Several electro-optic tunable filter structures which are currently being studied will be described. These include polarization conversion devices as well as interference filters. Transmission characteristics of these filters are electronically controllable. Some important properties of these devices will be discussed and compared.
An optical filter has been developed in which the operating wavelength, linewidth, line shape and peak transmission can be independently controlled. In this filter, the optical interaction is controlled electro-optically thereby requiring much less drive power than similar filters using the acousto-optic interaction. A filter has been fabricated and tuned through the visible portion of the spectrum. The programming features of the filter are described in this paper.
A narrow-band electro-optically tunable infrared filter is described and initial experimental results are reported. The filter is basically a multiple-cavity Fabry-Perot filter. Metal layers, which separate the cavities, serve as electrodes and optically decouple the two or more cavities. The structure has potentially high transmission and may be tuned over a wide range of wavelengths.
We have demonstrated operation of a tellurium dioxide non-collinear acousto-optic filter which has been placed under the programmable control of a SYM 1 microcomputer. One program provides for automatic scanning through the spectral range (450 to 750 nm), stopping at each line above a specified threshold for a scale expanded scan; the center frequency, bandwidth and relative intensity of each line is displayed. We can also program the RF power level to be normalized to the detector and filter response functions. A programmable stored replica can be used to compare the measured spectrum with specified characteristics of a spectrum stored in the computer, so that various comparison operations can be carried out. One of the advantages of the AO filter is its easy adaptability to computer interfacing while sacrificing none of its speed of operation, and these qualities have been incorporated in this demonstration.
This paper presents a discussion on the development of a spectrometer which utilizes an acousto-optic tunable filter (AOTF) as the wavelength dispersing component. The advantages of the AOTF in such a system are given, along with the design considerations for construction of the instrument. A breadboard system which utilizes a desktop calculator is discussed, followed by the description of a dedicated microprocessor-based spectrometer. Experimental results are given for each system and an evaluation of overall performance is presented.
For the past several years, we have been building mechanically tuned birefringent filters for the spectral range 4500 - 7500 Å with bandpasses as narrow as 50 mÅ and stability of better than 5 mÅ. The filters can operate over a spectral range of -5 to 60°C and one version has been successfully shaken to the shuttle bay vibration specification. The tuning techniques used are mechanically simple and require minimal demands on encoders and drive electronics. Tuning to a random wavelength can be accomplished in less than one second while adjacent wavelengths can be reached in a fraction of a second. Tuning also results in considerable relaxation of the environmental requirements of birefringent filters.
Acoustooptic beam deflectors are finding increased application at infrared wavelengths. For deflectors operating at, say, 10.6 μm, the large drive powers required for efficient operation will heat the deflector material and may cause degradations in beam quality. To determine the extent of such thermal degradation in practice, the performance of an 800-spot and a 200-spot germanium deflector have been measured in our laboratory. Test results have shown less than 1/2 λ/D beam-quality degradation over time constants of one minute or less. These laboratory results have confirmed our theoretical expectations, which were based on a time-dependent thermal model of the acoustooptic deflector. Extensions of our analysis to other candidate infrared materials will be discussed.
Experimental results achieved with a multiple-position random access switch for 10.6 μm radiation are described. A set of cascaded Fabry-Perot interferometers produces a two-dimensional array of beam positions with a wide field-of-view.
Optical processors employing feedback can perform operations which are difficult or impossible to achieve with conventional optical techniques. As in electronic systems, optical feedback can i) introduce versatility and additional degrees of flexibility in attaining a processing function ii) utilize gain to extend processing capabilities and iii) introduce new nonlinearities as well as enhance those inherent.
The coupling efficiency between a GaAlAs laser diode and a Ti:LiNb03 waveguide has been examined theoretically and compared to experiment. An estimate of the coupling efficiency likely to be achieved in practice is given.
Optical Transient Experiments allow one to probe the coherent interaction of light with matter, as well as measure the time for energy transfer, T1, and loss of phase coherence, T2, in atoms, molecules and solids. Coherent optical transients are induced when the interaction of light with the sample is changed in a time which is short compared to T2. The interaction can be modified by changing the amplitude, frequency or phase of the existing laser light. This can be accomplished with an electro-optic modulator which makes it possible to electronically control a wide variety of laser experiments. The method of electro-optic laser phase and frequency switching with an extra-cavity modulator is discussed below.
