In order to increase the storage capacity and the density of near-field optical disk drives, a flying pickup-head has to allow a slider to fly at a stable height above the disk surface with the use of near-field optics. Since both the precision of the track pitch and the flying height are of a nanometre scale, it is necessary to increase the motion accuracy of the pickup-head. In this study, a piezoelectric bender is used as an actuator of the pickup-head, and two quadrant photodetectors are used to sense the pickup-head displacement and the rotating disk deformation. Based on an optical lever method that magnifies a small displacement, the flying height variation of the pickup-head above the disk is measured. Further results show that using the proposed measurement method in the real-time control of flying height is feasible.

a b s t r a c tIn this paper, a new approach is proposed to realize a lens-less and 90°light-bending by a volumeholographic element, consisting of 3D sub-wavelength index-gratings throughout the sample. In this approach, a top incident plane-wave is diffracted by 90°and guided into a planar volume hologram. Conversely, a side-incident light may be guided and diffracted out of the top sample surface. The diffraction-efficiency η of light was studied in real-time during light-exposure and also in the dark after exposure to observe "dark-enhancement". It is shown that 90°light-bending at normal incidence can be achieved with a high efficiency, η$70%, and a small angular-tolerance, Δθ¼0.02°. It is further shown, as a proof-of-concept, that a 5°cylindrical-wave top-incident beam may be used to improve angular tolerance to Δθ¼5°, but with a reduced efficiency of η$0.80%. CrownHolographic elements have been widely used in optical system to replace traditional optical devices. Examples are compact holographic lens, holographic grating, and holographic concentrator for solar energy harvesting [1-3]. One of the key issues of a holographic element concerns optical properties of the recording materials. However, PQ/PMMA materials has recently been shown to have superior optical characteristics, such as high contrast refractive-index variation, low shrinkage after exposure, environmentally stable and no need for lithographic development [4][5][6][7]. Currently, there is an emerging interest in using holographic elements for compact display system with edge-lit hologram [8-10] and for solar harvesting to achieve high conversion-efficiency and enable an easy adoption to a building's non-planar structure. To achieve high conversion-efficiency, a transmission holographic element has been proposed to split solar radiation into regions according to the spectral sensitivity of the cells [11][12][13]. To enable adaption to building structure, several materials approaches have been developed that include the use of thin-film, polymer, or dyesensitized photovoltaic cells. Another optical approach is to combine low-cost concentrators with high-efficiency solar cells [14,15]. So far, there is no holographic approach that utilizes 90°h olographic recording geometry [16] to enable solar collection with a large collection angle, over the entire solar spectrum and with high concentration factor.In this paper, we demonstrate lens-less bending and concentration of light by a volume holographic element (VHE). The PQ/PMMA material has a good optical quality and was chosen as the holographic recording material. The recording of index-grating was performed using two different schemes-top incident and 90°i ncident configurations. The efficiency of light-diffraction was studied in real-time and also in the dark to observe "dark-enhancement". We show that it is possible to achieve 90°light bending with high efficiency η¼70%. Furthermore, this 90°b ending scheme leads to a concentration factor of 2.5 times, without the use of any external optics. With...