ИСТИНА

Kerr optical frequency combs in high-Q microresonators [1] are attracting growing interest, especially after mode-locking via dissipative Kerr solitons (DKS) has been demonstrated on a variety of platforms [2,3]. Such combs have promising applications due to low power consumption and possibility of chip integration. A traditional approach to obtaining DKS in microresonators relies on narrow-linewidth tuna-ble lasers for pumping. Independently the same type of microresonators could be used for significant line narrowing of diode lasers exploiting resonant Rayleigh backscattering [4] for self-injection locking [5]. Kerr soliton frequency combs have also been demonstrated with self-injection locked diode lasers [6]. Previously for self-injection locking only single frequency stabilized diode lasers were used with either Bragg-grating or distributed feedback configuration, having narrow linewidth comparable to the reso-nance linewidth of the high-Q microresonator. Surprisingly, we found that the initial stabilization is not required for soliton comb generation, and simpler but more powerful diode lasers may be used, and demonstrate a technique to stabilize, generate and control coherent low-noise soliton Kerr combs using commercial broad spectrum multi-frequency CW laser diodes, self-injection-locked to an ultra- high-Q crystalline whispering-gallery-mode microresonator. In this configuration the role of the microresonator is twofold: 1) it selects and narrows the linewidth of the laser via self-injection locking, and 2) soliton Kerr comb is generated in the microresonator. We manufactured a MgF2 resonator, 5 mm in diameter with com-puter controlled single-point diamond turning machine and polished it with diamond slurries, achieving Q > 109. For pumping, we used free-space laser diodes (Seminex, λ~1535, 1550 and 1650 nm, spectrum width ~10 nm, P~200 mW) coupled to the resonator with a total internal reflection prism. Generation of self-injection locking soliton combs stable for hours (beat note linewidth <1kHz) was observed when the laser current was adjusted [Fig.1]. By changing current it was possible to select the pumped mode of the resonator thus gradually changing the central frequency of the soliton by 10 nm. In several cases, we ob-served simultaneous excitation of two solitons with different central frequencies. In this case beat note spectrum demonstrated two narrow lines separated by ~ 10 MHz distance, corresponding to FSR differ-ence at central frequencies. The diode multimode spectrum (10 nm) was narrowed to single mode line with FWHM of only 5 kHz, comparable to the results achieved with self-injection-locked DFB lasers. The work was supported by the Russian Science Foundation project #17-12-01413