Howard J. Baker

CO2 large‐area discharge laser using an unstable‐waveguide hybrid resonator

A one‐dimensional unstable resonator combining free‐space and waveguide propagation is used to efficiently extract power from a large‐area gain medium in a slab waveguide CO2 laser. A power of 240 W in a near‐diffraction‐limited beam has been attained at 12% conversion efficiency from a 38 cm gain length in a compact sealed‐off laser head, corresponding to a specific power extraction of 14 kW m−2 of discharge area.

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.

Propagation characteristics of coherent array beams from carbon dioxide waveguide lasers

The characteristics of coherent array beams generated by waveguide carbon dioxide laser structures have been investigated, with particular emphasis on their propagation and transformation properties. Methods of array beam generation are presented, and measurements of array beam quality are given through the use of the M/sup 2/ parameter. Efficient transformation of an array beam from the antisymmetric to symmetric pattern is achieved through the use of a binary phase plate, and spatial filtering is shown to improve the beam quality at the expense of only a moderate loss of total beam power. Coherence effects in the near-field propagation of array beams are also explored.

High-power two-dimensional waveguide CO/sub 2/ laser arrays

A two-dimensional, 39 element array of waveguide carbon dioxide lasers has produced a total CW laser power output of 750 W, using RF excitation of a compact, close packed assembly. Thermal effects associated with close-packing in the array are evaluated in terms of discharge wall temperature rise and cavity mirror distortion. The array elements are shown to have a 30% power derating relative to a single isolated laser channel, caused mainly by internal temperature rise. Mirror distortion and waveguide constructional errors do not degrade the beam quality of the array emission, which matches the theoretical value. However mirror distortion is shown to be responsible for a large spread in laser frequency between channels in the array. Based on the results, prospects are good for the construction of larger, multikilowatt incoherent arrays.

High-average-power Nd: YAG planar waveguide laser that is face pumped by 10 laser diode bars

A planar waveguide Nd:YAG laser is pumped with 430 W of power from 10 laser diode bars to produce a multimode output power of 150 W at an optical efficiency of 35%. Use of a hybrid resonator of the positive-branch confocal unstable type for the lateral axis and of one of the near-case I waveguide type for the transverse axis increased the laser brightness by a factor of ~26 with only 12% less power than in the multimode case.

Single‐mode selection using coherent imaging within a slab waveguide CO2 laser

A technique for selecting a single high‐order lateral mode of a rf excited slab waveguide laser is described. An in‐cavity grid which matches the periodicity of the desired mode, together with slab waveguide dimensions which create coherent imaging by the Talbot effect, have produced a stable output power of 155 W in a high spectral purity, single‐resonator mode.

Planar waveguide, 1 kW cw, carbon dioxide laser excited by a single transverse rf discharge

A transverse radio frequency discharge excited, carbon dioxide slab laser has been operated cw with a hybrid waveguide / confocal unstable resonator to produce an output power of 1.06 kW at 12% efficiency. The active volume of 770×95×2 mm3 consists of a single discharge section with one cable feed.

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.

Similarity and scaling in diffusion-cooled RF-excited carbon dioxide lasers

The theoretical and experimental study of alpha RF discharges presented in this paper has resulted in the formulation of a set of similarity and scaling laws for diffusion-cooled alpha RF discharge-excited CW waveguide CO/sub 2/ lasers. For the first time, the parametric dependencies of the voltage-current-power characteristics of a transverse alpha RF discharge have been investigated over a range of excitation frequencies 100-160 MHz, for gas pressures 40-100 torr and for interelectrode distances 1-3 mm in a typical waveguide CO/sub 2/ laser gas mixture (He:N/sub 2/Co/sub 2/=3:1:1+5 percent Xe). Relative to dc discharges, the additional scaling law fD=constant is established, and the analysis indicates both high- and low-frequency limits to the operation of practical self-similar laser devices. >