Slab lasers offer advantages over rod lasers when higher output power is required. However, these advantages are realized only with careful engineering. In this paper some important design considerations needed to obtain high performance from slab lasers are reviewed.

A Nd:Glass laser has been used to generate laser produced plasmas (LLP). The plasma emission in the keV range useful for X-ray lithography has been measured. Lithography with sub-micron linewidths has been demonstrated with a thin absorbing mask. The Nd:Glass slab laser is operated in a Q-switched one-dimensional unstable resonator. The resulting beam quality is a few times diffraction limited and is focused to an area less than the lO cm2 in our vacuum chamber. We have operated at 5 J up to 4 Hz repetition rate without any degradation of the laser output. Injection mode locking of the Nd:Glass laser with 0.7 ns pulses increases the intensity of the Q-switched laser output by about a factor of 10 and allows us to achieve a total integrated pulse length of less than 10 ns. Optical damage limits the laser intensity. The damage threshold for injection mode locked pulses focused into a Nd:Glass slab outside the laser cavity is about 20 J/cm2. However, we have observed another damage mechanism at lower intensities in Nd:Glass slabs in use in the laser head. Brown discoloration occurs in filaments along the laser beam path and we believe solarization with the help of self-focusing and the ultra- violet flashlamp radiation may be occuring. We are still investigating this phenomenon but at present it is limiting the laser output to only 2 J per pulse. The focused laser intensity is 2.1013 W/cm2 on a solid copper target in our vacuum chamber. The plasma emission in the keV X-ray range has been measured through a variety of thin film X-ray filters with a Hamamatsu micro-channel plate detector. Using the published values for the detector quantum efficiency, the micro-channel plate gain, and the filter's transmission spectra, we estimate that the conversion efficiency in the plasma from laser radiation to soft X-rays of energy greater than 0.5 keV is around 2%. We have performed single-level demonstration exposures of PNMA resist through a 10 im thick aluminised Kapton debris shield and a 4 im thick Boron Nitride X-ray mask supplied by Piero Pianetta the the Stanford Synchroton Research Laboratory. The 1.0 pm linewidth gold absorber patterns on the mask are accurately reproduced in the resist. The PMMA resist exposure rate at a 5 cm working distance from the plasma has been measured as 0.2 micron per lO J of total laser energy so we obtain a single layer exposure in about 30 minutes. The potential for improvement is enormous. The X-ray signal measured through the mask increases exponentially with the laser energy on target so increasing the focused intensity will reduce exposure rates dramatically. Available improvements in higher average power lasers and sensitised resists both offer an order of magnitude improvement over this system . These results indicate excellent potential for commercial LPP X-ray lithography.

This paper is concerned with mechanisms in near infrared lasers in the 1 to 3tm region and mainly with flashlamp excitation. It explores two types of laser, based on rare earth doping in crystal host media- eg Er:YAG, Cr:Er:YSGG and Cr:Tm:Ho:YAG, or on rare earth doping into glassy media prepared by the sol-gel process.

New developments on Q-switched and mode-locked solid state lasers are reviewed. Both pulsed and continuous working systems are considered. Nd:YAG, Nd:glass, and ruby are still the most widely used pulsed short pulse solid state laser materials. Cw mode-locked and Q-switched Nd:YLF lasers gain importance as pump sources of pi- cosecond and femtosecond dye lasers. Short pulse tunable alexandrite and Ti:sapphire lasers compete with near infrared dye lasers and find application in the amplification of ultrashort dye laser pulses. Frequency tunable femtosecond pulses have been generated in color center lasers. Diode laser pumping opens the way to all-solid- state short pulse lasers. The coupled-cavity mode-locking technique (additive pulse mode-locking), the seeding pulse amplification, and the regenerative chirped pulse amplification are described.

The paper presents the results and the criteria followed in the design of a solid state laser operating in bust mode. Specifically addressed for a Thomson scattering diagnostic, the laser delivers at 1 urn a burst of 10 shots, each of 10 J, 15 nsec and pratically diffraction limited. Main problems are thermal effect which can cause un unaccettable change of divergence during the bust. It will be show that the simple scheme of nocooling turns out feasible. Effects of doping level, choise of Nd hosts together with choises of discharge electronic will be considered. An optimum choise proved to be the YLF oscillator and preamplifier, followed by a double pass zig-zag phosphate slab. Near field and far field pattern show a good point and divergence stability. Roughly speaking, we can say that an "overall induced thermal lens" longer than 40 m for the 2 cm output beam has been achieved.

