Adaptive optics aberrometer and wavefront corrector using a twisted-nematic liquid-crystal on silicon chip

Through the application of adaptive correction, both the permanent and changing aberrations of the eye that affect vision acuity are compensated for in order to provide diffraction-limited imaging. This thesis investigates the aspects of establishing an adaptive optics system for the correction of ocular wavefront errors using a twistednematic liquid-crystal on silicon (TN-LCoS) chip as the wavefront corrector in conjunction with a Hartmann-Shack device as the wavefront sensor. TN-LCoS chips are originally designed to be used as mass-manufactured, high resolution image generating elements in video projectors. With higher density of correcting elements than the conventional form of ocular wavefront correctors (different forms of deformable mirrors), LCoS chips are emerging as potential solutions for realising cost-effective AO systems. Through the application of phase-wrapping, the resolution and stroke of the LCoS chip are fully exploited to operate the AO system as a reliable aberrometer and wavefront corrector. One of the set-backs when using these LCoS chips is that they can generate a high proportion of unwanted beams, i.e., beams other than the first diffraction order. In particular, chips with TN alignment of the LC molecules will show a strong zeroth-order contribution. That affects both the chips’ true wavefront generation capabilities as well as the fidelity of the Hartmann-Shack wavefront sensor, creating an unpredictable correction scheme that does not achieve diffraction-limited performance. In this thesis the ability of the TN-LCoS chip to act as a light modulator has been thoroughly investigated. Based on this, modifications to the experimental adaptive optics set-up and the closed-loop control algorithm have been implemented to accommodate for the chip’s limitations. Experiments that were carried out include the generation of Zernike wavefronts, measurements of Zernike coefficients of elements with known and unknown optical properties, as well as open- and closed-loop corrections of static and dynamic aberrations. Parameters such as experimental and simulated point spread functions (PSFs), and residual wavefront RMS values in both the pupil and retinal planes were correlated. The closed-loop wavefront corrections with volunteers’ eyes have shown narrowed point spread functions (PSFs) and higher Strehl ratios (SRs). The system established has full functionality for ocular wavefront correction, permitting future extension of usage in retinal imaging systems or for psychological experiments