Narrowing of the linewidth of a diode laser was achieved by an external high-Q microcavity, which was a fused silica sphere 370 μm in diameter. Coupling of the radiation in and out, and matching to a 'whispering gallery' mode of this sphere, were ensured by the method of frustrated total internal reflection. Optical feedback was provided by the Rayleigh scattering. The width of the beat-note spectra of the investigated and reference lasers did not exceed 100 kHz.
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Vasilev et al. (1996) studied this question.
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