This PDF file contains the editorial “Editorial: SPIE's 40th Anniversary” for OE Vol. 34 Issue 09

This PDF file contains the editorial “Guest Editorial: Optical Science and Engineering in Finland” for OE Vol. 34 Issue 09

The fabrication of epitaxial compound semiconductors and optoelectronic devices at Tampere University of Technology is described. First, we determine the bandgap energies of technologically important quaternary alloys Ga_xIn_1-xAs_yP_1-y, grown by gas-source molecular beam epitaxy, which are lattice-matched to InP and GaAs throughout the entire composition ranges. Second, we discuss state-of-the art aluminum-free laser diodes intended for pumping light at λ = 980 nm into erbium-doped optical fiber amplifiers. Third, principles of a new kind of avalanche photodetector (APD), a "unipolar" APD, are discussed. Our attempt is to develop a compound semiconductor detector that could detect an event of single-photon absorption. Finally, we present a photovoltaic fiber optical power converter. This device is comprised of a series of photovoltaic cells and an integrated 980-nm data laser. It is designed to convert the fiber-coupled optical power launched by a transmitter laser at 780<λ<850 nm into electrical power.

We have fabricated InGaAsP/InP separate confinement heterostructure multiple quantum-well lasers emitting at 1.43 μm, which corresponds to a local absorption maximum of liquid water. We have carried out the modeling of the laser, including the calculation of gain, optical confinement factor, and threshold current. The laser structure was grown by atmospheric-pressure all-organometallic vapor-phase epitaxy. The study of the laser structure growth indicates that tertbutylphosphine partial pressure of 2.0 Torr or larger is necessary for the growth of layers with mirrorlike surface morphology. Photoluminescence measurements show that the optimum purge time between the growth of quaternary quantum-well and barrier layers is from 0.5 to 2.5 s. Lattice-matched 500-μm-long devices with three quantum wells lased at a threshold current density of 1.35 kA/cm² with external quantum efficiency of about 25% per facet. These results are comparable with the results reported for lattice-matched 1.3- and 1.55-μm devices, and are in agreement with the values obtained with laser modeling.

A preform process permitting 80-km drawable-fiber length has been developed. Important aspects of the preform deposition and collapsing are discussed. High-quality preforms have been manufactured with two types of synthetic tubes. For single-mode fibers having 9.0-μm mode-field diameter with slightly depressed-index cladding, the median attenuations on a production basis are better than 0.335 and 0.20 dB/km at 1310 and 1550-nm wavelengths, respectively.

The equipment and the process for soot sleeving of optical fiber preforms made by the modified chemical vapor deposition (MCVD) method has been developed. The equipment consists of a soot-sleeving lathe that is used for deposition of soot glass particles onto the surface of an MCVD core preform and a separate furnace that is used for drying and sintering the deposited porous glass layer. An outline of the equipment is presented. This equipment has then been used to study the basic parameters of flame hydrolysis deposition as well as sintering of the porous layer. The raw material and the fuel gas flow as well as the substrate diameter proved to be the most important parameters affecting the process. The basic knowledge achieved is used to optimize the process for three different preform sizes. In the soot-sleeving process for 80-km optical fiber preforms, an average deposition rate of 5.2 g/min is achieved with a double burner. The overall quality of the drawn fiber proved to be good. The typical attenuations were 0.330 and 0.215 dB/km at 1310- and 1550-nm wavelengths, respectively. The geometry of the drawn fibers was found to be very good.

A computer-aided design system for integrated devices is described, combining guided-wave circuit layout design and beam-propagation simulation of waveguide optics. The layout design includes efficient ways of generating various waveguide routing geometries, interfaced with generation of data on the lithographic mask geometry and with a simulator of optical propagation through waveguide devices. The optical simulator includes two alternative algorithms for the beam-propagation method algorithms. Additional modules connected with specific waveguide fabrication technologies are described. Calculation examples are included, applying the system to problems handled in published literature.

The feasibility of a passive fiber optic bus that uses bidirectional integrated optics bus access couplers is studied. The number of bus nodes can be increased when an optimized asymmetric bus access coupler is used instead of a symmetric one. A modified Ag-Na ion exchange process provides the possibility of manufacturing integrated optics asymmetric couplers for the bus. The process is suitable for the processing of integrated optics components that combine deep multimode waveguides with different core sizes on a single substrate. An access coupler is designed, modeled, and processed. The coupler is optimized for seven nodes; the asymmetry of the coupler is 5 dB.

