Innovative resonator concepts especially for high energy lasers with low amplification coefficients are numerically designed and experimentally validated. Coupling properties are evaluated with regard to atmospheric propagation and long-distance effect of the extracted laser radiation.
The resonator layout and the feasibility studies are performed numerically. The calculations are based on the integral equation of the Fresnel-Kirchhoff formulation of Huygens’ principle. Promising resonator architectures are applied among others to the chemical oxygen iodine laser for optimizing the resonator performance and the coupling behaviour. On the basis of the experimental results numerical scaling studies describe the resonator performances for lasers of higher and highest power classes.
Hybrid resonators (e.g. NBHR: Negative Branch confocal Hybrid Resonator) show optimum performance for lasers of low amplification in the low to medium power classes. These types of resonators combine a stable and an unstable direction that are orthogonally oriented. Hybrid resonators are optimally adaptable to rectangular cross sections of the active medium by cylindrical mirrors. The active cross section of the resonator mirrors can be efficiently reduced by folding the internal beam path in stable direction. The double-pass principle enables a larger overall amplification by doubling of the gain length. Double-pass hybrid resonators (D-NBHR) allow the extraction of high laser power at enhanced beam quality.
Unstable resonators can be applied to laser systems of the 100 kW class and above. Lasers of high power classes provide higher output coupling ratios even for laser-active media with small amplification coefficients. In consequence, the diffraction structures within the far field of the laser beam are smaller than in the case of low power classes. A further reduction of the diffraction structures can be achieved by a modified geometry of the unstable resonator. This "Modified Negative-Branch confocal Unstable Resonator" (MNBUR) is tested for different scraper geometries ([ - and L-form) at the 10 kW COIL system. These tests confirm the reliability of the numerical predictions and validate the results of the scaling studies.
Besides of the described resonator concepts, resonator types with optical mirror coatings of variable reflectivity are further promising alternatives.