G. D. Bacher

Self-scanning of a continuous-wave dye laser having a phase-conjugating resonator cavity

A continuous-wave dye laser having a self-pumped phase conjugator in place of its usual output mirror will slowly change its own output wavelength with time. The laser has a bandwidth of 1.5 GHz and can self-scan to either longer or shorter wavelengths over a 37-nm range. The phase conjugator uses self-pumped four-wave mixing in a BaTiO(3) crystal. A ring laser that uses two-wave mixing in the same crystal is also observed to have a frequency offset of a few hertz compared with the frequency of the pumping beam. These two effects are related; both are caused by a spontaneously moving photorefractive-index grating in the BaTiO(3) crystal.

Measuring photorefractive trap density without the electro-optic effect.

Two optical beams can couple in a photorefractive crystal without using the electro-optic effect. Beam coupling is due to a spatially modulated absorption caused by the rearrangement of trapped charges. We use these gratings to determine the effective photorefractive trap density for several barium titanate crystals.

Absorption gratings in photorefractive crystals with multiple levels

Two coherent light beams can couple in any absorbing material owing to a light-induced modulation of the material’s dielectric constant. In photorefractive crystals the coupling caused by these absorption gratings appears in addition to any electro-optic coupling, complicating the interpretation of data. However, in contrast with the electro-optic gratings formed by charge diffusion, absorption gratings do not necessarily vanish as the beam-crossing angle approaches zero if there is more than one absorbing level. We show that a plot of the coupling strength of the absorption gratings versus the beam-crossing angle is characterized by only three parameters, independent of the number of absorbing levels. We use absorption gratings with a two-level model to determine experimentally some important crystal parameters, including the relative density of donors and acceptors in a barium titanate crystal. Our values agree with those obtained from measurements of the bulk light-induced absorption of the crystal.

Measurement of the photorefractive phase shift.

We present a method to separate the effects of trap gratings and electro-optic gratings in BaTiO(3) crystals, and we determine the true spatial shift between the electro-optic grating and the optical intensity pattern. At small beam-crossing angles this spatial shift is strongly affected by a photogalvanic current in the crystal.

Powerful, diffraction-limited semiconductor laser using photorefractive beam coupling

We use semiconductor laser amplifiers and a photorefractive crystal to generate a high-power, diffraction-limited laser beam at 860 nm. Using a single flared amplifier, we obtain 1.09 W in a diffraction-limited beam from 2.2 W of pump power. Using an array of flared amplifiers, we also demonstrate efficient beam coupling, showing that this technique is easily extended to semiconductor amplifier arrays.

Self-bending of light beams in photorefractive phase conjugators

Stimulated photorefractive phase conjugators often exhibit well-defined curved beam paths that cannot be explained by simple beam fanning. We propose a model of these apparently curved paths as a series of straight-line segments, with beams propagating in both directions along these segments. These line segments spring from the amplification of scattered light between regions of the crystal already containing counterpropagating pump beams. As these line segments form, they create new interaction regions that generate new segments, thereby making the final beam path appear to be curved. Application of our model to a single-interaction-region mutually pumped phase conjugator shows that the threshold coupling strength required for the appearance of these new segments is only slightly higher than the threshold for the phase conjugator itself.

Photorefractive properties of rhodium doped barium titanate

Summary form only given. We have measured the effective trap density, the number of active trap levels, the sign of the dominant charge carrier, and the photorefractive two-beam coupling gain of four rhodium-doped crystals of BaTiO/sub 3/, all grown at Hughes Research Laboratories.<<ETX>>