The operation of optical narrowband reflection filters based on resonance anomalies of waveguide gratings is well established for gratings of infinite extent. We investigate the properties of finite-aperture waveguide-grating resonance filters by means of rigorous electromagnetic theory and an approximate model. The rigorous approach illustrates the scattering of optical energy from the guided mode at and near the edges of the element, which leads to a reduced diffraction efficiency into the backward-diffracted zeroth order. The approximate approach provides an optical-engineering model for the estimation of the minimum grating size required to achieve a high resonance-wavelength reflectivity.

With uniform partially coherent illumination of a sufficiently small coherence width, linear holographic axicons produce an extended axial line image of a desired length and a nearly uniform intensity. This result is demonstrated by numerical diffractive simulations based on generalized axicon phase functions and the space-frequency theory of optical coherence. The conclusions are also verified through analytical calculations employing the asymptotic method of stationary phase.

An optical surface roughness and waviness sensor for sufficiently smooth surfaces is introduced. The sensor is based on a computer-generated hologram , which focuses the specular reflectance to desired spots on the focal plane. Computer simulations were made to analyze the system and to compute the calibration curve. Contrast analysis was performed to estimate the average roughness of Gaussian surfaces. Results are compared with data measured form surface roughness standards.

An approach to quantitative evaluation of surface plasmon resonance (SPR) measurements is given. In order to determine thicknesses and refractive indices, measured SPR curves were fitted numerically with a theoretical model based on the Fresnel equations. The model also takes into account additional roughness layers, representing real properties of the interfaces, which significantly improves the fitting accuracy. The intermediate layers have physical thicknesses and effective complex refractive indices, responsible for light losses at the interfaces. This model has been verified by direct measurement of surface roughness with the help of an atomic force microscope (AFM). It has turned out that the fitting procedure can reveal microstructures of a substrate and deposited layer, and when combined with the AFM for roughness measurement, this model can find simultaneously both the thickness and the refractive index of the metal film.

Long optical fibers embedded inside a composite material enable on-line monitoring of the manufacturing process and can also be used to assess damage by monitoring strain and cracks in the structure. We report on measurements of the microbending loss in long single-mode and multimode telecommunication fibers that were successfully installed inside the walls of composite containers during the normal manufacturing process. The work was largely empirical, and theoretical aspects were not investigated thoroughly. The measurements were carried out using the optical through power technique and a commercial optical time-domain reflectometer that gives not only the attenuation but also spatial information. We found that the coating material and its thickness have an effect on loading sensitivity and on the applicability of the method for cure monitoring. Although we discovered that repeatability issues are a problem, the measurement data indicate that the end-of-curing process of the composite can be determined by optical sensing fibers and damaging external loading of the structure can be detected.

We introduce a new technique to measure the amplified spontaneous emission and signal gain of the fiber amplifier. We have constructed a measurement system that consists of a tunable signal source, a fast chopper, detectors, and a microcomputer. This amplifier measurement system can be used to characterize amplifier properties as a function of input signal wavelength and power.

Implementation and test results of a 3-D sensor based on time-of-flight (TOF) laser radar are presented. A sensor capable of measuring 3-D positions and orientations in a large working space is used for interactive teaching of robot paths and environments. It consists of a pointing device, a laser rangefinder, and a video tracker. The 3-D position and orientation of the pointer are obtained by measuring the distance from two separate points on the pointer arm to a tracking receiver and by using the tracking-camera image for detecting the angle of the pointer on the plane that is perpendicular to the optical axis of the tracking system. The rangefinder uses a new active target operating principle, including fiber-coupled transmitters attached to the pointer arm. The distance and angle measurement accuracies were measured to be better than ± 5 mm and ± 5 deg in the ranges of 2.3 to 4.7 &mum and ± 40 deg, respectively, using ordinary technology. The operating range is likely to be increased and the accuracy enhanced by using the latest state-of-the-art TOF rangefinding technique.

