The objectives of reactive chemical and nonreactive thermal processing with laser radiation are outlined giving indication that processing with laser radiation is governed by a hierarchy of time constants originating from photon-matter interaction, phase transition dynamics, laser source excitation fluctuations, and optical feedback in combination with the influence of beam delivery systems, processing/shielding gas flow configurations, robotics, production lines and environment. The minimization of losses by heat flow, reflection and transmission and the stringent need for quality assurance require as first approach the control of processing, which is mainly due to the capability of laser radiation source. The current status of laser radiation sources is reported giving information on the state of the art of processing with laser radiation in combination with subsequent demonstration of future trends and developments with respect to radiation sources, beam delivery, beam shaping, materials, processing and quality assurance.

We report on the development of high repetition rate, high average power excimer laser systems with projected repetition rates of several kHz and multi kilowatt outputs. An optimized system concept has been derived by theoretically and experimentally comparing the various system options. Preliminary results are presented for a compact spark-preionized KrF and XeC1 excimer laser system operating at repetition rates of up to 800 Hz and being capable of average output powers of more than 300 W. The laser is excited by a single-thyratron switched LC-inversion circuit, utilizing single or double stage pulse compression.

Operation of the discharge excimer laser in the "switchless" mode offers an interesting opportunity for high average power industrial excimer laser design. Its potential for high efficiency, energy and repetition rate has been recently investigated and fullfills early expectations. When properly used, U.V. corona preionization still remains attractive for active volumes below one liter and must be taken into account among the various trade off for future products in the 1kW average power range.

Rare-gas-halide lasers are receiving increasing attention because their ultraviolet wavelength optical outputs have promising applications in several fields. In order to establish a technical base for high average power excimer laser systems, a 10 1 active volume avalanche discharge laser device has been realized at the ENEA Frascati Research Center. In this paper, we describe some preliminary experimental results obtained with this system.

The operating characteristics of a pulse forming network (PFN) for large volume excimer lasers comprising paralleled waterline capacitors are described. The PFN uses different kinds of switches such as thyratrons and a multichannel pseudo spark switch. The X-ray pre-ionizer is composed of a low ohmic pulse generator and an X-ray tube with a hollow cathode. To investigate the evolution and damping of gas inhomogenities, caused by the short laser discharge, optical interferometry with high time resolution is used. Results and conditions are presented for operating the excimer laser at different wavelengths with high energy output.

From among the areas of excimer laser development at Los Alamos two are selected for further discussion: ultra-high brightness excimer laser systems and discharge-pumped XeF(C→A) lasers operating in the blue-green portion of the spectrum. Two different high brightness systems are described. One is based on small-aperture KrF amplifiers, while the other is based on a large-aperture XeC1 amplifier. The XeF(C→A) laser is tunable from 435 to 525 nm, and may one day become a viable alternative to pulsed dye lasers for many applications.

Excimer lasers emitting more than 200 W output power are not commercially available. A significant increase requires new technological efforts with respect to both the gas circulation and the discharge system. A due research project has yielded a laser which emits 0.5 kW at 308 nm when being UV preionized and operated at a repetition rate of 300 Hz. The laser, which is capable of operating at 500 Hz, can be equipped with an X-ray preionization module. After completing this project 1 kW output power will be available.

The excitation of excimer lasers in tubular discharges results in simple and compact devices needing no preionization. Optical output energies are in the millijoule range. We investigated XeF, KrF and ArF lasers for various operating conditions. The lasers consist of capillary glass tubes with two internal electrodes at the ends and an aluminium-foil wrapped around the tube as capacitive electrode. A maximum output energy of 0.3 mJ has been achieved for the XeF laser. The good quality of the discharge is indicated by the observation of spontaneous mode locking. The detailed study of the discharge for different polarities of the electrodes has shown that efficient operation with a high gas lifetime can be obtained by a purely capacitively excited discharge. A gas lifetime of about 10,000 pulses for 3 litres gas mixture has been observed. Using a two stage Marx generator to generate 100 kV excitation voltage a maximum output energy of 0.7 mJ was obtained for a gas mixture of Kr, F2 and He with an efficiency of 0.17%. The KrF laser operates also without the buffer gas. Laser action in ArF has been achieved with 15 μJ pulse energy and 10 ns duration.

