Abstract

Kerr optical frequency combs in high-Q microresonators [1] are attracting growing interest [23], especially after mode-locking via dissipative Kerr solitons (DKS) has been demonstrated on a variety of platforms [3,4]. Such combs are a promising source for compact applications due to its potential advantages of low power consumption and possibility of chip integration. A traditional approach to obtaining DKS in microresonators relies on narrow-linewidth tunable lasers for pumping. Independently the same type microresonators could be used for significant line narrowing of diode lasers exploiting resonant Rayleigh backscattering [5] for self-injection locking [6]. Kerr soliton frequency combs have also been demonstrated with self-injection locked diode lasers [7]. Previously for self-injection locking only single frequency stabilized diode lasers were used with either Bragg-grating [6] or distributed feedback configuration [7], having narrow linewidth comparable to the resonance 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 computer 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 observed 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 difference 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.

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