Spatial Light Modulator-Based Shack Hartmann Wavefront Sensor With Built In Influence Matrix Measurement

Abstract

The Shack Hartmann wavefront sensor (SHWFS) is used to detect the incoming aberrated wavefront for use with an adaptive optics system by using a 2-D lenslet array to produce an array of spots and recording the spot deviations near the focal plane. The size and number of lenslets within the array is closely related to the exposure time and resolution of the WFS and depends on the irradiance of the target, the atmospheric situation inside the FOV, and the optical performance of the adaptive optics system. Since a WFS normally has a fixed number of lenslets and aperture size, to make the WFS work properly, it requires a well-suited calibration process using a planar wavefront. It produces difficulties in the optimization of the system when varying circumstances are introduced. In this paper, we describe how by delaying the phase at each pixel, an SLM (Spatial Light Modulator) is used to build lenslets with different sizes and aperture counts. The multimode WFS has calibration data for each mode before operating the system and depending on the variable circumstances it is able to easily convert to another mode even as the system operates. This paper describes the setup of a multimode wavefront sensor using an SLM, the construction of a lenslet array using an SLM-generated Fresnel lens, and the theory of calibration and reconstruction. We also prove the capability of the WFS performance by measuring a wavefront aberrated by a phase plate.