When imaging a sample, inhomogeneities in refractive index cause blur in the image and decrease resolution. Adaptive optics (AO) is a technique that can correct for the resulting aberrations. The most common implementation of AO uses a single deformable mirror that is conjugate to the pupil. A single pupil-conjugate corrective device provides correction over a limited field of view owing to field-dependent aberrations. To overcome this limitation, an additional specimen-conjugate deformable mirror can be used. However, adding a second reflective correction device significantly increases system complexity. We have developed a closed-loop multiconjugate AO system for field-dependent aberration correction in a confocal fluorescence microscope. A 140-actuator deformable mirror is used in the pupil plane with a custom 37-element transmissive deformable phase plate inserted in a sample-conjugate plane. Both devices are calibrated and controlled in closed-loop using a Shack-Hartmann sensor in combination with an integral control law. The sensor consists of an EMCCD and lenslet array with a 500 μm pitch and a 47 mm focal length. Results from a Drosophila ovary and HeLa cells are presented.
We introduce an optimization-based open-loop control method for 2D wavefront modulators. The optimization problem is convex with inequality constraints and can be solved using an interior-point method in real-time. Compared to conventional influence matrix inversion, this new method takes into account the system limitations, such as the actuation polarity and voltage limits of the drivers. It searches for the global optimum of actuation signals within system boundary constraints. Consequently, while reducing the complexity of the hardware, it is more immune to systematic errors and guarantees optimality of the actuation signals. The control system is implemented on two different electrostatically-actuated phase modulators; a conventional deformable mirror and a novel refractive optofluidic phase modulator. We experimentally compare the performance of the optimizationbased controller with conventional methods for high order Zernike mode replication. It is demonstrated that the introduced technique enables more accurate control for both modulators, particularly at large correction amplitude and/or higher order corrections.
Two-dimensional spatial wavefront modulation in real-time is an essential tool for applications such as adaptive optics and laser beam shaping. Micro-mirror-based MEMS wavefront modulators have led to a major reduction in the cost of practical wavefront modulation, but the system complexities due to their reflective operation are still prohibitive. To address this issue, we demonstrate here a highly-miniaturized electrostatically actuated optofluidic transmissive phase modulator capable of positive or negative phase shifting through the use of hydromechanical coupling. The approach is based on a unique push-pull electrostatic actuation concept that exploits the inherent liquid-mechanical coupling in the design and is free of polarization and diffraction effects. This optofluidic phase modulator is able to correct aberrations up to 5th radial Zernike polynomial modes with high fidelity and, by use of sensorless wavefront estimation algorithms, allows for the realization of a completely in-line adaptive optics system.
A low-cost, compact electrostatic deformable mirror is developed using widely available polymeric materials. The fabrication method offers exceptional post-fabrication mirror flatness error of < 250 nm RMS, which can be reduced below 22 nm at best-flat. A custom real-time control system is also constructed and high fidelity reconstruction of Zernike modes up to the 4th radial order is presented. Real-time wavefront correction is also demonstrated using a spatial-carrier laser interferometer as a wavefront sensor.