Free-Motion Beam Propagation Factor Measurement by Means of a Liquid Crystal Spatial Light Modulator

摘要

In contrast to the mechanical scanning proceduredescribed in the standard ISO/DIS 11146, the use of electronically tunable focal length lenses has proved its capability for themeasurement of the laser beam propagation factor () withoutmoving components. Here, we demonstrate a novel experimentalimplementation where we use a low-cost programmable liquidcrystal spatial light modulator (SLM) for sequentially codifyinga set of lenses with different focal lengths. The use of this kind ofmodulators introduces some benefits such as the possibility forhigh numerical aperture or local beam control of the phase of thelenses which allows for minimizing systematic errors originated bylens aberrations. The beamwidth, according to the second-ordermoment of the irradiance, is determined for each focal length byusing a digital sensor at a fixed position with respect to the spatiallight modulator. After fitting the measured data to the theoreticalfocusing behavior of a real laser beam, the beam propagationfactor is obtained. We successfully validated the results in the laboratory where a full digital control of the measurement procedurewithout mechanical scanning was demonstrated.