Much confusion has arisen from the requirement that publications in Optical Engineering be "original." Coupled with my insistence that no paper is so original that it can stand without reference to prior work is the question: "How much originality is sufficient?" Surprisingly, few people ask. Virtually everyone "knows." Unfortunately, ther is no unanimity. Stating my standards of sufficient originality as clearly as I can should help explain my publication decisions to irate, or at least annoyed, readers and referees.
Laser damage of optical components is an important issue facing the optical engineer of today. The lifetime of the components not only affects the design of the laser system, but also can involve long downtime for replacement of components that may require careful alignment. This Optical Engineering special addresses several important issues regarding laser damage.
We review studies of single- and multiple-shot laser damage to plastics and theories of the damage mechanism. Single-shot damage is caused by absorbing defects, many of which can be eliminated by monomer purification. Multiple-shot damage is controlled by viscoelastic properties; plasticization improves damage resistance. Also, we review high-power laser studies of dye-impregnated plastics. Photobleaching is the most serious problem. We review progress in understanding the physics of that process. Dye-induced lowering of laser damage resistance and thermal bleaching are other important problems discussed. Plasticization to improve multiple-shot damage resistance may encourage thermal bleaching. Finally, we present a list of as yet untested and potentially damage-resistant commercially available plastics.
Features of multiple-shot laser-induced damage to optical materials are analyzed. The methodology of studying the accumulation effect responsible for catastrophic damage, based on the statistical approach, is suggested, and the basic characteristics of multiple-shot damage are introduced. Possible mechanisms of microdamage in materials under multiple laser irradiation are discussed. The experimental data on the laser-induced damage to optical polymer materials which demonstrate some basic properties of the accumulation effect are presented. The ways of suppressing the accumulation effect in transparent polymers which result in a considerable increase of laser damage resistance are proposed.
We report bulk damage measurements on single crystal quartz and two types of fused silica under single and multiple pulse laser irradiation at 1.064 um and 0.532 um. Whereas the single pulse damage threshold varies little among the materials or with wavelength, the multiple pulse induced damage behavior depends strongly upon material and wavelength. Laser calorimetry and measurements of light scattered during irradiation give no evidence of change prior to the pulse on which macroscopic damage occurs. However, the distribution of the number of pulses needed to produce macroscopic damage as a function of intensity is indicative of an accumulation process in which some minimum intensity is required to initiate the accumulation of microscopic damage.
This work surveys the properties of materials at 1.3 um. The interest in this wavelength has been generated by the development of second generation optical fibers and by the development of the iodine (I*) laser, which operates at 1.315 um. Most of the data reported here were taken by Nd:YAG lasers modified to operate at 1.319pm. Spectral scans of some less common materials are presented. The effective optical absorption coefficient Beff measured by laser rate calorimetry is given for all materials. The thermo-optic coefficient an/aT at 1.3 um is given for a number of materials. If 1.3 um results are lacking, values at 1.15 um are given. Among the lower absorbing materials are KCI, LiF, NaCI, CaF2, SiO2, YLF (LiYF4), sapphire (Al203), MgF2, BaF2, and Q-98 phosphate glass. Results on a number of Schott infrared and laser glasses are given, including LG-670, LG-660, LG-750, LG-810, LG-760, LG-680, IRG-7, IRG-9, IRG-3, and IRG-N6. Results are given also on ZnS (normal and water clear), YAG (Y3A1501 2), ZrO2, SrF2, ZnSe, MgO, and CdTe. Some other glasses studied were hafnium fluoride glass, As2S3, CORTRAN 9753 and 9754, and Barr & Stroud calcium aluminate glasses BS 39B and BS 37A. The ten lowest absorbing materials had effective optical absorption coefficients Leff between 0.13 and 0.5 X10-3 cm-1. The lowest a n/aT value measured was 0.06X10-5/°C on Schott LG-660 alkali-Zn-silicate glass.
The results of single and multiple pulse damage studies at 10.6, 1.06, and 0.532 um in diamond-turned bulk Cu and diamond-turned electro-deposits of Ag on Cu are presented. The single pulse damage threshold decreases or remains constant with increasing spot size, as previously reported. On the other hand, the multiple pulse threshold increases with increasing spot size. The difference in their behavior suggests that the two phenomena are the result of different mechanisms. Localized surface defects and impurities appear to be responsible for single pulse damage, while for multiple pulse damage a model is proposed based on the cumulative effect of plastic deformation induced by thermal stresses.