Experimental liquid crystal 8-12μm infrared modulators have been fabricated and operated utilizing the cholesteric-nematic phase change effect occurring with cyanobiphenyl liquid crystals. A one mil thick film of 4-cyano-4'-pentyl-biphenyl has an average transmission over the whole 8-12μm region of ≈87%. Experiments to determine modulator response times and contrast are discussed as well as some of the basic limitations of infrared modulation using liquid crystals.
Silicon substrate is attractive for many guided-wave optical devices because of its superb electronic properties and the broad spectrum of fabrication technologies available. Acousto-optic interaction can provide active electro-optical functions for those devices on the electro-optically passive substrate. Hybrid material technologies such as the deposition of oriented ZnO thin film and the deposition of thin films with high photoelastic coefficients are employed for the generation of surface acoustic waves and for the preparation of optical waveguides for acousto-optic interaction. The performances of such devices are limited by surface acoustic wave propagation loss, by the acoustic wave dispersion on multi-layered structure, by restricted optical aperture dimension, and by the electromechanical conversion efficiency of the thin film piezoelectric element. The discussion involves a number of guided-wave acousto-optic devices with applications ranging from RF spectrum analysis, acousto-optical filtering, to optical interferometry.
This article describes the implementation and performance of a Joint Transform Correlator (JTC) which uses a G.E. Coherent Light Valve (CLV), a Hughes Liquid Crystal Light Valve (LCLV), and a vidicon. System design considerations and system performance are discussed in relationship to the wavefront modulator characteristics.
We report the influence of liquid-crystal (LC) thickness on the optical performance of a reflection-mode image transducer utilizing a nematic LC with a 45° twist. Our analysis includes all multiple-reflection effects arising from the multilayer structure of the device. These effects must be considered to obtain a full understanding of transducer performance when coherent light is used for image readout. The off-state intensity, ideally zero, varies strongly with LC thickness. This dependence can be markedly altered by the optically isotropic layers in the transducer. If the LC thickness varies across the transducer aperture, these layers can produce interference fringes in the off-state intensity. In the on-state, transducer sensitometry (i.e., coherent readout light intensity vs. incoherent writing light intensity) depends critically on LC thickness. LC thickness variations across the aperture produce complex on-state intensity behavior. This behavior is analyzed and its consequences for real-time tracking of moving objects by optical correlation are discussed.
The theoretical resolution of an electrooptic spatial light modulator [such as the Pockels Readout Optical Modulator (PROM)] is a function of the electrostatic field distribution arising from stored point charges located within the active electrooptic crystal layer. The Fourier transform of the voltage distribution (which can be directly related to the modulation transfer function) is derived as a function of the charge location within the electrooptic crystal. In addition, the resultant analytic expression contains the dielectric constants of the blocking layers and electrooptic crystal, and the thicknesses of the three layers. This formulation allows the effects of charge trapping within the bulk of the electrooptic crystal to be modeled. In particular, the low spatial frequency response decreases linearly and the high spatial frequency response decreases exponentially with the distance of the point charge from the dielectric blocking layer/electrooptic crystal interface. Thus the overall sensitivity and resolution are degraded strongly by charge storage in the bulk away from the interface. Utilizing superposition, this formulation can be readily extended to accommodate arbitrary charge distributions arising from different exposure parameters. The implications of these results for device design and operation are discussed.
The photo-DKDP spatial light modulator is found to be an excellent candidate real-time and reusable 2-D optically-addressed spatial light modulator. Extensive sensitometry, MTF, and other device data are reported with emphasis on a new field dependent photo-sensitivity and the importance of linear MTF data. A new spatial birefringent modulation transfer function is described and its use in obtaining more linear device response, bias level suppression, phase modulation and 1 msec erase time are noted. New real-time image processing, optical pattern recognition, and optical signal processing applications of this SLM are described with emphasis on the accuracy of the photo-DKDP based real-time experiments.
The diffraction limited resolution of photodichroic materials when used for holographic recording make them desirable candidates as spatial light modulators (SLM) in the frequency plane of the Joint Fourier Transform (JFT) correlator. Since the photodichroics generally have photosensitivities which are highly wavelength dependent, KF:LiF is a particularly good choice because its write and read sensitivities coincide with the strong 514nm and 488nm lines of the argon laser. Experimental results are reported on the use of KF:LiF as a SLM in a JFT correlator. Sections of a linear maximal length pseudo-random sequence with time-bandwidth products of up to 2.56 x 106 are successfully cross correlated.
