We propose to determine the optical field in multimode circular fibers by u
sing a one-step method that measures the Wigner distribution function of a
section of the field in the fiber. This method allows an estimation not onl
y of the power carried by each mode but also of the relative phases of diff
erent modes in the fiber. An additional measurement with the same setup can
even determine the propagation constants of different modes. An example is
provided, and the connection of this method of field recovery to the coupl
ing coefficient between fibers and light sources is also discussed. (C) 200
1 Optical Society of America.