Envelope narrowing from photorefractive phase conjugate feedback to a semiconductor laser

The effects of envelope narrowing were confirmed experimentally. In our experiments a Hitachi HLP1400 laser diode is coupled to a ring PPCM that is constrained to be unidirectional. The photorefractive material used is barium titanate. The laser spectrum was compared to that of a laser diode coupled to an external cavity with a dielectric mirror, i.e., we compared the effects of phase conjugate feedback to conventional feedback. In both cases the laser is multimode when coupled to the external cavity. The envelope for the phase conjugate feedback is narrower than that for the conventional case. It can also be seen that in the conventional case the gains for the different longitudinal modes vary with the phase delay as each mode experiences a different round trip phase delay. This is not the case for the phase conjugate cavity because all of the modes experience the same round trip phase delay