A laser rangefinder-based experimental 3-D coordinate meter with vision guidance for pointing at visible features is presented, and its measurement performance on circular tags is tested. This is conceived of as part of an automated 3-D measurement system concept that comprises CAD-based graphical measurement planning producing a measurement model file (MMF), actual measurements executed accordingto the information in the MMF and with vision guidance, analysis of the results, and decisions regarding acceptance of the part measured, etc. The automated 3-D measurement will be used for dimensional control of large manufactured objects and in other industrial applications. The experimental system is based on the Acman 200 Automatic 3-D coordinate meter, in which the target aiming sensor based on a positionsensitive detector is replaced with the vision sensor and a control system. The control system guides the laser rangefinder to point at the nominal position of the target point according to the information in the MMF, then the vision sensor is used to guide the measurement exactly to the target point, and finally the range value and the two corresponding angle values to the target point are measured and the related x,y,z coordinates are computed. The vision sensor employs template matching and centroid methods for coarse and accurate tag finding, respectively, and it is situated on the same optical axis with the laser rangefinder and red pointing beam in the measuring head of the coordinate meter. Experimental results on the reliability, repeatability, accuracy, stability, and execution time for pointing operation and 3-D coordinate measurement are given in a measurement volume of 13 x 11 x 4 m. The repeatability of the vision-guided pointing was better than 0.02 mrad, and the worstcase pointing accuracy was 0.04 mrad. The repeatability of the 3-D coordinate measurements was 0.3 mm (standard deviation for x,y, and z) on the average, and the accuracy was better than ± 1 mm (for x,y, and z) for 81% of the measured coordinate values.

Responsivity calibrations of trap detectors with a cryogenic absolute radiometer and a power-stabilized 543.51 6-nm He-Ne laser are described. The trap detectors are used as transfer standards of optical power at visible wavelengths because they have a predictable spectral responsivity. New results on transmittance measurements of PRC Krochmann V(λ) filters are presented, especially the angle and temperature dependence of the transmittance. The characterized trap detector and the V(λ) filter are used for a novel realization of the SI base unit of luminous intensity, the candela. Test measurements indicate a good agreement with previous photometric scales.

Automated interferometers designed for calibration of gauge blocks and line scales are described. The gauge-block calibrator is a scanning interferometer with a white-light source and 633-nm He-Ne laser. The line-scale interferometer also uses a dynamic measurement method with a microscope and a CCD camera as the graduation line detector. The interferometers mainly use the same electronic units and meters for the environmental parameters. The combined standard uncertainty (1σ) is 52 nm for the gauge-block interferometer and 72 nm for the line-scale interferometer for 1-m steel standards

The interference pattern of a Fizeau interferometer, ideally a (nearly) self-imaging field, is examined under true measurement conditions. The quality of the higher longitudinal-order fringes is investigated by finite-aperture diffractive calculations. A specific symmetry property of the volume interference pattern, namely the invariance of the intensity structure under a rotation of the interferential wedge about its apex, is studied by nondiffractive simulations that take into account the instrument geometry and finite incident wave. The formation of the higher longitudinal-order fringes is verified experimentally. The possibility of transferring these fringes close to the device by a suitable non-normal incidence is demonstrated. The existence of various fractional Talbot planes is also confirmed.

We have applied electronically resonant coherent anti-Stokes Raman scattering (CARS) to the detection of nitrogen dioxide. The resonance CARS spectrum of NO₂ in a sample cell was recorded around a Raman shift of 1500 cm^-1 using a frequency-doubled injection-seeded single-mode Nd:YAG laser and a tunable dye laser. In particular, we demonstrate the use of a broadband Stokes laser to measure resonance CARS spectra of NO₂. Recordings at various pump laser frequencies were made by temperature tuning of the Nd:YAG master laser. The pressure dependence of the signal was also investigated. The resonance CARS results are compared with laser-induced fluorescence measurements. The work shows the high resolution achievable in complex molecular spectra using resonance CARS.

We report on an experimental study of cold rubidium atoms in a magneto-optical trap. Fluorescent light from the cloud of trapped atoms was collected onto a CCD (charge coupled device) camera. The number density of atoms and the lifetime of the trap are deduced from the recorded CCD images. Special emphasis is given to the calibration of the COD camera.

Numerical calculation of the two-wave mixing gain factor for arbitrary orientation of the holographic grating vector using the coupled wave theory developed for optically active electro-optic crystal enabling pump-beam depletion is presented. Two-dimensional angular dependencies of the gain factor are measured in Bi₁₂TiO₂₀ photorefractive crystal under an alternating external electric field using the fanning effect. It is found experimentally that the angular ditribution of fanning light is independent of the polarization state of the pump beam. This fact could not be explained by the existing theory.

