Analytical model for CO(2) laser ablation of fused quartz.

This paper reports the development of an analytical model, with supporting experimental data, which quite accurately describes the key features of CO2 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 MJ 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 CO2 laser wavelength optimized for maximum absorption coefficient. Material removal rates of 0.33 µm per J 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 CO2 laser ablation of fused silica or other materials of similar thermophysical properties.

Locking and wavelength selection of an ultra-collimated single-mode diode laser bar by a volume holographic grating.

Wavelength-locking by a volume holographic grating (VHG) is reported for a diode laser bar with 49 single mode emitters, fitted with a dual-axis collimation phase-plate for smile elimination and excellent beam pointing correction. The much-improved VHG feedback with the ultra-collimated array beam gives 100% wavelength locking at 975 nm over a 17°C temperature range and external cavity lengths up to 110 mm. This enables a folded cavity configuration to provide a fully-locked array with wavelength selection into 200 pm channels over an 8 nm band, suitable for multi-bar dense wavelength-combining.

Femtosecond pulses at 50-W average power from an Yb:YAG planar waveguide amplifier seeded by an Yb:KYW oscillator.

We report the demonstration of a high-power single-side-pumped Yb:YAG planar waveguide amplifier seeded by an Yb:KYW femtosecond laser. Five passes through the amplifier yielded 700-fs pulses with average powers of 50 W at 1030 nm. A numerical simulation of the amplifier implied values for the laser transition saturation intensity, the small-signal intensity gain coefficient and the gain bandwidth of 10.0 kW cm(-2), 1.6 cm(-1), and 3.7 nm respectively, and identified gain-narrowing as the dominant pulse-shaping mechanism.

Generation of microstripe cylindrical and toroidal mirrors by localized laser evaporation of fused silica.

We report a new technique for the rapid fabrication of microstripe cylindrical and toroidal mirrors with a high ratio (>10) of the two principal radii of curvature (RoC(1)/RoC(2)), and demonstrate their effectiveness as mode-selecting resonator mirrors for high-power planar waveguide lasers. In this process, the larger radius of curvature (RoC(1)) is determined by the planar or cylindrical shape of the fused silica substrate selected for laser processing, whilst the other (RoC(2)) is produced by controlled CO(2) laser-induced vaporization of the glass. The narrow stripe mirror aperture is achieved by applying a set of partially overlapped laser scans, with the incident laser power, the number of laser scans, and their spacing being used to control the curvature produced by laser evaporation. In this work, a 1 mm diameter laser spot is used to produce grooves of cylindrical/toroidal shape with 240 µm width and 16 mm length. After high reflectance coating, these grooves are found to provide excellent mode selectivity as resonator mirrors for a 150 µm core Yb:YAG planar waveguide laser, producing high brightness output at more than 300 W. The results show clearly that the laser-generated microstripe mirrors can improve the optical performance of high-power planar waveguide lasers when applied in a low-loss mode-selective resonator configuration.

Laser smoothing of binary gratings and multilevel etched structures in fused silica.

We describe a promising approach to the processing of micro-optical components, where CO(2) laser irradiation in raster scan is used to generate localized surface melting of binary or multilevel structures on silica, fabricated by conventional reactive-ion etching. The technique is shown to provide well-controlled local smoothing of step features by viscous flow under surface tension forces, relaxing the scale length of etch steps controllably between 1 and 30 microm. Uniform treatment of extended areas is obtained by raster scanning with a power stabilized, Gaussian beam profile in the 0.5 to 1 mm diameter range. For step heights of 1 microm or less, the laser-induced relaxation is symmetric, giving softening of just the upper and lower corners at a threshold power of 4.7 W, extending to symmetric long scale relaxation at 7.9 W, with the upper limit set by the onset of significant vaporization. Some asymmetry of the relaxation is observed for 3 microm high steps. Also, undercut steps or troughs produced by photolithography and etching of a deep 64 level multistep surface are found to have a polarization-dependent distortion after laser smoothing. The laser reflow process may be useful for improving the diffraction efficiency by suppressing high orders in binary diffractive optical elements, or for converting multilevel etched structures in fused silica into smoothed refractive surfaces in, for example, custom microlens arrays.

Dual-axis beam correction for an array of single-mode diode laser emitters using a laser-written custom phase-plate.

A single optical component for a diode laser bar combines fast-axis smile and lens error correction with slow-axis collimation. Produced by laser-machining/polishing, it provides 0.9 mm focal length, 200 microm pitch slow-axis collimation on the same surface that corrects fast-axis errors. Custom fabrication enables fill-factor optimization for the 49 single-mode beams and gives parallel collimation with rms pointing errors of 3% and 6% of the far-field divergence for the fast- and slow-axis array respectively. Sub-micron pitch mismatch between the slow-axis lens and emitter arrays, and beam pointing changes by thermal expansion of the laser bar are detected.

Efficient laser polishing of silica micro-optic components.

We report a study of the basic characteristics of laser polishing of fused silica with a protocol that is particularly suitable for surface smoothing of micro-optic elements fabricated by a laser ablation process. We describe a new, to our knowledge, approach based on scanning a highly controlled small size laser beam and melting areas of tens to hundreds of micrometers of glass using a computer-controlled raster scan process, which does not require beam shaping, substrate preheating, or special atmospheres. Special test samples of silica substrates with prescribed spatial frequency content were polished using a range of irradiation conditions with the beam from a well-controlled CO2 laser operating at a wavelength of 10.59 microm. An analysis is presented of the laser-generated reduction in surface roughness in terms of measurements of the spatial frequency characteristics, and the results are compared with the predictions of a simple model of surface-tension-driven mass flow within the laser-melted layer. This technique is shown to be capable of smoothing silica surfaces with approximately 1 microm scale roughness down to levels < 1 nm with no net effect on the as-machined net surface shape, at realistic production rates without a preheating stage, and with noncritical residual stresses.

Effect of vaporization and melt ejection on laser machining of silica glass micro-optical components.

A new regime for silica glass machining for micro-optical fabrication applications, which uses pulsed CO2 laser radiation in the 2.5-100-micros pulse width region that has been generated by an acousto-optic modulator, is investigated. A filamentary melt ejection process that generates fibers and significant melt displacement limits machining quality below 30-micros pulse width. Ablation and melt ejection thresholds are quantified relative to pulse width, and the region from 30 to 50 micros is identified for low-threshold, smooth machining without melt displacement and ejection effects.