Raman-Nath Diffraction Research Articles

Measurement of Leaky Lamb Wave Velocity Using Raman-Nath Diffraction

A method of measuring the phase velocity of a leaky Lamb wave using Raman-Nath diffraction is investigated. Each mode of the wave with the different phase velocity in a substrate is radiated at each radiation angle into a load medium that is in contact with the substrate. An incident optical wave into the load medium can be diffracted by the radiated acoustic wave, and the diffraction image is formed in parallel to a wave vector of the radiated acoustic wave, reflecting the phase velocity of the leaky Lamb wave. By observing and processing the diffraction image stored in the form of digital matrix data, the phase velocity can be estimated. This method can be used to observe the dispersion of the phase velocity, the separation of plural modes excited at the same frequency, and the generation of the third harmonics of the wave number. This technique has the merit of measuring the leaky Lamb wave phase velocity separately, simply and precisely.

Raman-nath diffraction by microparticles in water

In Raman-Nath diffraction by water, the intensities of even order light beams are selectively enhanced by suspension of microparticles. The diffracted beams remain for several seconds even after the ultrasonic oscillation stops. These phenomena are elucidated by assuming the laminar arrangement of micropaticles due to the ultrasonic wave.

Raman-Nath diffraction by microparticles in water

Raman-Nath diffraction by microparticles in water

Raman-nath light diffraction by ultrasound in gyrotropic cubic crystals in an external electric field

UDC 535.42:534.8 Investigation of light diffraction by ultrasonic (US) waves in gyrotropic cubic crystals in an external electric field is of scientific and practical interest [1 I, as a number of crystals with a structure of sillenite (Bi12GeO20, Bil2SiO20, Bil2TiO20, etc.) possess the high specific rotation of a light polarization plane and at the same time a substantial electrooptical effect. While in uniaxial and biaxial crystals the electric field-induced anisotropy is manifested only for light propagations close to the optical axes, in a cubic crystal the anisotropy must be taken into account for any geometry of acoustooptical (AO) interaction [2 ]. It is interesting to investigate the simultaneous influence of gyrotropy and electroinduced anisotropy on AO diffraction in cubic crystals. The AO Raman-Nath diffraction in gyrotropic crystals has been considered in an approximation of a given polarization [3, 4 I. For calculation of the complex vector amplitudes of diffracted waves, they were decomposed into circularly polarized components. In [5 ], a system of equations of coupled waves is given that describes specific features of the Raman-Nath AO diffraction in gyrotropic cubic crystals with the strong light-ultrasound interaction without allowance for the influence of electroinduced anisotropy of the crystal in an external electric field. In [6, 7 ], consideration is given of the influence of an external electric field on the Bragg light diffraction by ultrasound in gyrotropic crystals. It is shown that the combined influence of gyrotropy and acousto- and electroinduced anisotropy causes a substantial change in the polarization and energy characteristics of diffracted waves. Some peculiar features of the Bragg AO diffraction by confined light beams in gyrotropic cubic crystals are discussed in [8 ]. A promising use of biaxial gyrotropic crystals for creation of AO filters is shown in [9 I. In the present work we investigate the Raman-Nath AO diffraction in gyrotropic cubic crystals in an external electric field by solving numerically a system of reduced equations obtained by the method of slowly changing amplitudes. From the Maxwell and material equations 12, 10 ] of a gyrotropic crystal in an external electric field a wave

Non-Bragg orders in dynamic self-diffraction on thick phase gratings in a photorefractive polymer

We demonstrate that recording thick holographic phase gratings in photorefractive polymers can lead not only to very efficient Bragg diffraction but also to rather strong diffraction into non-Bragg orders. We show that this effect has features drastically different from those of Raman-Nath diffraction on thin gratings. We compare the experimental results with a model based on the theory of dynamic self-diffraction in a photorefractive medium. Applications of this effect in devices for optical image processing are proposed.

A numerical analysis and expository interpretation of the diffraction of light by ultrasonic waves in the Bragg and Raman-Nath regimes using multiple scattering theory

In this paper, we examine some of the fundamental properties of Bragg and Raman-Nath diffraction of light by ultrasonic waves by revisiting the well-known multiple plane wave scattering theory developed by Korpel and Poon in 1980. The purpose is to provide a clear and unambiguous insight into the variety of physical and geometrical configurations associated with the process of optical diffraction from Bragg and Raman-Nath ultrasonic cells, treating each domain separately. Despite well-established theoretical models, there is a tendency to sometimes erroneously associate general Bragg domain diffraction (as opposed to exact Bragg diffraction where the incident angle is Bragg-matched and the interaction width is infinite) with only two diffracted orders that vary sinusoidally with peak phase shift of the light and distance of propagation. In numerical analyses of the coupled equations, there is also a tendency to sometimes limit the number of orders to a few lower ones. With the enthusiasm to arrive at a solution, this truncation is sometimes applied in the Raman-Nath regime as well. In doing so, higher Raman-Nath-scattered orders are implicitly assumed to be progressively weaker and, therefore, negligible. In complex acoustooptic systems, such approximations can lead to serious errors. With an aim toward rectifying these and other common misconceptions, a thorough numerical analysis of uniform plane wave acoustooptic diffraction in the two well-known regimes is presented and the limits of such analysis are examined.

