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Optics Letters

Effects of laser coherence on coupling efficiency for the double phase-conjugate mirror

Mark T. Gruneisen, Edward D. Seeberger, John F. Mileski, Karl W. Koch

Published Apr 15, 1991• 20 citations

Abstract

We present the results of an experimental study of the effects of laser coherence on the operation of the double phase-conjugate mirror using barium titanate as the nonlinear medium. In our experiment the two input beams that drive the interaction originate from a single helium-neon laser, and the degree of coherence between the input beams at the crystal is varied through the use of a time-dependent phase delay in one of the beam paths. As the two input beams make the transition from being mutually incoherent to mutually coherent, the phase-conjugate reflectivity decreases dramatically and becomes unstable. These results are significant to laser phase-locking applications where changes in the coupling intensity can affect the quality of phase locking.

DOI: 10.1364/ol.16.000596

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We present the results of an experimental study of the effects of laser coherence on the operation of the double phase-conjugate mirror using barium titanate as the nonlinear medium. In our experiment the two input beams that drive the interaction originate from a single helium-neon laser, and the degree of coherence between the input beams at the crystal is varied through the use of a time-dependent phase delay in one of the beam paths. As the two input beams make the transition from being mutually incoherent to mutually coherent, the phase-conjugate reflectivity decreases dramatically and becomes unstable. These results are significant to laser phase-locking applications where changes in the coupling intensity can affect the quality of phase locking.

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Photorefractive self-pumped phase conjugation is attractive for its simplicity and high reflectivity with low-power light sources. In order to realize its potential applications, it is important to study its response. So far the response of a phase-conjugate wave has been described with its rise time. It has not studied how the phase-conjugate wave responds when the spatial structure of an input probe wave is changed dynamically. We investigated the response of the phase conjugate wave by using two mutually coherent input beams, which can be regarded as components of a probe wave, and by introducing sinusoidal phase changes into one of them with varied frequencies. Each input beam has a power of 6 mW at the wavelength of 515 nm. A BaTiO3 crystal was employed as a self-pumped phase-conjugate mirror.1 By measuring relative phase between the two output beams, which can be regarded as components of a phase-conjugate wave, we found the response characteristic represented by a time-lag of first order, which is similar to RC circuit. The response time thus determined was 0.32 s, which was much shorter than the rise time of 30 s obtained with the same condition.

Mutually pumped phase conjugators with picosecond pulse trains are experimentally demonstrated in rhodium-doped barium titanate. Stable phase-conjugate reflectivities are obtained when the counterpropagating pulses do not meet in the crystal. In contrast, when the pulses do meet in the crystal the reflectivities become unstable and unequal in value in two directions. This behavior is caused by the interference between two counterpropagating pulses. In addition, the reflectivities for the picosecond pulses are found to be half of those for continuous waves. The reasons for the small reflectivities of the pulses are discussed.