Contrasting conventional optical and phase-conjugate feedback in laser diodes

An experimental comparison of phase conjugate feedback to conventional optical feedback in a semiconductor laser is presented, which contrasts the optical frequency spectra and power noise spectra with varying levels of feedback. These spectra are correlated with the single frequency regimes III and V and the chaotically unstable regime IV. Conventional feedback is derived from a mirror and variable neutral density filter, and the phase conjugate feedback is derived from a self-pumped rhodium doped barium titanate photorefractive crystal. Both systems show two stable, single frequency operation regions analogous to regime III and V operation separated by a single region of unstable operation analogous to regime IV. It is found that phase conjugate feedback leads to distinctive behaviors including: differences in the relative intensity noise spectra; dynamically varying output frequency spectra close to the transition from regime IV to V; systematic power transfer from one laser diode longitudinal mode to a nearest neighbor through regime IV; and a very much larger range of feedback levels leading to unstable output. The level of optical feedback leading to a transition from regime III to IV is the same for conventional optical feedback and phase conjugate feedback when the correction for the different coupling efficiencies is made. The transition from regime IV to V occurs for much higher levels of feedback when PCF is used leading to the larger range of feedback levels giving chaotic behavior with PCF. The results are discussed in the context of existing theoretical models of laser diodes with phase conjugate feedback.