Figure 2: Measurement of the OAM moments of a superposition of 
with variable coefficients.
with variable coefficients.
Behaviour of the first (a) and second (b) moment and of the root mean square deviation (c) of the OAM distribution of the beam prepared using a q-plate, as a function of the q-plate tuning voltage controlling its retardation θ and for different values of the polarization helicity s3 before the q-plate. The two parameters θ and s3, together with the q-plate charge q, define the specific OAM superposition of states having 
and 
. Experimental data points for 
are represented by ○ (○), for 
by □ (▪), for s3=0.50(−0.50) by ◊ (♦) and for s3=0 by ▴. Error bars represent standard deviations due to misalignment errors, estimated by repeating the experiments after realignment. The curves give the corresponding theoretical predictions calculated from equation (7) for s3=±1 (dotted curves), s3=±0.87 (dashed curves), s3=±0.50 (dot-dashed curves) and s3=0 (solid curve).