The design and operation of a large, repetitively pulsed e-beam laser is described. The laser is capable of 100 Hz operation and has an average power of over 1 kW.

The effect of the addition of Ne as a buffer gas to an Ar/F2 laser gas mixture was studied in two different systems. With the largest device the total output energy was optimized, in the smaller system the pressure dependency of the addition of the Ne was studied. Futhermore the fluorescence signals of ArF* and Ar2F* were recorded as a function of the excitation length and pumping intensity.

The bandwidth of most excimer lasers allows the amplification of subpicosecond pulses. In this paper a novel method and experimental arrangement is reported for the generation of femtosecond pulses in the XeF, XeC1 and KrF laser wavelengths. This is based on the use of a hybrid excimer-dye laser arrangement, incorporating a twin tube excimer laser and a subpicosecond dye laser setup. With additional pulse compression, the shortest pulse durations obtained are 560 fs, 170 fs and 45 fs - with 1 mJ, 5 mJ and 4 mJ pulse energy - for the above mentioned three excimers, respectively. The study of the gain dynamics for the XeF, XeC1 and KrF laser transitions showed that only 22%, 38% and 75% of the stored energy can be extracted by a subpic9second pulse. The saturation energy density was measured to be 0.2 mJ/cm2, 0.85 mJ/cm z and 2.0 mJ/cm2, respectively.

An X-ray preionized discharge-pumped XeCl laser with two flat electrodes and a variable active volume of up to 1 1 is described. A charge-transfer circuit is employed with low-inductance water capacitors. The laser beam width is determined by X-ray collimators. Beam cross-sections up to 4x5 cm2 have been achieved. The laser pulse energy exceeds 3 J in a 40 ns pulse (FWHM). The dependence of output energy on gas pressure, charging voltage and beam width as well as the X-ray preionizer performance are presented.

The construction and operation of a long pulse, narrowband, e-beam pumped XeC1 Master Oscillator-Power Amplifier (MOPA) laser is described. The gas chemistry optimized for long pulse operation is discussed. The gain as a function of time for wideband, mediumband (15 GHz), and narrowband (300 MHz) conditions is presented, and the out of band loss determined.

The pulse duration of discharge-pumped excimer lasers in general assumes values between 10 and 40 ns. For several purposes, including pulse transmission through optical fibers, longer pulses are required. This can be accomplished by (i) optimizing the gas mixture aiming at longest possible pulse duration instead of maximum energy, (ii) by modifying the electric circuit of standard excimer lasers, or (iii) by designing special discharge circuits including the prepulse technology.

Laser energies of several joules per pulse are obtained in XeCl lasers with X-ray preionized discharge. High average power are accessible with this technique at high repetition rate, but reliability problems occur, especially for the electron gun of the X-ray source. In this work, a secondary mission electron gun is used for the X-ray source. This gun has a potential of more than 108 pulses at a pulse repetition frequency of 1 kHz. The maximum energy is 1.2 j/pulse from a 40 x 4.2 x 2.3 cm3 discharge. The minimum X-ray dose is shown to be nearly 40 mrad/pulse, i.e. more than one order of magnitude larger than in a similar smaller system.

Pseudo-spark switches are promising condidates to overcome the limitations of pulsed power networks, imposed by the commercially available thyratrons in use. The pseudo-spark switch uses a high power density, low pressure glow discharge, and is capable of withstanding up to 100 % current reversal. Metal-ceramic prototypes have switched up to 40 kV/30 kA at a rate of current rise of 8 x 1011 A/sec. At 32 kV/20 kA and a pulse duration of 90 nsec (FWHM), repetition rates of over 100 pps and a lifetime of over 5 x 106 discharges have been achieved in a discharge circuit with a short circuit load. Streak camera measurements confirm that the discharge in the switch remains a homogeneous glow. The switches seem to be scalable to higher power levels; in addition, paralleling of separate discharge channels in a common housing has been shown to be possible.

