This paper reviews the Smith Chart and the characteristic matrix for evaluating the reflectance of optical thin films. The methods are developed from fundamentals and brief examples are given. The use of an HP-67 programmable calculator to make computations based on these methods is also discussed.
The advent of very large laser systems with thousands of optical components has necessitated an investigation of methods for cleaning both coated and uncoated optical elements. To perform adequately in laser systems, optical surfaces must be free from both films and particulate matter. Films have undesirable absorption bands and particles scatter light. Furthermore, in high power laser systems contaminants on the surface or trapped between the layers of coatings are known to lower the damage threshold. All surfaces must therefore, be clean when the optical elements are coated and installed and must remain clean for years after installation. Accomplishing this goal of long term cleanliness requires detailed consideration of cleaning methods and all mechanisms of recontamination. Generally, the nature of contaminants are unknown and therefore several cleaning steps may be necessary. Many conventional cleaning methods have proven unsuitable because they damage the precision surfaces they are to clean. Once cleaned, optical elements must be handled in such a manner as to preclude or at least minimize recontamination by airborne particles, aerosols or direct contact transfer. When finally installed the optical housing should not shed particles from fasteners or seals and should not outgas, which could cause an undesirable film to condense on the optical element. This paper discusses many of the techniques currently being used for cleaning optics and several new methods under investigation. It also discusses clean room procedures for reducing recontamination after cleaning.
Maximum energy throughput, good image forming qualities, and low scattered light levels are all desirable in high performance optical systems. Mirrors need to have a good optical figure, high reflectance, and low scatter. Windows and other transmitting optics should have low intrinsic absorption, low surface absorption and scatter, minimum bulk scattering, and a good optical figure. Optical figure is usually readily evaluated, but microroughness and scratches and digs which affect optical scattering are more difficult to measure quantitatively. This paper presents a brief description of several optical characterization techniques which include methods for measuring surface microtopography, scattered light, absolute reflectance, change in reflectance with temperature, and optical absorption. In addition, an objective, nondestructive technique which can supplement or eliminate visual scratch/dig measurements is discussed.
The application of ellipsometry to optical measurement problems encountered in the optical coating industry will be discussed. Although ellipsometry has been demonstrated to be a sensitive and accurate technique for measuring the optical properties of surface films and bulk materials, it has not been widely used in this industry. Specific measurements that will be discussed are: thickness and optical constants of films, optical constants of bulk materials, and stress-birefringence. Special precautions and errors caused by thin film surface contaminants will be described. Finally the applicability of ellipsometry to in-situ measurements and deposition monitoring will be reviewed.
This paper discusses the source and magnitude of uncertainties in the measurement of practical laser-damage thresholds. Damage is defined to be a permanent induced change in a bare or coated optical surface which degrades the merit of the surface with regard to specifications such as scattering, reflection, or transmission. Bulk damage is not treated, and the discussion is limited to tests conducted with laser pulses having wavelengths of 694 nm or 1064 nm and durations less than 100 ns. The measurement of pulse energy, the spatial and temporal profile of the pulse, and determination of the occurrence of damage are described in detail. It is shown that the uncertainty in the measurement of absolute laser-damage thresholds can be as small as 10% and that, within such limits, there is no site-to-site variation of the threshold of optical surfaces.
The influence of various factors on the damage threshold of coated optical components used in high-power, pulsed CO2 lasers is reviewed. The factors considered are substrate roughness and polishing defects, coating defects, linear absorption, deposition parameters, standing-wave electric fields, pulsewidth dependence, multiple-shot conditioning, and optical performance. Experimental results of many researchers are used to illustrate the discussions.
A special instrument has been built for precision measurement of reflection and trans-mission of large optical components for high power lasers. These optical components are much too large to be measured in ordinary spectrophotometers normally used for this purpose. The coatings on such large components have traditionally been evaluated from measurements performed on smaller witness parts coated along side the large parts. Mea-surements performed directly on the large parts eliminate the uncertainty associated with reliance on witness parts. The instrument is designed to handle optical components as large as 500mm x 500mm up to 100mm thick and weighing over 50kg. The whole clear aperture of the part can be scanned and variations in the coating or substrate performance within the aperture are mapped on a graph produced by an x-y recorder connected to the instrument. Reflection and transmission can be measured at incidence angles up to 60° with either S or P polarization. Double bounce or double pass are used with high values for R or T. The measurement accuracy is ± .05% or better for high or low values of R and T. Coating bire-fringence can also be measured by using an orientation of polarization 45° from P or S. Birefringence as low as 2° can be measured.
This paper reviews infrared coating materials for reflectors and windows for use in high energy laser systems operating in the 2 μm to 10.6 μm spectral regions. The work discussed is the result of research performed at our laboratories during the last several years. The primary emphasis in the discussion is on coatings that have low optical absorption. A long-term objective of our work has been to reduce optical absorption in infrared coatings to less than 0.1%.