This PDF file contains the editorial “Guest Editorial: Optics in Switzerland, Part 3: Industries and Observatories” for OE Vol. 34 Issue 09

To exploit the enormous potential transport capacity of optical fibers it will be necessary to replace many of the functions that today are implemented electronically with optical technology. These optical realizations often call for optical modulators and switches, sometimes integrated with other optical components. Polymeric integrated optical circuits hold the promise of high performance paired with low cost. The static and dynamic properties of an exploratory polymeric optochip with several Mach-Zehnder interferometers were measured. It was found that the insertion loss for butt coupling to standard fibers is relatively high and that it increases drastically but reversibly when the chip is illuminated with white light. In addition, the required bias voltages drift away slowly when the device is either subjected to laser light or connected to a voltage. Nevertheless, if certain precautions are taken, the Mach-Zehnder interferometers are well suited for switching and modulation applications. Newer polymers and lens coupling will yield lower losses and improved stability.

The principle of the photolithographic mask transfer of high-spatial-frequency gratings under the Littrow angle is shown to be so tolerant that a broad spatial-frequency spectrum can be transferred under the same exposure conditions.

A miniature optical position sensor has been designed, developed, and constructed. It provides fast and contactless measurement of the lateral position of a 0.5-mm-diam cylindrical pin in two directions with an accuracy of ± 3 μm over a range of 1.4 x 1.4 mm². The sensor consists of an optomechanical measuring head, an optoelectronics circuit, and an interconnection cable including six optical fibers. It has been implemented on a robot arm and applied with success in an automatic microassembly operation.

The importance of on-line process control in chemical plants is increasing constantly as a result of the high level of automation in production, stricter customer requirements with regard to quality, and considerations of environmental protection. We have developed a fiber optical photometer system for the performance of a wide variety of online measurements. Specific sensors can easily be installed in chemical plants and explosive environments and are linked via glass fibers to an optoelectronic device, which can be more than 100 m away. Our photometers are designed for long performance without any service. Some of more than a hundred applications are discussed.

An overview of the main optical methods used today in the detection of fire is presented, including optical smoke detection by light scattering and obscuration, as well as infrared flame detection. Further, intrusion detection by detection of emitted infrared radiation of human bodies is discussed.

Laser welding as a means for optical packaging, providing long-term stability and submicron precision, has been implemented for fixing single-mode fibers or fiber arrays to integrated optical waveguides of GaAs and InP optoelectronic chips. Fixing accuracies better than 0.5 μm are achieved by applying a new method of corrective welding. Devices such as high-power LEDs featuring low spectral modulation, 980-nm laser diodes, integrated reflective Mach-Zehnder modulators (GaAs), and 1 x 2 InP switches have been packaged and used in system test beds.

The beam parameter product is the most important characteristic value of a materials-processing laser. Known formulas lead to expressions that show its relevance to kerf width, depth of focus, and taper. These new expressions are then applied to fiber optic beam delivery. A formula is presented to minimize reflection losses by specifying antireflection coatings with the right spectral and hence angular bandwidth. These optical principles were used in the engineering of two improved processing heads to image the fiber end face onto the workpiece. A welding objective offers 106 mm of free working distance for reliable welding at high power. A cutting objective offers a 0.3-mm spot size with a 400-μm fiber. Both objectives consist of three lens elements only, thus minimizing cost, mass, and reflection losses. They are corrected to transverse aberrations less than 1/5 and 1/20 of a 400-μm fiber core diameter, respectively.

By patterning the iron oxide film of a conventional iron oxide mask it is possible to obtain a binary phase grating of high diffraction efficiency and high transmission that can be used in high-resolution displacement sensors.

Accurate target detection is fundamental to many industrial measuring applications. One type of target uses retroreflective material for defining object surface points. In a photogrammetric procedure camera at different positions illuminate these targets, which are imaged onto the CCD array. By intersecting all the lines of sight, the targets are fixed in space. In the past, accuracy improvements have been concentrated mainly on algorithms or sensor calibration, such as better modeling of optical and electronic distortions, and less effort has been spent on potential targeting problems. It is conventionally assumed that the target center is physically fixed and independent of the angle a between the target normal and the line of sight. The author investigates this assumption in the case of retroreflective targets. Tests have revealed a systematic shift of the target center from -30 to +30 μm normal to the line of sight, when the target is viewed from different directions, i.e. -40 deg ≤ α ≤ +40 deg. There is no shift seen for punched targets.

