D. R. Hall

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.

1.8kW diode laser system for fibre-delivery using brightness-enhanced diode stacks and a novel final beam-shaper

A 1.8kW diode laser source made up of 100 W diode bars is designed for fibre delivery, using two new techniques to enhance the delivered brightness and equalise the beam-parameter product. Custom corrective phase plates in laser-cut silica are attached permanently to the two stacks of ten bars, correcting bar smile and restoring a factor of 2.5 in lost brightness. The two units are beam-compacted and polarisation coupled to a single array beam. As a final step, a novel confocal beam-slicer produces five segments from the slow-axis beam profile and stacks the segments in the fast-axis direction.

Enhanced peak power CO2 laser processing of PCB materials

Laser drilling has become a common processing step in the fabrication of printed circuit boards (PCBs). For this work, a recently developed enhanced peak power CO2 laser (~2.5 kW peak power, 200W average) or ultra-super pulse (USP) laser is used to drill alumina and copper coated dielectric laminate materials. The higher peak power and faster response times (than conventional CO2 lasers) produced by the USP laser are used to produce high speed alumina laser scribing and copper coated laminate microvia drilling processes. Alumina is a common PCB material used for applications, where its resistance to mechanical and thermal stresses is required. Here we present a comprehensive study of the melt eject mechanisms and recast formation to optimise the speed and quality of alumina laser scribing. Scribe speeds of up to 320 mms-1 (1.8 times current scribe rate) have been achieved using novel temporal pulse shapes unique to the USP laser. Also presented is the microvia drilling process of copper dielectric laminates, where the multi-level configuration presents different optical and thermal properties complicating their simultaneous laser ablation. In our experiments the USP laser has been used to drill standard thickness copper films (up to 50 μm thick) in a single shot. This investigation concentrates on understanding the mechanisms that determine the dielectric undercut dimensions.

High average power cw face pumping of a Nd:YAG planar waveguide laser with diode bars

Summary form only. Face pumped planar waveguide lasers can be efficiently power scaled by increasing the area of the waveguide proportionately with the number of diode laser pumps. High gain can be achieved and thermal fracture, lensing and birefringence problems are reduced significantly when compared to side-pumped rod lasers or thicker slab designs.

Beam reformatting of one- and two-dimensional arrays of CO/sub 2/ waveguide lasers

The conversion of a linear to rectangular array beam is demonstrated for multi-element RF excited CO/sub 2/ waveguide lasers using the two-mirror beam shaping technique introduced by Clarkson and Hanna. High efficiency conversion of a one-dimensional array into a rectangular array format has been achieved with the reformatting mirrors in the near-field and with no auxiliary optical elements. The brightness of a 2-kW 64-element two dimensional array has been enhanced by a factor of 2 using a modified version of the reformatting technique to improve the fill factor of the array beam.

Development of edge pumped Yb:YAG planar waveguide lasers

A waveguide with 150 μm core height of 2% Yb:YAG between sapphire claddings is core-pumped at 480W by diode bars coupled into the 13 mm long edge-facet. The pump unit has custom correction of collimation errors and lens aberrations. Using a 6mm width and 7° edge-facet angle, power is limited by competing ASE loss or parasitic oscillation along TIR-trapped internal paths, giving 40 W output for stable and 25 W for unstable resonators. Ray-tracing shows a 20° facet angle is necessary to successfully out-couple ASE from the core. Preliminary operation at 90W and an increased threshold for the parasitic oscillation are obtained.

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 gain and high peak power in planar waveguide lasers

We report enhanced peak power and gain for pulses in the 10-100 /spl mu/sec range, with specific peak power to 240 kW m/sup -2/ of electrode area at frequencies to 4 kHz. Pulsed small signal gain is increased by a factor of 7 relative to cw/long pulse excitation.

Operation of a cw 14C16O2 laser in the 12 μm spectral region

Abstract We report the cw operation of a sealed-off dc discharge excited carbon dioxide laser with a novel variation to the conventional grating-tuned resonator arrangement, which permits the achievement of useful laser power output levels in the wavelength range of 11.1–12.1 μm, using the 14 C 16 O 2 isotope in the laser gas mixture.

Mode-selective toroidal mirrors for unstable resonator planar waveguide and thin slab solid-state lasers

An effective method for efficient power extraction from planar waveguide or thin slab solid-state lasers is provided by the hybrid configuration, which combines an unstable lateral resonator with a stable or waveguide resonator in the transverse direction. A two-mirror configuration is preferred for ease of alignment and simplicity and previously we have used spherical optics [1], but transverse mode selection has relied on the use of an intra-cavity slit. More general 2-mirror resonators require at least one toroidal mirror surface, with large curvature (RL ∼ 0.2m to ≫1m) in the lateral direction and small curvature (RT ∼ 15 to 50mm) in the transverse direction. For a large core height planar waveguide as in [1], RT and the mirror distance can be chosen to obtain mode selectivity by low-loss coupling of the fundamental mode into the core, combined with spread of higher order modes into the cladding. However, additional selectivity may be available if the transverse size of the toroidal mirror is restricted to form a slit-shaped mirror. Such mirrors are difficult to fabricate by conventional means.