Real-time determination of laser beam quality by modal decomposition
  We present a real-time method to determine the beam propagation ratio M2 of laser beams. The all-optical measurement of modal amplitudes yields M2 parameters conform to the ISO standard method. The experimental technique is simple and fast, which allows to investigate laser beams under conditions inaccessible to other methods. 
Oliver A. Schmidt, Christian Schulze, Daniel Flamm, Robert Br\"uning,   Thomas Kaiser, Siegmund Schr\"oter and Michael Duparr\'e
physics.optics