Acousto-optic System Research Articles

Modeling of an acousto-optic laser beam steering system intended for satellite communication

High data rate of laser communication systems should be matched by wide operational bandwidth of beam positioning. This could be achieved by the application of nonmechanical beam steering technologies utilizing acousto-optic phenomena. Major components of acousto-optic beam steering systems, a Bragg cell, and a quadrant detector are subjected to comprehensive laboratory testing and characterization as components of a control system. A mathematical and computer simulation model of the entire steering system addressing system dynamics, cross coupling of azimuth and elevation channels, and nonlinearity, and facilitating synthesis of advanced control laws is developed.

Acousto-optic scanning system with very fast nonlinear scans

A very fast (>100 kHz) acousto-optic scanning system, which relies on two counterpropagating acoustic waves with the same frequency modulation, is proposed and experimentally demonstrated. This scheme completely suppresses linear frequency chirp and thus permits very fast nonlinear scans and nonconstant linear scans. By changing the phase between the modulating signals, this scheme also provides very fast longitudinal scans of the focal point.

Acousto-optical stabilisation of the laser beam direction

An investigation is reported of an acousto-optical system with optoelectronic feedback, ensuring stabilisation of the laser beam direction. Various modifications of the system based on isotropic and anisotropic diffraction of light are considered. An analysis is made of the influence of the acousto-optical interaction selectivity on the angular operating range and on the stabilisation coefficient. The results of an experimental study of a system with a paratellurite crystal cell are given.

Acousto-optic system for automatic identification and decoding of digitally modulated signals

University of PlymouthSchool of Electronic Communication andElectrical EngineeringDrake CircusPlymouth, PL4 8AAUnited KingdomE-mail: creeve@plymouth.ac.ukAbstract. A simple acousto-optic system is described that is capable ofdemodulating frequency- and phase-modulated signals using a bicellphotodetector. A summary of an analytical study of the system is pro-vided, describing its main performance characteristics. Results from aMathcad model of the system are presented showing that, by taking thesum and difference of the outputs from the two bicell elements, it ispossible to identify and demodulate a wide range of different types ofmodulation without any a prioriinformation. Thus the technique is wellsuited for surveillance receiver and electronic support measures (ESM)applications. In particular, it is shown that biphase shift keying (BPSK),quaternary phase shift keying (QPSK), offset-keyed QPSK, differentialQPSK, continuous phase frequency shift keying (FSK) [including mini-mum shift keying (MSK)], and ordinary FSK all give unique and easilyidentifiable signatures. Experimental results are presented, which are ingood agreement with both theoretical and modeled results.

Experimental study of small signal amplification in Bragg acoustooptic bistable system

The external small signal amplification ability of the Bragg acoustooptic system has been studied in this paper. It has been proven by experiment that there are saturation and resonance phenomena in the amplification. Bifurcation parameters at the bifurcation points are decreased by external simple harmonic signals. At the period-2 bifurcation point, small signal amplification energy is mainly from the period-2 component. External signals have the ability of frequency pulling and synchronising period-2 frequency. These phenomena have great significance concerning bifurcation and chaos.

Spectral Change in Acoustic Pressure and Cavitation Inception

Acoustic pressure spectra were measured against the excitation level of the piezoelectric transducer in order to investigate the relationship between acoustic spectra and cavitation inception at an excitation frequency of 20 kHz. Measurements were conducted on degassed and nondegassed distilled water using an acoustooptic detection system. The results of the measurements suggest that the observation of n/2 (n: 3, 5, 7…) order subharmonics is useful to monitor the inception of transient cavitation.

Two-dimensional Raman–Nath acousto-optic bistability by use of frequency feedback

From the frequency dependence of the optical transmission characteristics of Raman-Nath acoustic-optic diffraction, the dynamic equations and steady-state solutions of a two-dimensional Raman-Nath acousto-optic bistable system are derived. In this system, feedback is realized by use of a diffracted light beam to control the frequency of the acoustic-optic driver. The curves of light output versus input corresponding to several orders of the diffracted beams are obtained by theoretical analysis. They can all lead to bistability. The amount of hysteresis is a function of the order of the diffracted beam, the bias frequency, and the feedback coefficient. Experimental results are presented and compared with theoretical analyses.

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.

Acoustooptical system for imaging TV information by using a copper vapor laser

A laser acoustooptical system for projecting TV images onto a large-area screen is developed and produced. A copper-vapor laser with an average power of 20 W is used in it as a radiation source. Images are formed by using green and orange spectral lines. The exposure of a whole TV line on a screen in a time corresponding to the laser pulse duration is the principle governing the operation of the system. A TV line is formed by an acoustooptical modulator (AOM) designed on the basis of a paratellurite (TeO2) crystal. Frame scanning of an image is performed by using an electromagnetic galvanometer. A high line and frame resolution of about 2000×1000 pixels in the TV standard is obtained. The system makes it possible to form a TV image of size greater than 20 m2 with brightness of about 50 cd/m2 at a distance of 20 to 100 m. Operating principles of the system and requirements imposed on the laser, the AOM, and other elements are considered.

