Dietmar Ehrlichmann

Azimuthally unstable resonators for high-power CO/sub 2/ lasers with annular gain media

Stable-unstable resonators have proved suitable for the extraction of a high-quality beam from a gain area that consists of a rectangular slab. Such gain areas have two substantially different transverse dimensions, and the resonators are stable in the small dimension while unstable in the larger one. Using off-axis unstable resonators avoids a central beam obscuration and improves beam quality. The adaptation of stable-unstable resonators to annular gain areas is described in this paper. The resulting resonators are stable in the radial direction and unstable in the azimuthal direction. Different unstable resonators, wound to match the annular geometry, are presented. The resonator modes are calculated numerically using a 3D-diffraction code that considers gain and misalignment. Resonator design parameters are obtained from a geometrical description of the resonator. Experimental results from a diffusion-cooled CO/sub 2/ laser confirm theoretical predictions and show that the resonators are capable of extracting beams that are nearly diffraction-limited with high efficiency from an annular gain medium. Output powers of 2 kW have been obtained from a gain length of 1.8 m. >

Diffusion-cooled CO2 laser with coaxial high frequency excitation and internal axicon

A report on the design and performance of a coaxial laser system with a stable resonator with internal axicon is given. Efficient diffusion cooling is obtained by using an annular transverse high-frequency excited discharge. Experimental results are compared with the results of a loaded resonator model that takes tilted mirrors into account. The agreement between theory and experiment for extraction efficiency, alignment sensitivity, polarization and mode structure is very good.

Azimuthal mode discrimination of annular resonators

A diffraction formula for annular beam propagation is suggested. Significant computational savings are obtained without restriction to low azimuthal mode orders. Azimuthal mode discrimination is shown to exist in stable annular resonators. High-order azimuthal modes can suffer low diffraction losses with certain mirror parameters. These high-order modes are identified with azimuthal revolving rays that satisfy known geometric relations for multipass resonators.

Ring resonator for lasers with annular gain media.

A ring resonator for lasers with annular gain media is presented. The resonator consists of two annular mirrors. While the radiation is reflected back and forth between the two annular mirrors, diffraction effects induce an additional azimuthal radiation flux. Output coupling is obtained through a decentered coupling aperture on the circumference of one of the two mirrors. The azimuthal radiation flux permits the extraction of optical power from the whole gain volume through the coupling aperture. The azimuthal radiation flux can revolve in two directions. The associated modes are degenerate, and random jumping between unidirectional and bidirectional operation is observed. Unidirectional operation has been stabilized but remains very sensitive to mirror alignment. High extraction efficiencies have been demonstrated experimentally with this resonator with a diffusion-cooled CO(2) laser and 2 times diffraction-limited beams have been obtained. An empty resonator model that shows the effect of edge diffraction at the coupling aperture on the resonator modes is also given.

High-power CO/sub 2/ laser with coaxial waveguide and diffusion cooling

A diffusion-cooled CO/sub 2/ laser using a coaxial waveguide is analyzed theoretically and experimentally. The resonator used for extracting the laser beam consists of two annular plane mirrors enclosing the two ends of the waveguide. The beam exits through an aperture in one of these annular mirrors. The mirror tilt is shown to provide efficient beam extraction through this aperture. A theoretical resonator model based on the vector modes of propagation in a dielectric coaxial waveguide is presented. Experimental data show the feasibility of coaxial waveguide lasers and their ability to supply beams of high power and quality. Experimental data are discussed with respect to the presented theory. >

Simple annular resonators with toric and helical mirrors

Two simple annular resonators, each consisting of two total reflecting annular mirrors with radially concave curvatures, are described. The beam is guided to a coupling aperture in azimuthal direction by the azimuthal mirror shape. A resonator with tilted toric mirrors works like an azimuthal unstable resonator with two beams starting from a core oscillator of this resonator towards the coupling aperture. The function of this resonator is akin to conventional positive branch unstable strip resonators wrapped to an annular geometry. A resonator with helix-shaped mirrors emits one well defined beam. A numerical model of the resonators using a 3D-diffraction code and including effects of misalignment and saturable gain is presented. Numerical beam propagation techniques based on different approximations are compared. Due to the high azimuthal Fresnel number geometrical optics becomes applicable and turns out to be useful for a rough estimation of resonator design parameters. Experiments have been carried out using a diffusion cooled transverse RF-excited coaxial CO2-laser-gas discharge. Experimental data are compared with theoretical results concerning near-field and far-field patterns as well as misalignment sensitivity. Theoretical and experimental data correlate well. Both resonators yield more than 10 percent efficiency.

Coaxial CO2 lasers

Advantages of coaxial radio-frequency excited discharges for the design of slow-flow and fast- flow high power CO2 lasers are discussed. The crucial point of the development of such laser systems is the feasibility of resonators with high extraction efficiency and high focusability. Azimuthally unstable resonators prove to be suitable for these annular gain media. A theoretical description of such resonators is compared with experimental results with both slow-flow and fast-flow discharges.

Stable-unstable resonators for annular gain media

Annular gain media offer various advantages to the design of lasers. The advantages comprise efficient laser excitation by internal pumping sources for solid state lasers and large discharge surfaces for diffusion-cooling of high power CO2-lasers. To benefit from these advantages it is necessary to find a suitable resonator. The presented solution to the resonator problem consists of a hybrid resonator which is stable in radial and unstable in azimuthal direction. Basically, it can be considered as the annular equivalent of the well-known stable-unstable slab resonator. A geometrical model of the resonator indicates that for proper resonator function the mirror shapes must respect certain constraints. A diffraction model of the loaded resonator gives detailed information on the beam properties and extraction efficiencies. To reduce the computational effort for beam propagation an approximate diffraction kernel for annular beams has been used. Experimental data on a 1 kW diffusion-cooled CO2-laser are reported.

Development of an industrial CO2 laser with more than 40-kW output power: recent results

Results of power scaling experiments on high power CO2 lasers for industrial applications are given. Rf-excited systems with fast axial gas flow and fast gas flow in an annular discharge gap (coaxial flow) are compared. A resonator for coaxial lasers delivering a beam with high focusability is described.

Coaxial slow flow CO2 laser with 2-kW output power

An output power of 2 kW is attained with a compact coaxial slow-flow CO2 laser. The resonator consists of two toric copper mirrors, which are tilted a small amount. These mirrors form an unstable resonator in azimuthal direction, from which two laser beams are extracted with high efficiency through a coupling aperture in one of the mirrors. The beam divergence of a single beam is nearly diffraction limited in the radial and azimuthal direction.