Miao Yu (University of Maryland), Mikhail A. Vorontsov (University of Maryland), Svetlana L. Lachinova (University of Maryland), Jim F. Riker (AFRL/DESM), V. S. Rao Gudimetla (AFRL/DESM)
Keywords: Adaptive Optics
Abstract:
Propagation of optical waves over along horizontal path through continuously distributed or layered phase-distorting medium results in the development of intensity scintillations and phase singularities in the optical receiver system pupil. Both effects are highly undesirable for the traditional (based on phase conjugation) adaptive optics (AO) technique which requires direct reconstruction of the phase aberration function based on data obtained from a wavefront sensor. The intensity scintillations “propagate” to the wavefront sensor output resulting in a parasitic modulation of the sensor’s output and phase reconstruction errors. Wavefront phase singularities (branch-points) add an additional complexity to phase reconstruction computations. In this paper, we consider both the traditional adaptive optics technique and an alternative model-free control strategy (e.g., wavefront control based on a decoupled stochastic gradient descent (D-SPGD) technique). The latter does not require reconstruction of the phase. It is demonstrated in this paper that the model-free technique is more robust to intensity scintillations.
Optimization of adaptive compensation efficiency includes not only optimization of control algorithm parameters, but also identifying the optimal position for the wavefront corrector in the adaptive system wave-train. The recipe widely used in the multi-conjugate AO approach for wavefront corrector position suggests positioning the wavefront corrector in the conjugate (image) plane of the phase-distorting layer that the corrector intends to compensate. In this paper, both receiver system aperture diffraction effects and the impact of wavefront corrector position on phase aberration compensation efficiency are analyzed. As shown in the presented study, this recipe on multi-conjugate AO approach indeed results in optimal closed-loop compensation performance, but only if aperture-induced diffraction effects can be neglected. In the presence of aperture-induced diffraction and/or for the case of multiple phase-distorting layers separated by short distances, the optimal corrector position for both closed-loop phase conjugation and D-SPGD control algorithms corresponds to the conjugate pupil-plane. Any advantage that may arise from relocation of the wavefront corrector from the plane conjugate to pupil-plane disappears in the presence of aperture diffraction effects.
Because in most cases the geometry of the phase-distorting layers location is unknown or known with some degree of uncertainty, the results presented in this paper suggest that there is no compelling reason for relocating the wavefront corrector from the conjugate plane of the telescope pupil, unless phase aberrations are the result of a single phase-distorting layer with an accurately defined location and aperture diffraction effects neglected. These results and analyses are expected to provide important insight for the development of high performance adaptive optic systems over long horizontal paths.
Date of Conference: September 12-15, 2007
Track: Poster