Acousto‐optic system for the generation and control of tunable low‐frequency signals

An optically efficient interferometric acousto-optic architecture for the tunable generation and control of low-frequency signals is pro- posed using two radio frequency (rf) or microwave band Bragg cells in an in-line configuration. Critical beam deflection optics, such as a dove prism, ensure that the interfering beams are collinear and stationary, thus providing the high heterodyning efficiency required at the fixed op- tical detector. Tunable 1- to 15-MHz ultrasonic band signal generation is demonstrated with rf Bragg cells, showing high (107 dB/Hz) carrier-to- noise ratios. This free-space interferometric acousto-optic system with two collinear output beams with a relative color shift is required for mak- ing a compact, lightweight, and powerful optical platform for baseband/ low-frequency optical signal processing and control functions, such as required in advanced phased-array systems used in sonar, medical, and industrial applications. © 1996 Society of Photo-Optical Instrumenta- tion Engineers. Subject terms: acousto-optics; phased arrays; low frequency; ultrasonic signals; control.

Optoelectronic generator based on the acousto-optical interaction

A theoretical investigation is reported of an acousto-optical system with optoelectronic feedback of the hybrid type. The conditions for self-excitation of harmonic vibrations in this system are determined. Steady amplitudes of vibrations under soft and hard excitation conditions are calculated.

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.

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.

Specialised acousto-optical processor for input, display, and coherent-optical processing of multiparameter information from spaceborne telemetric systems

An acousto-optical system for input, display, and coherent-optical processing of information was implemented experimentally. The information transmission capacity, the structure of the information fluxes, and the efficiency of spaceborne telemetric systems were taken into account. The number of equivalent frequency-resolved channels corresponded to the structure of a telemetric frame of a two-step switch. The number of intensity levels of laser radiation corresponded to the scale of changes in the parameters. Use was made of the technology of a liquid optical contact between a wedge-shaped piezoelectric transducer made of lithium niobate and an anisotropic light-and-sound guide made of paratellurite with asymmetric scattering geometry. The simplest technique for optical filtering of multiparameter signals was analysed.

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.

Acousto-optical wideband correlator system: design, implementation, and evaluation

We describe the design and development of an acousto-optical wideband correlator that we integrated into a digital signal-processing system testbed. We report its measured performance and compare it with various theoretical performance measures, one of which compares the optical system with an equivalent efficient digital correlator. The measured performance of the optical system was 20 to 70 times that of a VAX 6410 computer using a fast-Fourier-transform correlation algorithm and a vector processor, even though the electronic interface system limited the performance of the optical system to less than 0.5% of its potential. We also compare the system with commercially available digital signal-processing boards.

Detection of short pulses by Fresnel preprocessing

Short pulses create unique Fresnel diffraction effects, as they propagate through an acousto-optic signal-processing system, that produce an interference pattern. By following the detection of the interferometric pattern with spectral analysis, we extract information about pulse width, pulse separation, and relative time of arrival. Laboratory models of the Fresnel-transform processing system and an optical spectrum analyzer were built and tested to verify the basic concepts. Pulses as short as the theoretical limit of 20 ns for this system and separated by as little as 60 ns or as much as 17 µs were detected and measured. Short pulses were also detected in the presence of cw signals by use of a frequency-variant matched-filtering operation.

Experimental demonstration of an acousto-optic system for two-dimensional phased-array antenna scanning

A novel in-line interferometric acousto-optic architecture for two-dimensional beam scanning of planar phased arrays is demonstrated experimentally for a 330-MHz carrier. The required modulo-2pi phase control along the two orthogonal antenna-array directions is achieved by using two independent control frequencies f(1) and f(2) with frequency variations of 0-170 and 0-210 kHz, respectively. A processor output carrier signal-to-noise ratio of 48 dB at + 1-MHz offset is measured in an analyzer bandwidth of 300 kHz, with a carrier tuning range of 90 MHz.

Acousto-optic wavelength division multiplexing system

A novel wavelength division multiplexing/demultiplexing (WDM) system using acousto-optic Bragg cell is presented. The system is adaptable for any wavelength deviation of light sources caused by temperature changes and allows changing the operating wavelengths over a wide range. High diffraction efficiency, high resolution, and a large number of channels are obtained. Computer simulation for a five-channel system in the 0.8-μm region is presented. © 1992 John Wiley & Sons, Inc.

Integrated-optics system for a read/write head in disk holographic memory devices

An investigation was made of the possibility of fabricating planar acoustooptic systems intended for operation in read/write heads of disk holographic memory devices. An optical system for such a device was made from homogeneous refracting waveguide lenses. The basic factors limiting the data recording density were identified. A prototype planar acoustooptic system for a 16-bit device was constructed and investigated.