Modern excimer lasers comprise feedback systems to stabilize the output at preselected values. The shortcomings of the conventional systems comprising a single microprocessor and a lot of signal and command lines are (i) low data transfer rate and (ii) sensitivity against electromagnetic interference which is always present within discharge-excited excimer lasers. These shortcomings can be removed by utilizing a local-area network (LAN) with microprocessors at each actuator and detector, which communicate by a single fiberoptic ring line.

This paper describes the development of a control and monitoring system for a high power excimer laser within the framework of the joint European research project called EUREKA. As very high power excimer lasers become more attractive for industrial use, it is necessary to address the question of detailed monitoring and control. Remote monitoring of a variety of laser parameters, such as flow rates and flow performance is required in fast gas flow, multi-kilowatt devices, and the ability to monitor component performance over long periods of time is also desirable. Since the excimer laser environment is electromagnetically noisy, an overview of an approach that uses optical sensors and fibreoptic communication networks will be given. The paper will discuss the necessity to measure and control the timing of the laser, the gas composition, the alignment of the laser resonator, the uniformity of the discharge and the performance of the power supply. Examples of fibre optic sensor designs which could be used in the laser environment will be given. Hybrid devices with conventional sensor heads but using fibre optic communication links would be a possible short-term alternative where they would significantly outperform the all-optical solution.

Recent results confirm new advances in XeC1 excimer laser discharge design have achieved major milestones in reliability - 1010 pulses without replacing the thyratron, electrodes, capacitors or power supply. Other data on standard KrF excimer lasers allow statistical analysis of numbers of merit, including MTBF and maintenance intervals. Projections are made of operating costs and maintenance for both continuous operation and low duty cycle of two different size lasers with all major gases: XeCl, KrF and ArF.

The beam properties of a typical excimer laser with plane parallel mirrors can be approximately described from the geometric optics point of view as a superposition of ray bundles with locally varying angular distributions. These distributions have been measured by recording the far-field intensity after a small aperture shifted transversally through the beam. The characteristic differences to the situation at e.g. an incoherent source and an ideal laser with a stable resonator and the consequences for imaging are discussed within this framework.

We present the results of a detailed experimental study of the operation of two Gen-eralized Self-Filtering Unstable Resonators applied respectively to two different UV-preionized excimer lasers. By applying a G-SFUR with magnification |M|=10 to a short pulse XeC1 laser, a diffraction limited laser beam of 3.5 mJ, 11 ns duration and with a brightness of 4.8 x 1013 W cm-2 Sr-1 has been obtained. With a G-SFUR having |M|=20 applied to a longer pulse XeCl laser a diffraction limited beam of 21 mJ, 18 ns duration and with a brightness of 2.1 x 1014 W cm-2 Sr-1 has been obtained. The effect of the filtering aperture dimension on the output beam energy and divergence has been also investigated.

A computer controlled experiment for damage threshold measurements on optical surfaces in the UV spectral region is described in detail. The spatial intensity distribution of the probe beam can be monitored in a plane equivalent to the sample surface using a UV sensitive video camera. A relatively smooth near Gaussian beam profile is obtained. The experimental techniques used for damage recognition involve Nomarski microscopy, He-Ne laser scattering, stereo microscopy with He-Ne laser illumination, fluorescent light analysis as well as photoacoustical measurements. A comparison between the different techniques is given with respect to damage threshold results on a 248 nm high reflection coating. The influence of experimental parameters on the measured threshold fluences such as the number of pulses per site and pulse repetition rate is discussed.

In our dynamic model 1 for the interaction of ultraviolet laser pulses with condensed organic matter, it was proposed that successive absorption of two or more UV photons by the chromophores in the solid resulted in excitation to high electronic excited states and was followed by decomposition and ablation. Since the population of the upper excited states would depend on the power density of the radiation at the surface of the polymer, the width of the laser pulse is emphasized in this model. In earlier work,2 the success of this model in fitting data which were obtained at one laser pulse width was realized. In the present work, the width of the laser pulse was varied from 4Ons to 100ns by using pairs of identical laser pulses (each of FWHM = 4Ons) which were separated by a set time interval. The response of PMMA to etching by pairs of pulses suggests that short-lived species which may be electronic states (e.g., triplets) and/or radicals play an important part in polymer ablation. The etching of polyimide by these extended pulses shows trends that are opposite to those observed in PMMA. In this instance, the shielding of the latter portion of the incoming pulse by the products that are ablated by the front portion is probably a serious effect.