A simple laser of titanium-doped sapphire in a plane mirror res9nator was pumped by a flashlamp pumped dye laser. The dye laser emitted pulses of duration 2 js (FWHM) and energy close to one joule at 490 nm. Values for the lasing threshold and slope efficiency (well above threshold) for the titanium :sapphire laser were derived by two methods. Firstly the experimental data were fitted by theory which took account of the spatial and temporal properties of the pump beam and had only the threshold and slope as adjustable parameters. Secondly, the parameters were calculated from the measured spectroscopic properties of the resonator mirrors and the crystal. The sets of derived parameters agreed within about I 5%. Two crystals were employed, with low figure of merit (27 for the higher-doped sample). The greatest overall efficiency was limited principally by the pump beam properties and not by losses in the crystals.

I summarize the recent progresses in laser cavity design, mainly applied to high power, pulsed solid-state laser systems. Evaluation criteria of laser cavities are considered and the main project tools as available today are reviewed. Emphasis is given on the use of unstable resonators, with and without variable reflectivity mirror outcoupling.

By using variable reflectivity mirrors of super-Gaussian profile in unstable resonators instead of traditional hard edge output couplers we have greatly improved the beam quality, also obtaining higher energetic efficiency. Mode profiles have been calculated within geometrical optics approximation and by means of numerical simulations according to the diffractive theory. The output energy has been predicted by means of a closed form expression resulting from an innovative theoretical model that takes account of the effects of mode profile on gain saturation. Several super-Gaussian mirrors have been fabricated by means of a quasi standard dielectric thin film deposition process and have been extensively tested in a pulsed Nd:YAG laser. The experimental results, in terms of output energy and focusing properties, confirm the superior performances of super-Gaussian resonators.

The properties of several resonator schemes, which provide high output power and good beam quality simultaneously were investigated both theoretically and experimentally. Experiments were performed with high power Nd:YAG rod systems and Nd:YAG slab systems. Best performance was achieved with unstable resonator schemes. Output power up to 230 Watts was obtained with 6 x 3/8 inch rod and unstable resonators with different reflectivity profiles of the output coupler. Destruction of the high-reflecting spot can be prevented by appropriate resonator design: the thermal lens must be exactly corrected for and a slight misalignment of the rod is necessary. The unstable resonator provides good focusability over the whole range of output power. Compared with optimized stable resonators, the output power was 25 % lower. Considerable improvement of beam quality of slab lasers was achieved by using folded stable resonators, plane- unstable resonators, on-axis unstable resonators and off-axis-unstable resonators. Common stable resonators operating near the limit of stability are too sensitive against the negative refractive power that occurs in the y-direction. Beam parameter products of 1 mm mrad and lower were measured with off-axis unstable resonators.

Since the invention of diode lasers in the early 1960's there had been continuous investigations in laser diode pumped solid state lasers as has been reviewed in detail by a number of papers ( see e.g. [1] ). There are two main advantages of using diode lasers instead of flashlaraps as a pump source for solid state lasers: First the emission of the diode lasers matches well with the absorption bands of several Rare Earth ions that are doped in laser crystals ( mainly Nd3+, but also Er3, Tm3, Dy3', and others ) . This summary will report only about diode lasers at a wavelength of around BlOnm, which fits to an absorptionband of Nd3t Second diode lasers provide the possibility of longitudinally pumped configurations and therefore an excellent mode matching with the solid state laser mode. For both reasons the efficiency of a diode laser puniped solid state laser is nuch higher than of a flashlamp pumped one. Since the early 1980's a much wider interest in diode laser pumped solid state lasers arose. It was stimulated by the improved performance of the new generation of diode lasers in terms of reliability , operational lifetime and output power [21. Two important steps in direction to the diode lasers at present time were the developments of double hetero (DH) structure- and graded index separate confinement hetero (GrInSCH) structurediode lasers. In the same way the development of new production techniques were necessary to ensure the reliability of the diode lasers. Starting with the liquid phase epitaxy (LPE) the (GaAl)As structures are now grown by the molecular beam epitaxy (MBE), mainly used for very high precision laboratory investigations, and metal organic chemical vapour deposition (MOCVD), mainly used for commercial production. As a first commercial product SDL introduced a 100mW array in 1984. Since then the output power of the commercially available diode lasers increased by two orders of magnitude to lOW. These diode lasers are multi stripe bar arrays like the 5W diode laser

