Design and implementation of multi-layer and multi-bit micro holographic optical data storage employing blue lasers

ABSTRACT A zoom objective lens for a multi-layer optical rec ording has been designed. A newly proposed optical power arrangement enables varying back focal length while focal length invariant by a linear movement of sin gle zooming component. The zoom lens is implemented into multi- layer and multi-bit holographic recording system em ploying 405 nm blue lasers. 1. INTRODUCTION Recently, multilayer micro holographic optical disc systems have been investigated to attain high capa city and high data transfer rate [1,2]. In the multilayer recordi ng, micro-holograms are recorded throughout the vol ume of recording material by changing the recording depth. For such multilayer recording systems, objective l ens accesses to multiple recording layers. The design of objecti ve lens needs to satisfy couple of constraints. Fir st, working distance, air spacing between objective lens and re cording disc, is unchanged while changing the recor ding layer. Numerical aperture (NA) must be an invariant for th e range of recording depth to equalize transverse s ize and modulation of index of refraction of the micro holo grams. Ideally, the number of moving elements shoul d be a minimum. Also, a linear relation between zooming of the element and recording depth is preferable. Typical way to design such optics is depicted in Fi g. 1. First, an intermediate image, a virtual or re al one, is formed by a movable zooming optics. Then, the inter mediate image is relayed to a final focal point ins ide a recording medium by a secondary optics consisting o f high NA objective elements. Moving the first opti cs along optical axis alters conjugate ratio between the int ermediate image and final image, therefore, the fin al image position also moves, which ends up with selection of writing depth. The approach is rather a simple one however we immediately notice several drawbacks of the approac h if we recall the design requirement for optical d ata storage applications. First, the system focal length varies as the writing depth changes. Consequently, NA va ries as selection of recording layers. As a result, a varia ble aperture and tuning of laser power is needed to equalize the transverse size and modulation of index of refracti on of the micro holograms. Second, the location of the final image nonlinearly moves with respect to the movement of t he zooming element. This would impose an additional requirement on actuators for selection of writing d epth. To overcome such issues, we propose an optica l power arrangement having constant focal length as well as linear movement of zooming element. Note that, eve n being able to implement such additional features, the pro posed optical power arrangement requires only one m oving component. Thus, our approach inherits the simple i mplementation of the conventional approach. Importa ntly, such a minimum optical power arrangement is possible onl y if specific combination of power is adopted for t he zooming optical component. In this paper, first analysis of the proposed power arrangement is described. Based on the power arrangement, we discuss design of all off-the-shelf write/read optics having a constant numerical aper ture (NA) of 0.4 for a range of writing depth of 0.2 mm. Design and implementation of multi-layer and multi-bit mic ro holographic recording system employing blue lasers will be addressed.