Measurement of multiplexed fractional vortices with integer mode interval

In recent years, light's orbital angular momentum (OAM) has been used as a new degree of freedom in both classical and quantum communications. Fractional vortex (FV) with non-integer topological charge (TC) has been used in optical communication to increase transmission capacity and improve security capability. However, it is difficult to detect multiplexed FVs accurately, which restricts the application of FV in optical communications. In this work, we reported a scheme to detect TCs of multiplexing FVs with integer OAM mode intervals. It started by raising an algorithm that can calculate the TC of a single FV with the intensities of two integer vortices and a correction function. Then, based on this algorithm, multiplexing FVs with discrete TCs were regarded as four (three) integer vortices and the corresponding TCs could also be measured by the intensities of two integer OAM modes. The experiments were employed to measure multiplexed FVs in the range from −6.9 to +6.9 for proving the strategy, and the results showed that the TC measurement error is less than 0.023. This multiplexed FVs detection scheme can increase the number of available vortex modes in a certain range, which might inspire wide optical communication applications of FV beams.