This is a descriptive survey of the special optical problems which arise in the design and use of lasers. The optical problems, and some of the solutions, related to achieving and preserving spatial and temporal coherence, working with high power beams, and diagnosing laser characteristics are treated.
Coherent laser radar, operating at 10.6 um wavelength, utilizes a CO2 laser oscilla tor and amplifier as well as an infrared isolator, mechanical duplexer, and a heterodyne mercury-cadmium-telluride detector. Although limited to clear weather conditions, the high Doppler sensitivity, 2 kHz/cm/sec, and narrow beamwidth, 10 microradians, result in extremely precise velocity and angle measurements. An operating system is described with application to lleasurements on the retroreflector-equipped satellite, GEOS-C.
The objective of this paper is to offer a working set of principles to allow anyone with adequate general background but no specific laser optical experience to be aware of the various parameter trade offs required to design a laser scanning system. A set of formulae are developed to determine the parameters involved in the specification of a laser scanning system and, unless stated, all units are centimeters and radians. The discussion assumes a simple Gaussian beam intensity distribution of the lowest order -- TEM00 The effect of higher order modes is considered briefly at the end of the paper.
Laser light can be used to cut and cauterize tissue. At the present time CO2 lasers at 10.6 km, argon ion lasers in the blue-green part of the spectrum, and Nd:YAG lasers at 1.06 μm have operational characteristics which make their use in surgery potentially valuable. In this paper, the optics necessary to safely deliver the light from these lasers to surgical targets are described along with some suggested areas for improvements.
Carbon dioxide laser systems with continuous beam powers in excess of 15 kW offer the material processing industries a new method of directing and controlling energy to do work. This paper summarizes some of the important characteristics of such a lasers ystem and describes laser material processing. Laser weld penetration obtained at speeds over 100 in. /min (42 mm/s) are similar to those of vacuum electron beam under identical conditions. The laser achieves kerf widths of less than 0.10 in. (2.5mm) in cutting titanium and Rene 95 alloys with thicknesses up to 2.2 in. (55 mm), using an inert gas assist to reduce surface oxidation. Transformation hardening is achieved in depths up to 0.060 in. (1. 5 mm) at hardness values of Rockwell C 60 with distortion of less than 0.002 in. (0. 05 mm) TIR. Laser surface alloying techniques can modify the surface chemistry to resist wear or corrosion.
For many applications present dye lasers must be improved with regard to output power, spectral brightness and beam quality. This development is hampered by significant optical problems, particularly thermal distortion effects. These problems can be minimized but not eliminated so that some compromises in laser performance are necessary.
The Total Internal Reflection - Face Pumped Laser (TIR-FPL), developed with the support of the Air Force Avionics Laboratory and Naval Missile Center, has demonstrated the capability of producing high brightness beams in a repetitively pulsed manner. Reported herein are the recent performance results of several laboratory models which utilize both Nd:Glass and Nd:YAG. With a Nd:Glass slab operating in a simple Q-switched oscillator cavity, performance has been demonstrated with a near diffraction limited beam of 0.5 joules/pulse, 10 pulses/second. With the same laser operating as an amplifier for a Q-switched Nd:YAG TIR-FPL oscillator, performance has been demonstrated with an output beam of 1.5 joules/pulse at 10 pulses/second. An Nd:Glass laser with a large slab has been operated with a Q-switched output beam of 9.5 joules/pulse at 3.0 pulses/second and 25 joules/pulse at 2.5 pulses/second. Also contained herein is a description of a preliminary hardware design for a high brightness FPL-TIR designator system operating with an output beam of 5.0 joules/pulse, 10 pulses/second. Optical design considerations unique to the FPL are emphasized.
A number of laboratories throughout the world are actively pursuing the development of pulsed high power Nd:glass lasers for use in their laser fusion and laser plasma interaction studies programs. Perhaps the most ambitious programs in this field have been undertaken by the Lawrence Livermore Laboratory where several irradiation facilities are in operation and where the 4-arm ARGUS and 20-arm SHIVA lasers are currently under construction. These systems are designed to provide the energy, power and wavefront uniformity required to spherically implode nuclear fuel targets such as the deuterjw-tritium (DT) solid sphere, in an effort to achieve thermonuclear fusion through inertial confinement.(1)
The development of high power electrically excited CO2, gas lasers is traced from their inception to their rather sophisticated present day orm. Particular attention is paid to the optical homogeneity problems which limit the degree of diffraction limitness attainable with a given laser.
