Experimental measurements and compensation of optical phase discontinuities

In this paper, we evaluate the effectiveness of adaptive optics (AO) compensation for optical phase discontinuities, akin to the spatial phase functions resulting from laser propagation through a shock wave encountered in free flight. In high subsonic to supersonic flow regimes, shock waves may form local to the beam director of aircraftmounted laser propagation systems. The discontinuous nature of the spatial phase functions imposed by shock waves may provide challenges in wavefront sensing and hardware correction. To experimentally evaluate the performance of conventional AO solutions, we utilized a benchtop turbulence generator and AO system. Here, we apply simulated phase sheets with optical phase discontinuities to a laser beam using a spatial light modulator; after which, we compensate for them in our AO system using a deformable mirror (DM). In this paper, we demonstrate AO correction informed by both a Digital Holographic wavefront sensor (DHWFS) and a Shack– Hartmann wavefront sensor (SHWFS). Testing of zero-thickness, step-like phase discontinuities shows that the SHWFS is unable to reconstruct the wavefront accurately, leading to poor AO compensation and as a result, no improvement in performance. The DHWFS-based correction, on the other hand, remains reliable and reveals significantly improved AO performance in the presence of phase discontinuities. SHWFS-based AO compensation improves when the phase function is more ramp-like rather than step-like (representing a shock wave of finite thickness); however, problems arise due to discontinuity alignment with DM actuators. The results of this study begin to illustrate the practical challenges of using AO systems to compensate for phase discontinuities.