Free-motion beam propagation factor measurement by means of a liquid crystal SLM

We propose a compact and robust method to measure beam propagation factor (M²) of continuous wave (cw) laser beams. In contrast to the conventional scanning procedure described in the ISO/DIS 11146 standard, our proposal takes advantage of the features of programmable liquid crystal spatial light modulators (SLMs) where a lens is codified. Then, by using a CCD at a fixed position, the beam width according to the second order moment of the irradiance is determined for each focal length of the codified lenses. After adjusting the measured data to the theoretical focusing behavior of a real laser beam, the beam propagation factor is obtained. The proposed method was successfully validated in the laboratory where a full digital control of the measurement procedure was implemented. The M² is from a very low time-consuming calculation. Its application to compact Optics systems is straightforward.