Acousto-optic Cell Research Articles

Performance of an acousto-optic Bragg cell under ion microbeam irradiation

An acousto-optic (AO) deflector composed of PbMoO 4 was exposed to 4 MeV protons while operating under Bragg angle conditions. An ion beam in air of 1 mm width was directed normal to the crystal face and laser beam. Between exposures, the approximately 13 mm × 8.5 mm AO deflector was mechanically translated in two dimensions in front of the fixed ion beam. The AO diffraction efficiency was mapped and was observed to change as a function of ion beam location and dose rate. These effects are attributed to the induced change in the temperature distribution of the crystal, which changed the sonic velocity and refractive index. Similar effects were observed when the ion beam was directed at the acoustic transducer.

Basic image-processing operations by use of acousto-optics

We describe some basic optical image-processing operations with acousto-optic (AO) Bragg diffraction. Instead of using frequency-plane filters, we place an AO cell behind the object. We then realize experimentally one-dimensional edge enhancement, which utilizes a high-pass filtering effect in the undiffracted order from the AO cell. A numerical simulation compares well with the experimental results. With two AO cells oriented orthogonally to each other, a second-order mixed derivative operation, evident from the four-corner enhancement of a square, is also demonstrated.

Acousto-optic and electro-optic modulators for stereoscopic laser videoprojection

An original combination of acousto-optic modulators and electro-optic cells for stereoscopic laser videoprojection is presented. It is based on the separation by orthogonal polarizations of the two left and right images of a stereoscopic pair. As several video lines are scanned simultaneously, the acousto-optic modulators operate in multifrequency mode. The polarization states are controlled by the electro-optical devices. A practical bifrequency acousto-optic modulator using the slow shear acoustic wave in paratellurite has been designed, and an electrooptic device with four ammonium dihydrogen phosphate crystals has been implemented. Experimental results are reported.

Analysis of phase relationships between radar signals in acoustooptic processing systems

The performance and the salient operation features of an interference correlator with a modified optical twin-wave Rayleigh interferometer system in the mode of measurements of phase shifts between radar signals are considered. Such measurements for signals received from two antennas provide information on the angular position of an object or its displacement. A system wherein radio signals are fed for processing into a correlator with the aid of a double acoustooptical cell with a paratellurite acoustic line is experimentally realized and investigated. We consider the modes of system operation when a signal is generated in the regions of the extrema of the correlation functions and in the regions of their strongest variation (extrema of the derivatives). The accuracy of measurements obtained experimentally for the phase shifts between radio signals reaches 0.003 rad to give an accuracy of determining the signal time delays of the order of 0.005 ns in the frequency region of 100 MHz.

Radiometers, autocorrelators, and matched-filter receivers by means of acousto-optic spectrum analysis

Acousto-optic spectrum analyzers provide a convenient means of separating wide-bandwidth signals into their frequency components. By a change in the rf input signal into the spectrum analyzer and by the provision of additional digital postprocessing, it is possible to perform radiometry, signal autocorrelation, and matched-filter reception. Although the acousto-optic device has a space-integrating architecture, the matched-filter receiver can be implemented for signals having time durations much longer than the acousto-optic cell. The resulting signal-to-noise ratio improvements from the receiver are consistent with the time-bandwidth product of the waveform, rather than the time-bandwidth product of the acousto-optic device. A mathematical foundation of the processor is presented along with specific receiver implementations. Computer-simulation and experimental results demonstrate key findings. In one experimental example, a linear-frequency-modulated waveform is matched-filter processed to recover a signal that is -24 dB with respect to the input noise floor.

TeO2 single-crystal acoustooptic 2×2 switch for light beams of different wavelengths

The principles of acoustooptic switching of two light beams of different wavelengths by means of an acoustooptic interaction with two acoustic waves are examined. A version in which the acoustic waves form parallelograms and a version with strictly orthogonal propagation are considered. The latter version is the more attractive one when a TeO2 single crystal is used as the acoustooptic cell. Experiments on the switching of two-color Ar laser radiation with wavelengths λ1=0.5145 and λ2=0.488 µm are carried out and demonstrate a high switching efficiency. On the whole, the experiment confirms the basic conclusions of the theory.

