Sub-milliwatt-level microresonator solitons with extended access range using an auxiliary laser

Abstract

The recent demonstration of dissipative Kerr solitons in microresonators has opened a new pathway for the generation of ultrashort pulses and low-noise frequency combs with gigahertz to terahertz repetition rates, enabling applications in frequency metrology, astronomy, optical coherent communications, and laser-based ranging. A main challenge for soliton generation, in particular in ultra-high-Q resonators, is the sudden change of circulating intracavity power during the onset of soliton generation. This sudden power change requires precise control of the seed laser frequency and power or fast control of the resonator temperature. Here, we report a robust and simple way to increase the stability range of the soliton regime by using an auxiliary laser that passively stabilizes the intracavity power. In our experiments with fused silica resonators, we are able to extend the pump laser frequency stability range of microresonator solitons by two orders of magnitude, which enables soliton generation by slow and manual tuning of the pump laser into resonance and at unprecedented low power levels. Both single-and multisoliton mode-locked states are generated in a 1.3-mm-diameter fused silica microrod resonator with a free spectral range of ~50.6 GHz, at a 1554 nm pump wavelength at threshold powers <3 mW. Moreover, with a smaller 230-μmdiameter microrod, we demonstrate soliton generation at 780 μW threshold power. The passive enhancement of the stability range of microresonator solitons paves the way for robust and low threshold microcomb systems with substantially relaxed stability requirements for the pump laser source. In addition, this method could be useful in a wider range of microresonator applications that require reduced sensitivity to external perturbations.

Advances in soliton microcomb generation

“…As a result, the dramatic decrease of intracavity heat when a pump laser tunes into a soliton existing range can be effectively compensated for by an auxiliary laser located at the blue-detuned regime. This principle has been verified recently, 15,16,28,29,34 and the requirement for a rigid tuning time (on the order of thermal lifetime) can be relaxed using the auxiliary-laser-assisted approach.…”

“…In another experiment, it is proved that the durations of soliton steps can be extended by two orders of magnitude under the assistance of a codirectional-coupled 1.3-μm auxiliary laser, enabling robust soliton generation by even manual tuning of the pump frequency into resonance with submilliwatt-level power. 29 Besides, SMCs are also realized in silica and high-index doped silica glass microresonators. 15,28 All of these experimental results suggest that using an auxiliary laser can contribute to intracavity thermal equilibrium.…”

“…Until now, based on advanced experimental techniques, SMCs have been realized in an MgF 2 whispering-gallery-mode (WGM) resonator, 10 silica disk 13 and microrod, 28,29 AlN, 30 LiNbO 3 , 31 and CMOS-compatible microring resonator of SiN, 12 Si, 32 and high-index doped silica glass. 15 Table 1 briefly summarizes some typical SMCs obtained on various material platforms with distinct cavity properties.…”

Ultra-efficient frequency comb generation in AlGaAs-on-insulator microresonators

“…Recently, several approaches provide other ways to overcome the thermal problem and to generate a low-noise comb state. These include using an auxiliary laser to provide temperature compensation 43 , or applying dispersion engineering in normal GVD region to eliminate the influence of thermal effect 44 . Further investigation along those directions can improve the potential of the ultra-efficient combs in this platform.…”

A nonlinear enhanced microresonator gyroscope

“…One of the key features of ultrahigh-Q optical microresonators is the strong Kerr nonlinearity that they exhibit at modest input powers of milliwatts or even microwatts, leading to important effects such as frequency comb generation [14][15][16]. Recently, Kerr interaction between counterpropagating light waves in a bidirectionally-pumped microresonator was found to give rise to spontaneous symmetry breaking [17][18][19].…”