James M. Zamel

Active tracker laser (ATLAS)

A high brightness diode-pumped, Nd-YAG solid state laser has been designed, fabricated, and tested. This phase conjugated master oscillator/power amplifier (MOPA) device produces 20-ns Q-switched pulses at 2500 Hz at an average power of 690 W and a beam quality of 1.1/spl times/DL when the pump diodes are operated at 27.5% duty cycle. With an external KTP doubler, this device has produced 175 W of green average power at a beam quality of 1.5 /spl times/ DL and a conversion efficiency of 45% over continuous operating times as long as one hour. This 1.06 /spl mu/m result is believed to be the highest average power brightness achieved, and the 532-nm performance is both the highest average green power and the highest average brightness ever reported.

Active Tracker Laser (ATLAS)

A phase conjugated Nd:YAG amplifier has demonstrated 690 W average power and 1.1 ×DL beam quality. The frequency doubled output was 175 W at 1.5 ×DL and 45% conversion efficiency. This is the highest average green power and highest average brightness reported.

High-average-power industrial laser for precision machining

High brightness, high average power, diode pumped Nd:YAG solid state lasers (DPSSL) are being developed by TRW as part of the Precision Laser Machining Technology Reinvestment Program. The use of diode pump arrays in place of flashlamps, and zig-zag slab geometry, allow lasers to be scaled to power beyond the current generation of lamp-pumped rod lasers while providing excellent beam quality. The efficiency is 3 - 4 times better using diode arrays in place of flashlamps resulting in less waste heat in the laser medium and reduced optical aberrations. The corresponding beam quality provides more than an order of magnitude increase in the average intensity available at the workpiece, thus enabling new machining capabilities.

Xenon Target and High-Power Laser Module Development for LPP Sources

This chapter gives an overview of LPP EUV source development work at Northrop Grumman Corporation (NGC). The chapter covers development of the laser module, xenon target, and overall system. The volume editor (V. Bakshi) prepared this chapter as a summary of information provided to him by NGC. Lasers for LPP EUV sources are expected to produce tens of kilowatts of high-pulse-rate, high-pulse-energy, short-pulse-width, near-diffraction-limited output. Such lasers will be focused onto a condensed jet of cryogenic xenon or tin targets to produce a plasma with sufficient temperature to generate EUV radiation. For the generation of the EUV-producing plasma, pulse widths of around 10 ns and pulse energies in the range of 0.5 to 1 J are required. High beam quality and low pointing error are required to maintain constant high intensity on the EUV source target so that the radiated EUV power and consequent exposure doses on the semiconductor wafer are uniform. Depending on the choice of target material, eventually pulse rates of at least 7500 Hz and laser powers of 10–30 kW will be required to ensure the required power collection at the intermediate focus (IF). In 1999, NGC constructed a 1700-W diode-pumped Nd:YAG phase-conjugated master oscillator-power amplifier (MOPA) laser, designated EUV-Alpha, which was used in a lithography testbed at Sandia Labs in Livermore (see Chapter 24 for further description). Later NGC built an EUV-Beta laser (Fig. 25.1) that produced 4500 W and was operated at NGCs EUV source development facility at Cutting Edge Optronics (CEO). The Beta laser, a modular design for better maintainability, was twice as efficient and had two-thirds the footprint of the Alpha laser. For this laser, NGC selected a MOPA architecture (Fig. 25.2) using stimulated Brillouin scattering (SBS) phase conjugation to compensate for aberrations, figure error, and thermal distortions in the Nd:YAG gain media. The output of a custom 12-W master oscillator (MO) was split in two with a polarizer and directed to two amplifier trains. Each amplifier train consisted of two diode-pumped zigzag slab amplifiers, image relay telescopes, shaping optics, and an SBS cell. After round trips through the two slab amplifiers, the two MO beams were brought to their full 750-W power in each train, and then polarization-combined for a total of 1500 W. In the Beta laser, there were three such 1500-W modules, which yielded a system total of 4500 W at 7500 Hz.

5.4 kW diode-pumped, 2.4x diffraction-limited Nd:YAG laser for material processing

A 5.4 kW diode-pumped laser has been demonstrated with a beam quality of 2.4X the diffraction limit. This laser has achieved breakthrough capabilities in laser welding, producing high-quality single-pass welds in 65-mm steel and 50-mm titanium and aluminum.

Kilowatt-class diode-pumped industrial lasers

TRW is developing two diode-pumped Nd:YAG laser designs (called DP1 and DP2) for high-brightness laser machining. Two DP1 lasers have been built; one has operated at its design limit of 1000 W with a stable resonator, and the other at 55 W with an unstable resonator at reduced duty cycle, in agreement with model predictions.