SINGLE-MODE OPERATION OF A LASER-DIODE ARRAY WITH FREQUENCY-SELECTIVEPHASE-CONJUGATE FEEDBACK

A new technique for single-mode operation of laser-diode arrays is pre sented. A gain-guided GaAlAs laser diode array is coupled to an extern al frequency-selective phase-conjugate feedback system that contains a photorefractive barium titanate crystal, a Fabry-Perot etalon, and a spatial filter. The etalon is the key component, and it forces the arr ay, which has low spatial and spectral coherence when it runs freely, to oscillate in a single spatial and a single longitudinal mode. At a drive current of two times the threshold current, the far-field patter n is reduced to only 1.4 times the diffraction limit, the spectral ban dwidth is less than 0.02 nm, and the coherence length is increased by a factor of 70. The technique has general validity and can be applied to various other multimode laser systems. (C) 1998 Optical Society of America.