Modulation-free frequency stabilization of a laser based on a confocal Fabry-Perot cavity

Locking laser to a proper reference frequency can obviously improve the laser s frequency stability. The optical path and frequencies are different for axial and paraxial beams traveling through a confocal Fabry-Perot (CFP) cavity, thus the frequency difference between them which is a dispersion-like signal can be used to stabilize the laser frequency. Thus the phase-sensitive detection via lock-in and frequency dither on the laser or cavity is no longer required. With this modulation-free stabilization method, a homemade 852 nm grating-external-cavity diode laser is stabilized to a CFP cavity frequency, whose frequency jitter is less than ±340 kHz in 30 seconds estimated from locked error signal. The long-term frequency stability was greatly improved compared with the frequency fluctuation of about 10 MHz under free-running condition in the same time scale. Moreover, the frequency shift and the zero-crossing slope of the dispersion-like frequency-discriminating signal can be conveniently changed by adjusting the angle between the two incident beams. The laser frequency stability locking with different frequency interval is investigated.