Spatial light modulators (SLMs) form the heart of several current and future optical technologies. These include but are not limited to optical memories, adaptive optics or wavefront correction, active optical elements, femtosecond pulse shaping, neural nets and, of course, optical computing. Over the past 20 years many interestng and technically viable modulators have been developed. However, as none of these areas have yet to breach into high volume consumer markets, affordable high quality SLMs have been euphemistically 'scarce'. The intent of this paper is to assimilate information on a variety of SLM technologies and to explore their advantages and disadvantages, be they real or perceived. We mention several new technolgies; however, the main emphasis is on devices which are currently commercially available.
This paper discusses the development of an optically addressed smart pixel spatial light modulator (SLM). The term 'smart pixel' refers to the ability to modulate the phase of a read beam with greater than eight phase levels. The modulation is a function of the output of four photoactive sites per pixel. In this design the four photoactive sights surround a modulator element and are addressed by four independent write beams. The modulator is controlled by the output of a circuit connected to the photodetectors. A novel addressing scheme which utilizes diffractive microlens arrays is also presented.
Pattern recognition algorithms make consistent measureable comparisons among image sets. In this application, normal patient patterns are recongnized and the degree of difference from normal indicates a medical diagnosis of either low- or high-probability of pulmonary embolism. The figure of merit for this study is the vector inner product between the Fourier transforms of each patient image and a filter. The medical application lends itself to implemenation in an optical correlator.
In this paper we have demonstrated a wavelength tuning scheme using a twisted nematic liquid crystal based LCTV operated in an amplitude coupled binary phase mode for three wave mixing devices. A cosine chirp that is written on the LCTV by electrically addressing its pixels through a personal computer, functions as a programmable spherical lens. Deflection of the input beam is achieved by introducing tilt aberration on the lens. A spot resolution of less than 1.65 arcsec, and a steering range of 2 degrees at a wavelength of 632.8 nm were achieved by continuously varying the tilt coefficient of the lens. The potential advantages of this scheme are speed, compactness, operational ease, and power consumption.
In this paper, we present our design of an integrated optics rotation sensor (IORS). Based on rugged glass-on-silicon technology, this gyro represents a versatile, low cost approach to high tech commercialization. The IORS is suited for a number of applications, including robotics, navigation, and guidance, and as an automotive safety device. In terms of compactness and ease of use, the IORS presents an improved approach to rotation sensing over other gyro technologies. In this paper, we present the basic operation of the gyro, along with a brief explanation of its use as an automotive yaw sensor.
Liquid crystal televisions (LCTVs) have become very popular spatial light modulators. Their polarization and phase modulation capabilities allow them to be used as inexpensive spatial light modulators in a wide variety of applications. The design of a dynamic Hermann wavefront sensor system is described. A LCTV is used as an aberration generator in an optical system. A LCTV is also used as a Harmann wavefront sensor to measure the aberrations. Experimental results characterizing the LCTVs performance as an aberration generator and Harmann wavefront sensor are presented.
Embedded optical fiber sensing (OFS) array, signal processing neural network (NN), and embedded micro actuators are three key techniques for optical fiber smart structures. On the basis of the former two techniques, a smart structure state monitoring system is described. The microbending OFS array configured with many rows and columns of fibers laid perpendicular to each other, and the NN signal processing based on the outgoing light intensity pattern is suggested. The actual series experiments on smart skins, gypsum members, and reinforced concrete members, in which the OFS array is embedded, are given and analyzed in detail. The experimental results show the effectiveness of the OFS array and NN signal processing method proposed by the authors. The system can monitor inner states of the structures, such as strains or stresses or cracks, and has potential applications in areas of space aeronautics, ship and civil engineering, etc.
The intent of this paper is to introduce sampled-aperture, coherent, laser imaging techniques for application to diverse technical and commercial problems that can benefit from the unique advantages of laser imaging. A brief overview is presented of the principles underlying sampled-aperture laser techniques and several example techniques are described along with a discussion of their attributes. Six potential applications are offered that could benefit from unconventional laser imaging methodologies, algorithm advances, or even direct implementation.
The design of a novel wavefront sensor is presented. The wavefront sensor is an extension of the classical Hartmann wavefront sensor. Analogous to the operation of the classical Hartmann wavefront sensor, each lenslet forms an image of the object, but shifted by an amount proportional to the average tilt across its subaperture. An optical information processor system is used to compute the location of each image relative to each other formed by the lenslets.
Recent advances in CCD fabrication techniques have brought on the omnipresence of low cost monochrome CCD cameras. Full color cameras are traditionally made in one of the following two ways: subpixelated color or multiple camera schemes. Each system offers a unique set of tradeoffs. Noteworthy among these is the prohibitive expense of high resolution full color image capture systems. We present a novel technqiue for transforming a single (monochrome) high resolution CCD camera into a full color camera via the use of a fast switching ferroelectric four state color filter. Using all off-the-shelf componentry and color balancing software, we present full color images of stationary scenes constructed from their respective RGB space images.
