Aberration Compensation Using Nematic Liquid Crystals

Abstract

We have developed a novel transmissive nematic liquid crystal devicewhich is capable of compensating spherical wavefront aberration thatoccurs during the operation of optical pickup systems. In order toincrease the storage capacity, next generation optical data storagesystems beyond CD and DVD will use according to the Blu-Rayspecification (BD) blue laser light and an objective lens with highnumerical aperture (N.A.) of 0.85. However, such high N.A. systems havean inherent higher sensitivity on aberrations. For example sphericalaberration is inversely proportional to the wavelength and grows withthe fourth power of N.A. of the objective lens. In an optical pickupsystem there are two sources for spherical aberration: The first one isthe variation of the substrate thickness due to manufacturing tolerancesunder mass production conditions. The second one concerns disks withmultiple data-layers, which cause spherical aberration when layers areswitched, as the objective lens can only be optimized for a single layerthickness. We report a method for effective compensation of sphericalaberration by utilizing a novel liquid crystal device, which generates aparabolic wavefront profile. This particular shape makes the devicehighly tolerant against lateral movement. A sophisticated electrodedesign allows us to reduce the number of driving electrodes down to twoby using the method of conductive ladder mashing. Further evaluation ina blue-DVD test drive has been carried out with good results. By placingthe device into an optical pick-tip we were able to readout a dual-layerROM disk with a total capacity of 50 gigabytes (GB). A data-to-clockjitter of 6.9% for the 80 gin and of 8.0% for the 100 mu m, coverlayer could be realized.