Properties of lasers with frequency shifted feedback are discussed. The analysis concentrates on the passive characteristics of the `cavity.' Particular emphasis is given to the case where the frequency shift is small compared with the free spectral range of a normal Fabry-Perot cavity with no frequency shift.

To improve the capabilities of excimer laser ablation processing apparatus, we have been developing an innovative Multiple Irradiation Optical System (multiple irradiator). In this system, we placed a reflector opposite the mask, and used a mask with a highly reflective coating. The laser beams therefore irradiate the mask repeatedly, and we can use the laser energy more efficiently than conventional optical systems. The experimental results show that our system is 12 times more energy efficient than the conventional optical systems. Also, we have made a prototype excimer laser material processing apparatus with the multiple irradiator, which is suitable for drilling viaholes of less than 100 micrometers diameters such as viaholes of print circuit boards. The processing speed is less than 1 minute when the material is 100 mm X 100 mm wide and 25 micrometers thicknesses polyimide film, and the laser energy is 40 mJ/shot with 600 Hz frequency.

In this paper we describe a different mathematical technique which derives from the MTF of a system. This technique evaluates the loss of flux density in the central lobe of the Airy Disc caused by aberration of the diffraction image caused by the mode shape of the laser cavity, the thermal-mechanical distortion of the optical components in the high power laser beam train and ultimately the focusing optical components. The theoretical model, which we call Central Lobe Energy Distribution Technique, provides the equations for evaluating optical distortion caused by the absorbed thermal flux density on the mirrors, the mechanical bowing and subsequent distortion of the mounting configuration and the criteria for sizing the optical components in relation to the laser beam diameter. A comparison is made for both the Strehl criteria and for the M² technique, showing examples.

A new mathematical model for the fundamental mode of a Gaussian beam is presented. Unlike the Kogelnik and Li model, this new model is an exact, closed-form solution of the scalar Helmholtz equation for beams with a Gaussian amplitude distribution. The model is the 0,0 order of a family of solutions to the wave equation in the oblate spheroidal coordinate system. A geometrical interpretation of the model, based on the concept of a skew line, is also presented that allows simple, straight-line modeling of intracavity and extracavity beam propagation.

We have created a program for the modeling of laser beams to answer systems questions. We have used the IMAGE program for wavefront modeling and far field modeling. In order to answer such questions as 'How much astigmatism is permitted in a Gaussian beam clipped at one-half the one-over-e-squared-radius with an obscuration ratio of 0.3 if 75% of the energy is to lie within 2*lambda*F#?' one might need to use a large lens design program. We have developed the IMAGE program so that the systems engineer has a good tool to answer such questions quickly. In fact, we use a large optical design program for our lens design work and still find that IMAGE is much more convenient and quicker than the more powerful program.

The ability to vary glass properties by adjusting composition continues to make glass a leading material for application as both active and passive elements, in bulk as well as in guided-wave laser systems. We consider here how glass is engineered for specific intracavity and extracavity laser applications. Mention also is made of process and manufacturing techniques which result in glasses with improved or special properties critical for applications involving laser systems.

Laser system performance and reliability depends on the related performance and reliability of the optical components which define the cavity and transport subsystems. High-average-power and long transport lengths impose specific requirements on component performance. The complexity of the manufacturing process for optical components requires a high degree of process control and verification. Qualification has proven effective in ensuring confidence in the procurement process for these optical components. Issues related to component reliability have been studied and provide useful information to better understand the long term performance and reliability of the laser system.

In CO₂-laser engineering, combining high output power with a good laser beam quality is a very challenging design problem. Due to residual absorption in the output coupler (O.C.), thermal aberrations are added to the transmitted beam which result in thermal focussing of the window and in a decreased beam quality. Using thermal finite element analysis in combination with the new concept of embedded Gaussian beam, the influence of the cooling geometry on beam quality can be studied in detail. In particular, the case is considered where the usual conductive cooling is complemented by convective cooling through a gas film blown over the outer surface of the output coupler. To confine the gas jet over this surface a diamond machined piece of ZnSe is used. We present experimentally obtained values of the laser beam quality on a 2.5 kW and a 4 kW resonator, provided with this actively cooled output coupler.

