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 b
oundary conditions of the particular cavity. This phase always acts to
reduce the threshold pin for the laser with feedback. This is true re
gardless of the external phase delay. Experimental verification using
semiconductor lasers is provided and the analysis is extended to the c
oupling of two lasers via photorefractive coupling.