Given the lack of an accuracy standard for measuring a laser’s M-square value, what can a laser user utilize that will instil confidence that the M-squared measurement being made is as accurate as possible? While the ISO 11146-1 provides a method for making a measurement, there is no means to insure the M-square measurement is the most accurate possible. Most mainstream M-square measurement systems have higher then desirable variability in their measurements and consequently puts into question the accuracy of the result. Variabilities of 5 to 10% are not uncommon and therefore undesirable for many users. As there is no perfect laser for which an accuracy standard could perhaps be utilized, the repeatability of a measurement can therefore be the next best metric in providing the upper and lower limits in the measurement’s accuracy. The predominate source of variability should be limited to that of the inherent stability of the laser and not the variability caused by the measurement device itself. The mainstream M-square measurement systems have higher than desirable variability due the constantly changing attenuation and camera exposure times from motion of optics through the beam caustic and the resulting time averaging of the measurement. Having an instrument operating in real-time with fixed attenuation/exposure time and thus eliminating any time averaging of beam caustic data points, would therefore provide the best repeatability possible and limit any variation to that of the inherent stability of the laser source.
Michael Scaggs and Gil Haas, "M-squared measurement with improved repeatability for CW to single pulse lasers," Proc. SPIE 10518, Laser Resonators, Microresonators, and Beam Control XX, 105181K (Presented at SPIE LASE: February 01, 2018; Published: 16 February 2018); https://doi.org/10.1117/12.2289122.
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