Appropriate Measures and Consistent Standard for High Energy Laser Beam Quality (Postprint).

Book excerpt: Along with power output of the laser system, laser optical quality or beam quality provides a suitable measure of performance. Power and beam quality are standards for the comparison of laser systems with each other and against mission requirements. An understanding of the meaning of beam quality is necessary to completely define laser performance capability. The current state of our community includes a multitude of different and not well understood beam quality measures, M (exp 2), Strehl Ratio, brightness, power in the bucket, "times diffraction limited" and mode content determined by a variety of beam radius measures: half-widths, second-moment radius, widths at 1/e or 1/e (exp 2) points, width of primary lobe, etc. Another complication is that different elements of the community use different measures to evaluate optical quality characteristics. We examine the assumptions behind common measures of beam quality and compare the various measures as they relate to beams from laser employing stable resonant optical cavities. We show how the mode composition of a beam depends on prior determination of beam radius and how the term "times diffraction limited"can mean different things depending on the method used to measure beam radius. We show the ambiguities that arise between certain classes of beams and measures of beam quality and advocate for a laser beam quality standard that relates directly to mission requirements.