Krzysztof Nowak

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.

Localized CO2 laser damage repair of fused silica optics.

A technique has been developed for the localized treatment of laser damage sites in fused silica optics by CO2 laser melt-flow smoothing, by using a 50 to 125 microm diameter beam in a regime that avoids mass removal by ablation. A detailed calibration of the laser irradiance for the threshold ablation of craters was carried out for a range of beam diameters and pulses in the 20 micros to 200 ms range. The results agree with a thermal model that also provides estimates of the melt depth for the different irradiation conditions. Smoothing trials for glass melting at irradiance values just below the ablation threshold irradiance were conducted to determine the optimum conditions and limits for the smoothing process. The technique has been found to remove damage pits up to a depth of 0.5 microm, while the small melt depth associated with localized treatment limits the smoothing to a <or=5 microm lateral size.

Correction of beam errors in high power laser diode bars and stacks

The beam errors of an 11 bar laser diode stack fitted with fast-axis collimator lenses have been corrected by a single refractive plate, produced by laser cutting and polishing. The so-called smile effect is virtually eliminated and collimator aberration greatly reduced, improving the fast-axis beam quality of each bar by a factor of up to 5. The single corrector plate for the whole stack ensures that the radiation from all the laser emitters is parallel to a common axis. Beam-pointing errors of the bars have been reduced to below 0.7 mrad.

A tunable microcavity

We present a miniaturized Fabry-Pérot microcavity for cavity experiments on optically active nanostructures. Spectral and spatial tuning of the Purcell effect (weak coupling regime) on a single InGaAs quantum dot is demonstrated.

Low-loss passive alignment of single-mode fibers in low-temperature cofired ceramics using CO2 laser fabricated U-grooves.

We report a step toward optoelectronic integration on low-temperature cofired ceramics substrates in the form of fiber alignment U-grooves. The precision of the CO2 laser machining of green state ceramic for this purpose is investigated. The groove-writing process with a speed of 1 mm/s was carried out in air at room temperature and ambient pressure. The process is to a large extent self-cleaning without any gas jet assist. By analysis of groove profiles after firing of a near-zero shrinkage green tape formulation, it is shown that the cutting accuracy is approximately 2 microm, which is at the fundamental limit set by the particle size (1-5 microm). We demonstrate low-loss butt coupling of <1 dB for single-mode fibers using the laser written U-grooves. The technique exhibits a potential for fabrication of low-cost fiber ribbon and fiber array connectors.

Brightness improvement for micro-lensed, laser diode bar stacks

The poor beam quality of multibar laser diodes currently limits the effectiveness of the associated beam shaping and coupling optics used for fibre or solid-state laser pumping, fibre optic beam delivery and for direct use of the diode light in material processing. Here we report a new technique which uses custom-made correction optics to provide a major improvement in beam pointing accuracy and beam quality for multibar diode stacks fitted with integral fast axis lenses.

High-brightness diode sources for waveguide laser pumping

A key design requirement for the realisation of high power solid-state planar waveguide lasers is the power-efficient usage of beams from multiple diode laser bars. Such bars are commercially available as stacks, providing up to 80 W per bar, with integrated fast axis collimation lenses on each bar. In principle, with good collimation and aberration-free relay optics, a high fraction of the power from the diode laser stack may be coupled into a waveguide of given core height and NA

Laser machining in ceramics manufacture

This paper reviews recent research at Heriot-Watt to develop laser-machining processes in ceramics. A particular focus is in applications where a high degree of customisation is required, for example in the manufacture of dental restorations.