Fast M2 measurement for fiber beams based on modal analysis

We report on a fast and experimentally easy technique for measuring the beam ropagation ratio M2 of light guided by optical fibers. A holographic filter enables us to determine amplitudes and phases of the excited fiber eigenmodes. The coherent superposition of modes allows the reconstruction of the optical field. With this information at hand, we are able to simulate the free-space propagation of the beam and to perform a virtual caustic measurement. Associated beam propagation ratios M2 accurately agree with ISO-standard measurements.