The intent of this article is to briefly review ruby (Cr³⁺ in Al₂0₃) and neodymium in yttrium-aluminum garnet (Nd³⁺ in Y³Al₅0₁₂ - "YAG") lasers by stating theii present operating characteristics in the most common and useful modes of operation, and then commenting on the factors that limit their performance and what, if anything, can be done to improve upon them. Other reviews of ruby (Refs. 1,2) are recommended for details concerning its operation (output characteristics, mode struc-ture, properties of ruby, etc.). Several reviews of other crystalline lasers have been written (Refs. 3-5) but the best review of Nd:YAG lasers is by Gallagher (Ref. 6); further sources of information on this laser can be found in the current literature (Refs. 7-19).

There are a number of characteristics which distinguish glass lasers from other solid laser materials. After a brief review of these properties, and of the lasing ions in glass, a number of recent glass laser systems developments will be described in more detail. A more complete review is given in reference 61.

A dye laser uses a fluorescent organic dye in liquid solution, or in a polymer, as the active medium. .The device may be excited either by a flashlamp or with a giant-pulse solid-state laser. The broad fluo-rescence linewidth of a dye and the large variety of dyes available permit the construction of a tunable laser operating throughout the visible spectrum. Furthermore, use of a liquid active medium facilitates cooling of the laser for operation at high repetition rates. The gain and power output performance of the dye laser is greater than that of the gas laser and is comparable to that, obtained from solid-state lasers. Factors which influence the gain of the dye laser and the excitation power required to reach threshold are reviewed. Tuning by means of dispersive elements in the dye laser cavity is discussed. The results of mode-locking experiments are described. Finally, consideration is given to the possibility of obtaining CW operation of the dye laser.

Important developments in gas laser technology have arisen in the past year or so. These developments include improvements in the performance of existing systems and the achievement of laser action in new systems. In the former category we would mention the construc-tion of compact CO₂ lasers with power outputs previously achieved only in very large devices and the commercial exploitation of CW ultraviolet laser outputs in noble gas ion lasers. New systems would include CW laser action in metal vapors in a buffer discharge of helium and CW laser action in pure helium itself.

This paper will review some of the important aspects of infrared lasers. By infrared, I choose to define a wavelength range which is longer than about 2μ, and extends to 1000μ. In this wavelength range, during the past 8 years a large number of laser sources have been discovered, and exploited in applications of investigation of physics and related fields. The most widely used are the gas lasers such as the CO₂ laser and the solid state lasers such as the PbSnTe injection lasers. The largest number of laser transi-tions occur in atomic spectra of noble gases. However, from the point of view of power output, the molecular lasers are more important. By molecular lasers, we mean those lasers which operate on transitions between vibrational-rotational levels of molecules. One such example is the carbon dioxide laser at 10.6μ. Since the discovery of the CO₂ laser in 1964, the art of molecular lasers has advanced rapidly. It has been shown that the carbon-dioxide laser is capable of producing large amounts CW powers as well as pulsed or Q-switched power, efficiencies of the order of 20%. In addition to the CO₂ lasers, other molecular systems have also become important in the last few years, including the HCN laser and the H₂O laser which havelaser transitions in the far infrared portion of the region, at wavelengths between 100μ and 1000μ. These laser transitions while not capable of high power output, are nonetheless very important since these are about the only sources of coherent radiation in this part of the spectrum. In the present paper, however, we shall restrict ourselves to a detailed discussion of only the molecular lasers in the 10μ range and in particular to the discussion of carbon-dioxide lasers. We will very briefly dis-cuss the H₂O and HCN gas lasers as well as the tunable PbSnTe diode lasers.

During the last few years, a five order of magnitude jump has been made in researchers' ability to generate short light pulses (i.e., from 10^-8 sec to 10^-13 sec) having peak powers in excess of 106 watts. During this same period, a three order of magnitude jump in our ability to generate high peak powers - (i.e., from 10⁹ w to 10¹² w) and to measure (10^-10 sec to 10^-13 sec) short duration light pulses. This paper will review the techniques utilized to generate and measure these powerful "bullets" of light.

