There has been considerable interest in recent years in the development of electronically tunable optical spectral filters. The applications for these filters range from rapid-scan spectrometry to multispectral imaging in remote sensor systems. The requirements on the characteristics of these filters include:
The Fabry-Perot interferometer is being applied increasingly as a tunable filter. The theoretical limits to its performance are considered, including the effects of phase change at multilayer reflecting coatings. Various forms of construction are discussed as are several ways of tuning Fabry-Perot filters. The review concludes with a description of a number of practical devices.
This article reviews the types and capabilities of birefringent filters. The general operating principles of Lyot (perfect polarizers), partial polarizing, and Solc (no internal polarizers) filters are introduced. Appropriate techniques for tuning each filter type are presented. Field of view of birefringent filters is discussed and is compared to Fabry-Perot and interference filters. The trans-mission and throughput advantages of birefringent filters are shown. Finally, the current state of the art in practical filters is reviewed.
This paper reviews the theory, techniques, and applications of acousto-optic tunable filters (AOTF). Two basic types of AOTF are described, i.e., the collinear and the noncollinear configurations. The important device characteristics of the AOTF presented include the bandpass response and spectral resolution, angular aperture, and the filter tuning relation. This review concludes with a discussion of various AOTF applications.
This paper describes the performance characteristics of an electronically tunable optical filter based on the electro-optic effect. Like the acousto-optic tunable filter, this device operates by selectively coupling principal polarizations in a birefringent crystal at a phase-matched wavelength by means of a spatially periodic refractive index perturbation. Instead of a traveling acoustic wave, however, the electro-optic tunable filter employs a temporally static electric field. The main advantages of this filter are its very low power consumption and its versatility of passband programming, by virtue of sepa-rately addressable voltages under microprocessor control. Two basic embodiments of the filter structure are described, one using longitudinal fields collinear with the light beam, the other using transverse fields in a "thick waveguide" configuration. Various examples of passband synthesis are described that result from apodizing the amplitude of the applied electric field perturbations. Experimental results of filter transfer characteristics at visible wavelengths are presented for the basic transfer function, for passband broadening, and for sidelobe suppression, together with theoretical computer plots showing excellent agreement. Typical driver voltages are 50 to 100 V.
Electro-optically tunable spectral filters have been developed over the years and now include a rather diverse collection of devices. These devices, which are of increased importance for remote sensing applications, employ linear electro-optic (Pockets) materials such that tuning is accomplished via externally controlled voltages. A brief review of the properties of linear electro-optic materials is presented followed by descriptions of tunable birefringence interference filters, Fabry-Perot devices, and other novel filter structures. Recent developments in electro-optic tunable filters are included. Issues relating to the further development of infrared tunable spectral filters are discussed.
A new approach to the study of ground clutter radiation in electro-optical missile seekers has been developed that utilizes a comparison between seeker responses for artificial and measured infrared (IR) scenes. Seeker responses were statistically very much the same for both types of input except at a few isolated points thought to be the commonly measured false targets. The result is that it seems false targets and general random fluctuations caused by background features can be treated separately.
Several techniques are described for boresighting 1 .06 Lm designa-tor lasers to visual, TV, and forward-looking infrared (FLIR) sights. One concept can be used to boresight all three types of sights, even in flight with relative motion between the laser designator system and the boresight module. Special factory hardware incorporating a CO2 laser, a reflective beam expander telescope, and a phosphor thermal imaging disk was developed for precision alignment of the laser input and FLIR output lines of sight of the boresight module. The operation and effectiveness of the boresight module is illustrated by airborne sensor video imagery and missile firing test results.
A computer code has been developed to predict the detector output from a translation-scanning optical tracker with realistic input scenes. The approach is based on Fourier transform techniques with some new manipulations being introduced in frequency space. The code has been validated by use of a laboratory tracker with complex extended targets and point sources.
