Spectrally pure semiconductor lasers produced via self-injection locking to high quality factor monolithic optical resonators demonstrate sub-kHz instantaneous linewidth. The lasers are used in photonic sensor systems and microwave photonic oscillators benefitting from the improved spectral purity, the stability and the reduced environmental sensitivity of the lasers. The laser frequency stability is defined by both the optical resonator and the optical path of the entire system comprising the laser, the resonator, and the miscellaneous optical components. The impacts of the various destabilization factors are usually convoluted, and it is hardly possible to separate them. In this paper, we report on an experimental study of an influence of the variations of the optical path on the laser frequency stability. We have created a whispering gallery mode optical resonator having the record small thermal sensitivity, on the order of 0.1 ppm/
C, and demonstrated a self-injection locked laser based on this resonator. The measured laser stability is characterized with 1 s Allan deviation of
, limited by the thermal sensitivity of the optical path between the laser and the resonator. The thermal stabilization on the order of 10
K at 1 s is achieved using a standard thermo-electric element. The long term drift of the laser frequency is determined by both the fluctuations of the atmospheric pressure in the laboratory impacting the monolithic resonator and by the optical path instability.
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