Optoelectric devices for optical memory systems

Citation

Wang, Xu (2000) Optoelectric devices for optical memory systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/kf9g-xq27. https://resolver.caltech.edu/CaltechTHESIS:10052010-160806470

Abstract

This thesis describes several optoelectronic devices developed for the optical memory system. It consists of three parts, which are liquid crystal beam steering devices, modeling and measurement of optical diffraction from subwavelength structure at focused spot, and CMOS detector design. In liquid crystal beam steering devices, two types of liquid crystal beam deflectors have been successfully demonstrated. The essential working idea of those two deflectors is the same, which is utilizing a blazed grating (either permanently fabricated or virtually built up) to deflect the incident beam and the liquid crystal is used as an electrically controllable birefringent medium. The reflective type deflector exploits the liquid crystal on silicon (LCOS) technology to build up a virtual blazed grating (dynamic grating) and vary this virtual grating's period to achieve multiple steering angels. Seven addressable angles with as high as 93% steering efficiency have been demonstrated. A numerical modeling tool is also developed to analyze liquid crystal molecule director's distribution under nonuniform electrical field (fringe-field-effect). The transmission type deflector is based on a custom-fabricated PMMA blazed grating (fixed grating) and dynamically address multiple angles by stacking several layers of PMMA/LC composite gratings. 16 steering angles are obtained with the contrast ratio of 18. A fabrication-compensation trick is proposed to greatly improve the device's performance. In modeling and measurement of optical diffraction from subwavelength structure at focused spot, we describe a technique for studying scattering from subwavelength features that used a focused ion beam system to generate subwavelength, submicron features and a simple scatterometer to measure the scattering from single subwavelength scatterers. A two-dimensional model that handles arbitrary profiles is described. The model is shown to agree quite well with the experimental measurements. The model is then used to demonstrate ways in which the aspect ratios of subwavelength ridges and trenches can be obtained from scattering data and how ridges can be distinguished from trenches over a wide range of aspect ratios. We show that some earlier results on distinguishing pits from particles do not extend to low aspect ratio features. Finally, in CMOS detector, we have designed a CMOS detector to be used in the compact memory module. The chip features 64 x 64 array of 17µm x 17µm pixel with 23% filling factor within a 2mm x 2mm die, correlated double sampling (CDS) signalconditioning circuit in each column, as well as the software-controlled window-of-interest readout. Even more, two pixel designs (active photodiode and direct readout photo diode) are incorporated with the same chip for performance comparison.

Thesis Files