Thermal bistability-based method for real-time optimization of ultralow-threshold whispering gallery mode microlasers

Abstract

A method based on thermal bistability for ultra-low threshold microlaser optimization is demonstrated. When sweeping the pump laser frequency across a pump resonance, the dynamic thermal bistability slows down the power variation. The resulting lineshape modification enables a real-time monitoring of the laser characteristic. We demonstrate this method for a functionalized microsphere exhibiting a sub-microwatt laser threshold. This approach is confirmed by comparing the results with a step-by-step recording in quasi-static thermal conditions. c 2018 Optical Society of America OCIS codes: 140.3945, 140.3410, 140.3530, 140.6810, 130.3990 Optical microcavities have drawn a large interest in the last two decades and received numerous applications, including Cavity-QED and photonic devices, like light emitting diodes or microlasers. A special attention has been devoted to whispering gallery mode (WGM) microcavities induced by surface tension, like microspheres and microtoroids. They benefit from the sub-nanometer roughness of molten silica, which results in very high quality factors Q, enabling laser operation with a pump power in the sub-microwatt range [1]. These ultra-lowthreshold lasers have been obtained by silica functionalization with embedded rare earth ions. Different doping techniques, like fiber doping, sol-gel coating, ion implantation, co-deposition or co-sputtering have been successfully used with Nd 3+ , Er 3+ , and Yb 3+ or mixtures [1][2][3][4]. In this paper, we report on a new method for the fast measurement of the laser light-light characteristic based on thermal effect, and allowing real-time optimization. In monolithic microcavities, self-heating by the minute dissipated optical power induces a negative frequency shift, resulting in a bistable behavior when this shift exceeds the cold cavity linewidth [5][6][7]. For decreasing laser frequency, the heating pushes cavity resonance frequency in the same direction, and they remain close to resonance, as long as the thermal shift compensates the laser offset. The loss of the resonance results in a rapid drop of the coupling. In reverse direction, the thermal shift and the laser scan have opposite directions, and the line is narrowed. Due to the high-Q and the small heat capacity of the cavity, the bistability threshold is very low, typically in the microwatt range or less. At low scanning rate (typically up to 100 Hz) the heating affects the cavity as a whole, and only one cooling time is involved, while for increasing scanning rate a dynamical thermal effect appears which depends on the heat diffusion details. Our method exploits the low frequency regime, with a decreasing pump frequency so that the resonance shifts in the same direction as the laser. In our experiment, the microlaser is optically pumped by a laser injected in a WGM. In contrast to usual techniques, its frequency is not fixed, but is continuously swept across the resonance, at a nearly constant power. The thermal effect converts this frequency modulation into a smooth sweep of...

On the dispersion management of fluorite whispering-gallery mode resonators for Kerr optical frequency comb generation in the telecom and mid-infrared range

“…These parameters are particularly important for WGM applications. The self-thermal locking method can be used for WGM pumping for microlasers [27] and combs [1]. However, it is only applicable for the material that possesses the same signs of these two parameters, such as MgF 2 , fused silica and Sapphire.…”

“…The tunable cw pump laser with a sub-KHz spectral linewidth at 1552 nm was used. It was then thermally locked to excite different WGMs [1,27]. Depending on the pump condition, both primary Kerr combs and full developed Kerr combs were observed.…”

Phase-locking transition in Raman combs generated with whispering gallery mode resonators

“…Since the thermo-optic and thermal expansion effects have opposite signs, self-thermal oscillations occur [27]. Therefore, it is impossible to apply the self-thermal locking method that has been successfully used in silica resonators [28]. Nevertheless, the PDH locking technique was successfully applied in this experiment.…”

Spectro-temporal dynamics of Kerr combs with parametric seeding

“…This can be achieved using an active locking method such as the Pound-Drever-Hall method or a passive method. Concerning the latter, one can use either self-injection locking [27], which can be very compact, or self-thermal locking, which has also been demonstrated for microlaser characterizations [28]. Since self-thermal locking is an easy and simple method, it is utilized throughout our experiments.…”