A comparison of passive coherent beam combining architectures: Experimental results

Modal properties of two architectures for coherent beam combining are theoretically analyzed and experimentally verified. The supermodes of a two-laser spatially filtered cavity exhibit two distinctly different types of behavior depending on the path length error. When the error is small, the two modes present different cavity loss values and can be differentiated by gain. However, cavities containing path length errors greater than a critical value produce modes with identical losses and different resonant frequencies. A diode-bar side-pumped plano-concave Nd:YAG laser cavity is built for experimental verification of the theory. Experiments have shown two distinct regions as predicted by theory. In the small path length error region, the cavity runs in one single mode; however, when the path length error goes beyond a critical value, the cavity lases in two modes simultaneously or alternates between two modal states. Detailed loss versus phase error curves are presented and compared to theory. The modal behavior in this spatial filtering architecture is quite different from that found in a superposition architecture for coherent beam combining where the fundamental mode always has smaller loss per round trip than the second mode. The modal curves for a superposition architecture are provided for comparison with those from spatial filtering.