Experimental Measurements of Path Length Sensitivity in Coherent Beam Combining by Spatial Filtering

The supermode behavior of lasers coherently coupled by a passive spatially filtered cavity has been experimentally measured as a function of path length error between laser cavities. A custom laser-diode pumped Nd:YAG laser is constructed containing two laser channels coupled by a spatial filter. The supermodes are seen to exhibit two different types of behavior depending on the magnitude of the relative path length error. When the path length error is small, the two supermodes present different cavity loss values and can be differentiated by gain. However, when the error exceeds a specific value, the two supermodes have identical loss and different resonant frequencies. Experiments show the two distinct regions predicted by theory, and quantitatively verify the loss, oscillation frequency, and modal power distribution of the supermodes as a function of path length error.