Measurement of the M~2 beam propagation factor using a focus-tunable liquid lens

摘要

We demonstrate motion-free beam quality M~2 measurements of stigmatic, simple astigmatic, and general astigmatic (twisted) beams using only a focus-tunable liquid lens and a CCD camera. We extend the variable-focus technique to the characterization of general astigmatic beams by measuring the 10 second-order moments of the power density distribution for the twisted beam produced by passage through multimode optical fiber. Our method measures the same M~2 values as the traditional variable-distance method for a wide range of laser beam sources, including nearly TEM_(00) (M~2 ≈ 1) and general astigmatic multimode beams (M~2 ≈ 8). The method is simple and compact, with no moving parts or complex apparatus and measurement precision comparable to the standard variable-distance method.