High Frequency Electro-Optic Modulators for Integrated Optics

[摘要] This thesis encompasses the theoretical design and experimental evaluation of a novel type of high speed phase modulator using LiNbO3 and GaAs technology suitable for guided wave optics. A brief review of the development of Integrated Optics and in particular of optical modulators is given in Chapter 1, along with an outline of the aims of this project. Chapter 2 provides a comprehensive survey of the methods available for determining the theoretical propagation constant of a guided wave in a variety of dielectric waveguide configurations. A comparison between the various methods is made to provide an indication of the accuracy and complexity of each procedure for a particular waveguide geometry. The effect of metal films in close proximity to optical waveguides on the propagation characteristics is derived through the effective index method. The results obtained are then applied to minimise absorption losses due to the metal overlay in active devices. The main theoretical design of the proposed electrode structure is developed in Chapter 3. The input impedance of the device is matched to the characteristic impedance of the transmission line drive system at the operating frequency using the standard transmission line equations for low-loss lines. The response of the device is modelled by a computer program which calculates the input impedance of two short circuited transmission lines of unequal length connected in parallel. The program allows the asymmetry of the line lengths, the attenuation constant and characteristic impedance to be independently varied so that their influence upon the overall response may easily be determined. An expression is derived for the theoretical depth of modulation and the power/bandwidth ratio for such a device. Finally a description is given of the numerical methods used to obtain the characteristic impedance and attenuation constant of symmetric and asymmetric coplanar microwave waveguide. The chapter concludes with a brief mention of the analysis procedure for Schottky contacts on GaAs. The general photolithographic and fabrication procedures for forming optical waveguides in LiNbO3 and GaAs are introduced in chapter 4. This chapter also gives a detailed account of the modified lift-off technique which is successfully employed to pattern metal films up to 2mum thick - a technique which is required for the standing wave modulator electrode. The use of proton-exchange waveguides as a suitable optical waveguide for high speed active devices is explored in chapter 5. An initial experimental investigation is presented with the theoretical determination of the refractive index profile for waveguides formed in X- and Z-cut LiNbO3 from the observed optical properties. The fabrication parameters required to produce monomode stripe optical waveguides are determined and verified experimentally for a variety of exchange conditions. Other techniques are discussed which offer more control over the refractive index profile and address the problem of index stability. A brief description of the micro-analytical techniques available to monitor the hydrogen concentration is also given. Chapter 6 describes the methods available for characterising the small signal response of the modulator. A comparison is given between the experimental results and the theoretical predictions. Factors affecting the discrepancies in the response are also mentioned. In particular the uncertainties of the high frequency dielectric constants of LiNbO3 are quantified. Chapter 7 concludes the thesis with an outline of the future work required and the conclusions that may be drawn from the work carried out so far on proton exchange and the standing wave modulator.

[发布机构] University:University of Glasgow

[关键词] Electrical engineering