Currently, in most adaptive optical systems, the control loop between the wavefront sensor and the deformable mirror involves intense mathematical calculations, both during calibration and operation of the system. Although thorough research has been done to optimise the control loop, some issues like error propagation and system bandwidth will always be ultimately limited by the coupling between the mirror and the wavefront sensor. Closed-loop by direct feedback from the wavefront sensor to the deformable mirror was proposed by F. Roddier in his well-quoted curvature wavefront sensing paper. However, due to the natural properties of the defocused-image, this direct feed-back method is limited to bimorph mirror applications only. Recently, M.A.A Neil et al proposed a new modal wavefront sensor (MWFS), which can detect several Zernike modes by a simple intensity subtraction operation. One drawback of this method is that it can only handle a limited number of modes. However, in this paper, we refine this method to detect the orthogonal modes of a deformable mirror instead of Zernike modes in a to-be corrected wavefront. Since the number of actuators of a deformable mirror limits the number of mirror modes, the drawback is minimised in this application. Considering the mirror modes can be directly transformed to the deformable mirror control command set by a proper gain coefficient, it is reasonable to construct a direct-feed back adaptive optical system with the modal wavefront sensing. We will report our first stage investigation on direct feedback adaptive optical system which is to understand the response of MWFS to mirror modes.
This paper is a revision of a paper presented at the SPIE Conference on Micro-Optics, VCSELs, and Photonic Interconnects, Apr. 2004, Strasbourg, France. The paper appears (unrefereed) in SPIE Proceedings Vol. 5453.
An adaptive beam position control system is reported, for use in alignment correction for free-space parallel optical interconnects at the board-to-board level. A liquid-crystal spatial light modulator (SLM) displaying binary phase gratings is used as a diffractive optical element to steer the beam. The design and implementation of a feedback controller for the SLM are presented, and shown in experimental results to completely correct for step disturbances within 1 to 2 sample periods. Extension of the system to tolerate random vibrations is considered, in terms of both the required SLM technology and the design of the control algorithms. The discussion highlights some of the trade-offs in choosing components when using this technology to correct for different levels of vibration.
Free-space optical interconnects have a wide range of potential applications in the field of telecommunications, computing, and data storage. Of particular interest is their use at the printed circuit board-to-printed circuit board level to either augment or replace a conventional electronic backplane. Although it is possible to design a free-space interconnect capable of operating at data rates equal to, or greater than conventional electronic backplanes, acceptance of free-space technology has been slow due to the perceived difficulties in aligning the system, and in maintaining alignment during the operational cycle of a product. In this paper we describe the implementation of a free space optical interconnect that uses a programmable diffractive element for active beam steering in order to maintain the link between the two printed circuit boards. Moreover, by using a modal wave front sensor, the diffractive element corrects for static aberrations. These static aberrations, such as field curvature and coma, are due to intrinsic imperfections in the optical system related to the lenses used. In addition, aberrations introduced by misalignments can also be compensated for. The diffractive structure is displayed on a ferroelectric liquid crystal spatial light modulator. Simulations and theoretical discussions of the performance of the system are shown and analysed.
A demonstrator system for a free space adaptive optical interconnect is reported in which beam position at the receiver plane is maintained using feedback control, given disturbances to the alignment of the optical system. A Liquid Crystal Spatial Light Modulator (SLM) displaying binary phase gratings is used to steer the beam. The design and implementation of the controller is presented, and shown in experimental results to completely correct for step disturbances within one to two sample periods. Extension of the system to tolerate random vibrations is also considered, both in terms of the required SLM technology, and design of the control algorithms. The discussion also highlights some of the trade offs in choosing components when designing for tolerance to different levels of vibration.