Measuring laser beam quality using spatial light modulators

We report on a fast and easy technique to measure the beam propagation ratio M2 of laser beams by using a spatial light modulator (SLM). The SLM is used to digitally propagate the light by various distances to a fixed plane, where a CCD camera records the beam intensity. Accordingly, no moving components are involved, avoiding the traditional scan in propagation direction. Two different approaches are presented, first, using the SLM to function as a lens of variable focal length, and second, using the SLM to manipulate the angular frequency spectrum of the beam, yielding an artificial beam propagation. In both cases the beam diameter is determined as a function of the variable focal length, or the (artificial) propagation distance, and the M2 value is inferred from a fit of the measured data. Either technique yields accurate results as shown by measuring the beam propagation ratio of different Laguerre-Gaussian beams and comparing to the expected theoretical M2 value. Deviations are found to be less than 0.15. Both presented techniques have the potential of real-time measurements due to the high refreshing rate of the SLM of 60 Hz.