A majority of the scene radiance detected at satellite altitudes can be due to atmospheric scattering by highly variable concentrations, compositions, and size distributions of aerosols. These same aerosols are implicated in the anomalous enlargement of laser beam images and can influence effective use of laser beacons as navigational aids, photometric standards, or probes of boundary-layer air masses. A rapid field method for aerosol particle collection and spectral analysis now allows determination of such aerosol sources, strengths, and sinks within the atmosphere, and can aid in providing required "ground-truth" for truly remote identification of aerosol characteristics. Specific illustrations of the striking changes of aerosol composition are provided for continental, marine, and coastal regions, highlighting the unique enrichment of nitrate particulates in areas of coastal upwelling. Spectra characteristic of these differing atmospheric particulates, as collected in numerous field studies, are included for the mid-infrared range. A special feature of these data is their direct revelation of chemical species of covalently bound elements in a completely nondestructive manner.
A technique for building a high resolution linear focal plane using a single low resolution area detector array is described. A silicon area imager is fabricated with an opaque light shield layer having small clear apertures located over the detector elements which define the picture elements. By displacing the apertures sideways from one row of detectors to the next and using the array in conjunction with a multiline data buffer, a line scan resolution equal to the total number of detector elements in the area array is achieved. The design consider-ations and tradeoffs involved in using such a technique will be discussed. A practical realization of such a system will be shown which uses an array built with a 4 X 1024 element configuration to achieve 4096 resolution elements per line. Multispectral capability is achieved by using four such arrays fabricated on the same device with spectral filtering provided by patterned interference filters on the cover slip. Array output and results of device characterization will be shown.
The hole grating beam sampler consists of a uniform array of small circular holes in a highly polished mirror. When used as a turning flat in a high-energy laser (HEL) optical train, it produces a multiplicity of angularly separated low-power replicas of the HEL beam in both the transmitted and reflected directions. It thus provides the opportunity to perform many simultaneous diagnostic tests such as power monitoring, bore-sight alignment, jitter sensing, wavefront sensing, power-in-the-bucket measurements, and spectral purity measurements in real time during HEL operation on a noninterference basis with the intended mission. The principle of the hole grating beam sampler will be discussed in detail, and several relationships useful to the HEL systems engineer will be derived. The capabilities and limitations of this versatile HEL diagnostic tool will then be discussed in the context of several specific applications.
Various planar configurations of the luminescent solar concentrator (LSC) are discussed including the uniformly doped, the stacked plate, the thin film, and the multilayered film LSC. Radiation which is lost from the luminescent plate by falling within the critical angle for total internal reflection is examined in terms of the above configurations. A ten parameter efficiency profile has been developed to better evaluate the collector performance and to allow direct comparison with other photovoltaic devices. A collector efficiency of 3.2% is reported and the feasibility of 8 to 10% efficient devices discussed. Several optical distribution factors have been defined and experimentally evaluated for the luminescent plate and associated photovoltaic cell. These factors have been used to calculate a plate-to-cell optical coupling coefficient for two commercially available solar cells. LSCs based on organic and inorganic phos-phor systems are compared and the problem of dye staility discussed.
Thermo-optical tests were undertaken to evaluate optical effects of aircraft internal-compartment air turbulence. The turbulence sources affecting camera performance included a cold vertical vehicle window and an oscillating camera tube. A mock-up of the camera tube, compartment, and window was fabricated and interferometric data taken under simulated flight airflow and temperature conditions. Test results were initially obtained separately with a heated tube and with a simulated cold vehicle window. Test results of the combined turbulence sources indicated an rms wavefront error due to turbu-lence effects controllable in the range of 0.054 A and 0.071 A at 0.6328 um, depending on vehicle window temperature levels and compartment airflow conditions.
This paper examines the problem of registering a recon-naissance side-looking synthetic aperture radar (SAR) image to a three-dimensional reference map. The registration technique developed in the present work is based on computing an image-to-data base correspondence in terms of a SAR sensor model as a function of such parameters as altitude, range, scale, etc. If their exact values are known, the model can precisely predict the two-dimensional image coordinates for any three-dimensional data base point, thereby accomplishing registration. However, the platform ephemeris data usually provide only model parameter estimates. The objective, then, is to improve them so that the model can predict the image location of any data base point within a desired accuracy range. Preliminary investigations demonstrate the feasibility of achieving location accuracy within 50 m.
