The article "Propagation factor quantifies laser beam performance" (Laser Focus World, Dec. 1999, p. 119) neglects several key issues in the measurement of M2 values. First by its very composite nature, while the M2 value can be used to compare lasers, it frequently offers little insight into the precise nature of the laser adjustment required to achieve a higher-quality beam. Of greater concern to many users is that the optical setup required to measure lasers is strongly dependent on the laser wavelength, beam diameter, divergence, and approximate M2 value. Thus, marketing pitches notwithstanding, no universal measurement box for all lasers is possible.
Second, pulsed lasers represent a unique challenge to M2 measurement, not mentioned at all by the author. Virtually all M2 measurement systems measure only the average M2 value over a series of pulses—if they can measure pulsed lasers at all. The multiplexed M2 optic we introduced in 1992 was the first to measure a beam at 16 points in space for a single pulse. We simply relied on the long field time (16 milliseconds) of a camera compared to the short (typically nanosecond) pulse length of a laser. This approach allowed pulse-to-pulse variation in M2 in YAG pump lasers to be identified as a contributor to variation in the output of titanium-doped sapphire lasers and nonlinear laser converters.
Finally the author implies that M2 is a complex measurement, yet our $999 LaserTest beam profiler includes an M2 measurement technique compliant with ISO standards that only requires a simple lens to create the beam waist. Even the lens selection is simplified by using a built-in M2 simulator to predict the best lens focal length of a given set of laser parameters.
Gary Forrest
SensorPhysics
Oldsmar, FL 34677
www.sensorphysics.com