High Bandwidth Integrated Optical Ridge Guide Modulators

Abstract

Project description High Bandwidth Integrated Optical Ridge Guide Modulators Electro-optic modulators are key elements in today's optical fiber communication networks and their further development is crucial to match the near future requirements of higher bandwidth and lower power consumption as well as of lower cost. This is the motivation to perform this project which aims to develop new types of electro-optic modulators made in lithium niobate (LN). Mach-Zehnder (MZ) type modulators are integrated optical devices in which the continuous input light is split into two independent waveguides where electric fields are applied. Due to the electro-optic effect of the LN, different changes in the refractive index of those waveguides occur and, therefore, a phase-shift is created between the light traveling through them. At the output, both waveguides are joined again and the phase-shift determines whether there is a constructive or destructive interference, giving place to an amplitude modulation effect. The project started with a deep study of the working principles of MZ interferometric electro optic modulators in LN. This included the modeling and understanding of titanium in-diffused waveguides in LN, of the interferometric structures to be developed, of the relevant characterization techniques and of radio frequency (RF-) conducting microstrip waveguides to be used as electrodes. As a result of this study, it was theoretically demonstrated that new types of optical waveguides, in particular ridge waveguides, are able to increase the performance of modulators. Ridge waveguides have a better confinement of optical fields leading to lower driving voltages and, as a result, to lower RF-power consumption of the device. Furthermore, the velocity matching condition necessary to get a high modulation bandwidth is easier to achieve. As a consequence, faster devices of lower drive power have been designed [Ref. 1]. Subsequently, the development of ridge waveguide interferometers was started yielding already promising results. Special techniques such as the wet-etching process for Z-cut LN, recently developed in Paderborn, were used. The fabrication of modulators with ridge guides in LN implies the development of radio-frequency traveling-wave electrodes on top of the ridges. This is a challenging task, for which different fabrication techniques are being developed in order to obtain high quality thick electrodes with good transmission characteristics [Ref. 2]. Photolithographic techniques are being used with different types of photo resist. Also different etching procedures, such as ICP plasma etching (Inductively Coupled Plasma) and several wet chemical etching procedures have been tested. Moreover, lift-off techniques and different materials for the electrodes are currently under evaluation. After the best materials and procedures are found, optical and electrical testing will be made and results will be compared with the previous simulations. This will include optical mode behavior, radio-frequency waveguides performance, modulating driving voltage measurements and frequency dependence of all parameters. As a final goal, new structures of electrooptic modulators based on original calculations and tested with experimental measurements will be demonstrated. The project has been carried out during the last three years within a collaboration established between the Applied Physics/Integrated Optics Group of the University of Paderborn (Prof. Dr. W. Sohler) and the Laboratory of Integrated Optics and Optoelectronics (Prof. Dr. J. Rodríguez) of the University of Oviedo (Spain). At the University of Oviedo, mainly theoretical and simulation work has been done, whereas at the University of Paderborn, the technological facilities for the fabrication and test of integrated optical devices are being used. In particular, clean room lithography equipment, thin layer deposition and etching machines, laser sources, optical test benches and radio-frequency measurement equipment are essential for the modulator fabrication and testing. During that time, the main funding for the research activity came from the Spanish Ministery of Education and Science under a PhD grant awarded to Mr. M. García Granda. In order to successfully conclude this research, the final experimental steps need still to be completed. This will include the fabrication of radio-frequency electrodes on new samples applying the different techniques mentioned above and the corresponding electro-optical testing of the resulting devices. These activities are expected to last up to about six months. References [1] M. García-Granda, H. Hu. Design of Broadband Electrooptical Modulators Using Ti:LiNbO3 Ridge Waveguides. Spanish Meeting of Optoelectronics, OPTOEL'07, Bilbao, Spain. July 2007. ISBN: 978-84-95809-30-8, pp. 69-74. [2] M. García Granda, H. Hu, W. Sohler, J. Rodríguez García. Novel structures for broadband electrooptic modulators in LiNbO3. Submitted to European Conference on Integrated Optics, ECIO'08, Eindhoven, Netherlands.