Propagation invariants of twisted Gaussian Schell-model beams

The second-order moments method is used to study the M2 factor and intrinsic astigmatism of twisted Gaussian Schell-model (GSM) beams. It is shown that the M2 factor of twisted GSM beams defined by the determinate of the 4×4 variance matrix is a propagation invariant and is independent of the beam twist, whereas the twist affects the intrinsic astigmatism of twisted GSM beams.