Nath Diffraction Research Articles

Probing of liquids by Bessel light beams under conditions of acousto-optical interactions

The acousto-optical interactions of Bessel and Gaussian light beams with the ultrasonic field in liquids under the condition of Raman–Nath diffraction for the standing-wave mode are experimentally investigated. The main differences of acousto-optical interactions for Bessel and Gaussian light beams are revealed. The various phenomena of refraction and focusing of Bessel beam in liquids owing to sound-field spatial modulation are demonstrated.

Optical Visualization of Ultrasound Pressure Fields in Turbid Media Based on Coherent Detection Imaging Technique

An optical method for visualizing ultrasound fields in turbid media is described. A laser beam is modulated by ultrasound waves in a light-scattering medium and is detected using a coherent detection imaging technique based on the optical heterodyne method. We demonstrate for the first time the ability to visualize the profile of an ultrasound field generated by a focus-type ultrasound transducer. This is achieved by utilizing forward-scattered light originating from a laser beam that is phase modulated by Raman–Nath diffraction and is also modulated in amplitude through the density variation of scatters in a test medium and/or a small amount of beam deflection. In this study, we demonstrate the two-dimensional determination of a sound field, even in an optically turbid medium.

Sub-Picosecond Optical Pulse Generation with Repetition Rate of Several Tens of Gigahertz Using Electrooptic Standing-Wave Phase Grating with Periodic Domain Inversion

We propose and demonstrate a novel electrooptic spatiotemporal modulator for sub-picosecond optical pulse generation with a repetition rate of several tens of gigahertz. The pulse generation is directly accomplished with an electrooptic standing-wave phase grating with periodic domain inversion that operates over 10 GHz. This device produces a phase distribution that is like a sinusoidal standing wave in the beam cross section. The output optical beam undergoes Raman–Nath diffraction with diffracted beams whose intensities vary temporally. When we extract a certain diffracted beam, we can obtain a pulse train. A near-Fourier-transform-limit pulse train with a repetition rate of twice the modulation frequency can be obtained using the zero-order undiffracted beam. In the experiment, we found that our new device operates as theoretically predicted and that we can obtain a pulse train with a pulsewidth of about 1 ps from the fabricated device at the maximum modulation power. By improving the modulation efficiency of the device or by further increasing of the modulation frequency, a sub-picosecond optical pulse train with a repetition rate of several tens of gigahertz can be generated by the new device.

Beam coupling ratios and diffraction efficiencies of a layer-structured liquid crystal device; the effect of injection temperature and layer thickness

Beam coupling ratios and diffraction efficiencies of a photorefractive liquid crystal cell were measured while changing the injection temperature and the layer thickness of the liquid crystal in a layer-structured fabrication. A high beam coupling ratio and first-order Raman–Nath diffraction efficiency were observed at low injection temperature and small cell gap, which showed high order parameters. However, as the cell gap was reduced, the shear stress at the interfaces between the liquid crystals and the photoconducting layers increased, so that the reorientation of the liquid crystals by an internal field could be restricted by the geometry and interrupted by a highly-induced surface energy, reducing photorefractive effects. An optimum exists in the neighborhood of 7.5 μm when LCs are injected at 25 °C under our experimental conditions .

Acoustooptical Spectral Processing of Radar Signals Taking into Account the Signal Spectra of Local Heterodyne and Optical Radiation

We describe the formation of a radio signal designed for acoustooptical processing based on the radar signal analyzed which is characterized by a spectrum, a heterodyne signal with a spectrum of known structure, and a mixer. The signal formed fits the working frequency of the acoustooptical cell. In the cell, the signal produces a traveling acoustic wave due to propagating inhomogeneities of the dielectric constant or refractive index. Within the framework of physical optics, we consider how the acoustooptical cell used for processing the signal works in the regime of the Raman–Nath diffraction. The cell is illuminated by quasimonochromatic radiation of known arbitrary spatial and spectrum structures. One works in the first and higher diffraction orders. Using an original method proposed earlier, we analyze the functioning of the acoustooptical cell in the Bragg diffraction regime at small radiation transformation coefficients. The description is done for a probing signal of known spatial and spectrum structures. The method of acoustooptical analysis of the spectrum structure of a radio signal or radar signal is considered under the decreased influence of the spectra of the probing optical radiation and heterodyne signal. The effect of the spectrum structure of the probing radiation used when processing the radio signal which propagates within the acoustooptical cell is studied in complete analogy with taking into account the exciting radiation spectrum in spectroscopy. For signals with a low level of noise, we use the method based on the Fourier transformation of the intensity distribution registered and on that of the intensity distribution of the initial probing radiation spectrum. Effects of the width and spectrum structure of the heterodyne can also be taken into account similar to the method of excluding the spread function used in investigating spectrograms. The processing uses Fourier transforms of the registered spectrum intensity distribution and of the known spectrum intensity distribution of the heterodyne signal.

Holographic recording in thin C 70-doped polymer organic films

The dynamic and static thin holographic gratings have been recorded in C 70-doped photosensitive polyimide 6B and 2-cyclooctylamino-5-nitropyridine. The drastic change in diffraction efficiency has been observed under the Raman–Nath diffraction condition using the second harmonic of pulse Nd laser. The threshold of the transition from phase grating to amplitude one has been determined. The results obtained have been explained by the absorption spectral peculiarities of the fullerene-doped films and by their thermal properties.