The assembly of optoelectronic devices in their bare chip form into fully functional modules and the performance of a 1 x 2 optical space switch, combined with a dedicated high-speed digital-driver integrated circuit, is reported. The specifications of the packaged module closely approach the data on the unpackaged devices. The peak digital modulation rate of the switch is 1.5 Gbit/s, a performance compatible with many broadband uses. Successful testing in a system test bed demonstrates the usefulness of the realized module from an application point of view. High-speed optoelectronic assembly technology without performance degradation requires a number of packaging skills. These include rugged submicron-accuracy fiber fixing, suitable microwave electronic interconnections, and appropriate thermal-load management

The 3-m KOSMA telescope at Gornergrat (Switzerland) is dedicated to millimeter- and submillimeter-wave astrophysics with observations mainly of interstellar atomic and molecular lines. This includes measurement of the large-scale structure and dynamics of molecular clouds in our galaxy for a better understanding of the processes connected with star formation. The present research activity of the observatory is described with a special scope on the description of the techniques and instrumentation used. The site at Gornergrat has superb atmospheric transparency, thus allowing operation up into the highest-frequency atmospheric window at around 850 GHz. We discuss the rapid development of sensitive superconductor-insulator-superconductor mixer elements, now covering frequencies well up into the submillimterwave range with close to quantum-limit detection. As frequency-analyzing devices we exclusively use acousto-optical spectrometers at KOSMA. Progress into the submillimeter spectral range is complemented by the now mature technique of building radiotelescopes with sufficient surface accuracy to guarantee operation into the terahertz region (λ = 200 μm). A new 3-m reflector for the KOSMA telescope with an expected surface accuracy of 10 μm is under construction.

The use of plastic as a substrate material for optical lenses has been increasing in importance due to its smaller density, simpler machining, and lower price. However, scratches present a major problem wherever the lens surface is exposed to the environment. Various processes have thus been developed to protect the surfaces with hard coatings. The most common technique is dipping, but spinning and spraying are other methods to apply lacquer. Common to all these techniques are the required posttreatments such as drying and heat or UV curing. Safety considerations, as well as the long processing time and the handling required between the different steps, have led to the development of new technologies. Plasma polymerization, a technology relying on plasma-enhanced chemical vapor deposition, is used to deposit dense organic layers with reproducible, well-defined surface properties. In contrast to dip coating, plasma polymerization coats even strongly curved structures with good uniformity. This is important for bifocal or trifocal lenses, which have sharp edges on the surface. Due to the wide range of refraction indexes (about n = 1.5 to 1.8) available by using different monomers, even the latest high-index materials can be coated without disturbing interference fringes. After a survey of plasma polymerization basics, some aspects of industrial equipment and processes for ophthalmic applications are presented. An outlook into the future of plasma polymerization for optical applications concludes the paper.

Testing of surlace shapes by automatic interferometry is exact, fast, and comfortable, and therefore popular. It however suffers from drawbacks: the equipment is often quite voluminous and expensive, and the attainable accuracy is limited by the precision of the reference surface (relative testing). Using the phase-shift software of BIFO, which has been improved over the years, Fisba Optik AG recently developed a matchboxsized, cost-reduced interferometric surface shape sensor to overcome the first drawback. Furthermore, this sensor was combined with methods for absolute calibration to solve the reference-surface problem.

Holographic mask aligners represent the latest addition to commercially available lithographic technologies. Their combination of very high resolution (< 0.5 μm) and very large exposure field brings a new capability to the microelectronics industry, especially for the manufacture of flat panel displays. The machine is fully automated and includes a scanning laser illumination system, a dynamic focus system permitting patterns to be printed over poor-flatness substrates, and an alignment system providing 0.3-μm overlay accuracy. A higher-accuracy alignment system under development demonstrates 50-nm measurement accuracy. A step-and-repeat hologram recording method enables tighter control of feature linewidth.

We describe the general characteristics of the TIRGO infrared telescope, located on Gornergrat (Switzerland), and its most recent instrumentation. This telescope is specifically designed for infrared astronomical observations. Two newly designed instruments are presented: the imaging camera Arnica and the long-slit spectrometer LonGSp, both based on two-dimensional array detectors.