Neural network (NN) architectures provide a thousand-fold speed-up in computational power per watt along with the flexibility to learn/adapt so as to reduce software life-cycle costs. Thus NNs are posed to provide a key supporting role to meet the avionics upgrade challenge for affordable improved mission capability especially near hardware where flexible and powerful smart processing is needed. This paper summarizes the trends for air combat and the resulting avionics needs. A paradigm for information fusion and response management is then described from which viewpoint the role for NNs as a complimentary technology in meeting these avionics challenges is explained along with the key obstacles for NNs.
The interferometric mode of the ESO very large telescope (VLT) permits coherent combination of stellar light beams collected by four telescopes with 8m diameter and by several auxiliary telescopes of the 2m class. While the position of the 8m telescopes is fixed, auxiliary telescopes can be moved on rails, and can operate from 30 stations distributed on the top of the observatory site for efficient UV coverage. Coherent beam combination can be achieved with the 8m telescopes alone, with the auxiliary telescopes alone, or with any combination, up to eight telescopes in total. A distinct feature of the interferrometric mode is the high sensitivity due to the 8m pupil of the main telescopes which will be compensated by adaptive optics in the near-infrared spectral regime. The VLT interferometer (VLTI) part of the VLT program is conceived as an evolutionary program where a significant fraction of the interferometer's functionality is initially funded, and more capability may be added later while experience is gained and further funding becomes available. Major subsystems of the present baseline VLTI include: three auxiliary telescopes, three delay lines which permit combining the light from up to four telescopes, and a laboratory which contains an imaging beam combiner telescope, and enough space to accomodate a number of experimental setups. This paper presents a general overview of the recent evolution of the project and its future development.
We present an application of an iterative deconvolution algorithm to speckle interferometric data. This blind deconvolution algorithm permits the recovery of the target distribution when the point spread function is either unknown or poorly known. The algorithm is applied to specklegrams of the multiple star systems, and the results for (zetz) UMa are compared to shift-and-add results for the same data. The linearity of the algorithm is demonstrated and the signal-to-noise ratio of the reconstruction is shown to grow as the square root of the number of specklegrams used. This algorithm does not require the use of an unresolved target for point spread function calibration.
The large binocular telescope (LBT) will have two 8.4 m apertures spaced 14.4 m from center to center. Adaptive optics will be used to recover deep, long exposure diffraction-limited images in the infrared. The LBT configuration has a diffraction-limited resolution equivalent to a 22.8 m telescope along the center-to-center baseline. Using simulated LBT images and an iterative blind deconvolution algorithm (IBD--Jefferies and Christou, 1993) a sequence of three exposures, at sufficiently different parallactic angles, allows recovery of imagery nearly equivalent to that of the circumscribing 22.8 m circular aperture. To establish a credibility basis for these simulations we have studied the performance of IBD for image constructions of several examples of atmospherically perturbed and partially corrected stellar and galactic data. IBD is robust against influences of real, non-ideal data obtained from large astronomical telescopes, including partial anisoplanicity and Poisson noise from object, sky, and thermal background. For faint objects, which are sky-background and photon-statistics limited, the use of adaptive optics is presumed in these simulations. IBD removes the dilute aperture point spread function effects in the set of parallactic angle-diverse images linearly combined to produce the circumscribed aperture result. Optimal image combination strategy is considered for multi-aperture imaging array configurations.
The very large telescope interferometer (VLTI) will relay stellar beams from each individual telescope to the combining facility through an air path, as opposed to vacuum. Internal air turbulence will induce optical path fluctuations which have been taken into account in the global VLTI error budget and in the expected performance of the delay line control system. This paper presents experimental data used to validate the assessment of these turbulence effects. A comparison with theoretical models developed for the free atmosphere is included in order to investigate their applicability in the VLTI delay line tunnel. Optical path fluctuations were measured by a laser interferometer working in the 0-80m range, in an underground tunnel representative of the VLTI beam transport facilities. The derived index structure coefficient, Cn2, inside the tunnel has been compared with high sensitivity temperature measurements.
The Air Force Phillips Laboratory is in the process of demonstrating an advanced space surveillance capability with a heterodyne laser radar (ladar) system. Because coherent detection is used and the range resolution is obtained by the narrow pulse widths, satellite images can be reconstructed in two ways. The first is to convert the ladar returns into intensity projections, and then use tomographic techniques to create the image. The second approach is to use synthetic aperture radar methods which reconstruct the E-field image of the object and then square the result to get an intensity image. In this paper, these two methods are compared using computer simulations. It is shown that images reconstructed from intensity projections are of much higher quality than images reconstructed from E-field projections. This is shown to be true for both high and low light levels, and for full or limited sets of views.
