Real-time measurement of beam quality factor M2 based on defocus gratings

Crossed phase defocus gratings are designed and a new technique for real-time measurement of beam quality factor M2 is achieved. We can get the beam spots at 9 various positions to be measured simultaneously by using the defocus gratings closely connecting with a short-focal-length lens. The lens provides the majority of the focusing power and the grating provides 9 separate optical axes and effectively modifies the focal length of each axis. The value of M2 is determined by calculating the beam diameters using the second moment method and curve-fitting. A He-Ne laser beam is measured by using the gratings designed and a M2 value of 5.775 is achieved. Compared with a conventional measurement, the error is 5.1%.