Photoablation of Polystyrene (PS) and Polymethylmethacrylate (PMMA) by 193 nm radiation of an excimer laser was studied in two types of experiments. 1) Time resolved absorption measurements show that the ablation of PS starts within the first few nanoseconds during the laser pulsa. 2) By laser-mass-spectrometry of the ablated neutral species their chemical nature and velocity distributions were measured. At low fluences, monomer molecules are the main product, at high fluences many smaller fragments are observed. The velocity distributions of the molecules are similar to that of adiabatic gas expansions. For comparison experiments with condensed CH3I were performed. Extremely sensitive detection of CH3I, CH3 and I by resonant multiphoton ionization combined with time-of-flight (TOF) mass-spectrometry allows to distinguish CH3 radicals from "direct" dissociative UV-photodesorption, "thermalized" dissociation products and thermal desorption of parent molecules at low fluences. The more efficient, "explosive" photoablation is observed at fluences above 1,5 mJ/cm2.

UV-excimer lasers have proven to be interesting sources for applications in the field of nonlinear optics, photochemistry and spectroscopy because of both their high pulse energy and high average power. In addition, they allow tuning over a wide range due to the relatively broad fluorescence bandwidth of several hundred wavenumbers. We report on generation of tunable radiation around 193 nm (ArF) and 248 nm (KrF) by Raman-shifting of dye laser radiation (ArF, KrF) or second harmonic generation (KrF) and subsequent amplification in excimer laser sections. Measurements of resulting energies and tuning ranges are presented. The beam quality and power density of the amplified radiation is suitable to the generation of tunable VUV radiation by stimulated anti-Stokes Raman-scattering in H2 down to 116 nm. This VUV radiation allows many applications in atomic and molecular physics. As an example, we present results of concentration- and velocity distribution measurements of 0-, Be- and C-atoms by laser-induced fluorescence spectroscopy. An alternative method to generate high-power VUV radiation with the aid of excimer lasers is two-photon pumping of H2 with 193 nm-radiation (ArF). Emission of laser radiation on more than 20 transitions is observed in the Lyman and Werner bands between 160 nm and 117 nm. We compare results obtained using the same set-up as for the Raman shifting mentioned above with results obtained using a commercial oscillator-amplifier ArF laser source.

Novel polymer ageing treatments based on 276 nm and 308 nm excimer laser radiation are reported. Comparison of laser ageing with natural and conventional artificial ageing (weatherometer) treatments has begun. Preliminary results indicate that the laser treatments offer increased acceleration factors 3200:1 to 120,000:1 over natural ageing and 50:1 to 3500:1 over weatherometers. Thus the laser treatments offer the prospect of reducing testing times from several months to a few tens of minutes.

Excimer laser (KrF, ArF) radiation has been used for the surface modification of Al - Si and Ni alloys with the aim to improve their corrosion-errosion and hardness properties. A variety of experimental techniques including direct laser surface treatment and laser assisted chemical vapour deposition (CVD) have been employed and the parameters for process optimization have been determined.

Excimer laser material processing of technical parts leads to results which, in many cases, differ strongly from previously known effects. This paper is a review on processing of metals, plastics, glass, ceramics, and compound materials. Since the effect of radiation is determined by beam parameters, beam handling will be described as well. Starting with the beam properties of interest, the technique and results of excimer laser processing will be summarized.

Cutting of fibre reinforced polymers with lasers gives rise to some problems. The different thermal properties of the fibre and the matrix make it difficult to achieve cuts with minimal thermal damage at the edges. The excimer laser radiation promises advantages over a treatment with the CO2 laser or the Nd:YAG laser. The formation of pores, grooves and cracks and the redeposition of ablated material in the kerf are shown in SEM micrographs. In case of alumina ceramics, a minimum pulse width - depending on the workpiece thickness - should be used to minimize recondensation of vapour in the kerf. The ablation depths per pulse and the feasible feedrates using excimer lasers nowadays available are given.