Phase-modulated beams technique for thin photorefractive films characterization

The phase-modulated beams technique is developed for nonlinear thin photorefractive films characterization. In the Raman–Nath diffraction approximation, the formulas are deduced, allowing us to measure the amplitude of phase grating recorded in a film and its nonlinear refractive index n2. The method is applied for studying Langmuir–Blodgett multilayer thin (∼0.6 μm) films of Bacteriorhodopsin at wavelength 633 nm.

Enhanced wave mixing in photorefractive rhodium-doped barium titanate crystals

We present an experimental study of the self-diffraction in Rh:BaTiO 3 and BaTiO 3 crystals for small angle between the interfering laser beams. Much larger diffraction efficiencies and beam amplifications are measured in the self-diffraction pattern of Rh:BaTiO 3 than in the corresponding pattern of undoped BaTiO 3 crystals. The enhancement increases with the Rh concentration in the barium titanate crystals. This effect cannot be explained by a simple Raman–Nath diffraction model, although it gives acceptable predictions for the undoped crystals, at small modulation index. In the same crystals, the results of self-diffraction in the Bragg regime remain in a good agreement with the existing theory.

Diffraction of light by two ultrasonic beams of frequency ratio 1:2, beyond the Raman–Nath diffraction regime

Light diffraction by two ultrasonic waves of frequency ratio 1:2 is examined beyond the Raman–Nath diffraction regime. Measurements were taken of ±1 order diffracted light intensities for some Raman–Nath parameter values, as a function of the phase shift between the sound beams. The experimental results obtained are discussed and compared with relevant theoretical predictions. Within the experimental conditions, a quite good quantitative agreement between the experimental data and the appropriate theoretical expectations is observed.

Near field of light diffracted by an ultrasonic wave, beyond the Raman–Nath diffraction regime

The near field of ultrasonically diffracted light is examined for a wide range of Raman-Nath (v) and Klein-Cook (Q) parameter values up to the region of total light rediffraction phenomenon occurrence to the zeroeth diffraction order. The behavior of the secondary interference plane position is studied, both theoretically and experimentally, for vari- ous Q and v values. In addition, the relationship is shown between dif- fracted light intensity modulation minima localization shift and rediffrac- tion phenomenon. A comparison is made of the results of the general Fresnel region theory, including the influence of complex light amplitudes over the diffracted light intensity distribution, with those obtained from Cook's simplified near-field description. The same, limited validity range of the latter theory is revealed. For all the experimental conditions con- sidered, quite good agreement is observed between experimental data obtained and the appropriate theoretical predictions. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00507-3)

Diffraction of light by two parallel adjacent ultrasonic beams of the frequency ratio 1:1, beyond the Raman–Nath diffraction regime

Light diffraction by two adjacent ultrasonic waves having the same frequencies is examined experimentally beyond the Raman-Nath diffraction regime. Measurements were taken of the 0 and 61 order diffracted light intensities for a few (settled) Raman-Nath parameter val- ues, as a function of the phase shift between the sound beams. The experimental results obtained are discussed and compared with rel- evant, generalized, theoretically numerical predictions, as well as with the ones obtained by means of the 0 and 61 order approximation only. In both cases, however, within all the experimental conditions consid- ered, a quite good quantitative agreement between the experimental data and the appropriate theoretical predictions is observed. © 1999 So- ciety of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00607-8)

Optical amplification with high gain in hybrid-polymer–liquid-crystal structures

Efficient coherent light amplification with a very high net exponential gain Γ≈2600 cm−1 has been observed in sandwich-type hybrid-polymer–liquid-crystal structures. The amplification is a result of an energy exchange between two beams in a two-beam coupling experiment. The reported gain coefficient has been measured in a tilted sample configuration within a Raman–Nath diffraction regime at electric fields of the order of 1 V/μm.

Raman–Nath diffraction caused at the focal point of a concave type transducer

To estimate the intensity of sound pressure in the vicinity of the focal point of a concave type transducer, Raman–Nath diffraction caused at the focal point of a concave type transducer is theoretically and experimen- tally discussed. Sound field, near the focal point, including the acoustic nonlinearity, is theoretically calculated by using the KZK equation. Light intensity of zeroth order Raman–Nath diffraction is also calculated as a function of the sound pressure at the focal point. In the high intensive sound region, it is found that the acoustic nonlinearity makes the light intensity curve disappear at the local peaks. The He–Ne laser whose beam spot diameter is 1.2 mm is supplied as a light source. Light intensities of the zeroth-order Raman–Nath diffraction caused at the focal point are observed by changing the driving voltage of the transducer. The experimentally obtained graph shows the relatively same pattern compared with the theoretically calculated one. The sound pressure at the focal point is estimated by fitting those two results. It is confirmed that the sound pressure estimated by the optical method shows good agreements with the estimated one by the hydrophone measurements.

Orientational photorefractive effects observed in poly(vinyl alcohol)/liquid crystal composites

) as a photoconductive sensitizer under an applied dc field. Orientational photorefractivity was demonstrated by observing Raman–Nath diffraction beams with an external dc field. The photorefractive gratings were partially memorized even in the absence of the applied dc field.

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.

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.

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.

Monte Carlo simulation of the Feynman diagram approach to strong acousto-optic interaction

In previous papers it has been shown that acousto-optic interaction of arbitrary light and sound fields is solvable by means of Feynman diagrams and path integrals. In this paper we use a quantum-mechanical interpretation of these path integrals, based on probability-amplitude densities, for Monte Carlo simulations on a computer. This simulation is applied to a rectangular sound field (moving grating). Results agree with the well-known solutions for Raman–Nath, Bragg, and near-Bragg diffraction.