Acousto-optic photonic crossbar switch Part I: design

We present the design of a 12 × 12 photonic crossbar interconnection network constructed using a single three-dimensional acousto-optic crystal. Previous crossbars based on bulk acousto-optic cells require multichannel deflectors with one deflector per optical input; in contrast the design presented here angularly multiplexes these independent deflectors into a single-transducer acousto-optic device. A Fourier-optics analysis of an acoustically lossy Bragg deflector is coupled to a momentum-space analysis that permits the derivation of complete design equations for the switch. As a concrete example, the complete design of a 12 × 12 crossbar is presented. Finally, a coupled-mode analysis of the first- and second-order diffractions in the angularly multiplexed Bragg cell reveals the fundamental efficiency bounds of the switching network.

Polarization changes and beam profile deformation of light during acousto-optic interaction in isotropic media

A spatial Fourier transform approach is proposed to investigate the effects of polarization changes and beam profile deformation of light during acousto-optic (AO) interaction in isotropic media. The behaviour of the total scattered optical fields inside the AO cell can be properly described by a vector wave equation of which the permittivity is perturbed by an acoustic wave propagating inside the medium. In the Bragg regime, using a spatial Fourier transform approach, two coupled differential equations can be derived from the wave equation to depict AO interaction in the spatial frequency domain. Analytic solutions, which comprise the effects of changing polarization, beam deformation and propagating diffraction, can be found from the coupled equations. Detailed numerical simulations, including Fourier transforming the incident light profile to calculate the spectra of the scattered light beams and, hence, their profiles in space using the inverse transform, are presented.

Holographic real-time imaging of standing waves in gases

Stroboscopic holographic interferometry has been developed at an advanced undergraduate level allowing real-time imaging of standing sound waves in gas filled, closed tube resonators. A heterodyne Mach–Zehnder interferometer was first built by students to show the feasibility of interferometric detection of sound waves in a small cell. In the subsequent holographic study, the laser irradiance is modulated by an acousto-optic cell at frequencies near that of the standing wave, and a video camera records the fringe motion due to sound pressure changes. Fractional fringe shifts are observed for an air filled cell, and multiple fringe shifts are imaged for the case of freon. Sound reflections from the cell ends are easily observed, with nonsinusoidal waveforms dominating at high intensities due to superposition of resonator harmonics.

Correlator with a modified optical Rayleigh interferometer system used to measure small phase shifts of rf signals

An experimental investigation was made of the operation, and estimates were obtained of the characteristics, of a correlator based on a double-beam optical Rayleigh interferometer with a doubled acousto-optical cell. This correlator was used to measure small phase shifts of radar signals. The experimental results were compared with a theoretical analysis of the correlator operation. The precision deduced from measurements reached 0.003 rad for the phase shift and 0.005 ns for the time delay. The theoretical analysis and the experimental results showed that the proposed and implemented interferometric correlator is quite promising for radar systems.

Effects of acousto‐optic correlator bandwidth on coded pulse compression

The effects of limited frequency bandwidth on the response of space-integrating acousto-optic correlators are considered. They are studied in the classical scope of pseudorandom sequence correlation, used in spread spectrum systems, in terms of the SNR enhancement in the correlator compared to the ideal (infinite bandwidth correlator) enhancement figure. An analytical approach using filtering of both useful signals and noise at the correlator input is developed. Numerical computations are drawn that show a loss of 4 dB when the code chip rate used is equal to three times the correlator bandwidth. Experimental results obtained for an acousto-optic cell are also presented and agree well with the theoretical results.

High-performance guided-wave acoustooptic Bragg cells in LiNbO/sub 3/- and GaAs-based structures

We present a theoretical study of guided-wave acoustooptic (AO) Bragg cells produced by a single surface acoustic wave (SAW) in PE:LiNbO/sub 3//LiNbO/sub 3/- and ZnO/GaAs/AlGaAs-based structures. A rigorous modeling of the SAW propagation in multilayered structures has been used and the analysis of the AO interaction has been performed by using a generalized multimode formulation of the coupled-mode theory. High-performance configurations, particularly advantageous in wide-band optical communication and signal processing applications are presented.

Modeling the noise figure of an acousto-optic receiver

By defining the processing gain of an acousto-optic receiver as the ratio of the signal-to-noise ratio at the output of the detector to the signal-to-noise ratio of the intermediate-frequency input, one can model a noise figure for the acousto-optic receiver. The noise figure has a minimum of 0 dB and depends on the ratio of the noise power (internal to the acousto-optic cell) to the intermediate-frequency input noise power multiplied by the frequency and the spatially dependent exponential factor.