CO lasers are attracting increasing attention as the result of demonstrated advantages in power conversion and volumetric efficiency, and projections of favorable atmospheric propagation characteristics. A high energy pulsed electric discharge laser has demonstrated efficiencies in excess of 60%, and a small (~1 liter) supersonic CO EDL has produced over 100 kW under quasi-cw conditions (~2 milliseconds). Further improvements in performance are anticipated for optimized devices. Balanced against these advantages are a number of technological problems which must be addressed before practical devices can be realized. Of particular significance are the problems associated with obtaining diffraction limited performance. Beam quality may be degraded by many factors, e. g. , flow induced medium inhomogeneities, non-uniform heat addition, boundary layer effects, turbulent fluctuations, and component limitations. The effects are particularly pronounced for CO EDL's because of the high density of the active medium, and devices must be designed to minimize or compensate for these disturbances. This paper briefly reviews the status of CO EDL technology and the considerations which must be taken into account in the design of this class of device.
A study was performed to determine the performance of a wideband chirp-driven waveguide amplifier. Included in the study are the closed-form solutions of the rate equations with a chirped input, the calculation of the frequency errors generated by dispersion and the necessary waveform needed to overcome dispersion and thirdly the effects of a saturation and a nonuniform susceptibility profile across the aperture of the waveguide on the mode quality. The topics discussed in detail are:
1. The absence of saturation-induced power broadening of the amplifier bandwidth in fast chirped radar applications.
2. The excellent mode discrimination properties in waveguide devices, particularly when compared to the more conventional free-space device.
3. The mechanisms that cause degradation in performance of waveguide amplifiers in laser radar applications.
If one defines an optical component as an assembly of two or more optical elements, then the collimating telescope is probably the most used optical component in laser systems. This paper reviews some concepts which indicate why this might be so.
Laser source and detector characterization necessary for laser systems design is reviewed. Power, energy and temporal development of sources are discussed. Also, the spatial response of detectors is addressed.
The design of optical components for high average power lasers requires some additional and unique considerations not present in low power systems. Most common components simply cannot survive in the hostile environment of a high power laser beam. The most successful approaches are those which most constructively deal with the problem of the absorption of laser radiation and the removal of waste heat, while staying within an allowable beam distortion budget. This paper contains a discussion of the problems and promise of four high power components: solid windows, aerodynamic windows, high reflectance mirror coatings, and pulsed damage resistant mirrors. The state of the art performance of these optical elements is discussed. Finally, some crucial issues which must be resolved if applications of high average power lasers are ever to become routine, are enumerated.
The demands on the performance of mirrors used in high energy laser applications are severe. They may be summarized as follows: (1) excellent optical figure even under high thermal load, (2) high catastrophic damage threshold, and (3) low scattering level. Light scattering, particularly backscattering, is important, and can result in cavity depumping or damage to components in the cavity. Scattering is a consequence of microirregularities, localized imperfections, and contamination at the mirror surface. For many surfaces total integrated scatter (TIS) for visible wavelengths follows a 1/λ2 dependence, as predicted theoretically for distributed microirregularities. However, at infrared wavelengths, TIS levels often are much larger than predicted by this theory. Evidence that this additional scatter is a consequence of dust and other localized imperfections rather than distributed microirregularities will be presented. Comparison of TIS levels and angular scattering distribution measurements for several surfaces with the results that a newly derived theory using directly measured FECO surface statistical information will be made. Low absorption under thermal load is also a very desirable characteristic of laser mirrors, both because mirror heating causes loss of optical figure and because the damage threshold of the mirror surface is related to its reflectance, R, as a function of temperature, T. Typically, R 1 so that even large percentage changes in absorption with increasing tem-perature result in very small changes in dR/dT. However a new instrument has been designed to measure these small changes and measurements on a number of mirror coatings made with a precision of better than 1 x 10-4 will be presented and interpreted.
During the past ten years there has been an increasing interest in the imaging properties of point-source holograms. Now, the capabilities (and difficulties) of holographically-formed optical elements are fairly well defined, and several applications have been discussed. In this article I survey the current technology and applications of Holographic Optical Elements (HOEs), viewed as components of a general optical system. The recent development of HOEs has followed Gabor's principle of wavefront reconstruction (holograms).(I) The important aspect of this development was the study2yf imaging (3) properties and aberrations of point source holograms, notably by Meier' ' and Champagne. It was the ability to analyze and compensate aberrations,Oat permitted a quantitative consideration of holograms as optical elements by Latta.") Practical HOEs became possible with the development of appropriate ray tracing techniques,(5-7) and high-efficiency hologram recording materials.(8,9)
Coherent Optical Adaptive Techniques offer promising solutions to problems of phase distortions occurring in optical systems or produced by the atmosphere. A review of several basic COAT system concepts is presented along with a discussion of the basic hard-ware components required to implement such systems. Experimental and computer simulation results are presented that graphically demonstrate the ability of COAT systems to compensate propagation distortions in both imaging and transmitted-beam applications.
