Well known for their use in adaptive-optical (AO) systems, wavefront sensors—for example, the Shack-Hartmann configuration, which consists of a lens array in front of a camera, allowing wavefront tilts to be measured across the field and then integrated into a wavefront shape—are also important for ophthalmology, imaging through turbid media, and laser-beam characterization. However, a Shack-Hartmann-type lens array is not the only type of optical element that can be placed in front of a camera to measure wavefront error; a diffuser (usually placed relatively far away from the camera) can be used instead, simplifying the optical system. The downside of a diffuser-based wavefront sensor, which creates a speckle pattern that can be analyzed, is the complex calculations required to retrieve the wavefront data.
A broadband, compact, and low-cost diffuser-based wavefront sensor has now been developed by Pascal Berto, Hervé Rigneault, and Marc Guillon of the Université Paris Descartes Neurophotonics Laboratory (Paris, France) and the Institut Fresnel (Marseille, France), who simply place a thin diffuser near a camera to allow sensing of the local wavefront gradient by measuring the translation of the speckle pattern (in reference to a previously recorded speckle pattern of a flat wavefront) as a function of the position across the wavefront. Local speckle grains are shifted laterally in proportion to the local wavefront tilt; a so-called diffeomorphism algorithm combined with a 2D gradient integration determines the retrieved wavefront shape. In an experimental verification, a holographic diffuser was placed a few millimeters from a monochrome CMOS camera and speckle pattern shifts were recorded and analyzed via algorithms implemented in MATLAB. The λ/300 sensitivity of the setup was limited by the 8-bit camera used. Phases sensitivity is tuned by changing the distance of the diffuser from the camera. Reference: arXiv:1710.03797v1 [physics.optics] (Oct. 1, 2017).