A multijoule Nd:glass laser system for laser fusion research is currently under construction at the University of Rochester's Laboratory for Laser Energetics. The twenty-four beam system will consist of hundreds of optical elements. The design of optical components for use in lasers operating in the multikilojoule energy regime must include considerations neglected in more conventional applications. The relevant component design factors include resistance to damage caused by laser radiation, contribution to non-linear phenomena, retardance, beam break-up and loss of focusable energy. This paper describes variable ratio beam splitters that permit flexible balancing of beam line energy content. Several options are considered in light of the relevant design parameters. The result consists of plane plate, coated beamsplitters used in conjunction with half wave retardation plates in the presence of plane polarized light.
A brief outline will be given of several different numerical methods for the synthesis of interference filters with rather complicated spectral characteristics. With these it is possible now to design coatings having almost any desired transmittance curve. Unfortunately, the practical realization of such multilayer systems is often not easy. Frequently the thicknesses of the various layers of the coating assume different values. This necessitates the use of special thickness monitoring techniques during the construction of the filter. Also, the designs often call for refractive indices that are intermediate to those of known materials. Ways must be found to simulate materials with such indices. Other complications arise when the resulting multilayer filter is to be used in unusual environmental circumstances. Apparatus and methods will be described which are used at the NRCC for the construction of such filters.
Increased interest in the ultraviolet portion of the spectrum in recent years has resulted in considerable improvement in interference filters for that area. The purpose of this paper is to describe various types of bandpass filters for use in the 200-nm to 400-nm region. Achievable specifications are presented for signal-to-noise ratio, peak transmittance, depth of blocking, etc., and manufacturing techniques are discussed. Examples are given of general classes of filters used in different areas of the ultraviolet, and specific advantages and disadvantages of each are considered.
A special technology is required for research to be done in the vacuum ultraviolet spectral region. Because optical components are for the most part reflecting optics, considerable effort has been expended in a search for highly reflecting mirror coatings. For wavelengths down to about 1000A, coatings with reflectances of 80% are obtained by overcoating Al with LiF before Al203 has a chance to form. MgF2 is perhaps a more useful overcoating for Al because it is not affected by atmospheric moisture during storage in air, although its region of high reflectance extends only to about 1200A. For wave-lengths below 1000A, coatings of Pt and other members of this group, Rh, Re, Os, and Ir, and other heavy metals, are very useful but provide reflectances of only about 20-35% down to about 500A. Polished SiC mirrors, originally formed by chemical vapor deposition, have the highest reflectances thus far measured from 1000A to about 600A. Transmitting filters of LiF, MgF2, and quartz transmit from the visible region to cut-off wavelengths of 1050A, 1140A, and 1450A, respectively. For wavelengths below 1000A, filters can be made of thin, unbacked, metal films. For example, a film of Al 1000A thick transmits from about 800A to L2 3 x-ray edge at 170A. The techniques for producing and using reflecting coatings And transmitting filters will be discussed, and a short description of diffraction gratings for use in the vacuum ultraviolet region will be given.
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. Materials limitations, more severe than those in the visible and IR, have limited broadband reflectors to 80-85% average reflectivity in the 1200-2000Å 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 1460A. Dichroic coatings with maximum reflectance in one spectral region, and high trans-mission 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 narrowband interference filters, multi-layer 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 commercial 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.
Inhomogeneous metal/dielectric films have been investigated for use as selective absorbers of solar energy. For solar energy conversion systems, the effectiveness of these films compares favorably with the best selective absorbers, but inhomogeneous films have a basic mechanical simplicity that could make automatic production both feasible and economical.
Since the introduction of commercial sputtering equipment in 1 964 both users and manufacturers have made significant, if not spectacular, contributions to the development of the technology. The Symposia cosponsored by Consolidated Vacuum Corporation and the University of Rochester in 1966, 1967 and 1969 represent a composite of most of the early work that was done. The earliest equipment configurations available and in use were D. C. in both the diode and triode modes. These were followed by the introduction of R. F. in both modes for the deposition of dielectric materials. Although applications work was initially undertaken by people in a number of industries, ultimately the greatest progress came from the microelectrics industry. While some applications work continued in the optical, decorative and solid film lubricants fields, the disadvantages of sputtering continued to outweigh its advantages. A number of unrelated events occurring over the last several years however, has created a new burst of interest and enthusiasm for sputtering as a thin film deposition technique. Magnetically enhanced sputtering is central to this vitalization. A brief review of the development of sputtering is followed by an indepth discussion of magnetically enhanced sputte ring; its advantages, disadvantages and application opportunities.
Data are provided to demonstrate that metals, semiconductors, and insulators can be sputter deposited to obtain preselected properties. Semiconductors and insulators such as CdTe, CdS, In2-x SnxO3, TiO2 and a-Si have been controllably and reproducibly deposited to obtain a wide spectrum of electrical and optical properties. Electrical resistivity and optical absorption have been related to compositional and structural properties. Metals such as Cu and Ag have been deposited for the purpose of achieving damage-resistant high reflectivity surfaces for high energy laser applications. A sputter deposited, mechanically polishable, damage-resistant mirror material with high reflectivity was developed.