Elsevier

Optics Communications

Volume 188, Issues 5–6, 15 February 2001, Pages 291-299
Optics Communications

Fast switching liquid crystal lenses for a dual focus digital versatile disc pickup

https://doi.org/10.1016/S0030-4018(01)00989-0Get rights and content

Abstract

We investigated two types of liquid crystal (LC) lenses, the convex LC lens and the adaptive LC lens. Both lens types were designed and fabricated for integration in a novel pickup head for dual layer digital versatile discs (DVDs). The polarization dependent focal length of LC lenses was used to read both DVD information layers simultaneously. For fast and near diffraction limited performance, a new circular electrode structure with 64 ring electrodes was studied. Short response times of 50 ms for blazed Fresnel lens profiles and less than 10 ms for binary lens profiles could be realized using the transient nematic effect. Both lens types were characterized in terms of diffraction limited operation, focus efficiency and switching time.

Introduction

Interest in using liquid crystals (LCs) for adaptive optical systems has strongly increased during the last 10 years. The growing market in optical communication, optical data storage and optical measurement techniques has encouraged LC research in areas other than pure display applications. Elements for beam steering [1], [2], focusing [3], [4], filtering [5], phase modulation [6] and optical data processing [7] have been demonstrated. To keep pace with the growing demands of miniaturization in optical storage devices, new techniques which allow further integration of optical elements are needed. In such areas LC based elements can provide competitive solutions. They can be made fast and compact. Low driving voltages combined with very small power consumption make them favorable for battery driven products. They can be integrated in sensors, small cameras, CD/digital versatile disc (DVD) players, etc. Our aim was to design fast and diffraction limited LC lenses which could be integrated into a new dual layer DVD pickup head. Several LC lenses and lens designs are already available either commercially or in literature. None of these, however, met the DVD specific requirements which will be discussed in Section 2. New concepts that combine high optical quality and fast switching were necessary. For this purpose, two LC lens types, the convex liquid crystal lens (CLC lens) and the adaptive liquid crystal lens (ADLC lens) were investigated. The CLC lens was designed to provide high efficiency in focus. For the ADLC lens, a circular electrode scheme with 64 rings was designed to achieve high focus efficiencies in the considered focus range. An easy but fast switching technique was used to obtain switching times below 10 ms with nearly diffraction limited spot sizes. The lens designs and parameters will be discussed in 3 The convex liquid crystal lens, 4 The adaptive liquid crystal lens.

Section snippets

DVD-pickup requirements and the resulting lens parameters

DVDs are the common optical storage media for video, data and audio applications. Their high storage capacities of up to 17 Gbyte have been achieved by a reduction of the wavelength from 780 to 650 nm, by reducing the pitch and track sizes (compared to CDs) and by using two information layers on each side of the disc. We introduce a concept [8], where a LC lens is used for on-axis and simultaneous focusing on two information layers. On-axis simultaneous focusing works because LC lenses provide

Design

The CLC lens shown in Fig. 2 can be described as a glass lens with variable refractive index. The plano-convex lens cell was fabricated using a plano-concave lens and a glass plate separated by 7 μm mylar spacers. Before assembly, each interior surface was covered with a transparent electrode made of a indium–tin-oxide (ITO) and an additional layer of polyimide. The polyimide layers were rubbed in opposite directions to ensure a homogeneous orientation throughout the LC bulk. The opposite

The adaptive liquid crystal lens

The ADLC lens used a planar cell geometry (Fig. 6). The glass plates were covered with ITO electrodes. One glass plate is structured with an electrode pattern of 64 single addressable circular electrode rings. The electrode structure is explained in greater detail in Section 4.1. The surfaces were coated with polyimide layers to ensure parallel orientation of the LC medium. The cell was filled under vacuum and sealed using a two component epoxy adhesive. A thermo-compressive bonding technique

Performance of LC lenses in a DVD pickup system

Both lens setups, the dynamic CLC lens on its own as well as a static CLC lens in conjunction with an ADLC lens were tested in a DVD pickup demonstrator. It was shown that both information layers could be accessed simultaneously and layer distance errors of ±25 μm could be corrected. The control frequency was dependant on the switching times of the dynamic CLC lens in the first case and on the switching speed of the ADLC lens in the second case. Detailed results on the pickup performance will

Conclusions

We have demonstrated the near diffraction limited operation of two types of LC lenses. The lenses were fabricated and tested for applicability in terms of speed, efficiency and diffraction limited performance. Conductive layer meshing was used to reduce static phase aberrations due to the non-linear phase declinations between the electrodes. Experimental results showed that the CLC lens had a nearly diffraction limited focus spot with an efficiency of over 75%. It was shown that the response

Acknowledgments

We gratefully thank L.L. Wang, W. Haase, T. Weyrauch, E.P. Pozhidaev, B. Kley, D.A. Mlynski, B. Cramer, S. Dickmann, F. Reichel, S. Breitfelder, P. Gussek, W. Becker for their support, their helpful discussion and cooperation. The work was supported by the BMBF (project no: 01 BS 609/0 – Neuer optischer Schreib-/Lesekopf für die optische Disk) and by the Deutsche Thomson-Brandt.

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