A significant narrowing of the longitudinal-mode spectrum of continuous-wave visible gain-guided diode lasers was achieved by using an external cavity bounded by a phase conjugate mirror. Longitudinal-mode discrimination was further improved by inserting a thin étalon in the external cavity. With this technique, the coherence length of the laser beam has been increased by a factor of 4 and the emission wavelength was tunable over a range of 1.4 nm, at constant temperature.
Amplifying continuous wave phase conjugate mirror with strontium barium niobate