Since 1958, the University of Liège (Belgium) has been maintaining a laboratory at the Jungfraujoch International Scientific Station, in the Swiss Alps, at an altitude of 3580 m. Equipped with two high-performance Fou rier-transform spectrometers, this laboratory is devoted to the study of the solar spectrum in the visible and in the infrared, at very high resolution. Its first objective was to improve the knowledge of the chemical composition of the sun's outer layers, but since 1977 interest has moved progressively towards atmospheric research. The Jungfraujoch laboratory is now one of the primary stations of an international network to monitor the changes in the abundances of various constituents of the Earth's upper atmosphere.

The paper describes the design and construction of an improved form of cat's-eye retroreflector for use with a 3-D tracking laser interferometer. This reflector can accept an incoming interferometer beam within 60 deg of its main axis of symmetry and can provide a high-quality return beam at ranges up to 25 m.

We report on a novel transducer devised for sensitive detection of the binding of molecules in the immediate vicinity of an optical surface. The transducer consists of a chip with a surface coated by an extremely thin waveguide film of amorphous TiO₂ structured with a submicron grating relief. This microrelief is composed of two superimposed, uniform diffraction gratings of different periodicities, forming a bidiffractive grating with a frequency spectrum composed of two fundamental spatial harmonics. This bidiffractive grating serves as both an input and an output port for coupling and decoupling light beams to and from the planar waveguide. The bidiffractive coupler has translation-invariant coupling efficiency and allows background-free detection of the light decoupled from the waveguide. We outline the fabrication of the transducer and discuss the material properties of the waveguide film, which are very important for achieving optimum detection sensitivity. We assess the key factors affecting the detection limits of the transducer and comment on the resolution and the dynamic range for the measurement of changes in interfacial mass loading.

The optical 3-D measuring system described measures the shape of objects at high speed and in a true noncontact fashion. The system evaluates and displays the object points visible within the field of view of the CCD camera integrated within the system. A description of the system and some applications is given, and the potential for further evaluation or processing of the measured data is highlighted.

The Physikalisch-Meteorologisches Observatorium Davos and World Radiation Centre (PMODIWRC) is responsible for the worldwide homogeneity of measurements of the meteorologically important radiation: the short-wave radiation that originates from the sun and is manifested as total and spectral radiation in the direct solar beam and the diffuse radiation scattered by the atmosphere, and the long-wave radiation, which is infrared radiation emitted by the atmosphere and the Earth's surface. The total solar irradiance measurements are performed by electrically calibrated absolute radiometers of high accuracy. Improved methods have to be developed to transfer the scale of irradiance accurately to space to ensure the necessary continuity of solar monitoring programs. Results of such transfers by comparison with highly accurate cryogenic radiometers are presented. For the measurements of the direct solar spectral irradiance, filter radiometers-so-called sun photometers (SPMs)-are used. Highly stable SPMs have been developed at PMOD/WRC to measure the solar spectral irradiance from the ultraviolet to the near infrared. Results from the space experiments are presented, with special emphasis on the problem of degradation. A third field of research is the accurate measurement of the surface radiation budget and the determination of its dependence on the elevation in Alpine regions and on increasing concentration of greenhouse gases. The implementation of a network in the Swiss Alps is described, and some results are presented.

The realization of the α-trimmed mean filter on the threshold decomposition architecture is described. The threshold decomposition architecture is developed for facilitating realization of nonlinear, order-statistic filters. In this architecture, an input sequence is thresholded at different levels to form a number of binary sequences. The filter output is formed by combining the decision results made at various levels; at each level it is determined whether the signal should be above the associated threshold level. The decision rule used is the distinguishing feature of an individual filter; the current implementation of the α-trimmed mean filter is based on using a threshold logic function as the decision rule, thus allowing the decision rule to output multiple values. The statistical properties and performance of the a-trimmed mean filter are analyzed on the basis of this realization.

The problem of detection of aircraft at long range in a background of evolving cloud clutter is treated. A staring infrared camera is favored for this application due to its passive nature, day/night operation, and rapid frame rate. The rapid frame rate increases the frame-to-frame correlation of the evolving cloud clutter; cloud-clutter leakage is a prime source of false alarms. Targets of opportunity in daytime imagery were used to develop and compare two algorithm approaches: banks of spatio-temporal velocity filters followed by dynamic-programming-based stage-to-stage association, and a simple recursive temporal filter arrived at from a singular-value decomposition analysis of the data. To quantify the relative performance of the two approaches, we modify conventional metrics for signal-to-clutter gains in order to make them more germane to consecutive frame real data processing. The temporal filter, in responding preferentially to pixels influenced by moving point targets over those influenced by drifting clouds, achieves impressive cloud-clutter suppression without requiring subpixel frame registration. The velocity filter technique is roughly half as effective in clutter suppression but is twice as sensitive to weak targets in white noise (close to blue sky conditions). The real-time hardware implementation of the temporal filter is far more practical.