Coherence-multiplexed acousto-optic correlator for signal processing: theory and applications

A method of multichannel processing to compute correlation products simultaneously is introduced. The signals that process are encoded on a single light beam through the use of an electro-optic modulators that induce optical delays greater than the coherence length of light. The coherence-modulated light beam thus obtained is then spatially and temporally modulated through an acousto-optic Bragg cell. The potential number of channels is estimated to be approximately 5-10. The method can be combined with other existing systems, such as time-, space-, or frequency-multiplexed correlators, to increase the channel number. The method also applies to high-resolution spectral analysis.

Multistability in an acousto-optical system with a frequency feedback

An investigation was made of an acousto-optical system with hybrid feedback used to control the frequency of ultrasonic waves excited in an acousto-optical cell. An amplitude transparency, placed in front of a photodetector, ensured a nonlinear dependence of the intensity of the diffracted radiation reaching the detector on the ultrasound frequency. Conditions were found under which this nonlinearity gave rise to multistable states differing in respect of the amplitude, frequency, and direction of propagation of the diffracted beam. An analysis was made of various uses of such a system as an optical channel switch and in stabilisation of the direction of propagation of a light beam.

Limitations in acousto-optic FM demodulators

The ability of an acousto-optic cell to deflect a light beam through an angle proportional to the frequency applied to its transducer has been exploited to construct a simple yet effective FM demodulator. By measuring the position of the deflected beam with some form of detector assembly, the deviation from a given centre frequency can be obtained. The performance of the system is analysed and expressions for the system linearity, bandwidth and minimum frequency resolution are obtained. Two different types of detector assembly are considered: a single detector with a knife-edge spatial filter and a bicell detector. The performance of the bicell system is found to be superior by virtue of its ability to reduce/cancel the effects of amplitude modulation on the signal and variations in laser intensity. It is shown that systems can be designed to operate at centre frequencies from tens of MHz to a few GHz, with maximum frequency deviations and modulation frequencies from a few kHz to tens of MHz and with the ability to detect minimum frequency deviations down to tens of Hz. Experimental results are presented to demonstrate the validity of these expressions. It is concluded that an acousto-optic system may be used to measure the FM characteristics of a signal. There is also potential for applications in other areas where fine measurement of frequency deviation is required, such as Doppler analysis.

Incoherent optical image processing with acousto-optic pupil-plane filtering

We describe an incoherent image processor that uses orthogonally oriented one-dimensional acoustooptic cells to implement dynamic, arbitrary bipolar point-spread functions (PSF's). Arbitrary PSF's are implemented as a linear superposition in time of separable PSF's. The use of incoherent illumination increases the input field of view over that provided by coherent illumination, and implementation of the PSF by a pupil-plane filter yields a simple, compact single-lens imaging system. The acousto-optic cells offer a faster PSF update rate than that of conventional spatial light modulators, which is a critical issue for the implementation of a bipolar PSF as a subtraction between its positive and rectified negative parts. Initial experimental results are presented that demonstrate the realization of an arbitrary nonseparable PSF, image convolution with a bipolar PSF, two-dimensional image correlation, and an increased processor field of view.

Spatial phase coding for incoherent optical processors

In this paper we introduce spatial phase coding of incoherent optical signals for representing signed numbers in optical processors and present an experimental demonstration of this coding technique. If a diffraction grating, such as an acousto-optic cell, modulates a stream of light, the image of the grating can be recovered from the diffracted beam. The position of the grating image, or more precisely its phase, can be used to denote the sign of the number represented by the diffracted light. The intensity of the light represents the magnitude of the number. This technique is more economical than current methods in terms of the number of information channels required to represent a number and the amount of post processing required.

Acousto-optic modulator withwide bandwidth and large angular aperture

A new type of acousto-optic (AO) modulator with enhanced performance is described. By using phased array transducers in a birefringent AO cell, it is possible to simultaneously satisfy the tangential and noncritical phase matching conditions and achieve both large bandwidth and angular aperture. The new design is optimum for modulators and low resolution deflectors with the important advantage of fast input optics.

Time-delay detection of short pulses by Fresnel and Fourier transformations

The Fresnel transform of a signal, when used as a preprocessing step to a subsequent spectrum analysis, helps to detect the time delay between short-pulse signals. Light from the short pulses disperses spatially as it propagates through free space to create an interference pattern. As the pulses move through an acousto-optic cell, a point photodetector produces a signal whose temporal frequency is directly proportional to the time delay between the pulses. When this signal is fed to a spectrum analyzer, the frequency domain represents a time-delay domain.