M. D. Ewbank

Photorefractive properties of strontium-barium niobate

We have grown and optically characterized strontium‐barium niobate crystals, including both undoped and cerium‐doped crystals having two different Sr/Ba ratios (61/39 and 75/25). By measuring the coupling of two optical beams in the crystals, we have determined the following photorefractive properties: the effective density, sign, and spectral response of the dominant charge carrier, the grating formation rate, dark conductivity, and carrier diffusion length. We find that electrons are the dominant photorefractive charge carriers in all of our samples; the typical density of photorefractive charges is ∼1×1016 cm−3 in the undoped samples. The grating formation rate increases with intensity, with a slope of ∼0.3 cm2/(W s) over an intensity range of ∼1–15 W/cm2 in undoped samples. Cerium doping improves both the charge density (increased by a factor of ∼3) and the response rate per unit intensity (∼5 times faster).

Mechanism for photorefractive phase conjugation using incoherent beams

Two mutually incoherent extraordinary laser beams, incident upon opposite a faces of a BaTiO(3) crystal, generate a set of photorefractive holograms that channel both beams toward the +c face of the crystal, where they are internally reflected into the opposite channel. The resultant cross coupling of the two incident beams produces a pair of phase-conjugate (Phi*) beams. This new photorefractive interaction is characterized by reasonable Phi* reflectivities (~10-25%), good Phi* fidelity, and no image cross talk. The variations in Phi* reflectivity with crystal orientation, angles of incidence, and intensity ratio are presented. The formation times of the photorefractive gratings responsible for the phase conjugation are significantly shorter than those of other self-pumped phase conjugators.

Photorefractive properties of SBN:60 systematically doped with rhodium

Strontium barium niobate (SBN) was doped with rhodium to enhance its photorefractive behavior. Crystals with six different Rh concentrations, ranging from 0.015 to 0.20 wt. %, were grown and characterized. With a higher Rh concentration the following trends were observed at a wavelength of 514.5 nm: (i) a larger linear absorption coefficient α, (ii) a larger maximum two-beam-coupling coefficient Γ, (iii) a higher net coupling coefficient (Γ − α), (iv) a longer two-beam-coupling time response, (v) a shorter fixed-level-gain time response, (vi) a constant photoionization cross section, and (vii) a smaller photorefractive sensitivity. With extraordinary polarization used to invoke the large r33 electro-optic coefficient, two-beam-coupling coefficients exceeding 60 cm−1 were measured in thin (∼1 mm) plates of the heavily Rh-doped crystals, which is consistent with the expected coupling by inference from measurements with ordinary polarization in thick (∼5 mm) crystals. Contradirectional two-beam coupling in SBN along the ĉ axis (independent of polarization, since r13 = r23) gave a coupling coefficient of almost 14 cm−1 for the 0.20-wt. % sample; the contradirectional coupling coefficient decreased approximately in proportion with the decreasing Rh-doping concentration in the other crystals. Estimated photorefractive charge densities ranged from ∼4 × 1016 to ∼8 × 1017 cm3, which constituted approximately 1% of the total Rh-doping concentration.

Frequency shift and cavity length in photorefractive resonators.

Photorefractive resonators exhibit an extremely small frequency difference (Deltaomega/omega~10(-15)) between the oscillating and pumping beams. The observed frequency difference is proportional to cavity-length detuning. This dependence is explained by a photorefractive phase shift that is due to slightly nondegenerate two-wave mixing that compensates for cavity detuning and satisfies the round-trip phase condition for steady-state oscillation. The measured onset or threshold of oscillation as a function of photorefractive gain and intensity agrees with theory.

Photorefractive conical diffraction in BaTiO3

Abstract A laser beam incident on BaTiO 3 can cause a cone of light to exit the crystal. If the incident beam is polarized as an extraordinary ray, the cone of light is formed by ordinary rays. The cone angle is fixed by a phase-matching condition for the incident and cone beams. Measurement of this cone angle as a function of the incident angle is a simple and sensitive method for determining the birefringence of a BaTiO 3 crystal over the entire range of wavelengths where the sample is photorefractive.

Time reversal by an interferometer with coupled phase-conjugate reflectors

A new type of interferometer has been constructed that uses a beam splitter and two self-pumped BaTiO(3) crystals as phase-conjugate reflectors in place of the usual interferometer mirrors. Counterpropagating beams of light are spontaneously generated between the two crystals, coupling the pair of phase conjugators. This optical oscillation locks the relative phase of the two phase-conjugate reflections such that they recombine at the beam splitter to form only a single beam as though they were truly time-reversed waves.

Photorefractive properties of doped strontium-barium niobate

Abstract Transition-metal dopants of chromium and rhodium altered the photorefractive properties of strontium-barium niobate. Beam- coupling experiments were performed to determine photorefractive parameters such as the effective charge density, the grating formation rate, and dark conductivity.

Salt-based approach for frequency conversion materials

A salt-based approach for the development of frequency conversion materials is presented. Salts are generally rugged materials due to their strong ionic bonding. A high degree of chemical substitution (both of anion and cation) is possible, including highly nonlinear organic ions. This flexibility is consistent with the use of empirical survey techniques to identify promising candidate materials. We describe in detail our experimental approach and present examples of nonlinear optical crystals we have identified in our materials surveys.

Possibility of relative position sensing by using double phase-conjugate resonators

Abstract We propose and analyze the possibility of using double phase-conjugate resonators as linear relative-position sensors. Such position sensors are linear analogs to ring laser gyros, but require optical phase conjugation for their operation. In principle,a position sensor as proposed allows measurement of the linear position of a platform relative to an inertial frame in which an optical standing wave has been established. The relative position sensing is based on the inertial nature of the electromagnetic radiation.

Efficiency of phase conjugation for highly scattered light

Abstract We present experiments demonstrating reciprocity for conjugation of highly scattered light; if a fraction η of the total scattered light is incident on a phase-conjugate mirror having reflectivity ϱ, then a fraction ϱ η2 returns back through the scatterer as a phase-conjugate wave, with the remaining fraction ϱ (η−η2) being lost due to scattering. Low efficiency results from inability to gather a large fraction of scattering, but the efficiency is higher than is possible without conjugation.