Phase delay in phase-conjugate external cavity lasers

The boundary conditions of phase-conjugate external cavity lasers are considered. It is shown that the light returning to the laser from the external cavities has a specified phase, which is determined by the boundary conditions of the particular cavity. This phase always acts to reduce the threshold gain for the laser with feedback. This is true regardless of the external phase delay. Experimental verification using semiconductor lasers is provided and the analysis is extended to the coupling of two lasers via photorefractive coupling.