A miniature holocamera is described which utilizes cantilever beam vibrations of a ruby rod as a Q-switch mechanism. Up to three Q-switched pulses are obtained per pumping cycle. Triple-pulsed holograms provide information from which particle sizes, velocities, and accelerations are directly measured. The holographic record provides an image of a volume equal to a 35 mm still frame area in cross section and approximately 15 cm deep. Hand carrying ease of portability makes this camera especially convenient to move from site to site for short-term measurements.
This paper discusses a technique for converting digitized oblique low-altitude aerial photography to gray level imagery as seen from other viewing perspectives. The technique makes use of one- and two-camera perspective transformations. Inputs to these transformations are camera model information and a stick figure representation of the scene. The required camera model information is extracted from the imagery in conjunction with limited map data. Three-dimensional stick figure models are constructed interactively using the camera model and the digitized photograph. The perspective transformations are applied to the gray level image, pixel-by-pixel, and hidden surface corrections are performed to produce the new perspective views. The resulting images maintain the noise and resolution qualities of the original digitized imagery.
Fundamental waveguiding properties of amorphous As2S3 film and some applications to passive optical grating devices are presented. The propagation losses of amorphous As2S3 film waveguides were measured to be 4.4 dB/cm at 0.633 ,um and 1.4 dB/cm at 1.153 um. Chalcogenide amorphous semiconductors are characterized by the photoinduced refractive index change. Utilizing this phenomenon, a grating with the period of 0.35 um and the length of 0.7 mm was fabricated in a As2S3 film waveguide as the grating deflector with a two-beam interference technique. A guided wave of 1.153 um, TM0 mode, has been deflected with the deflection angle of approximately 90° and with the efficiency of 100%. With the same technique, a grating coupler with the period of 0.55 um and a slant angle of 10.5° was fabricated, and the coupling efficiency was 1.3% at 1.153 Am.
The number of data points transmitted by an optical waveguide can be greatly increased by assigning one narrow wavelength region to each data point. The technique, called "wavelength or spectral multiplexing," allows hundreds of data points to be transmitted simultaneously over a few fibers. This paper outlines the concept of a two-dimensional-grating multiplexer (and demultiplexer) and includes factors governing the design, test techniques, and experimental results derived after constructing such a system. This experimental system simultaneously transmits and reconstructs 5 X 103 high quality data points from a two-dimensional object through 50 fibers in a linear array. Information concerning resolution, system efficiency, distortion, wavelength range, and the selection of components is included.
The digital image analysis system (DIAS) is a software-oriented, computer-based approach to automatic image analysis. In this paper, some aspects of DIAS are discussed that can be performed with commercially available image pickup equipment attached to an optical microscope and linked to a sensor-based small computer (such as a System/7 or a Series/1). Through the application of an optical edge detection program, the system is adapted to precision measurement of linewidths and other parameters on semiconductor and garnet wafer patterns.
Scaling presently available excimer laser systems to lasers designed to operate at high average power and high pulse repetition rates for long periods of time requires advances in many areas of engineering technology. For economical application to industrial processes, the efficiency must be increased. This leads to more stringent requirements on preionization techniques, energy delivery systems, and system chemistry. Long life operation (>109 to 1010 pulses) requires development of new pulse power components, optical ele-ments, and flow system components. A broad-based program underway at the Los Alamos National Laboratory is addressing these key technology issues, with the help of advanced component and systems development programs in industry. A prototype XeCI laser meeting all requirements for efficiency, system performance, and life is scheduled for completion in 1985.
A quarter-meter diameter, modified Ritchey-Chretien camera is described for spacecraft based observation of Halley's Comet. The system will operate over an unusually broad spectral region, from 0.121 6 um (the hydrogen Lyman-alpha line) to 1.1 pm using reflective primary and secondary mirror optics and a thin UV-grade lithium fluoride (LiF) refractive element as a field corrector near the image plane. The f/12, 3000 mm focal length camera has a 0.4° full field of view for operation with a 15 X 15 mm charge-coupled device (CCD) detector array. The UV-enhanced silicon CCD detector is a 1000 X 1000 array of 15 um pixels which establishes the system resolution requirement. The optical design and performance are discussed, with an emphasis on the image analysis with respect to pixel resolution elements used for viewing extended objects such as comets.