The laser-induced damage (LID) thresholds of fused silica and single crystal NaCI were studied at wavelengths of 0.5 and 1µm for pulses as short as 4 ps for a variety of focal spot sizes. The problem of sample-to-sample variation was minimized by performing parametric studies on a single sample at a time. Beam distortion measurements and polarization dependence studies of the LID thresholds demonstrate that the contribution of self-focusing to the LID measurements in this work was negligible. The damage threshold field EB was found to increase as the pulsewidth was decreased in both materials at both wavelengths. The strongest pulsewidth dependence observed was approximately an inverse square root proportionality observed in NaCI at 1 µm for pulses shorter than 10 ps. For conditions of equal pulsewidth and the same focal spot size, EB was less at 0.5 um than at 1 um for both materials.
We used 20 ns, 248 nm pulses to study surface and bulk damage in bare, polished samples of UV-transmitting glasses, fluoride crystals, and isomorphs of potassium dihydrogen phosphate (KDP). The intent was to test enough samples to establish the safe operating limits for currently available windows and crystals finished by routine techniques for optical fabrication. In fluoride crystals, thresholds for bulk inclusion damage and for rear-surface damage were comparable. The median rear-surface threshold was 16 J/cm2. The fluence limit in fused silica windows was determined by the threshold for rear-surface damage, which ranged from 5 to 9 J/cm2. For the isomorphs of KDP, the limiting fluence was the threshold for front-surface damage, which ranged from 3 to 9 J/cm2.
Continuous wave laser damage on optical components is a major problem encountered in the operation of high energy chemical lasers. Although the damage phenomena on cooled and uncooled optical components are very similar to those on pulsed laser devices, mechanisms of damage are different. Unlike pulsed laser systems, maintenance of optical figure due to thermal distortion becomes an important aspect of the complete optical train for a device that may be required to operate for a number of seconds. This paper deals with damage mechanisms on cooled and uncooled optical components and how the damage may be predicted. Further, predictive tools for determining the thermal and mechanical forces that operate on optical components to induce distortions in the optical train are evaluated.
A beam of 500 to 1200 eV argon and oxygen ions, neutralized with an equal number of electrons, strikes a target, and the sputtered particles form a deposited layer of SiO2, Ta205, or TiO2. A second beam at lower energy is aimed at the substrate and used for stress modification and predeposition sputter cleaning. Deposited films are mechanically stable and extremely adherent; internal stress is compressive. The primary contaminant is carbon, which is less than one percent. Refractive indices near 1 µm are 1.47 for SiO2, 2.03 for Ta205, and 2.27 for Ti02. Optical absorptance of half-wavelength films at 1.06 Am is near 10-4 for SiO2 and Ta205 and near 4 X 10-4 for TiO2. Damage tests with a 114 um radius, 5 ns pulse length, 1.06 um wavelength laser show an onset energy flux for visible damage in the 12 to 30 J/cm2 range for single-layer films, and slightly less for a three-layer structure.
The neutral solution process (NSP) for chemically producing antireflection coatings has been scaled up from 24 cm and used successfully to treat components as large as 90 cm diameter. Since the process is primarily determined by diffusion through the surface, it has proven to be relatively insensitive to fluid flow effects which influence the chemical concentration buildup at the surface. Large parts visually appear more uniform than small parts processed in model tanks. The major problems encountered have been associated with using plastic support components beyond their normal temperature limits and with handling parts to the required cleanliness outside of a clean room environment.
An instrument is described that has been used to measure the spatial and temporal distributions of lightning strokes as seen from a high-altitude aircraft platform. It consists of two separate sensors. One has a lens, selectable filters, and a 50 X 50 element array of photodiodes. The second has an identical lens, filters, and a single detector that covers the same total field of view.
