Narrow-line-width diode laser with a high- Q microsphere resonator

A new modification of external optical feedback (OFB) was used to narrow the line of a diode laser (DL). A `whispering gallery' (WG) mode of a high- Q microsphere was excited by means of frustrated total internal reflection while the feedback for optical locking of the laser was provided by the intracavity Rayleigh backscattering. The ∼600 MHz beatnote of the two laser diodes optically locked to a pair of orthogonally polarized modes of the same microresonator had the indicated spectral width of 20 kHz, and the stability of 2×10 ^-6 over averaging time of 10 s. The feasibility of miniature sub-kHz-line width laser is discussed.