Stabilization of the coupled resonant cavity without dither

A simple scheme without dither to obtain an error signal for the stabilization of the coupled-cavity configuration for efficient fourth harmonic generation of continuous-wave lasers is proposed and demonstrated. This scheme is an analogue of the noise immune cavity enhanced optical heterodyne molecular spectroscopy (NICEOHMS) technique. In the proposed scheme, the robust stabilization is expected in comparison with the dither lock technique. In the demonstration, the stabilization of the coupled resonant cavity at the resonance is successfully achieved without the noise caused by the dither.