  Control of orbital angular momentum (OAM) in optical fields has seen
tremendous growth of late, with a myriad of tools existing for their creation
and detection. What has been lacking is the ability to arbitrarily modify the
OAM spectrum of a superposition in amplitude and phase, especially if a priori
knowledge of the initial OAM spectrum is absent. Motivated by a quasi-mapping
that exists between the position and OAM of Laguerre-Gaussian modes, we propose
an approach for single-step modulation of a field's OAM spectrum. We outline
the concept and implement it through the use of binary ring apertures encoded
on spatial light modulators. We show that complete control of the OAM spectrum
is achievable in a single step, fostering applications in classical and quantum
information processing that utilise the OAM basis.