Steady-state four-wave mixing in photorefractive quantum wells with femtosecond pulses

Photorefractive AlGaAs–GaAs multiple-quantum-well structures perform as ultrafast field autocorrelators during steady-state four-wave mixing by use of femtosecond pulses. The photorefractive thin films operate in the Raman–Nath diffraction regime with thin transmission gratings. The thin gratings permit degenerate and nondegenerate four-wave mixing without the need to satisfy phase-matching conditions. Nondegenerate mixing has several advantages over degenerate four-wave mixing. The autocorrelation is broadband and is viable for photon energies larger than the band gap of the photorefractive sample, and the diffraction efficiency is unaffected by the bandwidth of the femtosecond pulses. In addition, the readout laser can be a low-power low-cost cw diode laser.

Volume-diffraction analysis using bound-state perturbation theory

One-dimensional volume diffraction may be analyzed in a manner similar to the problem of an electron subject to a periodic potential. The quantity analogous to the diffraction Nath parameter ρ is then the ratio ΔW0/V, where V is the strength of the electron potential and ΔW0 the spacing between the free-electron-energy eigenstates. The crossover between the Bragg and Raman–Nath diffraction regimes is marked by a violation of the (1/ρ)≪1 condition, and corresponds to the requirement that (V/ΔW0)≪1 in order to preclude electron eigenstate mixing. Bound-state perturbation theory as developed for use in quantum mechanics may be applied to the diffraction problem to obtain a (1/ρ) expansion for the intensities of diffracted grating orders.

Correlation of quantum intensity fluctuations in Raman-Nath diffraction

A new method of generating correlated fluctuations in optical fields, based on Raman-Nath diffraction by a fine grating, is discussed. A nonclassical effect is predicted: a suppression of quantum fluctuations below the level of full coherence in the sum or difference photon numbers for the transmitted and scattered waves. The physics of this phenomenon involves a correlation of quantum fluctuations in the intensities of two modes that are coupled nonlinearly. This correlation is most effective in the limit of parametric interaction between the waves in the medium.

Optical diffraction tomography applied to airborne ultrasound

This paper describes the application of a light diffraction measurement method for airborne ultrasound. The method is based on Raman—Nath diffraction of a light beam combined with tomographic algorithms. Light intensity measurement data on acoustically diffracted light are used to characterize airborne ultrasound. The light intensity measurement data contain information on the integration of sound pressure along a specific light path. This integration allows the reconstruction of ultrasonic pressure in the cross-section of an ultrasonic beam. If the simplifying assumption of plane phase wavefronts is accepted the method gives absolute values for acoustic pressure. Reconstructions of phase surfaces are possible but are not discussed in this paper. The tomographic reconstruction of the measurement plane is done with an Algebraic Reconstruction Technique (ART) combined with nearest neighbour averaging. Three transducers were measured; results with two non-focussed air transducers with frequencies 200 kHz, 410 kHz and a focussed 1 MHz transducer are presented. Beam profiles, three-dimensional pressure maps and the optically detected acoustic ‘pulse-shape’ with tone-burst excitation are presented. The optical method offers good spatial resolution, bandwidth at least up to 1 MHz, and acoustic near-field measurements.

Direct optical characterization of ultrasonic waves using Raman-Nath diffraction of convergent light

An acousto-optic method for visualizing ultrasonic pulses based on Raman-Nath diffraction of a convergent light beam is presented and its basic features are experimentally confirmed. The laser light intensity of the far field diffraction orders becomes dependent on time, with a period of 1/ f ∗ p (where f ∗ p denotes the repetition frequency of fundamental tone of the diffracting pulse). The spectrum of the modulated light measured by a photodetector in one specific direction behind the acousto-optic cell contains all the relevant information about the ultrasonic wave. Experimental verification has been obtained for continuous and pulsed ultrasound in a TeO 2 crystal.

Active mode locking of a neodymium-doped fiber laser using intracavity pulse compression

Active mode locking of a Nd(3+)-doped fiber laser with piezoelectrically induced Raman-Nath diffraction modulation is demonstrated. By using intracavity pulse compression, stable pulses of 2.4-psec length are generated at a wavelength of 1054 nm.

Optical modulation by a traveling surface acoustic wave and a holographic reference grating

The interaction of an optical beam with a traveling surface wave and a stationary holographic reference grating is studied theoretically and experimentally. The spatial phase modulation produced by the traveling wave results in an intensity modulation in the diffracted orders of the optical beam. The intensity modulation is at the fundamental frequency of the traveling wave, and its amplitude depends periodically on the separation distance between the plane of the traveling wave and the plane of the stationary holographic reference grating. Experiments were carried out with 87-MHz surface acoustic waves traveling on a LiNbO3 substrate separated from a holographic reference grating in film. The amplitudes of the detected modulation in the m = 0 and m = ±1 diffraction orders of a probing beam are in close agreement with a plane-wave analysis in the Raman–Nath diffraction regime.