The effects of intracavity etalons on the temporal mode structure of a regenerative amplifier have been studied both theoretically and experimentally. The analysis predicts the output temporal pulse shape given the input pulse shape, the laser-cavity parameters, the etalon parameters, and the number of round trips the injected pulse makes in the laser cavity. Experimental results agree well with the theory.

Channeling a laser beam by multiple reflections between two closely spaced, parallel or nearly parallel mirrors, serves to reshape and homogenize the beam at the output gap between the mirrors. Application of this device to improve the spatial overlap of a copper laser pump beam with the signal beam in a dye laser amplifier is described.

We have designed and built a number of doublet and triplet lenses intended for focussing laser light. The wavelength range extends from 220 nm to 1550 nm. An ordinary achromat, sometimes called a telescope objective, is suitable for some applications, but its cemented surface will not tolerate high irradiance levels. We discuss the performance of the designs at widely spread wavelengths and show which types of designs can be used without refocussing (achromatic) and which types perform satisfactorily only with refocussing as the wavelength is changed. In particular, we describe the cemented telescope objective, the air-spaced telescope objective, a cemented doublet without axial color correction, an air-spaced objective of a single glass type, an air-spaced doublet for high power lasers, and a three element lens.

Mirrors with a super-Gaussian reflectivity profile are extremely useful for high power laser design with nearly diffraction limited output beams. The super-Gaussian mirrors are usually realized by dielectric mirrors with one or more tapered layers. Taking advantage of the thermo-optical effect we propose to build tapered mirrors and windows, the optical parameters of which can be electrically modulated. The most relevant result is that these devices can be easily employed to realize super-Gaussian mirrors with an electrically adjustable reflectivity profile.

The engineering calculation methods of complex crystal—optical elements consisting of anisotropic crystals with accidental direction of system compo nents optical axes are developed. The practical results of birefrigent crystal— optical elements ( BCE ) consisted of two one - axes crystals complex resear— ches are given. The BCE analysis and synthesis during propagation and interference of polarised *zaves are involved in researches. We present in report our BCE - based apparatures. Investigations in such directions as laser optical and optoelectronic instruments making, laser optical systems for detection and ranging, communication and information processing call for the development of an optical element base with extended function capabilities. In this connection, of current concern are investigations in the field of analisys and synthesis of complex crystal—optical elements of anisotropic crystals, which have the property of electromagnetic wave splitting into orthogonally polarized components with the possibility of their subsequent control. In this case of particular interest is studiyng interference-polarization properties of anisotropic crystal—opti— cal elements, since it is possible on their basis to propose new methods of optical wave grouping ( heterodyning ) and measuring the degree of radiation coherence as well as to develop new variants of laser interferference systems and devices. The main difficulty in calculation of electromagnetic wave transmission through complex crystal—optical elements consisting of anisotro— pic crystals with random orientation of optical axes in components is the necessity of taking into account the pecularities of extraordinary wave propagation: first, in an anisotropic crystal the vector of energy flux density, that is, a ray is not directed along the wave vector in the general case — meanwhile, it is precisely the ray direction that is directly observed and determined in practice and the basic requirement is the necessity of taking into account tracing of an extraordinary e— ray, that is, calculation of its energy trajectory; second, in the general case the e - ray does not lie in the plane of incidence.

This paper is solved the the coherent combination of several laser beams with the high-efficiency multi-beam grating which is optimized by specific requirement. The coherence between laser beams is controlled by injection locking feedback circuits.

Numerical simulation of high average power 3-wave mixing is applied to mid-IR parametric generation and compared with experiment. Aberration compensation and efficiency improvements are discussed.

We present results of an optical parametric oscillator based on lithium niobate pumped by a multi-longitudinal and multi-transverse mode Nd:YAG laser. Two 5 cm crystals were used in a walk-off compensated arrangement with a 14 cm overall cavity length. The oscillation threshold was 30 mj with a measured slope efficiency of 34% and an overall efficiency approaching 28%. We were capable of continuously tuning the OPO from 1.5 to 3.8 microns. At 2.1 microns, the maximum measured output energy was 58 mj in a 10 ns pulse. A comparison was made with an OPO using two 1 cm KTP crystals.

Continuous tunability and high efficiency are obtained from an intra-cavity doubled optical parametric oscillator (OPO) pumped by a frequency-doubled diode-pumped Nd:YAG laser. The OPO is tunable from 760 - 1040 nm with 30% efficiency, giving 380 - 520 nm tunability after intra-cavity doubling with 40% efficiency.

