Optical Head Design Using HOE's

Abstract

ABSTRACT We describe an optical head design using two spherical lenses and two HOE's. This study was intended to show the feasibility of using HOE's to improve the performance of optical head for Read/Write optical disc drives. The two HOE's correct the spherical aberrations of the lens, change the elliptical beam to a circular one and compensate for wavelength shift up to +, - 5 nm. Experiments have been performed to confirm these concepts. The HOE's were made optically in the visible wavelength using an intermediate CGH. Ideally the HOE's are to be generated by a computer and preferably written directly by an e-beam machine. Since at that time all the commercially available e-beam shops we contacted lacked the confidence in making such high resolution CGH, we decided to use the Optical-CGH approach. Actually this approach offers several advantages. First we can make large and high resolution ir HOE immediately. Secondly \ve can even make Bragg (or volume) ir hologram using visible light. Basically we create a pre-distorted CGH from which the final HOE can be made using visible light. The final HOE can then be reconstruct at the ir wavelength without aberrations. The techniques used here can be extended to optical head designs using aspherical lenses. The HOE's function can then be, in addition to beam shaping, to achieve a larger wavelength compensation, to correct the residual chromatic and aspherical aberrations of the aspherical lenses. The astigmatism of the laser diode can also be corrected by the HOE's. The possibility of an optical head using only HOE's (no lenses) will be discussed. Computer simulations and experimental results are shown.1. IntroductionThe HOE's are compact, light weight and low cost. Also a single HOE can often replace many conventional optics. Thus we expect the use of HOE's in place of conventional optics can reduce system complexity, size, and cost and at the same time improve performance and reliability.However, in order to make HOE practical, we must first overcome several technical obstacles. The first and probably the most important is to make HOE's less sensitive to wavelength changes so that laser diodes can be used as the light source. The wavelength of a laser diode changes with temperature and driving current. Even a so-called single mode laser may be driven into multi- mode. It is therefore very important that the optical system using HOE's must be compensated at least for the wavelength drift of laser diodes. The wavelength drift due to driving current and mode hopping cannot be controlled by stablizing the laser temperature.