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 multi-soliton 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-{\mu}m-diameter microrod, we demonstrate soliton generation at 780 {\mu}W 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.
Highlights
Over the past two decades, optical frequency combs based on mode-locked lasers have revolutionized the field of precision spectroscopy with an unprecedented frequency measurement precision [1,2]
We show a detailed study of the passive enhancement of the soliton access range by using an auxiliary laser
One part of the 1.5 μm light is sent to an optical spectrum analyzer (OSA), and the rest is sent into a fiber Bragg grating notch filter to separate the generated comb light from the pump light
Summary
Over the past two decades, optical frequency combs based on mode-locked lasers have revolutionized the field of precision spectroscopy with an unprecedented frequency measurement precision [1,2]. Direct soliton generation can be achieved by optimizing laser tuning speed and stopping at the right frequency This method works in materials with weak thermo-optic effect, such as MgF2 [14]. The auxiliary laser compensates sudden intracavity power changes when the microresonator enters the soliton regime Using this method, the length of the soliton steps is extended from 100 kHz to 10 MHz, which enables access to single-soliton states without specific requirements for pump laser tuning speed or power kicking techniques. Soliton states can be reached by arbitrary slow tuning of the laser into resonance, which significantly simplifies the soliton generation process This enables access to soliton states in ultrahigh-Q resonators with flawless mode spectra (no mode crossings), which has been previously challenging. Low power consumption of microresonator solitons is important for out-of-the-lab applications of frequency combs, e.g., in battery powered systems [53]
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