Laser-mode selection by a combination of biprism-like reflectors with narrow amplitude masks

In recent work the laser mode selectivity induced separately by a biprism-like reflector and by an absorbing strip was investigated by numerical analysis. It was shown that each of these elements in an otherwise conventional resonator was suitable to cause the laser to oscillate preferentially on the first odd mode that contains a line singularity, which is a useful dark beam (i.e., a laser beam with a dark central region) for high-resolution metrological applications. We study the combined effect of these two mode-selecting elements and show that the unified analysis leads to much better performance than could be expected from a simple superposition of the performance with each element alone. The results indicate that the mode selectivity can be enhanced by at least a factor of 3 compared with that of laser resonators with biprism-like reflectors alone. Thus a laser equipped with such a combined element will oscillate on a pure first-order mode with high power efficiency. Moreover, calculations show that the resultant dark beam, focused for metrological applications, has a significantly improved shape compared with the dark beam obtained by external modulation of a fundamental Gaussian beam.