A variant of the Hänsch–Couillaud technique for locking the pump laser frequency to a cavity-enhanced optical frequency doubler is presented. In contrast to the original scheme in which the pump laser light reflected by the enhancement cavity is monitored, the error signal for locking is obtained by polarization analysis of the light transmitted by the cavity. The cavity serves as a spectral and spatial filter for the pump beam, making the transmission-mode locking method particularly useful if the beam quality is poor. Experimental validation of the method is demonstrated by generating more than 190 mW of stable 461 nm light from a cavity-enhanced frequency doubler that is pumped by a diode laser system.
Vainio, M., Bernard, J. E., & Marmet, L. (2011). Cavity-enhanced optical frequency doubler based on transmission-mode Hänsch-Couillaud locking. Applied Physics B: Lasers and Optics, 104(4), 897-908. https://doi.org/10.1007/s00340-011-4437-z