Mitigation of inter-modal crosstalk in the holographic wavefront sensor through the implementation of a multi plane light conversion

Free space optical communication (FSOC) represents a promising technology to enable a secure transmission with high data rates over long distances. Ensuring proper operation in turbulent atmospheric conditions by adaptive optics (AO) is essential for the general feasibility of FSOC. A bottle neck of the AO performance is the limited speed of control loops based on Shack-Hartmann wavefront measurements. Alternatively, holographic wavefront sensors provide the potential for an increased measurement speed by enabling a direct measurement of individual spatial mode amplitudes of the turbulence. So far, their performance is limited by inter-modal crosstalk. In this work, a design algorithm to build a holographic sensor out of a sequence of holographic plates is developed. To this end, an adapted wavefront-matching mode sorter algorithm is implemented, which is based on the use of Zernike-polynomials. The algorithm allows for designing multiple holographic phase plates in sequence with a built-in optimization of crosstalk reduction. The phase profile of the received beam is converted through interaction with each phase plate before the beam hits a detection plane. The designed computer-generated-holograms are characterized by simulation and the mitigation of crosstalk is shown for a higher number of holographic phase plates. Additionally, a proof-of-concept experiment is performed to demonstrate the desired behavior of the holographic sensor.