Adaptive optics systems are used in various application areas to enhance the performance of optical systems. Typically, adaptive optics systems make use of active optical elements, i.e., wavefront correctors, to introduce appropriate adjustments in the properties of light waves. These adjustments can lead to the removal of image distortions in imaging systems, hence providing clear images of the object being imaged, or allowing for the efficient delivery of a laser beam in various applications including laser systems, manufacturing, free space optical communications, etc.
Recent developments in optomechatronic system design targeted at adaptive optics applications cover two main areas. The first area addresses the development of optical components for use in adaptive optics systems, namely the development of novel wavefront correctors and wavefront sensors. The second area addresses system level issues related to the integration of the various components in an adaptive optics system, as well as the development of advanced control and signal processing algorithms. This special issue presents examples of recent advances in the optomechatronic design aspects of adaptive optics systems, and covers topics related to the wavefront corrector design, the development of computationally efficient control approaches for adaptive optics systems, and application examples of such systems.
With regard to the development of wavefront correctors, Rodrigues et al. present a novel approach to the design of deformable mirrors which allows for higher dynamic range and scalability. The proposed design is based on considering segmented hexagonal bimorph mirrors, where each group of segmented mirrors is mounted on a rigid platform. A two stage actuation approach is proposed where fine surface deformations are produced by the segmented bimorph mirrors, and surface continuity is maintained using rigid body actuation of different groups of bimorph mirrors. Extensive simulation studies indicate the optical performance of the proposed design closely matches that of a continuous bimorph mirror. In Diouf et al., an approach to connect MEMS type of actuators to drive electronics through the use of through-wafer interconnects is presented. The proposed design approach allows for the development of large density actuator arrays suitable for future applications of adaptive optics systems. The article outlines the fabrication steps in the development of the through wafer via technology. It also discusses the design of the actuator array, the thermo-compression bonding of the actuator array to an interposer die, and the evaluation of the performance of the resulting device in terms of the actuator performance. The presented approach to the fabrication of MEMS deformable mirrors and associated interconnection to drive electronics has the potential of significantly increasing the actuator count in deformable mirrors.
The development of computationally efficient control strategies for adaptive optics systems are presented in two articles. Iqbal et al. present an approach to the design of a decentralised PID controller for magnetic fluid deformable mirrors. The controller design approach simultaneously accounts for modeling uncertainties in the plant as well as for performance constraints on the transient response of the system. A controller synthesis approach is developed based on the fact that the controller has a fixed structure. The proposed control algorithm is implemented on an adaptive optics setup and experimental results are presented to illustrate the performance of the resulting closed loop system. Fraanje et al. discuss the design of distributed controllers for deformable mirrors. The deformable mirror is modeled as a decomposable system consisting of an interconnection of identical dynamic components. A distributed controller is developed for the decomposable system, where each component of the distributed controller interacts only with its immediate neighbours. As such, the implementation complexity of the controller and the computational cost associated with the control algorithm are reduced. These features could be of significant benefit in adaptive optics systems involving very large arrays of actuators.
Two application examples of adaptive optics systems are presented in this issue. In Scott et al., an approach to compensate for aberrations introduced during off axis scanning in optical telescopes is presented. The proposed system allows for high resolution imaging with an increased field of view for the telescope. The compensation of off-axis aberrations is performed using a deformable mirror. An algorithm that allows for the adjustment of the mirror shape is proposed. The performance of the resulting system is compared based on considering two different mirrors and experimental results are presented. Nash et al. discuss the position control of a trap in an optical tweezer. The proposed approach involves the use of a feedback system based on a deformable mirror and a wavefront sensor to adjust the position of the trap. The use of feedback allows the optical tweezer to handle unknown and possibly time-varying aberrations. The design of the optical tweezer is discussed. An experimental setup is developed and used to test the stiffness of the trap.
The performance specifications for adaptive optics are becoming more and more stringent, both at the hardware level (mainly concerning the performance of the wavefront correctors and sensors), and at the algorithmic level (mainly concerning control and signal processing algorithms). These evolving specifications, emanating from the needs of various existing and emerging application areas, will continue to drive further innovation into the optomechatronic design aspects of adaptive optics systems.