Photorefractive beam clean-up using mutually pumped phase conjugators or ring resonators, and the gain spectrum of a semiconductor optical amplifier

By creating volume holograms, photorefractive barium titanate crystals can couple the energies of two light beams without crosstalk. These crystals can also produce the phase-conjugate replica of light beams. We use these two effects to determine the gain spectrum in high-power semiconductor optical amplifiers and to improve the beam quality of high-power semiconductor lasers and amplifiers.;Using mutually pumped phase conjugation, light from a single-mode laser is injected into an array of four lasers on a single semiconductor chip. The injection causes the four lasers to switch from multi-mode to single-mode operation and causes all the lasers to emit coherently with each other. We show for the first time that a mutually pumped phase conjugator can simultaneously conjugate more than three beams.;We choose the mutually pumped phase conjugator geometry based on our thorough investigation of how phase conjugators respond to changes in the mutual coherence between their incident light beams. We find that backscattering gratings form with mutually coherent beams, and that these gratings enhance the performance of some types of conjugators (bird-wing, British, bridge) while ruining the others (double phase-conjugate mirror).;A photorefractive crystal placed in a ring resonator can transform a spatially aberrated light beam from a high-power optical semiconductor amplifier into a Gaussian (TEM;Finally, we measure the calibrated small-signal gain spectrum, over a range of eight orders of magnitude, of a single-mode, flared semiconductor amplifier. By measuring the amplified spontaneous emission spectrum and using a simple theory, we deduce the spontaneous emission spectrum. We find that the detailed-balance theory of semiconductor lasers is not consistent with our data. A self-pumped phase conjugator helps determine a key calibration constant in our experiments, namely the coupling efficiency of the amplifier.