It is with great pleasure that I present as guest author this issue one of my long-time colleagues, John D. Lytle. Mr. Lytle is a well-known senior optical design engineer, deeply acquainted with optical fabrication techniques and problems. For about the last two years, he has geared himself particularly to state-of-the-art fabrication of plastic lenses, while being associated with rapidly growing M.U. Engineering & Manufacturing, Inc. In this article, he presents quite useful and interesting insights and solutions regarding optical tolerancing for glass and plastic optical systems, particularly emphasizing the differences involved between working with one or the other.
An optical time domain reflectometer is described which combines in one instrument the basic capability to analyze several important fiber characteristics. The device uses the polarization properties of light to enable high sensitivity fault detection close to the fiber input end; its probe pulse temporal characteristics and high gain photodetector in combination provide excellent discontinuity location resolution in long lossy fibers, and give an indication of fiber dispersion at large bandwidths.
The recent development of low-loss single-mode optical fiber waveguides for light has made possible a new class of inertial reference devices built on the principal of a closed loop interferometer. Light circulating through the loop in both directions experiences a relative phase delay proportional to rotation rate about the loop axis. This paper derives the phase delay and discusses signal detection, signal processing techniques and error sources. It is concluded that synchronous modulation and demodulation and an active gain control at the signal calculation level are required to eliminate drift errors. Potential performance is extraordinarily good; rotation rate sensitivity of a few milli-arc seconds per second and angular position random walk errors of an arc second per square root hour appear feasible.
Integrated optoelectronic devices will be considered in which optical fibers are coupled to optical channel waveguides and in which both optical slab and channel waveguides are coupled to detectors. Use of Si02/Si substrates allows formation of low loss waveguides, smooth V-groove channels for both channel waveguides and fiber support channels and sophisticated detector configurations such as charge-coupled device (CCD) linear arrays. We will discuss a slab waveguide-CCD detector coupling structure which has been experimentally characterized by a high degree of uniformity along the linear array, minimal excessive optical scattering and very low charge transfer inefficiency. Coupling of light from optical fibers to channel waveguides and from an array of channel waveguides to a CCD linear array is demonstrated. Applications of the various device structures considered to integrated optical spectrum analysis, optical transversal filtering, and time multiplexing of a number of fiber optical communication channels are discussed.
Reductions in power consumption and improvements in reliability have made AlGaAs stripe-contact heterojunction lasers well suited for many fiber optic data systems of present interest. Although the beam pattern of some of these lasers is in the form of a stable single lobe, instabilities in the lateral direction are often observed, and require further attention for fibers with small acceptance angles. For long-distance, high-data-rate fiber systems, heterojunction lasers with emission wavelengths beyond 1 ptm are being developed. Such lasers of InGaAsP/InP have many attractive properties, but suffer from carrier leakage at short wavelengths (1.0-1.2 µm) and elevated temperatures.
Real-time pattern recognition as applied to 2-D imagery has been limited to trivial or near trivial algorithms due to the severe constraints on processing speed and information bandwidth. Object identification and tracking applications of pattern recognition at video rates is a problem of wide interest, with previous attempts limited to very simple threshold or correlation (restricted window) methods. New high-speed algorithms together with fast digital hardware have produced a system for missile and aircraft identification and tracking that possesses an "intelligence" that far exceeds previously implemented real-time tracking capabilities. Adaptive statistical clustering and projection based classification algorithms are applied in real time to identify and track objects that change in appearance through complex and nonstationary background/foreground situations. Fast estimation and prediction algorithms combine linear and quadratic estimators to provide speed and sensitivity. Weights are determined to provide a measure of confidence in the data and resulting decisions. Strategies based on maximizing the probability of maintaining track are developed. This paper emphasizes the theoretical aspects of the system and discusses the techniques used to achieve real-time implementation.
The theory of signal detectability is used to evaluate the effects of instrument aberrations on a total visual optical system with the human eye included. A quantitative measure of grating detectability is obtained using a single-stimulus, random-presentation procedure, with multiple-rating responses. The results indicate that the detectability of square-wave gratings, viewed through a telescope, decreases as aberrations are introduced in the instrument. The amount of degradation is determined for each of six aberration conditions. This degradation index can be used to obtain the degraded modulation transfer function of the total system for each aberration condition.
An electronic analog device has been constructed which displays synthesized interferograms on a cathode-ray tube (CRT) for real time viewing. First- and third-order aberrations are generated individually or in combination. The exact amount of each aberration (in waves) appearing in the simulated interferogram is also displayed. A radial line cursor adjustable over 360 degrees in the artificial exit pupil provides profile information about the wavefront along the vector. A 3-D presentation of the wavefront is also included.
A technique is described which converts in real time the fringe pattern resulting from two interfering beams into a wavefront map. The interferometer incorporates, as part of a servo system, a piezoelectrically driven mirror that is capable of applying both a known optical phase offset and a periodic optical phase modulation to one beam. The ac signal detected by a single photodiode is processed to generate an error signal for the servo system and a signal proportional to the optical phase difference between the two beams. Three instruments built on this principle will be discussed. The instruments are capable of detecting phase differences with a precision approaching A/100 and may be used for measuring surface topography (roughness, irregularity) or transparent phase objects such as biological samples, microballoons and gradient index materials.
