Improved linear response in a modal wavefront sensor

The modal wavefront sensor provides a direct and less computationally intensive way, relative to a zonal wavefront sensor, of estimating the various aberration modes present in a beam of light. Such wavefront sensors are particularly useful when the optical system concerned is affected by a limited number of aberration modes. Unfortunately, the basic design of the modal wavefront sensor suffers from reduced linear response and intermodal crosstalk. As a result of the reduced linear response, the magnitude of an aberration mode estimated by a modal wavefront sensor may differ from the actual strength of the aberration mode. This difference is even more if other aberration modes, in addition to the aberration mode to be detected, also referred to as the sensor mode, are present in the beam. In this paper we propose a modal wavefront sensing scheme where the sensor output has a significantly enhanced linear response. The proposed scheme also provides superior intermodal crosstalk immunity, especially for a few selected aberration modes. We first show theoretically how by incorporating a variable magnitude of the sensor mode into the beam the precise strength of the same mode can be estimated. Results from numerical simulation and an experiment using a proof of principle setup demonstrate the improved performance by the proposed scheme.