The concept of nearest unlike neighbor (NUN), proposed and explored previously in the design of nearest neighbor (NN) based decision systems, is further exploited in this study to develop a measure of confidence in the decisions made by NN-based decision systems. This measure of confidence, on the basis of comparison with a user-defined threshold, may be used to determine the acceptability of the decision provided by the NN-based decision system. The concepts, associated methodology, and some illustrative numerical examples using the now classical Iris data to bring out the ease of implementation and effectiveness of the proposed innovations are presented.

Digital phase demodulation is being developed as a flexible heterodyne demodulation technique for on-line dynamic displacement detection. The digitized modulated carrier and reference carrier signals from a heterodyne interferometer are demodulated for surface displacements using computer algorithms. Since the signal demodulation is performed in the computer, the heterodyne beat frequency or center frequency of the carrier signals can be easily optimized for the desired detection center frequency and the heterodyne beat frequency does not have to be stable. The resulting measured displacements are calibrated with respect to the wavelength of light used in the interferometer and remain accurate for displacements larger than the wavelength of light. No assumptions are made about the sideband frequency content or the amplitude of the surface displacements. The research presented here introduces digital phase demodulation concepts and shows initial experimental results from a heterodyne interferometer while monitoring vibrational and ultrasonic displacements. The initial results show that the heterodyne sensor system is capable of measuring displacements greater than three wavelengths of light and has a single-shot resolution better than 7 nm. Thus digital phase demodulation will allow a single instrument to monitor displacements produced by low-amplitude, high-frequency ultrasound and by high-amplitude, low-frequency vibrations.

An interferometric displacement sensor with useful properties has been built based on a laser with optical feedback from light that is backscattered by a moving object. Information about the object's motion is encoded in the phase of the backscattered light, which in turn influences the phase and the amplitude of the laser via injection-locking physics. We derive the properties of the amplitude and phase modulation of the laser from standard injection-locking relations augmented by a self-consistency condition. These predictions are then confirmed by experimental results. An off-the-shelf two-mode frequency-stabilized laser is used in two different interferometric configurations. First, the amplitude modulation of the laser is utilized for displacement measurements in a homodyne setup. Second, the phase modulation of the laser is used in a pseudoheterodyne interferometer. In both cases, the backscattered light from the object can be injected into the laser cavity without the help of any focusing optics. Thus the injection-locked sensor combines the advantages of readily available equipment and a straightforward optical setup without need for intricate alignment, and thereby meets two important conditions for industrial applications.

The Air Force Phillips Laboratory is in the process of demonstrating an advanced space surveillance capability with a heterodyne laser radar (ladar) system. Notable features of this ladar system include its narrow (< 1.5 ns) micropulses, contained in a pulse-burst waveform that allows high-resolution range data to be obtained, and its high power (30 J in a pulse burst), which permits reasonable signal returns from satellites. The usefulness of these range data for use in reflective tomographic reconstructions of satellite images is discussed. A brief review of tomography is given. Then it is shown that the ladar system is capable of providing adequate range-resolved data for reflective tomographic reconstructions in terms of range resolution and sampling constraints. Mathematical expressions are derived which can be used to convert the ladar returns into reflective projections. Image reconstructions from computer-simulated data which include the effects of laser speckle and photon noise are presented and discussed. These reconstructions contain artifacts even in the absence of noise, due to the inadequacies of the standard tomographic problem formulation to accurately model the reflective projections obtained from the ladar system. However, object features can still be determined from the reconstructions when typical noise levels are included in the simulation.

This PDF contains the communication "Phase shifting: six-sample self-calibrating algorithm insensitive to the second harmonic in the fringe signal."

This PDF file contains the editorial “Book Rvw: Elliptical Fiber Waveguides," by R. B. Dyott for OE Vol. 34 Issue 09

This PDF file contains the editorial “Book Rvw: Coherent Quantum Optics and Technology," by Motoichi Ohtsu for OE Vol. 34 Issue 09