Abstract

We describe a phase-locking technique for combining two and three separate commercial laser diodes (Hitachi 1400) by using phase conjugate injections. The injections come from a double phase conjugation mirror (DPCM). The photorefractive crystal is a single crystal of BaTiO₃. Two phase-locking schemes were experimentally demonstrated: unidirectional injection and bidirectional mutual injection. Because the phase conjugate wave is backward going, this injection is more effective. Only a fraction of a microwatt injection from the DPCM is sufficient for phase locking. The visibility of the interference fringe between the two locked lasers is about 0.95. For the unidirectional phase conjugate injection, the curve of the dependence of locking bandwidth on the injection power was measured. The intermodal injection locking was studied. In this case the mode-coupling effect was included in the coupled wave equations. Our theoretical curve is in agreement with the experimental results. For the bidirectional phase conjugate mutual injection, the phase locking between two lasers is stable only when the injection level is less than −40 dB. When the injection level is larger than −30 dB, the output spectra of the lasers are broadened and the case is driven into multimode operation.

© 1992 Optical Society of America