From Photonics Rules of Thumb: Optics, Electro-Optics, Fiber Optics, and Lasers, Second Edition

LASER BEAM QUALITY

Beam quality is defined as


where

BQ

= (unitless) the wavefront error WFE expressed in waves

S

= Strehl ratio (unitless)

WFE

= RMS wavefront error in waves for errors less than about 1/5 of a wave

Discussion

The best beam focusing and collimation that can be obtained is derived from the diffraction theory for plane and Gaussian beams encountering sharp-edged apertures, as described in virtually every optics and laser book. In those analyses, it is assumed that the wavefront is ideal and that there are no tilt or higher-order aberrations in the phase front. The concept of beam quality has been developed to deal simply with the additional impact of nonuniform phase fronts in those beams. In a wide variety of applications, this definition is used to characterize the spreading that will be encountered in focused or parallel beams, beyond that associated with diffraction.

There are many other definitions of beam quality, so the reader is cautioned to understand what is meant by "BQ" in a particular application.

These characterizations of a laser beam are effective measures when the beam is nearly diffraction limited. A typical guide is that Strehl ratio and beam quality can be related to the RMS wavefront error (regardless of the composition of the aberrations) if the errors are less than ?/5 (some references use ?/2 ? as the standard). For highly aberrated beams, it may be difficult to establish the beam quality. For example, using...

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