A two-step fabrication process was developed which has produced previously unobtainable high frequency surface acoustic wave (SAW) devices. In addition, the design-to-test cycle time has been shortened significantly to allow an effective interactive design procedure. The SAW structure places some of the most stringent precision requirements on current electron-beam lithography, since finger placement errors are directly related to phase errors in the electrical performance of bandpass or pulse compression filters. A form of double precision is implemented in the interface software to enable successful patterning of the monotonic variation in line width from less than 0.4 pm to more than 0.9 pm required for a 1 to 2 GHz pulse compressor. Compensation for proximity effects due to the electron beam profile was also implemented. Alignment accuracy within a field is controlled to within ±250 A and field placement is accomplished via a laser interferometer controlled stage. Several alternative processes, including direct slice writing, reverse liftoff, and x-ray lithography are compared. Once the E-beam master is generated, large area contact replication is achieved using a modified conformable mask printer. This process has extended the range of SAW device performance beyond 2 GHz in a fundamental mode which, represents a significant advancement in microfabrication. Three-day turnaround from design to packaged devices was demonstrated using this technique.
The exposure of certain positive photoresists has been shown by Dill and co-workers to be modelable in terms of a local inhibitor concentration which results in a local development rate. The development process is assumed to be a surface etching reaction in which the surface velocity is the local development rate. The characterization of resists for the purpose of line-edge profile simulation therefore involves the measurement of development rate for controlled exposure dose profiles. In this paper a technique is described in which the resist thickness is continuously plotted during development. The low frequency capacitance is measured using a conducting substrate as one plate, and the highly conductive developer as the other plate of the capacitor. The inverse capacitance, proportional to the composite resist-oxide thickness, is obtained using an analog divider. Examples of standing wave effects in Shipley AZ 1350 resists, and resist development rate modification using chlorobenzene are presented.
A simple noncontact optical profilometer is described. Two light spots are scanned respectively over the shape to be measured and over a reference surface. The difference of elevation between the two surfaces is calculated from the time delay between the two pulses received by an image plane detector. Carefully calibrated, the method offers the possibility of a fast, precise and easily automated measurement. Feasibility experiments are presented which describe the application of the method to the control of small mechanical pieces (~cm) and the measurement of large objects (~m).
A nested array of Wolter type II grazing incidence telescopes for use in the extreme ultraviolet is described. It is demonstrated by an example that it is possible to design the array with all subsystems having matching focal lengths, matching field radii, the same grazing angle at the primary elements, and only slightly different grazing angles on the secondary elements.
Ronchi rulings may be used for fast, accurate measurements of Gaussian beam diameters. In this paper, we present a theoretical analysis of the Ronchi measurement method. The major theoretical result is a very simple relationship between beam diameter and the measured parameters, P max and P min. The Ronchi method is then compared with the more conventional pinhole, slit, and knife edge methods. The major advantages of the Ronchi method are discussed, and some notes on the application of the method are given.
Images whose properties are spatially variant must often be processed locally. Statistical techniques may be required to do this if the image is noisy. These may be difficult to apply when local regions are so small that means, variances, and similar quantities are unstable. We demonstrate how a practical statistical segmentation algorithm may be constructed which operates locally and gives satisfactory global results. The size of the local area over which computations are made has an important effect on the segmentation quality.
The separation of half-maximum intensity points of a focused truncated coherent Gaussian beam is evaluated. The results differ appreciably from the conventional values used in the past. This result is important in areas where the resolution of two optical beams is required, as, for instance, in determining the optimal separation of elements in a detector array used to identify these beams.
M.I.T. Lincoln Laboratory is supporting the Electronic Systems Division (ESD) of the Air Force Systems Command (AFSC) in the development of the Ground-based Electro-Optical Deep Space Surveillance (GEODSS) system. As part of this program, an Experimental Test Site (ETS) has been established in the White Sands Missile Range, New Mexico, to serve as a test-bed for the system. After a brief description of the GEODSS network, the ETS and its role are defined and illustrated. Next, the matter of sensor selection for the ETS is discussed. This is followed by the consideration of the general problem of point-source detection. Experimental data are discussed which indicate the point-source response of the ETS sensors. Then, a semi-empirical model for the point-source detection capabilities of these background-limited sensors is presented. Short dicussions of the satellite population and the peculiar problems associated with the passive detection of space objects by reflected sunlight are followed by conclusions based upon brightness measurements made at the ETS during approximately two years of operation.
Interest in vacuum ultraviolet optical coatings has increased in recent years, due to applications in space research, laser fusion, photochemistry, and analytical chemistry. A number of new high-efficiency coatings have been developed to satisfy the requirements of these applications. Material limitations, more severe than those in the visible and IR, have limited broadband reflectors to 80-85% average reflectivity in the 1200-2000 A region. A new technology of multilayer dielectric coatings for the vacuum UV, beginning in 1973, has led to laser reflectors with reflectances as high as 95% at wavelengths as short as 1460 A. Dichroic coatings with maximum reflectance in one spectral region and high transmission in another were developed for applications in UV pumping of longer-wavelength lasers, Raman shifting of UV laser lines, and generation of UV harmonics. Very narrow-band interference filters, multilayer dichroic reflective filters, and neutral density filters have been fabricated for rocket spectroscopy applications, as have high-efficiency UV AR coatings for lenses and laser optics. A summary of the current commerical UV coating technology and specific applications is presented, with graphs of reflectance and transmission versus wavelength. A description of measurement equipment and methods is given.