The coherent light valve (CLV) processor is a video input signal processor which employs a CLV tube as a two-dimensional optical modulator . This CLV tube is a modified version of one used in a commercial General Electric TV Video Display Projector. CLV optical processors have been used for some time for wideband spectrum analysis and other specialized signal processing operations. They can provide large time-bandwidth processing with a 200 Hz or less spectral resolution over a 10 to 20 MHz bandwidth. Processor configurations range from the usual optical bench and optical table installations to electronic enclosures. Continued work has been directed to a system configuration more suited to the user environment. This paper discusses a current project development, termed the compact CLV Processor. In this configuration the processor is packaged to fit into a standard relay rack, almost the size of the present CLV projector unit. This configuration and extended versions will be described.
Characterization results of the photo-addressed thermoplastic device as an optical spatial light modulator in terms of gray scale and MTF are reported in this paper. Image recording is carried out with the use of a spatial carrier. Measured results indicate MTF response well in excess of 200ℓ/mm.
A time-integrating optical processor is configured for one-dimensional chirp correlation spectrum analysis. A unique optical implementation, utilizing TeO2 shear wave acousto-optic cells, is designed to operate in a configuration which allows wide bandwidth diffraction to both the positive and negative first diffraction orders. The result is high optical throughput efficiency, excellent fringe visibility, and a coaxial system design. Chirp linearity experiments confirm coherence requirements for chirp transform processors.
A time-integrating optical correlator was designed and built to detect the presence and relative delay of a digital signal buried in both synchronous and random noise. The system accepts a digital reference signal at data rates from 1 Mb/s to 100 Mb/s and cross correlates this with an analog signal of up to 200 MHz bandwidth. The output correlation covers a relative delay of 1 microsecond with a resolution of 3 ns. The correlation is performed in an optical subsystem employing two acousto-optic Bragg cells. Integration time can be varied from 2 ms to several minutes. Preprocessing of the incoming data, combined with digital postprocessing of the output from the optical subsystem results in correlation gains of over 50 dB. Greater data rates, delay range and bandwidths can be readily achieved
A novel wideband programmable filter using acousto-optical devices for both signal and filter function is described. The salieht features of this technique are the electronically programmable filter function, the wide bandwidth capability and the fine frequency resolution filter potential. The filter employes a traveling acousto-optic lens together with acousto-optical spatial modulators such that both the input signal, the lens system , and the filter travel at the same acoustic velocity. Programming of the filter function is achieved by modulating the acoustic signal delievered to one of the spatial modulators, which serves as the spatial filter. Thus, the temporal frequency filtering function is performed by the spatial frequency filter in conjunction with a traveling acoustic signal. The traveling acousto-optic lenses as well as filter enables the use of state of the art acousto-optic spatial modulators as filter element, providing fine frequency resolution and simple addressing. Resolution in the filter domain of a few thousand is easily obtainable. Simple demonstrative experiments have been conducted successfully.
Acousto-optic devices have been analyzed and developed for use as modulators, filters and correlators, and in spectrum analyzers. The ability of the acousto-optic unit to amplitude-modulate, frequency-modulate, and deflect laser beams, makes them suitable for these applications. The various signal processing and control functions provided by the acousto-optic devices are optimized by controlling the ratio of divergence angles of the optical beam and the acoustic beam in the acousto-optic interaction region. Another application of acousto-optic devices is to simulate moving targets to test fuzing altimeters and active missile control systems. The frequency modulation, amplitude modulation, and deflection properties of acousto-optic components are all utilized in a single acousto-optic unit to provide the variable range and Doppler simulation. Development of a suitable acousto-optic component for this simulation application is described, as is a description of a simulator employing this acousto-optic device.
Studies on polarization switching in shear wave Te02 Bragg cells indicate that background rejection through polarization filtering is possible. Theoretical analysis reveals frequency bands with near homogeneous eigenstates suitable for polarization filtering. Experimental results confirm a background scatter reduction of 13 dB with polarization filtering. A total dynamic range of 80 dB has been achieved in an acousto-optic device operating from 40 to 80 MHz.
Acousto-optic transducers are the obvious candidates for real-time high bandwidth signal processors. Several new applications and system architectures for acousto-optic signal processors are described. These include: a passive ambiguity surface processor, a space-variant acousto-optic system and a self-synchronizing spread spectrum system.