Measurement of the light orbital angular momentum spectrum using an optical geometric transformation

We recently demonstrated a new method to efficiently analyse the orbital angular momentum (OAM) states of light by application of an optical geometric transformation (Berkhout et al 2010 Phys. Rev. Lett. 105 153601). Here we study the performance of such a system to the change in the observed OAM spectrum, as the input beam is misaligned with to the analyser. We present modelled and experimental results which show that our approach does correctly measure the OAM spectrum for lateral and tilt of the input beam.