Turnkey soliton frequency combs from a linear fiber cavity with microresonator mirror

We demonstrate robust turnkey soliton frequency combs using a fiber Fabry-Pérot cavity laser. This believed to be novel linear fiber-laser-based approach leverages Rayleigh scattering from a microresonator to serve as a partial reflector to facilitate both lasing and soliton generation. Numerical simulations are used to optimize the gain-maximum wavelength of the fiber cavity to match the target wavelength, at which the microresonator exhibits strong backscattering. A fused silica microrod resonator with an intrinsic Q-factor of 2 × 10⁸ simultaneously acts as partial reflector and filter for the laser cavity. Based on this fiber cavity laser, we successfully generate soliton crystal frequency combs in a fused silica microresonator with a mode spacing of approximately 109 GHz. We also validate soliton comb generation using different microresonators with varying dimensions. Our laser system exhibits self-injection locking to one of the microresonator modes. Turnkey performance is evaluated through laser current switching tests. The laser power conversion efficiency from 980 nm to 1550 nm is 25%. As a complement to chip-based systems, our work provides insights into soliton generation using extremely low-loss laser cavities and narrow linewidth fused silica microresonators. Our soliton frequency combs are expected to advance various microwave photonic applications that demand long-term stability and turnkey performance.