A cw-lamp-pumped Nd:YAG injection-mode-locked seeded master oscillator-amplifier beamline is used to pump various optical parametric oscillators (OPOs) using KTP, KNbO₃, AgGaS₂, and AgGaSe₂ crystals as the nonlinear materials. A comparison of these materials is presented and limitations on their ultimate power scaling are identified. This program has demonstrated tunable output in the 3 - 5 mm wavelength spectrum using a mode- locked and Q-switched (MLQS) pulse waveform at low power. A novel injection-mode-locked seeded Nd:YAG laser was developed to pump the OPOs that provided MLQS pulses at > 20 W and up to 10 kHz.

Results of thermal conductivity measurements are reported for several of the more recently developed nonlinear optical crystals. New or substantially revised values of thermal conductivity were obtained in six materials. Notable thermal conductivities measured were those for AgGaS2 (0.014 W/cmK and 0.015 W/cmK), AgGaSe2 (0.010 W/cmK and 0.011 W/cmK), BBO (0.016 W/cmK and 0.012 W/cmK) and ZnGeP2 (0.36 W/cmK and 0.35 WfcmK), with all values quoted for directions respectively parallel and perpendicular to the optic axis for each material. Accuracy was estimated to be within 10%. These new data are of potential importance in the design of high power optical frequency converters.

Optical parametric oscillators (OPO) of beta-Bariumborate pumped by pulsed single-mode Nd:YAG lasers and seeded with the light of narrowband tunable lasers are powerful coherent light sources which provide high efficiency (40 - 60%), wide tuning range (300 - 3000 nm) and narrow linewidth (0.02 cm^-1). In this contribution we also report on the efficient operation of noncritically phase matched cw mode-locked ps- and fs-OPOs of KTP synchronously pumped by Ti:Sapphire lasers. These OPOs generate stable transform limited pulses in the near infrared (1.05 - 2.9 micrometers ) with power conversion efficiencies of up to 40%.

Nonlinear difference frequency generation in KTP using a tunable room temperature diode laser near 810 nm with either a Nd:YAG at 1064 nm or a HeNe laser at 633 nm produced tunable infrared to detect CH₄ at 3.4 micrometers and N₂O at 2.9 micrometers . A scan of a germanium etalon showed an infrared scanning range of 2 cm^-1 without mode hops.

Utilization of the parametric down—conversion provides a unique possibility of absolute standardless brightness calibration ofhigh—temperature visible, infrared and near UV radiation. The calibration method has been suggested in Moscow State University in 1977 and then was applied there to experimental measurements of radiation spectral brightness for laser and luminiscence sources of visible range and filament lamps radiating in IR range. The method is based on the comparison between two signals: the signal p, corresponding to the measured radiation parametric up-conversion and the "noise" signal P corresponding to parametric scattering of the pump; this "noise" signal is observed both in the presence of the measured radiation at the converter's idler channel entrance and in the absence of it. The noise signal intensity is determined by the effective brightness of zero vacuum fluctuations at the idler frequency which equals to one photon per mode, or, in the energetic units, Bvac = hc2/A, where h is the Plank constant, c —the light velocity, )2 —the idler wavelength. Measuring the ratio of signals P" P' + P and P at the exit of the crystal—converter at frequency w' = wo — w (w0 being the pump frequency, and w —the frequency of the flux being calibrated), one can calculate the absolute value N of energetic brightness spectral density for the radiation which fills the idler channel of the converter: N = k[(P"/P )— 1] in the units "photon/mode" or B = BvacN in the energetic units (here icis a correction coefficient describing the non-ideal transparency of the converting crystal). No additional standard sources or detectors are required here; the method uses as a reference the effective value of zero vacuum fluctuations brightness. The reference effective brightness temperature depends on the radiation wavelegth: T0 = hc/Ak in 2, k being the Boltzmann constant, so that for A2 10 p Tvac 2000 K, and for ' 0.5 Tvac 40 000 K. In this work the accuracy of the method is analysed and the resolution for different schemes of the parametric photometer is calculated.

A laser power meter has been developed which can measure absolute power output to better than 0.1%. The power meter has a flat spectral response from ultraviolet to far infrared and is essentially 100% linear over its full dynamic range. The power meter has long term stability better than 0.001% per day. The operation of the power meter is discussed and performance measurement data are presented.