A simple process for optimizing laser efficiency is presented. While the basic ideas are generally applicable, most of the graphs and equations apply specifically to four level, solid state, Q-switched devices. Efficiency, in the form of a conservation of energy equation, connects output to input through a series of factors. These are related to the system parameters and optimization curves are given. The conservation of energy equation separates naturally into two parts, the pumping and lasing phases. The latter is considered first, in terms of rate equation solutions presented on a P-N diagram. Energy stored in the rod, fluorescence and output energy are related to mirror reflectivity R,resonator losses, and rod constants. Q-switched pulse length and allowed power densities in the resonator are related to the optimum R. Next, pumping efficiency is considered. After a brief discussion of pump head efficiency, the relationships between lamp output and circuit and lamp parameters are given. Based on recently published work on arc expansion dynamics, the energy delivered, charge, voltage and spectral factors are discussed. Finally, a curve of the pumping fluorescence factor is found and the overall optimization procedure is summarized.
In industrial applications of solid state lasers, such as materials processing, it is necessary to surround the laser with auxiliary optical devices. Discussed in this paper are the optical peripherals, which include appropriate optics for shaping, directing and focusing the laser beam, optics for viewing of the worksite, components to protect optics from contamination due to ejected material, beam monitors, radiation enclosures and safety shields necessary to provide for the safety of operating personnel and passersby.
A new approach to controlled thermonuclear fusion has recently become of interest. Laser light is used to compress and heat a small pellet containing deuterium and tritium. Energy is released in short times at high density, in contrast to the magnetic confinement approach utilizing long times at low density. Initial experiments are now underway to determine if this method is scientifically feasible. At the Lawrence Livermore Laboratory, a series of ever-larger Nd3+ glass lasers is being built in support of this effort. The culmination of this program will be the SHIVA laser system, a $20 million device which will put well over 1013 watts uniformly on a 200 pm target sphere. The design of such a laser system is governed by the desire to maximize the power per dollar, while avoiding damage to the laser. In addition, the target requires extremely uniform illumination and must be protected against destruction before the arrival of the main pulse. We consider the target requirements, laser design problems, point and focus system, and focusing optics of the SHIVA system.
Long range optical communication systems have a difficult pointing and spatial acquisition problem to overcome. Receiver acquisition is generally accomplished by some type of search to determine the direction of arrival of the transmitted optical beam. In this study, several optical acquisition techniques are compared in terms of time to acquire, receiver complexity, desired accuracy, aryl signal and noise power levels. Computed curves indicating the relative trade-offs are presented. The advantage of acquiring with arrays of detectors placed in the receiver focal plane is shown. Although some problems exist concerning their fabrication, such arrays would allow for the implementation of sequential and parallel search algorithms that can theoretically greatly reduce acquisition time. Such a capability can make arrays an extremely powerful device in future generation optical trackers. In general, improvement increases as the array size increases, but it is shown that the size of the array need not be very large before a significant reduction in search time is achieved over standard spatial search techniques. Implementation difficulties associated with detector arrays are discussed. The use of arrays to obtain rapid zoom capability is also suggested and examined.
A liquid biased, liquid crystal, infrared radiation detector has been devised for the purpose of determining the time integrated output spectrum and power spatial distribution of infrared radiation. The device has been specifically used to measure the spectrum and spaital power distribution of the output of a quasi-cw carbon monoxide gas laser. The detector recording medium is composed of thin mylar coated with temperature sensitive, encapsulated, cholesteric, liquid crystals and energy absorbing black paint. A uniform spatial color distribution of the liquid crystals is established and maintained by immersion in a controlled temperature liquid bath. The resulting uniform color distribution is modified, at various spatial points, by the addition of laser energy obtained either indirectly from the laser output coupler to yield the spatial dependence of the near or far field laser output or from a spectrometer output to yield the spectrum of the laser output. The addition of less than 15 mJ/cm2 of incident energy is adequate to modify the color distribution. The uniformity and precise control of the liquid crystal temperature and hence color, by the use of a liquid bath, makes it possible to compare colors, and hence laser output powers, at various spatial points. This permits the mapping of the laser near field output spatial distribution and the distribution of the laser spectrum with greater sensitivity than has been possible with more conventional tech, niques. Results showing the near field energy distribution and output spectrum of a supersonic flow, electric discharge CO laser obtained using the detection technique will be presented.
Some of the first uses of Lateral Interferometry, L.I., have been in applications in the areas of Laser Doppler Velocimeters (Ref. 1). During the past few years several new applications for L.I. have been proposed, some of which are aimed at filling important gaps in industrial instrumentation and precision noncontact measurement. The present paper summarizes progress that has been made in these and other newly discovered applications during the past year. Novel applications of Lateral Interferometry include determination of Optical Transfer Functions (OTF) of optical components and systems, and development of a new technique for noncontact monitoring submicroscopic surface defects of semiconductor blank surfaces. The paper also describes details of hardware development in these areas, unique optical design problems and their solutions. System characteristics of a working prototype of a fiber diameter monitor is given, together with a description of experience gained during the past months at demonstrations in the plants of different man-made fiber producing companies.