Self-giant-pulsed operation of ruby lasers at 77 and 300°K and Nd ³⁺:YAG lasers at 77°K pumped by the 5145-R output of a pulsed argon ion laser is described. Self-giant-pulsed operation was obtained by: (1) static mirror misalignment, and (2) static misalignment of the fila-ment of the pumped laser material with respect to the mirror resonator axis. An output that consisted of a single "giant" pulse could be obtained for ruby by either method; for Nd3+:YAG, the output always consisted of one or more "giant" pulses and characteristic relaxation oscillations. These results are compared with the predictions of the theoretical model of E. R. Peressini in which the self-giant pulsing of these lasers is described as a limiting case of spiking. The relaxation oscillations in the output of ruby lasers are compared with the theoretical predictions.

Many of the characteristics of solid-state lasers depend to a large extent upon the quality of the individual laser rod and its optical properties in the presence of pumping raidation. This paper su-marizes the input-output properties of typical neodium-YAG rods used in (moderate power, tungsten-pumped, continuous lasers operating at 1.0641 nicrons and 1.32 microns. A sinple method of determining the optimum reflectivity of the output mirror is described, and output-versus-reflectivity curves are given for a number of laser rods. Several features of the spectrum, polarization, and mode structure of the laser are related to the optical properties of the pumped laser rod.

Output power and efficiency as a function of tube length is determined for small CO₂ lasers. Discharge lengths from 5 to 25 cms were studied and all systems were run with static gas fills. Outputs greater than 3 watts in the TEM₀₀ mode and efficiencies greater than 7% were obtained. The effects of tube bore diameter and cavity insertion losses are also assessed.

Dye laser action has been observed for a number of scintillators to wavelengths as low as 3550 Å in the near ultraviolet spectral range. The compounds that have been lased in this region are sodium salicylate, 2, 5 diphenyloxazole (PPO), 2, 5 diphenyl furan (PPF), 2, 5 di- 1-naphthy-1, 3, 4 oxadiazole (PPD), 2 phenyl-5-1, 3, 4-oxadiazole (PBD), and aminobenzoic acid. These fluors are of current interest because laser action in these materials has been difficult or impossible to achieve by other dye laser pumping techniques.

Laser interferometry has rapidly moved from the research phase to practical application in the laboratory and factory. Principal uses are for measuring lengths or coordinates of objects to superior accuracy and for measuring parameters or quality of optical components or systems. The long coherence length of the laser source and its high intensity are principal advantages but they also lead to confusing multiple fringe patterns. A laser interferometer has been used to measure the radii of surfaces and to position the elements of high quality lenses so that they are centered and spaced to high accuracy. A polarization system is used to eliminate unwanted fringe patterns and balance intensities in the arms of the interferometer. I have been asked to talk about laser interferometry, a topic that has been extensively discussed in the past few years. I am sure however, that you have all heard that laser beams can be coherent over many miles and at the same time can have more than enough intensity for almost any type of interferometry. The predictions for applications have been impressive and they are coming true. I believe that the practical realities of the use of the laser in interferometry can be better considered in terms of actual applications since the real growth of the field is the total of many small almost independent uses. I will therefore describe an application that I am familiar with - the use of laser interferometry in the assembly of very precise lenses for use in making integrated circuit masks.

Measurements now being made with the aid of lasers cover a broad range of scientific problems. The most unique new capability is precise distance measurement over ranges of many meters using the stable wavelength of the laser light as a resolution limit. This falls in the category of inter-ferometry and has been covered very well by Herriott (Ref. 1). The laser has also become very useful for measuring long ranges with high relative precision but with resolution considerately less than that of the light wavelength itself. There are a number of ways to use the laser frequency as a carrier of much lower frequency information. This yields another kind of interferometry which uses "fringe" detectors that are quite different physically from the mirror interferometers used for very precise measurements. But the detectors for modulations put on laser light are true interferometers also, and merely work at a much longer wavelength. These techniques yield lower absolute resolution, but still give very meaningful and useful measurements with high precision when necessary.

