Orbital momentum mode generation by a tunable diffractive optical element based on lithium niobate

Tunable LiNbO₃-based diffractive optical elements (DOEs) to control orbital angular momentum spectrum of the beam are proposed. Tunable binary spiral axicons for generation of the beam with orbital angular momentum (topological charge l = 1) are fabricated and investigated experimentally by optical methods. The intensity distribution in the far field under a half-wave voltage (3400 V) applied to the DOE was in good agreement with the theoretical prediction. It was shown that utilization of two oppositely-rotating DOEs leads to appearance of a correlation peak, thus demonstrating ability to realize the correlation filtration for orbital momentum mode detection in telecommunication system. The response time of the DOEs was evaluated using an element designed for generating a TEM01 mode and fabricated by the same technology. The measured response time below 200 ns was limited by the bandwidth of the used photodiode. The proposed elements can be used in optical telecommunication systems with mode division multiplexing.