Direct-fiber connected SiN-based microresonators for stable soliton comb generation

Abstract

Microresonators, which are the latest addition to optical frequency comb (OFC) generators, have piqued the interest of numerous researchers and became the subject of many investigations due to its versatility and small form factor. OFCs from microresonators are generated via Kerr nonlinearity of a lossless, highly nonlinear material like silicon nitride (SiN). Despite its known advantages including mass producibility and compatibility with Si electronics and Si photonics, the existence of microcomb generated from a microresonator is limited. Moreover, the excitation frequencies to generate OFCs vary largely from experiments to experiments resulting in the difficulty and unpredictability of microcomb generation. In this study, we address these limitations by directly connecting a single mode fiber (SMF) and a polarization maintaining fiber (PMF) to Si₃N₄ microresonators via a UV curable optical adhesive and a fiber array composed of glass substrates. For OFC generation, a continuous wave (CW) from an external cavity laser diode (ECLD) (TSL-550) was used as the pump and a tunable CW laser (TLX1) was used as the auxiliary laser source. The auxiliary laser counteracts the shift of the microresonator's resonance frequency brought by the increase of the temperature in the resonator during optical pumping. Compared to the free-space coupled microresonator, operation duration was improved by 2 and 167 when the microresonatos were directly connected to SMF and PMF, respectively. To lessen variation in the employed pump and auxiliary laser frequencies, a Peltier device which acts as a thermoelectric cooling (TEC) device and a thermistor which serves as the thermal sensor were integrated on a copper microchip holder. The temperature was controlled by a temperature controller via a proportional-integral-derivative (PID) device. Microcombs were generated for three random instances and the pump and auxiliary frequencies were recorded. Standard deviations of the employed pump and auxiliary laser frequencies using a temperature controller were lessened by 41% and 92%, respectively. Additionally, using a temperature controller, the generated soliton comb is not disrupted even when an object emitting high temperature (~70°C) was brought in proximity to the Si₃N₄ microresonator.