Although removal processes with excimer-laser radiation are especially adapted for organic materials, enabling nonthermal photochemical processes due to high photon energies and single or multiphoton interactions, excimer-laser seem to be suited for processing metals as well, interacting thermal. Shortest pulse durations with high fluence cause a minimized thermal input by mainly vaporizing the irradiated material and less heat conduction into the solid. The subject of this paper is to work out the removal process of amorphous metals with excimer-laser radiation. The effect of beam parameters and thermal influence on removal process and machined workpiece and estimations for capability of excimer lasers at such applications will be discussed.

The structured removal of coatings by excimer laser irradiation was investigated for different coating/substrate systems. High structure quality and pm dimensions were attained for polymer coatings. Coating materials which melt under irradiation, show deterioration by melt flow, reactive or alloying action with the substrate, or limited wettability, which makes cleaning difficult. But in most cases the requirement of total removal can be met by the reduction of structure quality or machining speed.

Excimer lasers are finding increasing use in the area of laser materials processing. Of particular importance is the fact that excimers can remove material through ablation rather than by any purely thermal mechanism such as those associated with YAG or CO2 processing. This paper reviews briefly the basic mechanisms associated with excimer laser ablation and presents several examples of practical applications of this new technique.

The paper concerns the surface modifications induced by high energy U.V. laser pulses on metals and alloys. SEM observations were carried out to evidence the surface morphologies after irradiation. Microhardness measurements showed that it is possible to increase the surface hardness of pure iron, Ti and Ti6Al4V alloy by formation "in situ" of finely dispersed precipitates. Residual stresses measurements were also carried out by X-ray diffraction technique to evidence the tensile state of the surface after laser irradiation.

Laser glazing, using a KrF excimer laser (λ = 248 nm, τ = 22 ns), has been used to improve the corrosion resistance properties of crystallized Fe40Ni38Mo4B18 (Metglas 2826MB) and the permanent magnet material Nd15Fe77B8. The formation of an amorphous layer was confirmed by conversion-electron Mossbauer Spectroscopy (CEMS), and the thickness of the amorphous layer was determined from the attenuation of the X-rays diffracted from the underlying crystalline material. The variation with laser fluence of the amorphous layer thickness on the Metglas was measured. The corrosion properties of the original, crystalline and laser glazed Metglas were measured potentiodynamically in acid electrolyte. Similar voltametric characteristics were obtained for the laser glazed surface and original Metglas, both showing a lower current in the passive region than the crystalline material. Laser glazing of the Nd15Fe77B8 also produced a significant reduction in the rate of corrosion in base and neutral solutions.

Laser radiation can be used efficiently to generate X rays for lithographic imaging of submicron patterns, e.g. for VLSI device fabrication. Due to their short wavelength and high average power, excimer lasers show much potential for this application. Results are presented of scaling studies for high repetition rate excimer laser application, using the frequency doubled output of a low repetition rate Nd:YAG/Glass laser. Spectral and spatial characteristics of X-ray emission of the laser plasma are shown. The power density in the laser focus was 3x1012 W/cm2. With this source Si X-ray masks with submicron Au absorber profiles are imaged into high sensitivity X-ray photoresist. For the exposures 80 lasershots sufficed to yield high quality submicron structures. Extrapolation of the results to a high power excimer laser reduces the exposure time of the photoresists to several seconds, enabling a wafer throughput at an industrial level.

Holographic generation of gratings with periodicities down to 140nm in thin polymethyl methacrylate (PMMA) layers on Si-substrates using a narrow band KrF-excimer laser (λ=248 nm) is reported. At low single pulse energy densities (<3OmJ/cm2) the gratings are prepared by conventional photolithography. At higher single pulse energy densities (>70mJ/cm2) we directly write gratings in the PMMA by photoetching (ablation).

