Krzysztof M. Nowak

This paper reports the development of an analytical model, with supporting experimental data, which quite accurately describes the key features of ${\mathrm{CO}}_2$ laser ablation of fused silica glass. The quantitative model of nonexplosive, evaporative material removal is shown to match the experimental data very well, to the extent that it can be used as a tool for ablative measurements of absorption coefficient and vaporization energy. The experimental results indicated that a minimum of $12\mathrm{MJ}{\mathrm{kg}}^{-1}$ is required to fully vaporize fused quartz initially held at room temperature, which is in good agreement with the prediction of the model supplied with input data available in the literature. An optimal window for the machining of fused quartz was revealed in terms of pulse duration 20–80 μs and ${\mathrm{CO}}_2$ laser wavelength optimized for maximum absorption coefficient. Material removal rates of 0.33 μm per $\mathrm{J}{\mathrm{cm}}^{-2}$ allow for a high-precision depth control with modest laser stability. The model may also be used as a parameter selection guide for ${\mathrm{CO}}_2$ laser ablation of fused silica or other materials of similar thermophysical properties.

In this Letter we describe in more detail a solid-state seeded, nanosecond pulse, multiline ${\mathrm{CO}}_2$ oscillator designed and built for the extreme ultraviolet (EUV) laser-produced-plasma (LPP) source. Our oscillator featured quantum cascade laser seeders, a diffraction-type seed beam combiner, and a radio-frequency-discharge-excited, diffusion-cooled, slab-waveguide ${\mathrm{CO}}_2$ gain cell in a compact multipass regenerative amplifier configuration. The oscillator generated pulses of exceptional stability in terms of envelope, energy, and spectrum. Excellent stability of output was achieved without any additional techniques. The output spectrum consisted of two laser lines of a ${00}^{0}1-{10}^{0}0$ band of a ${\mathrm{CO}}_2$ molecule, P20 and P22, with a target of four lines P18-P24. The pulse duration was electronically adjustable between 11 and 35 ns at a repetition frequency from a few hertz to hundreds of kilohertz. Electronic adjustment of the pulse duration was achieved by relative timing offsets of individual seeders, opening an avenue to a range of on-line adjustments of pulse shape and spectral content timing. The jitter-tolerant operation allows for easy synchronization with an external event, such as a droplet target in an EUV LPP source. A resistance to parasitic seeding of more than 40 dB was recorded. The oscillator produced up to 20 W of average output power at a repetition rate of 100 kHz in a near-diffraction-limited beam of $M^2<1.3$ and a pointing stability below 50 μrad.

Quantum cascade laser (QCL) is a very attractive seed source for a multikilowatt pulsed ${\mathrm{CO}}_2$ lasers applied for driving extreme ultraviolet emitting plasmas. In this Letter, we investigate output beam properties of a QCL designed to address P18 and P20 lines of 10.6 micron band of ${\mathrm{CO}}_2$ molecule. In particular, output beam quality and stability are investigated for the first time. A well-defined linear polarization and a single-mode operation enabled a use of phase retrieval method for full description of QCL output beam. A direct, multi-image numerical phase retrieval technique was developed and successfully applied to the measured intensity patterns of a QCL beam. Very good agreement between the measured and reconstructed beam profiles was observed at distances ranging from QCL aperture to infinity, proving a good understanding of the beam propagation. The results also confirm a high spatial coherence and high stability of the beam parameters, the features expected from an excellent seed source.

In this Letter, we investigate, for the first time to our knowledge, the spectral properties of a quantum-cascade laser (QCL) from a point of view of a new application as a laser seeder for a nanosecond-pulse high-repetition frequency ${\mathrm{CO}}_2$ laser operating at 10.6 μm wavelength. The motivation for this work is a renewed interest in such a pulse format and wavelength driven by a development of extreme UV (EUV) laser-produced-plasma (LPP) sources. These sources use pulsed multikilowatt ${\mathrm{CO}}_2$ lasers to drive the EUV-emitting plasmas. Basic spectral performance characteristics of a custom-made QCL chip are measured, such as tuning range and chirp rate. The QCL is shown to have all essential qualities of a robust seed source for a high-repetition nanosecond-pulsed ${\mathrm{CO}}_2$ laser required by EUV LPP sources.