The superior electronic properties of GaAs, as compared with silicon, make possible the achievement of much higher performance levels in GaAs signal processing devices than have been demonstrated with silicon. Only recently, however, have advances in GaAs materials and processing technology made possible the fabrication of such devices as sub-100 ps propagation delay, high density planar GaAs in-tegrated circuits with large-scale integration (LSI) compatible power levels,1 and high transfer efficiency GaAs charge-coupled devices2 which should be capable of multi-gigahertz clocking rate operation. These high performance device technologies should have major impact on the high speed signal processing area, making possible, through their much higher speeds and lower power requirements, system approaches which could not be practically realized with existing silicon technology. In this paper the advantages of GaAs for high speed signal processing are reviewed and laboratory results obtained with high speed GaAs devices are reported.
A method is described for high-resolution remote three-dimensional mapping of an unknown and arbitrarily complex surface by rapidly determining the three-dimensional locations of M x N sample points on that surface. Digital three-dimensional (3-D) locations defining a surface are acquired by (1) optically transforming a single laser beam into an (expanded) array of M x N individual laser beams, (2) illuminating the surface of interest with this array of M x N (simultaneous) laser beams, (3) using a programmable electro-optic modulator to very rapidly switch on and off specified subsets of laser beams, thereby illuminating the surface of interest with a rapid sequence of mathematical patterns (space code), (4) image recording each of the mathematical patterns as they reflect off the surface using (a) a wavelength-specific optically filtered video camera positioned at a suitable perspective angulation and (b) appropriate image memory devices, (5) analyzing the stored im-ages to obtain the 3-D locations of each of the M x N illuminated points on the surface which are visible to the camera or imaging device, and (6) determining which of the laser beams in the array do not provide reflec-tions visible to the imaging device. Space coding of the light beams allows automatic correlation of the camera image (of the reflected spot pattern from the surface) with the projected laser beam array, thus enabling triangulation of each illuminated surface point. Whereas ordinary laser rangefinders aim and project one laser beam at a time and expect to receive one laser beam reflection (bright dot image) at a time, the pres-ent system is optical (nonmechanical and vibration-free) and can collect all the data needed for high-resolution 3-D topographic mapping (of an M x N sample of surface points) with the projection of as few as 1 + log2N light patterns. In some applications involving a rapidly changing time-dependent environment, these 1 + log2N patterns can be projected simultaneously in different wavelengths to allow virtually
Gas breakdown, or the ionization of the air in the path of a high power laser, is a limit on the maximum intensity which can be propagated through the atmosphere. When the threshold for breakdown is exceeded, a high density, high temperature plasma is produced which is opaque to visible and infrared wavelengths and thus absorbs the laser radiation. The threshold in the atmosphere is significantly lower than in pure gases because of laser interaction and vaporization of aerosols. This aspect of air breakdown is discussed in detail. Parametric studies have revealed the scaling laws of breakdown as to wavelength and laser pulse duration, and these will be discussed and compared with existing models. A problem closely related to breakdown is the plasma produc-tion when a high intensity laser interacts with a surface. In this case, the plasma can be beneficial for coupling laser energy into shiny surfaces. The plasma absorbs the laser radiation and reradiates the energy at shorter wavelengths; this shorter wavelength radiation is absorbed by the surface, thus increasing the coupling of energy into the surface. The conditions for the enhancement of laser coupling into surfaces will be discussed, particularly for cw laser beams, an area of recent experimen-tal investigation.
Experimental liquid crystal 8 to 12 Am 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 to 12 Am 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.
The potential for developing infrared imaging systems with both and passive capabilities is examined. Active and passive systems are red and several approaches to dual active/passive systems are outlined. le systems are postulated and their performance is analyzed. Technol-d utilization considerations are discussed.
The Kingslake Awards The Rudolf Kingslake Medal and Prize for the most noteworthy original paper to appear in Optical Engineering Volume 19 (1980) has been awarded to G. Ferran° and G. Hausler of the Physikalisches 'Institut of the Universitat Er-langen, Germany, for their paper, "TV Optical Feedback Systems," which appeared in the July/August 1980 issue, pages 442-451.