Remote focusing with dynamic aberration elimination by model-based adaptive optics

Abstract

Remote focusing using a deformable mirror (DM) provides a rapid and responsive technique for performing axial scanning. However, to maintain the laser focusing quality, the aberrations within the system must be adequately controlled. Accordingly, the present study proposes a low control complexity model-based adaptive optics (AO) approach for remote focusing with zero aberrations. Based on the Zernike coefficients measured by a self-built Shack-Hartmann wavefront sensor (SHWS), 15 independent closed-loop controllers (one for each Zernike mode according to Wyant expansion scheme) execute a three-step DM identification process to determine the control vectors required to actuate the DM in such a way as to restore the Zernike coefficients of the wavefront to their required values. The experimental results show that the controllers converge within four time steps (20 ms) and reduce the converged wavefront variance by nearly 475 times. Moreover, the controller used to manipulate the Z3 (defocus) Zernike coefficient converges within just three time steps (15 ms). Given a static disturbance of the wavefront, the proposed AO control method enables the defocus laser spot to be driven precisely along a target sinusoidal trajectory while maintaining all the other Zernike modes at zero. The control system shows a similar performance in compensating the dynamic aberrations produced by a low-frequency thermal disturbance within the system. Overall, the results indicate that the proposed model-based AO controller facilitates remote focusing with simultaneous aberration elimination and improves the central intensity of the focus laser spot by around 3.1 times compared to the case in which the controller is not applied.