Laser communications may be divided into four general application classes; (a) terrestrial short range paths through the atmospheres (b) closed pipe systems for sending high data rate between major metropolitan centers, (c) near space communications for relaying high data rates, and (d) deep space communications from the outer planets.

The application of high energy laser radiation to the problem of controlled thermonuclear fusion is discussed. Consideration of the radiation absorption process in a high temperature plasma, which serves as the working fluid for a fusion reactor, leads to an optimum laser power and pulse shape. Single laser pulse energies required approach 107 joules for a working reactor. A review of present high power experimental laser plasma work is presented, together with a series of concepts in which the method of laser ignition of a fusion reaction is applied to a stationary power plant and a spacecraft propulsion system.

The title of my talk is somewhat misleading in that if you expect to hear about new exciting non-linear materials of proven quality then you will, I fear, be disappointed. Having searched the literature for many months and having attended many conferences in the past year, I am forced to come to the conclusion there are very few, if any, new exciting non-linear materials on the scene. In fact I think its true to say that there are only two materials which seem to be reliable, and even these are to some extent suspect. All is not gloomy, however, and during the course of this talk I will have occasion to mention a few new materials and discuss some of their properties. Before we discuss specific materials I think it would be worthwhile to review briefly the subject of non-linear optics and to discuss some of the uses of the non-linear materials. In particular I would like to discuss what qualities are required of them before they are declared useful.

Lasers are being applied to an increasing extent in biology and medicine. Studies have been carried out at various wavelengths, from the ultraviolet to the infrared, at energy levels from the milli-joule region to more than 100 joules per pulse and at power levels from the milliwatt to the multimegawatt region. Biological studies have been carried out at the molecular level, on cellular components and isolated cells, on microorganisms, viruses, and tissue culture, on isolated physiologic systems, individual organs, and on intact animals. Studies in man have been oriented towards the use of the laser in ophthalmology, exploration of its potential as a surgical tool and for tumor eradication. The coherence property is being used in exploratory studies directed towards radiology and diagnosis with ultrasonic holography. Hazards associated with laser radiation are being determined and methods and procedures for minimizing these hazards developed. In this survey attention will be directed towards representative studies to provide the reader with some appreciation of the use of lasers in biology and medicine.

The development of laser display technology is traced and the results which have been obtained with the most recent systems are discussed. The important role played by components such as light beam deflectors, light modulators, and visible lasers are described. The unique features of laser displays which distinguish them from more conventional displays such as the cathode ray tube are emphasized. Problems which remain to be solved before this display technique becomes broadly applicable are also considered.

It is now more than five years since the great increase in holo-graphic activity began. As we take an overview of the current state of holography, two observations become evident. First, the level of activity has continued unabated- -certainly no diminution of activity is noticeable. Possibly the number of papers on holography this year will be higher than ever before. The second observation, perhaps not entirely compatable with the first, is that commercial applications for holography are yet rather limited.

The results of frequency locking experiments with coupled ruby lasers are presented. The single axial mode characteristics of an individual saturable dye-switched laser can be understood without requiring spectroscopic hole burning in the dye. For coupled lasers in which frequency locking is observed, the existence of a spectroscopic hole seems necessary to explain the experimental results. The widths of the holes are inferred to be about 0.10 cm^-1 using pyridine and 0.04 cm^-1 using cyclohexanol as solvents. A relatively slow time dependent perturbation of the dye by thermal density fluctuations in the solvent yields a fluctuation in the transition energy of about 0.12 cm^-1, which is in qualitative agreement with the inferred widths.