Active imaging techniques are described that have minimum transmitter aperture redundancy and maximum transmitter intermittency. The proposed techniques are variants of Fourier telescopy. These techniques largely overcome conventional signal limitations by encoding the image information in the time domain. The basic approach combines long-baseline interferometry with phase closure to obtain high resolution images with very low average transmitter power, by proper choice of phase closure strategy. Several strategies are discussed and simulation results are presented.
Theoretical problems are discussed and experimental results are presented with regard to the recording of large holograms of moving objects by the aperture synthesis method at optical wavelengths. It is shown, that for the recording of a hologram by a linear array of unphased coherent receivers, the phase distortions of the signal field can be compensated by a post- detector signal processing, based on the construction of joint statistical estimates of the phase distortions and the image of the object. As a result of the processing of the recorded holograms, an image of various distant extended objects is obtained with very high angular resolution.
This paper concerns analysis of imaging sensor called the passive synthetic aperture imaging (PSAI) sensor that is useful for deep-space imaging. The PSAI sensor concept is similar to some other interferometric imaging systems, except that a grating interferometer, which provides improved achromaticity, is used to interfere the light. In this paper we discuss features of the PSAI sensor concept including the optical system and image formation process.
Optical designs for phased-array imaging telescopes, and for interferometers with sparsely filled apertures, using simple two-mirror subtelescopes are discussed. Limitations to the field of view caused by Petzval curvature and distortion are discussed. Methods for reducing or correcting Petzval curvature in these systems by the use of off-axis paraboloidal mirrors are presented and design examples are given. It is shown that a 2.4 times increase in the field of view is possible for 10m diameter arrays, but that residual distortion precludes application of the method to the 50m diameter interferometers considered.
The problem of the short-exposure image restoration for post detection turbulence compensation in minimum-redundancy linear optical arrays is considered for general a priori limitation. A decision procedure for solving the problems of this class is proposed that consists in formalized registering the a priori information, finding a stable (regularized) solution and devising a numerical restoration algorithm, which based on the global optimization methods. It is shown that this optimization procedure can be used for image restoration in arbitrary optical aperture synthesis systems. An example of the undistorted test restoration is given.
The theory of operation of a coherent microscope based on synthetic aperture imaging concepts is described. It is a holographic approach that consists of sensing the 3D Fourier transform components of an object in time sequence and reconstructing the image by inverse transforming these components. The process results in complex-valued images in either two or three dimensions. The synthetic aperture microscope sensing method is to construct a synthetic lens composed of a multiplicity of optical plane wave transmitter and receiver pairs. Each of the pairs senses a diffraction grating in the object defined by a Bragg scattering condition. By using array transmitters and receivers, parallelism can be introduced that provides greater sensing speed. Because the data from the synthetic aperture microscope is digitally recorded, it is easy to correct for systematic phase errors in the sensing and to implement common image processing and rendering algorithms.
The application of stellar interferometry to the observation of laser illuminated artificial satellites is investigated for a relocatable imaging system. Because of the satellite surface roughness, the light scattered from the surface can be considered spatially incoherent, creating laser speckle at the pupil of the array. The speckle noise is manifested in loss of fringe constrast because of the intensity distribution of speckle is predominately dark for aperture areas small compared to the characteristic speckle dimension. Speckle mitigation by use of aperture diameters larger that the speckle size result, on average, in fringe visibility errors because each aperture resolves the satellite. We quantify these effects in terms of incoherent imaging theory and discuss situations where speckle can be mitigated by the motion of the satellite. A conceptual design of a relocatable interferometer is shown with results of simulations which exclude the effects of the atmosphere.
This paper presents a system for performing real-time vehicular self-location through a combination of triangulation of target sightings and low-cost auxiliary sensor information (e.g. accelerometer, compass, etc.). The system primarily relies on the use of three video cameras to monitor a dynamic 180 degree field of view. Machine vision algorithms process the imagery from this field of view searching for targets placed at known locations. Triangulation results are then combined with the past video processing results and auxiliary sensor information to arrive at real-time vehicle location update rates in excess of 10 Hz on a single low-cost conventional CPU. To supply both extended operating range and nighttime operational capabilities, the system also possesses an active illumination mode that utilizes multiple, inexpensive, infrared LEDs to act as the illuminating source and retroreflectors as the system targets. This paper will present the design methodology used to arrive at the system, discuss the overall system concept and process flow, and will briefly discuss actual results of implementaing the system on a standard commercial vehicle.
We describe a nematic liquid crystal spatial light modulator which can be used a high quality wavefront controller. We present results showing the open loop correction of wavefront aberrations and demonstrate wavefront shaping, by the production of the first 12 significant Zernike modes.