Thermal gratings and coherent phonons generated by periodic optical excitation in nematic MBBA

Generation of enhanced coherent phonons (ultrasonic waves) and thermal gratings by periodic excitation with light pulses (532 nm) from a Q-switched and mode-locked laser was observed in a numatic liquid crystal MBBA by using Raman-Nath diffraction of a cw probe light (633 nm). Annomalous behavior was found when an extraordinary probe light ws used; the diffracted light formed a sharp pulse (width ≈ 100 ns) superimposed by high-fequency phonon signals whose phase was shifted by л at the pulse peak. This is due to the creation of a large thermal grating originated from the change of molecular order by heating with light pulses. Experimental results are compared with calculated ones. Sound velocity and the decay time of the thermal grating were measured to be 1.51 × 10 3 m/s at 24 °C and 0.18 ms for the grating pitch of 18.5 μm.

Acousto-optic control and modulation of optical coherence by electronically synthesized holographic gratings

We introduce a new technique to modify the coherence properties and the far-field intensity distributions of optical fields. Use is made of a combination of Bragg diffraction and Raman–Nath diffraction in an acousto-optic deflector. The high-frequency Bragg grating in the deflector is phase modulated by a synthetic low-frequency Raman–Nath regime grating that has a structure of an optimized phase-only computer-generated hologram. We demonstrate, e.g., electronic control of the state of spatial coherence of light, Gaussian to flat-top conversion, and the generation of arrays of equally intense but mutually incoherent light spots.

Local detection of magneto-surface-acoustic-wave in highly magnetostrictive amorphous film by light-probing technique

we present an investigation on local properties of magneto-surface-acoustic-waves (MSAWs) propagating on Fe-Co-B amorphous sputtered films supported on soft glass substrates. The local detection of MSAW was performed by means of a light-probing technique based on the principle of Raman-Nath diffraction. The mode of MSAW in the elements was confirmed to be Rayleigh(P+SV)-type and the amplitude of MSAW was subjected to large change from place to place by applying a small field. The attenuation constant of MSAW also varied depending strongly, not only on the field strength, but also on the direction of the field relative to the direction of MSAW propagation. Theoretical analysis explains the experimental results very well in terms of the change of micro-eddy current loss in the conductive magnetic film, which reflects the change in the degree of magnetoelastic coupling in the film with external magnetic field.

Enhanced detection of acoustic waves using thick and thin reference gratings

Propagating acoustic waves typically produce small effective refractive-index variations and can thus be difficult to detect by simple optical probing. It has been shown that use of stationary reference gratings can greatly facilitate the detection of these waves. In this paper, the application of thick (Bragg diffraction regime) and thin (Raman–Nath diffraction regime) sinusoidal-phase reference gratings is considered for enhanced detection of acoustic waves, and their relative performance is compared. Simple, analytical expressions are obtained that describe the diffraction characteristics in each case. It is shown that thick reference gratings produce a purely sinusoidal temporal-intensity modulation, whereas thin gratings, in general, produce nonsinusoidal modulation.

Acoustooptical properties of ferroelectric Kd2Po4atT>Tc

Abstract Acoustooptical tensor components of DKDP single crystal have been measured at the frequency 107 Hz by the Raman-Nath diffraction reference method in the paraelectric phase. Different types of the temperature dependence for P 11, P 33, P 66 coefficients were revealed. The latter was shown to obey Curie-Weiss-like law.

Various Aspects of Ultrasonic Light-Diffraction and Domains of Parameters Pertaining to These Aspects

Various aspects of the ultrasonic light-diffraction phenomenon, including the Bragg reflection, Raman-Nath diffraction, and ray-optica aspect like mirror-type Bragg reflection up to higher order spectra are discussed in relation to their model mechanisms and the domains of parameters producing them.

Diffraction characteristics of planar absorption gratings

Planar (co)sinusoidal conductivity (absorption) transmission gratings are analyzed using rigorous coupled-wave theory. The first-order and higher-order diffraction efficiencies are determined over the entire range of possible conductivities and Bragg angles of incidence (or equivalently, grating periods) for H-mode polarization incident plane waves. The maximum possible first diffracted order efficiency is found to be 5.26%. Rigorous results are compared to approximate results from the Raman-Nath theory and the two-wave first-order coupled-wave (Kogelnik) theory. A regime parameter, ϱσ, is defined which delineates the regions of Raman-Nath diffraction behavior (ϱσ 1). Likewise, the angular selectivity characteristics of conductivity gratings are determined from rigorous theory and are compared with corresponding results from approximate theory.