The sensitivity to path length changes is analyzed in coherent beam combining architectures based on Fourier plane spatial filtering. Unlike superposition architectures based on 3-dB couplers and Dammann gratings, 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. Cavities with more than two elements exhibit similar behavior. Alternative filtering techniques are also discussed and compared. It is found that complementary filtering, where absorption spots are placed in the nulls of the intensity pattern in the Fourier plane, results in reduced power loss while preserving the same degree of modal discrimination.
Bibliographical noteFunding Information:
Manuscript received November 18, 2010; revised February 9, 2011; accepted February 14, 2011. Date of current version April 12, 2011. This work was supported in part by Cymer Inc., and Corcoran Engineering through the Small Business Innovation Research Award from the U.S. Navy under Program OSD05-D01.
- Coherent beam combining
- laser beam combining
- spatial filtering