The paper summarizes preliminary results obtained by surface modifications of various metal substrates with high power Excimer lasers. It was found that metals with different crystal structures exhibit different behavior, if small surface layers were melted. For fcc. metals deformation twins were found after exposure, while in bcc. metals twinning could not be observed. In hcp. metals twinning or cracking occurs depending on crystal orientation. Resolidification starts epitactically on crystalline substrates resulting either in a grain size comparable to the substrate, or in the formation of an amorphous layer, if a glass forming composition is present. Treatments of vitrified materials did always lead to amorphous resolidification. Surface roughness increased with increasing power flux density, roughness of the starting material and with the number of exposures. For some Ag-based alloys the laser treatment improved the properties of contact materials.

KrF laser ablation of Y-Ba-Cu-O targets has been studied using time resolved visible-uv spectroscopy and ion collector probes. These techniques have allowed the principal luminous species, ablation velocity and extent of ionization in the plume to be deduced. Comparative work using the TEA CCD, laser shows that similar spectral signatures are generated by the plume but that higher electron temperatures may exist because of stronger plasma coupling effects with the long wavelength laser.

The technique of fiber-coupling for high power laser beams have recently become a theme of considerable interest (1). While in medical applications new fields of use will open with higher efficiency of beam transmission, due to higher flexibility of beam delivery systems also industrial applications may become profitable, which so far have seemed unworthy to think about. Excimer lasers pose a particularly challenging task due to the high peak powers in the ultra-violet spectral range, where a low number of photons may suffice to overcome the local threshold energy for charge carrier generation. With two new techniques of pumping the excimer gas, longer laser pulses have been achieved, effectively lowering the beam peak power. While in standard systems pulse durations of 15 ns are typical, these new devices deliver 60 and 250-300 ns, respectively.

The use of optical fibers to transmit high power UV pulses from an excimer laser is desirable for many applications. We have tested different tapered and untapered fibers with regard to power transmission properties and lifetime at 308 nm from a XeCl excimer laser with unstable resonator (8 nsec pulse length). The transmitted power density in a 600 μm tapered fiber decreases from initally 1.25 to 1.0 GW/cm2 over a period of 105 pulses. This is sufficient for a lot of applications, such as medical surgery or etching of selected types of ceramics.

The aim of surgical therapy for osteoarthritic cartilage is the removal of the arthritic areas while maintaining the healthy tissue. Removal of calified areas by arthroscopy is preferably used in knee joints. The following investigations were performed to obtain the ablation rates during laser application in order to improve the ablation ratio of the calcified cartilage. For this purpose, specimens were immersed in tetracycline solution which has an absorption maximum at the laser's wavelength.

Laser energy has the potential to recanalize obstructive atherosclerotic vessels as an alternative or an adjunct to either bypass surgery or balloon angioplasty. But conventional lasers cause thermal side effects which may lead to extensive damage to neighboring layers. In contrast, excimer laser irradiation in the far ultraviolet range has proved to minimize or avoid these injuries to vessel walls. To evaluate the clinical feasibility of excimer laser angioplasty (ELA), we have performed basic investigations including histologic examination by light microscopy, scanning and electron microscopy, and temperature measurements, and later on in vivo animal trials. Using 308 nm irradiation (XeCl) we have treated the first patient ever to undergo ELA, and the procedure was successful: after recanalization of a total occlusion of a superficial femoral artery, dilatation resulted in sufficient blood supply to the periphery.

Excimer lasers operating at 193 nm are being used experimentally in a special type of materials processing wherein the central portion of the anterior surface of the human cornea is selectively ablated so as to change its refractive power and, hopefully, improve impaired vision. Research to date has demonstrated recontouring as a potential means for reducing myopia and hyperopia of cadaver eyes while studies of ablations on the corneas of living monkeys and of blind human volunteers show promise of prompt and successful healing. The procedure has also shown merit in removing superficial scars from the corneal surface. In this paper, we describe the electro-optical system used to deliver the UV laser beam in these experiments and report some preliminary results of the ablation studies.

Excimer laser technology has rapidly developed in the last few years. Commercial products are available since 1977, so that considerable knowledge on applications could be accumulated in the laboratories.