Raman-Nath Regime Research Articles

Experimental testing of truncated Raman-Nath system solutions

After a historical survey, the physical aspects of acousto-optics and selected topics of application are described. The mechanisms of the interaction of light and ultrasonics in diffraction, scattering, formation of Schlieren images, and the intensity distribution of diffraction spectra in Raman-Nath and Bragg regimes are discussed. The fundamentals of applying optical interferometric holography for the examination of ultrasonic fields and transducer vibrations are described.Finally, ultrasonic light diffraction phenomena for single and for two ultrasonic beams are considered from the point of view of their dependence on characteristic parameters.All phenomena and applications are illustrated to emphasize the wide range of possibilities of acousto-optics in research and application.

Some results on wavefield backpropagation

After a historical survey, the physical aspects of acousto-optics and selected topics of application are described. The mechanisms of the interaction of light and ultrasonics in diffraction, scattering, formation of Schlieren images, and the intensity distribution of diffraction spectra in Raman-Nath and Bragg regimes are discussed. The fundamentals of applying optical interferometric holography for the examination of ultrasonic fields and transducer vibrations are described.Finally, ultrasonic light diffraction phenomena for single and for two ultrasonic beams are considered from the point of view of their dependence on characteristic parameters.All phenomena and applications are illustrated to emphasize the wide range of possibilities of acousto-optics in research and application.

Diffraction regimes of phase gratings

The occurrence of Raman-Nath regime and Bragg regime diffraction by planar phase gratings is rigorously analyzed. It is shown that the published indicators for a Raman-Nath regime are unsatisfactory. An attempt to redefine the Raman-Nath region is given in this paper.

Surface acoustic wave acousto-electro-optic effect

A new device which combines both acousto-optic and electro-optic effects in one integrated manner using surface acoustic wave technology has been implemented and offers unique advantages. In particular, this permits an additional input port for information which leads to compact implementation and simplified architecture in optical signal processing applications. The objective of this letter is to present some theoretical and experimental results in the Raman–Nath regime. These are found to agree over the range of investigation.

Imaging methods for the observation of plasma density fluctuations

Imaging techniques offer attractive alternatives to small-angle Thomson scattering for scale lengths of density fluctuations causing diffraction in the Raman-Nath regime. This is the case for fluctuations with wavelengths above about 3 mm for Tokamak sized plasmas, when a CO2 laser probe beam is used. Four methods ('phase scintillation', phase contrast, wavefront shearing and interferometry) are compared, on the basis of their transfer properties. They offer new means of studying density fluctuations of magnitude up to the dimensions of the plasma, such as those associated with turbulence, magnetic islands, convective cells or driven waves. The long wavelength limitations are discussed in detail and are related to those encountered for far-field techniques. An experimental comparison of 'phase scintillation' and phase contrast is given, and the criteria relative to the depth of field of imaging instruments in this context are considered.

Analysis of Acoustooptic Diffraction by Adjacent Ultrasonic Beams Using Multiple Plane-Wave Scattering Techniques

Abstmct-Plane-wave multiple-scattering theory is used to analyze scattering in the Raman-Nath regime for adjacent ultrasonic beams of frequency ratio 1:m. The variation of the scattered intensities is obtained numerically as a function of the phase difference between the beams for the case m = 2. Results are compared with the work of earlier investigators.

Criteria for Raman-Nath regime diffraction by phase gratings

Three alternative criteria for the occurence of Raman-Nath regime diffraction by planar phase gratings are presented and evaluated.

Holographic evaluation of resolution in amorphous chalcogenide inorganic photoresists

Amorphous chalcogenide films are evaluated holographically as inorganic resists. In particular, the resolution of As(2)S(3) films is determined experimentally. The exposure characteristics of As(2)S(3) films for holographic storage and lithographic pattern generation are studied. An empirical model is discussed. Light diffraction for in situ monitoring of process conditions is employed, covering the Raman-Nath and Bragg regimes. The modulation transfer function of As(2)S(3) inorganic photoresists is presented. A minimum feature dimension of 100 nm delineated in As(2)S(3) is obtained.

Ultrasonic imaging by acousto-optic interaction in the Raman-Nath regime

The behaviour of sound imaging systems by diffraction of light is analysed for the Raman-Nath diffraction region. The study is based on a geometrical interpretation of the phenomenon. Imaging relationships are established for plane objects and extended to the 3-D case.

Criterion for Bragg and Raman-Nath diffraction regimes

The idea is well entrenched in the literature that thin phase gratings (whether holographic or acoustically induced) should exhibit Raman-Nath behavior (and thus give several diffracted waves), and that thick phase gratings should show Bragg behavior (one diffracted beam and that only for Bragg angle incidence). The parameter Q of Klein and Cook, which is a normalized measure of grating thickness, has been extensively used as a criterion for deciding which regime will apply. It is perhaps not generally realized that Q is not a reliable parameter for this purpose but requires, as indeed Klein and Cook noted, a limitation on grating strength. This limitation is a matter of practical concern. For example, we have observed Raman-Nath behavior with Fe-doped LiNbO(3) even for very large values of Q. The purpose of the present paper is to note that a parameter rho (first defined by Nath) is an effective replacement for Q, since rho is reliable and Q is not. rho is defined as lambda(0)(2)/Lambda(2)n(0)n(1), where lambda(0) is the vacuum wavelength of the light, Lambda is the grating spacing, n(0) is the mean refractive index, and n(1) is the amplitude of the sinusoidal modulation of the refractive index. The grating thickness does not enter rho, so the terms thin and thick are, strictly speaking, irrelevant to the question of which regime is operative. However, thin enough gratings will tend to operate in the Raman-Nath regime because the index modulation must be large for a thin grating to produce appreciable diffraction.

Optical diffraction studies of sound waves in lithium niobate

The acousto-optic interaction in the Raman-Nath regime was used to study sound waves propagating in the principal planes of piezoelectric lithium niobate. Complete cuts of the inverse phase velocity surfaces were obtained for surface waves on reflection and for three bulk waves and one surface wave on transmission through the surface acoustic wave (SAW) propagation surface. It is also shown how simple diffraction can be used to measure the group velocity direction and the electro-mechanical coupling coefficient for surface waves. A modified schlieren technique was used to visualize bulk waves generated by a finite transducer and to observe shear waves reflecting and mode converting to longitudinal waves at an interface.

Simple Acoustic Grating Modulators

Simple low-cost modulators based on the Raman-Nath regime of acoustooptic diffraction have been demonstrated. Experimental results on internal laser modulators constructed from some dense flint glasses and fused quartz are presented. 100% modulation of He-Ne lasers with rise times on the order of 1 musec are obtained with 10-MHz modulator drive powers of 100 mW. The theory necessary for the design of this class of modulators is reviewed and compared with the experiments.

Optical processing with twin ultrasonic light modulators

A collinear optical heterodyne system with twin ultrasonic light modulators (ULM) is considered within the Raman-Nath regime. The zeroth order beam of light emerging from the first ULM enters the second ULM and splits into the diffraction spectrum with the frequencies shifted by integral multiples of the driving sound frequency. For the purpose of photomixing, the positive or negative first order beam in this spectrum can be superposed on the first order beam of light from the first ULM which passes through the second ULM without deflection, and comes to the same location as the other. Experimental verification is successfully carried out by producing a beat signal of light with a difference frequency between two sound frequencies. It is demonstrated analytically that a convolution or correlation function between two time varying signals can be obtained, according to whether the beat signal of light has the sum or difference frequency of the two driving sound frequencies. The sum or difference frequency can be generated as one of the sound waves proceeds in the same or reverse direction as the other.