George C. Valley

Photorefractive screening solitons of high and low intensity

We analyze self-trapping of optical beams in photorefractive media for low irradiances that are typical of cw lasers and high irradiances that are typical of short-pulse lasers and obtain bright and dark soliton solutions in one transverse dimension. Whereas in the low-intensity regime photorefractive screening solitons are trapped by the refractive-index change that is related to the electric field associated with ionized donors or acceptors, in the high-intensity regime the index is related to the field that is also associated with free carriers. Since the time necessary to form the soliton scales inversely with the intensity, one expects a very fast response time in the high-intensity regime, and this suggests attractive applications in optical switching and reconfigurable optical waveguides. We show that throughout the entire intensity range dark solitons require lower voltages and less energy per pulse for applications than do bright solitons.

Simultaneous electron/hole transport in photorefractive materials

The steady‐state space‐charge field and the response time of a photorefractive material illuminated with a sinusoidal interference pattern are derived for two models in which both electron and hole transport are important. In the first model in which electrons and holes are produced from a single set of recombination centers, the sign of the steady‐state space‐charge field depends on the relative value of the conductivities and/or absorption coefficients of the electrons and holes, and a single response time proportional to irradiance is obtained. In the second model in which electrons are photoionized from one set of recombination centers and holes from another, the sign of the space‐charge field depends on the relative concentrations of the empty hole and electron traps, and two time constants, each inversely proportional to irradiance, are obtained.

Photovoltaic spatial solitons

We analyze self-trapping of one-dimensional optical beams in photorefractive, photovoltaic media for open- and closed-circuit realizations. We show that a passive load (resistor) in the external circuit can be used for switching of dark photovoltaic solitons. Dark solitons in a short-circuited crystal can be obtained for a much smaller nonlinearity than in open-circuit conditions. Shorting the crystal affects bright solitons very little.

Two-dimensional steady-state photorefractive screening solitons

We study experimentally steady-state photorefractive screening solitons trapped in both transverse dimensionsand measure their beam profiles as they propagate throughout the crystal. The solitons are observed to be axially symmetric, and they self-bend. We characterize the soliton dependence on the optical intensity, appliedelectric-field strength, and beam diameter.

Beam coupling in BaTiO3 at 442 nm

Steady‐state beam coupling is used to measure the concentration of empty traps, the sign of the dominant photocarrier, and the effective electro‐optic coefficient in seven crystals of BaTiO3. The measured value of the effective electro‐optic coefficient gives the product of the fractional poling of the crystal, the relative conductivity of the crystal, and the electro‐optic coefficient. Anomalously low beam coupling gain measured in an eighth crystal suggests that this crystal is nearly perfectly compensated; i.e., the photoconductivities due to electrons and holes are nearly equal.

Two-wave mixing with an applied field and a moving grating

The theory for two-wave mixing in a photorefractive material with an external electric field and a moving grating is developed. Large gain enhancements are predicted for grating periods of 10–100 μm and compared with recent observations in Bi12SiO20. Predictions are made for enhancements in GaAs, which has recently been found to be photorefractive in the near infrared.

Picosecond photorefractive and free-carrier transient energy transfer in GaAs at 1 mu m

The strength, formation, and decay of photorefractive and free-carrier gratings written in GaAs by 43-ps pulses at a wavelength of 1 mu m are investigated using picosecond-time-resolved two-beam coupling, transient grating, and degenerate-four-wave-mixing techniques. Photorefractive weak-beam gains of a few percent are measured at fluences of a few pJ/ mu m/sup 2/ (0.1 mJ/cm/sup 2/), and gain from transient energy transfer is observed at fluences larger than approximately 10 mJ/cm/sup 2/ in the beam-coupling experiments. The roles of saturation and two-photon absorption in determining the final electron, hole, and ionized-donor populations and the roles of drift and diffusion in determining the quasi-steady-state photorefractive and free-carrier index modulations are discussed. >

STEADY-STATE DARK PHOTOREFRACTIVE SCREENING SOLITONS

We present an experimental study of steady-state dark photorefractive screening solitons trapped in a bulk strontium barium niobate crystal. We compare experimental measurements with theoretical calculations of the soliton properties and find good agreement between theory and experiments. We confirm the shapepreserving behavior of the dark soliton by measuring its beam profile as it propagates throughout a specially cut crystal and by guiding a beam of a different wavelength.

Gamma and proton radiation effects in erbium-doped fiber amplifiers: active and passive measurements

Commercially available Er-doped fibers were irradiated with 5.6 and 28 MeV protons and /sup 60/Co gamma rays, up to levels of 50 krad. White-light transmission spectra under passive conditions (no pump or signal) were measured at several radiation levels for the six types of fibers that were tested. The spectra were used to evaluate the relative radiation sensitivity of the fibers and compare gamma versus proton-induced damage for two fiber types. The amount of radiation damage for the fibers was observed to scale inversely with the Ge concentration. Samples from three of the fiber types were configured as optical amplifiers using 980-nm and 1550-nm pump and input signals. In situ measurements of the gain, noise figure, and amplified spontaneous emission (ASE) were made as a function of pump power at several levels of radiation. A computer code, based on a conventional Er-doped fiber amplifier (EDFA) model, was written to simulate performance, using input data provided by the fiber vendor and anchored to measurements made prior to radiation. A comparison between the simulations and experimental data shows that, in certain fibers where the damage is significant, the radiation-induced loss determined from amplifier measurements can be substantially less than that determined from passive transmission spectra.

Picosecond photorefractive beam coupling in GaAs

We report the first observation to our knowledge of the photorefractive effect on picosecond time scales. Photorefractive beam coupling in GaAs with picosecond, 1.06-microm pulses is observed owing to charge separation between electrons and the ionized defect EL2(+) at low fluences and to separation between free electrons and holes created by two-photon interband absorption for high fluences. The accompanying processes of linear absorption, two-photon absorption, and transient energy transfer are also observed.