Study On Microwave Electrode Of Polymer Electro-optic Modulator

With the requirement for high speed and high bandwidth communication system in modern society, fiber communication system is developing very fast. The electro-optic modulators are very important equipments for the fiber communication system, and the wide bandwidth of fiber can not be used without the modulators. So the research of high-speed electro-optic modulators is paid more and more attentions. All kinds of materials are used to produce good electro-optic modulators, such as LiNbO3,semiconductor and polymer. Meanwhile the research of modulator structure design,waveguide design and microwave electrode is done for different materials. The polymer materials are paid more attentions because of its advantagesthan other materials. The advantages of polymer materials are as follows:(1) low microwave dielectric constant(2) high EO coefficient(3) good methods of fabrication(4) low prices of the materialsAmong all kinds of polymer modulators, M-Z polymer modulators are widely used, because the structure is easy in use and fabrication, and we also choose this structure for our EO modulaors. The structure of M-Z polymer EO modulator consists of waveguide and electrode. The features of a EO modulator is decided by its electrode system when the part of waveguide has been made. There are three regions in a electrode system. Every region has different structures which influence several features of a modulator. But the design of interaction region is the most important thing. Once the structure of this region is decided, other regions can be calculated by the software to produce a complete modulator. So we have to know the features of electrode system to make good design.We need to calculate the features, such as Vπ,Nm,BW,Zm according to the width,thickness and electrodes gap of electrode to design the electrode system, and all the calculations are the fundamental of electrode design.Take the design of CPW electrode system for our waveguide structure forexample, we got the following conclusions:1. Vπwill increase when G increases, W doesn't influence Vπ.2. Nm is mainly influenced by the dielectric constant.3. We can match Zm by changing G and W.4. BW is limited by No-Nm.5.α0 will decrease when G or W increases, but it will slow down when G or W is bigger than certain value.6. We can minimum Pdri by changing W.Through calculation and analysis, we found that we can not get good features with substrate Si when we design electrode system. This is because the Si substrate make No and Nm mismatch greatly. So we can get good features by changing the substrate material. But Pdri will increase when we change the substrate, and this condition can be improved by changing the size of the modulator.α0 will decrease when the dielectric constant of substrate decrease.Through testing the electrode samples with the same structure but different substrates, we found that the propagation losses of Si samples are very high. So the bandwidth of Si sample is much lower than 20GHz.