  We present an approach for measuring the orbital angular momentum (OAM) of
light tailored towards applications in spectroscopy and non-integer OAM values.
It is based on the OAM sorting method (Berkhout et al., Phys. Rev. Lett. 105,
153601 (2010)). We demonstrate that mixed OAM states and fractional OAM states
can be identified using moments of the sorted output intensity distribution and
OAM states with integer and non-integer topological charge can be clearly
distinguished. Furthermore the difference between intrinsic OAM and total OAM
for fractional OAM states is highlighted and the importance of the orientation
of the fractional OAM beam is shown. All experimental results show good
agreement with simulations. Finally we discuss possible applications of this
method for spectroscopy of semiconductor systems such as exciton-polaritons in
microcavities.
