Articles you may be interested inStudy of laser beam propagation in microholes and the effect on femtosecond laser micromachining Laser parameters, which significantly influence laser-material interaction processes, are the wavelength, the energy, and the power density. Additionally, there are parameters, like the pulse length, which also strongly influence processing speed and quality. Studies where different types of lasers have been used indicate that long pulses are beneficial for processing speed. However, when different types of laser systems are used to study the effect of the pulse length, a direct comparison of the results is difficult because the use of different lasers involves a simultaneous variation of other parameters ͑e.g., wavelength͒ as well. In this study a technique of pulse length variation is used in which the pulse length is the only varied parameter and thus enables the desired direct comparison. Pulses with different lengths are sliced out of pulses of a long pulse XeCl excimer laser, keeping all other laser parameters unchanged. Results are shown of hole drilling experiments in 125 m nickel, 25 m aluminum, and 125 m aluminum foil with pulse lengths between 9 and 150 ns. The influence of the pulse length on material processing is discussed in connection with energy and power of the pulses. The experiments show that both for pulses with the same energy and the same power long pulses remove more material than short pulses and, moreover, long pulses can yield higher quality of the drilled holes.

Studies of the influence of pulse length on material processing with different lasers have shown that a long pulse is beneficial for processing speed. In this paper a technique of pulse length variation is used in which the pulse length is the only varied parameter. Pulses between 5 and 150 ns length are sliced out of the 175 ns pulse of a long pulse excimer laser. The beam quality for each sliced pulse length is similar. In this paper the results are shown of hole drilling experiments in 125 µm aluminium foil with pulses of 10 and 100 ns length. The influence of the pulse length on material processing is discussed in relationship with equal energy and equal power density of the pulses. This study shows that in both cases long pulses remove more material than short pulses.

The net gain is measured for various pump power densities in a discharge-pumped long-pulse KrCl laser (222 nm). For this, a three-electrode laser system is employed, which is operated at high gas pressures of around 3.5 bar with pump power densities of 300−650 kW cm−3. The net gain reaches a maximum of 1.6% cm−1. Using the experimental results, values are calculated for the small-signal gain, the losses, and the saturation intensity. The losses and the saturation intensity are found to increase much stronger with the excitation rate than the small-signal gain. Fluorescence measurements at 222 nm show that the upper laser level depopulates faster when the power density increases. The influence of collisional quenching and the halogen consumption on the laser performance are discussed.

The discharge quality and optimum pump parameters of a long-pulse high-pressure gas discharge excited KrCl laser are investigated. A three-electrode prepulse-mainpulse excitation circuit is employed as pump source. The discharge volume contains a gas mixture of HCl/Kr/Ne operated at a total pressure of up to 5 bar. For a plane-plane resonator, the divergence of both output laser beams is measured. A low beam divergence of less than 1 mrad is measured as a result of the very high discharge homogeneity. A maximum laser pulse duration of 150 ns (FWHM) is achieved for a pump duration of 270 ns (FWHM) and a power density of 340 kW cm −3 . Pumping the discharge under optimum conditions employing a stable resonator results in a maximum specific energy of 0.45 J/l with a laser pulse duration of 117 ns and an efficiency of 0.63% based on the deposited energy. High-pressure gas-discharge pumped rare-gas halide excimer lasers are sources of very powerful ultraviolet (UV) radiation. The wavelength range covered and the high pulse energies have made excimer lasers an extensively used tool for applications in science, industry and also in medicine [1,2]. Gas discharge pumping opened the way to UV-lasers with pulse repetition rates up to several kHz with output powers up to the kW-level. However, excimer lasers pumped by a simple charge-transfer circuit [3,4] are restricted to low efficiencies and short pulse durations of only a few tens of nanoseconds. Voltage matching between the storage capacitance used to pump the laser and the discharge steady-state voltage is hardly achievable with such circuits. The discharge current tends to oscillate under these conditions. Discharge instabilities grow rapidly due to the voltage mismatching and the current oscillation. The beam quality of these lasers is poor, as the required time to build up a beam with a low divergence is longer than the stability time of the discharge.An important step to increase the efficiency and pulse duration of excimer lasers has been the development of u Fax: +31 53 4891102, E-mail: l.c.casper@tnw.utwente.nl the prepulse-mainpulse excitation technique [5,6]. The prepulse, a very fast rising high-voltage pulse, is applied to the discharge electrodes within a few nanoseconds after preionization to initiate the discharge homogeneously. A lowimpedance circuit generates the mainpulse in order to sustain the discharge and to pump the laser. The mainpulse voltage can be chosen independently from the prepulse voltage. Therefore, matching between the sustaining voltage and the steady-state voltage can be realized. As a result, this excitation technique leads to an optimization of the pump efficiency as the voltage matching is possible and current ringing can be avoided. The discharge stability improves considerably and leads to an extension of the laser pulse duration. In the case of XeCl lasers (308 nm), laser pulse durations of several hundreds of nanoseconds with efficiencies up to 5% have been realized [7,8] and raised strong interest in science and indu...

Spatially very homogeneous gas discharges with long-pulse duration have been realized in HCl-based rare-gas halide gas mixtures at over-atmospheric pressures. A low inductive three-electrode prepulse-mainpulse configuration with two discharge volumes has been used as excitation circuit. The energy transfer efficiency of the pump circuit is very high and reaches 87%. Furthermore, the rise time of the discharge current is very short, in order to improve the discharge stability for high pump powers. Experiments in Xe/HCl/Ne mixtures reveal spatially very homogeneous discharges for up to 370 ns (FWHM) with a power deposition of 260 kW cm −3 . For discharges in Kr/HCl/Ne mixtures, we also observe very homogeneous discharges for pump power densities of more than 500 kW cm −3 . Discharges in mixtures with a krypton concentration of 100 mbar and 0.5 mbar HCl are homogeneous for 300 ns against 200 ns when 1 mbar of HCl is used.