Robert McGraw

Studies of the dynamic range of photorefractive gratings in ferroelectric crystals

The dynamic range, defined as the range of intensity ratios for which two writing beams can induce a detectable photorefractive grating, is measured for ferroelectric crystals BaTiO3, KNbO3, SrxBa1−xNbO6, BaxSr2−xK1−yNayNb5O15. Our results show a large dynamic range in all these crystals, 70–100 dB, which should prove these crystals useful for signal processing and computing applications. The observed limit on the dynamic range was due predominantly to the detector noise and therefore was not the fundamental limit imposed by the underlying physics of photorefractive noise. The calculations of noise based on limited photon density flux (shot noise) and thermally induced grating fluctuations (thermal noise) show the dynamic range to be 4–5 orders of magnitude higher than that observed experimentally.

Light-scattering-noise limits to two-wave-mixing gain in Kerr media

The coupled-mode equations for two-wave mixing are extended to include light-scattering noise and are solved using the stochastic noise model. Solutions are given for amplitude, phase, and output-power fluctuations during weak-signal amplification. Minimum power requirements for low-noise signal amplification in Kerr media are determined.

The thermodynamic barrier to nucleation near a critical point

The revised capillarity approximation (RCPA) of McGraw and Reiss [J. Stat. Phys. 20, 385 (1979)] is reexamined using established liquid drop models to evaluate the effects of physical cluster excluded volume on the thermodynamic barrier to nucleation near a critical temperature Tc. A significant heightening of the nucleation barrier is predicted, and the anomalous behavior observed experimentally near Tc is examined within the framework of classical nucleation theory using the RCPA. Definitive methods for experimentally separating this pure nucleation effect from those dependent on slowing down of the post‐nucleation completion times for phase separation are described.

Microwave phase conjugation via degenerate four-wave mixing in an artificial Kerr medium

Collinear microwave phase conjugation was observed in an artificial Kerr medium consisting of short graphite fibers suspended in a binary liquid mixture. Using an 18 GHz pump beam with up to 20 W continuous power, characterization of the changes in the 94 GHz refractive index were made by interferometry. A nonperturbative method for describing the response of the medium was used to analyze the phase-shift measurements for the static berefringence and the time response as functions of microwave intensity.

Microwave Phase Conjugation In Artificial Kerr Media

One dimensional phase conjugation of microwaves via degenerate four-wave mixing in an artificial Kerr medium has been demonstrated. The medium consisting of a liquid suspension of elongated graphite particles is confined within a single mode waveguide. The decay of the nonlinear refractive index grating is shown to agree with previous relaxation time measurements.

Light-scattering noise limits to optical signal processing in photorefractive media

This paper investigates the effects of thermal light-scattering fluctuations and demonstrates that these are the dominant noise source inherent in photorefractive and electro-optic media. Fundamental noise limits to dynamic range and channel capacity are determined. Two sources of light-scattering fluctuations are examined: (1) thermal fluctuations in the space-charge field, which induce corresponding fluctuations in the dielectric constant through the electro-optic effect, and (2) fluctuations associated with the optical Kerr effect. Calculations are present for BaTiO3 and several other materials and are discussed in light of recent experimental measurements of dynamic range. Our results suggest a very large dynamic range for photorefractive materials (120 - 140 dB) that should prove useful for optical signal processing applications.

Four-Wave Mixing In Liquid Suspensions Of Microparticles

Saturation characteristics and response times are examined for a liquid suspension of Brownian microparticles subjected to electromagnetic fields in a four-wave mixing configuration. Calculations are presented for the evolution of particle density in the field of two degenerate plane waves of arbitrary intensity and angle of incidence. The analysis is then extended to include the additional presence of weak probe and conjugate-wave fields. Nonlinear field particle interactions are shown to give rise to the formation of higher order particle gratings and to novel pump grating modulation effects which greatly enhance the phase-conjugate characteristics of the microparticle suspension.

Noise in nonlinear optics

This paper examines thermal (light-scattering) fluctuations as a dominant noise source in nonlinear optical processes. Fundamental limits to conjugate wave fidelity and signal power requirement are obtained through the statistical thermodynamic treatment of light-scattering noise in four-wave mixing based on the fluctuation-dissipation theorem. Several types of nonlinear media are examined including artificial Kerr suspensions, isotropic Kerr media, and fluids near a critical point. Where measurements are available, excellent quantitative agreement is obtained between theory and experiment.