A method of calculating the self-radiance of the sky based on the LOWTRAN-4 computer code is described. The method is a significant improvement on that included as an option in LOWTRAN-4 itself as a result of two modifications: a) Our method, unlike the one included in LOWTRAN-4, takes into account the radiance scattered into the line of sight. b) The model of the atmosphere in the computer code must be divided into a certain number of layers, each one having approximately uniform conditions. Our method uses a correct optimal division. Consequently, our method gives a better agreement with the experimental data, especially for long optical paths in the lower layers of the atmosphere.
This paper describes a prototype computer-controlled digital optical disk memory system that was designed and built for an office-of-the-future application and also to track the technology, assess issues, and provide answers to a host of application-oriented requirements. The system uses gas lasers for writing and reading. Unique features of the system are simultaneous eight-channel parallel readout, write channel addressing, and a readout channel incorporating a single custom photodetector array for extracting data as well as focus and tracking error signals. The problems of media sensitivity and archivability are discussed, and sensitivity measurement techniques are described with typical examples shown.
Optical disk technology today finds widespread application in laser vision, compact disk, and data storage equipment. The required specifications of lenses which meet these requirements are discussed. Their image quality is given by rms of wave aberration, by point spread function, by encircled energy, or by modulation transfer function (MTF). Due to the automatic servo focusing technique, a certain amount of field curvature can be tolerated. Measured results of image quality are compared with theoretical values.
A variety of imaging procedures generate artifacts which have the visual appearance of a set of parallel lines across the image. In this paper we present a simple frequency-domain algorithm which considerably reduces that type of artifact. It has been found to be effective in a number of applications, and its performance will be illustrated by way of examples.
This electrically alterable magneto-optic device can be used as a two-dimensional spatial light modulator in an optical image processor or a display system. A thin magnetic garnet film is epitaxially deposited on a transparent nonmagnetic garnet substrate, in the manner of magnetic bubble memory films. Semiconductor photolithographic techniques are used to etch the film into a two-dimensional array of mesas and to deposit X-Y drive lines for matrix-addressed current switching of the mesa magnetization. Electromagnetic switching provides higher speed switching than a previously reported thermal switching method. The axis of polarization of polarized light transmitted through the film is rotated by the Faraday effect in opposite directions where opposite magnetic states have been written, and a polarization analyzer converts this effect into image brightness modulation. The resulting high speed random access light modulator is applicable to all three planes of the classic coherent three-plane correlator. Although the basic effect is binary, there are at least four possible configurations which achieve gray scale rendition.
The V-slit, N-slit, and crossed slit sun sensors that are normally used for sun elevation angle measurement on spinning spacecraft suffer from two defects, namely, nonlinearity of response and limitation in range. This paper describes a concept which overcomes both of these defects. Two versions of sensors based on this concept which give a linear output and have a range of nearly ±90° of elevation angle are described. The first version has already been flown on three Indian spacecraft, and a flight model of the second improved version is under testing for another satellite to be flown shortly.
Arbitrarily prescribed space variant operations can be carried out with high efficiency by multifacet holographic optical elements recorded in dichromated gelatin. In this paper, such elements are used to perform coordinate transformations from Cartesian to polar coordinates. Theoretical calculations of resolution limits and experimental results are given.
A technique for multiplexing holographic and speckle interferometric images utilizing a narrow angle holographic construction scheme is presented. The image reconstruction process utilizes a white-light processing system for color encoding of interferometric fringe patterns. The property of color encoding reveals interesting interferometric aspects of the multiplexed fringe patterns. A color video system is used to provide the advantage of displaying the output interferometric images for detailed study. Experimental results of time-aver-aged and double exposure interferometric processes are given. This technique is ideally suited to aid in the location of the J0 order holographic fringe.
Second generation infrared imaging systems require high-density focal plane arrays for staring applications. To meet this need, a focal plane structure using HgedTe photodiodes for detectors and Si charge-coupled devices (CCDs) for signal processing has been developed. Although conventional ion-implanted hybrid arrays have successfully been interfaced to CCD multiplexers, hybrid arrays fabricated on liquid phase epitaxial (LPE) layers. offer some inherent advantages with respect to performance, processing and yields. It has been determined that heterostructure diodes fabricated by a Hg infinite-melt LPE technique give superior performance relative to conventional ion-implanted devices. The devices exhibit high RoA products and good compositional uniformity. Data are presented on devices fabricated for both 8 to 12 um and 3 to 5 um applications.