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We study improvements of the Self-Injection Locked (SIL) distributed feedback (DFB) semiconductor laser phase noise as well as integral linewidth on the amount of the SIL feedback and show that SIL reduces the laser noise nearly equally in a broad frequency range, spanning from 1 Hz to 10 MHz. In our experiment SIL operation is supported by a tunable resonant reflector consisting of a high quality factor (high-Q) monolithic whispering gallery mode resonator (WGMR) supplied with a controllable attenuator. Using the experimental setup we validated theoretical formula describing the SIL spectral purity and identified practical parameters of the system allowing reaching the phase noise limit resulting from the fundamental thermodynamic noise of the resonator. We show that the linewidth improvement due to SIL effect is drastically impacted by the frequency spectral dependence of the phase noise of the laser and that it drops significantly in a flickering laser. We also confirmed that the thermorefractive coefficient as well as mode volume of the WGMR imposes the major limitations on the SIL performance. Low intracavity absorption is needed to suppress the attenuation-related thermal effects in the cavity. [ABSTRACT FROM AUTHOR]

We have designed and fabricated a Kerr frequency comb oscillator with 27.5 GHz repetition rate by pumping a whispering gallery mode crystalline resonator with a laser at 1560 nm center wavelength. The oscillator is stabilized to the Rb^87 D $_2$ line using frequency doubling with a periodically poled lithium niobate (PPLN) waveguide. Allan deviation of the repetition rate of the frequency comb was measured to be on the order of 10^{-11} at 1 s to 1000 s integration time. The long-term stability limitation is attributed to the frequency drift of the phase matching window of the PPLN structure as well as to mode interaction and associated drift of the resonator spectrum. [ABSTRACT FROM PUBLISHER]