Study Of Polymer Electro-Optic Waveguide Modulators

Because polymer electro-optic waveguide modulator has lots of excellent performances and its potential large applications, it was given lots of interest since 90s on last century. So far the studies of polymer modulators have been made great progress because of the developments of researches of materials, for instance, the polymer modulator with half-wave voltage of 0.8 V and with bandwidth of 150 GHz have been reported. However, the polymer modulators are not realized because of the problems of stability and high loss. So far as the problem of high loss, it can be solved by chemosynthesis of the material with low loss by molecular design, but this is very difficult to find a material satisfied the requirement of the waveguide. In this thesis, we try to avoid the way and wish reducing the loss of device by designing the structure of modulators. So the author putted forward a concept of modulation in claddings, i.e. the electro-optic polymer with high loss was used in the cladding and the organic or inorganic material with low loss and nonelectro-optic activity was used in the core. The microwave field couples with the evanescent waveguide mode in the cladding by the electro-optic effect and at the same time loads the energy of the signal onto the light carrier. Then the modulation is fulfilled. It can be anticipated that the loss from the material will reduce due to weak power of the guide mode in claddings in the case of modulation in claddings. The result showed the loss is only about 1/3 of its counterpart. Also, optimizing design and analysis showed that the decrease of modulation efficiency due to modulating in claddings can be compensated by appropriately reducing the size of cores. In the thesis it is also illustrated that the way of modulation in claddings helps to decrease the half-wave voltage of modulators. In the thesis the formulas of the modulator's half-wave voltage and 3-dB optical bandwidth have been deduced. These formulas reflect the effects of electrode's conductor loss, impedance mismatching and phase velocity mismatching between microwave signal and light carrier on half-wave voltage and bandwidth. Thus they are better than those formulas given in literatures. In the text the extents of effects of these factors on half-wave voltage and bandwidth are also discussed. So the formulas deduced in the text offer the basis of precise design of devices and quantitative discussion about the effects of conductor loss, velocity mismatching and impedance mismatching on performance of the device. Because the technology of information has been developed at very fast speed and people pursue unendingly bandwidth of communication and also the bottleneck of the system of optical fiber communication in fact lies in electro-optic modulation, the demand of modulator's bandwidth becomes more and more imperious. Considering the fact the author designed a kind of new type electro-optic polymer waveguide modulator with ultrahigh bandwidth. Its design of electrodes integrated embedded microstrip line (i.e. the upper strip of microstrip line is embedded in claddings) and trapezoidal or T type structure. The former can offer perfect velocity matching between microwave and light carrier and the latter has potency in reducing conductor loss. So combination of them not only fulfills the perfect velocity matching but also reduces efficiently conductor loss, which causes the bandwidth being increased greatly. Numeric result showed that the modulation bandwidth of embedded trapezoid or T type microstrip line increased 2~3 times. Considering the fact that the modulator is often sensitive to polarization of modes, which will limit the application to optical fiber communication, a kind of electro-optic polymer modulator with polarization insensitive modulation basing on TE/TM mode converter was putted forwarded. It make up of two microstrip lines linked with TE/TM mode converter. The modes modulated in this modulator always include TE and TM polarization whether the TE or TM mode inputted because of the conversation of polarization mode. This makes the polarization insensitive modulation become possible. In the text this polarization insensitivity was demonstrated and at the same time the effects of mode conversion efficiency on modulator performance, such as extinction ratio and modulation depth of small signal, are analyzed. Results showed that the extinction ratio of the device was deteriorated because of incomplete conversation of modes. It requires the conversion efficiency of TE/TM mode converter is at least larger than 95% in order to achieve extinction ratio of exceeding 20 dB.