We present experimental results and a model of Nd:glass disk amplifiers which are used in inertial confinement fusion research. We first review our previous measurements on pulsed xenon flashlamps. We then discuss out measurements on the enhancement of the Nd fluorescence decay rate in laser disks by amplified spontaneous emission. Using these data, we have constructed a model of flashlamp pumping which treats the transfer efficiency of pump light from the flashlamps to the disks as an empirical function. We have found a simple description of this cavity transfer function which provides an excellent fit to the amplifier results for various pump pulselengths. We discuss the concept of the pump area ratio for describing the flashlamp packing density and show that amplifier performance is optimized for values of this parameter near unity. We finally present results for both a singlesegment and a multisegment disk amplifier. We have used these devices to investigate new amplifier designs for a large scale fusion driver.

We present results of an experimental investigation of the optical losses produced by bending large core optical fibres, typical of those used in power beam delivery systems. Experiments have been conducted over a range of core diameters for both plastic clad silica and all silica fibres, as a function of bend radius. A theoretical model has been developed for predicting the magnitude of the bend loss, and agreement was obtained with the experimental results. The study thus yields design information for fibre beam delivery systems.

The YAG laser has an excellent characteristic that makes beam transmission with optical fibers. In addition, the oscillator of the 2 kW class has recently been developed, and so its applications are changing largely from conventional fine material processing. The authors have adopted the YAG laser welding that uses optical fibers favorable to application to narrow, complicated areas. First, the authors have analyzed the beam intensity distribution near the focusing point using ray tracing method, and manufactured the compact condensing optical lens systems, and metal mirrors which have high reflectivity and durability that can be inserted in the tubes of about 16 mm I.D. to weld them. And also have developed the beam transmission system that can transmit beam for a long distance of about 200 m using small diameter optical fibers. In addition, with these systems the welding conditions to get about 2 mm penetration for stable welding has been obtained and applied YAG laser welding of steam generator tubes in practical nuclear power plants.

Solid state lasers gain importance in the field of material processing due to the recent increase of output power up to more than 1 kW. In comparison with the C02-laser the main technological advantages are the shorter wavelength, respective the higher beam absorption of metal surfaces and the possibility of beam transmission through a fibre. With regard to both benefits the effects on cutting and welding are investigated and prospects to the technological and economical advantages of the solid state laser are presented.

Technology advances are described which have led to improved 1 kW average power solid state lasers. These developments lead to genuine processing advantages in terms of manufacturing flexibility, and in the case of welding, metallurgical quality. Results of various brief welding trials are reported on a number of different materials, with some preliminary mechanical weld test results.

The application of YAG-SLAB lasers operating in fundamental and low order mode to metal cutting has been investigated. The high beam quality and the relative insensitivity of the beam shape to average power variations make slab lasers strong candidates for high precision cutting applications. The advantage of very fine focusability (focused beam diameters down to 30,um) and high intensity (100 MW/cm2) for cutting applications are shown. Three limiting factors are derived for high quality cutting : The threshold intensity for efficient laser beam absorption, the depth of focus of the laser beam and the limited aspect ratio (cutting thickness to kerf width ratio) achievable by material removal. Experimental results of stainless steel cutting with fundamental and low order mode beams are given for material thicknesses of 0.5 mm to 1.5 mm. The cuts show kerf widths of 35im (aspect ratios of 21:1) and roughness values of Ra 1.6 pm for 0.8 mm thick steel samples.

In the field of the EUREKA program n° 249 which relates to high power YAG lasers, L'AIR LIQUIDE studies the comparative performances of C02 and YAG lasers in the cutting and welding of materials. Stainless steel cutting is the second step of those comparative studies. The experiments have been done on a 1 ,5 kW continuous wave ( CW) C02 laser and on a pulsed 600 W YAG laser, prototype of the 1 kW series that LUMONICS is launching now ( 1) , ( 2) ( see C02 and YAG lasers p . 9 top fig .1).

