcavity. The size and location of the blue-detuned modulation instability region can be varied by tuning the coupling coefficient between the two cavities, the loss of the auxiliary cavity or detuning between the two cavities. By using the blue-detuned modulation instability, we propose a soliton comb generation scheme by tuning the coupling coefficient in a coupled microcavity system instead of tuning the wavelength of the pump laser. We find that the soliton generation region (bistability region) depends on the coupling coefficient between the coupled cavities. We showed that the auxiliary microcavity introduces a new optical path which makes "blue-detuned" soliton comb generation possible. We numerically demonstrate soliton comb generation by tuning the coupling coefficient and design a Sagnac loop like structure to show that the same phenomenon is applicable for the coupling between the clockwise and counterclockwise propagation modes in a single microcavity. We further show that if the auxiliary cavity provides gain, then it is possible to generate soliton combs in the main cavity with higher loss, which will reduce the difficulties in fabricating low loss microcavities and expand the choices of cavity materials. Our study in this thesis provides a theoretical understanding and experimental guidance for optical frequency comb generation in coupled nonlinear microcavities. The results will benefit the development of chip-scale comb sources.