Results are reported from a composition study of forty-one phosphate glasses that are derivatives of a commercially available Nd-doped laser glass. Systematic variations have been made in the quantities of alkali and alkaline earth components in the glass. Correlations are developed relating the compositionally-averaged electric field strength and certain thermal, mechanical, optical and laser properties of these glasses. The compositionally-averaged field strength is found to be a good parameter for predicting the effects of complex mixtures of alkali and alkaline earth components in the glass. However, it is inadequate for explaining the effects of glass components that are predominantly covalently bonded with oxygen. Some possible explanations are given for the observed trends in glass properties with certain compositional changes.

Measurements of energy storage and heat deposition in flash-lamp pumped Nd:Yb:Er: glass, and Cr:Nd:Yb:Er: glass lasers are reported. Energy storage is determined indirectly by measuring output vs. input energy characteristics of erbium lasers operating in a free oscillation mode. A thermal camera is used to measure temperature increase following isolated flash-lamp pulses. Contributions of different sensitizers are distinguished by spectrally filtering flash-lamp pump radiation. It is determined that in Nd:Yb:Er: glass ytterbium sensitized pumping contributes 85% to the total erbium inverted population energy storage. Long flash-lamp pulse durations are therefore required for efficient Nd:Yb:Er: glass laser operation. Direct erbium, and neodimium sensitized pumping is determined to be relatively inefficient, although the corresponding visible part of the pump spectrum contributes in Nd:Yb:Er: glass as much as 65% of the total deposited heat. Chromium co-doping is observed to significantly increase energy conversion efficiency of pumping in the visible, allowing shorter flashlamp pulse durations in Cr:Nd:Yb:Er: glass lasers. In addition, analysis shows the ratio of the total heat generated per unit stored energy to be in Cr:Nd:Yb:Er: glass lower than in Nd:Yb:Er: glass.

Asdex Upgrade (AUG) is a toroidal magnetic plasma confinement experiment for nuclear fusion research. It is of the tokamak type, having maximum values for toroidal plasma current and magnetic field of 2 MA and 3.9 T respectively. The experiment will go into operation this year. Measurement of electron temperature and density are of extreme importance for the understanding of basic plasma phenomena. One of the most unambiguous measuring methods is Thomson scattering with pulsed high power lasers as a light source. The scattering system for AUG described here is an improved version of the existing device on ASDEX. Special emphasis has been put on wide flexibility to match different discharge conditions, and operating reliability combined with high total optical transmission and spacial and temporal resolution without losing the capability of recording the long term development of plasma parameters during a discharge.

Thomson scattering for measuring the electron temperature and density has been a well known diagnostic in plasma physics used since the invention of the laser (1). The method itself is therefore only very briefly described. This paper is mainly concerned with a very advanced Thomson scattering system developed at Garching to investigate the high temperature plasma ASDEX. A selection of measurements is shown at the end.

By combining the time-of-flight or LIDAR principle with a Thomson backscatter diagnostic, spatial profiles of the electron temperature and density can be measured with a single set of detectors for all spatial points. The technique was demonstrated for the first time on the JET tokamak and has been in routine operation since July 1987. Originally a ruby laser (3 J pulse energy, 300 Ps pulse duration, 0.5 Hz repetition rate) was used together with a 700 MHz bandwidth detection and registration system which yielded a spatial resolution of about 12 cm. A large filter spectrometer with 6 spectral channels covering the wavelength range 400 - 800 nm gives a dynamic range for the temperature measurements of 0.2-20 keV. The original system is described, examples of measurements are given and compared with the results of other diagnostics. The system is being upgraded to make measurements at 10 Hz and a major component of the new system is an Alexandrite laser (1-2 J pulse energy, 350+1-50 ps pulse duration, 10 Hz repetition rate) which is currently being constructed. The new laser and other technological improvements being incorporated into the upgraded diagnostic will also be described.

This study was undertaken to produce crack free seam welds of aluminium-magnesium alloys [5083 and 5754] using pulsed Nd:YAG laser. All the experiments in similar alloy welding without filler material addition resulted in welds with cracks. Electron micro probe analysis of the welds were carried out and it was found that the percentage of magnesium was reduced by 1 to 2 percent. Experiments with different filler material additions were carried out to avoid cracking. Using 4047 or 4043 as a filler material crack free welds can be obtained provided the weld metal contains at least 4% silicon.