Study Of Polymer Optical Switches Based On PMMA

Fiber has been widely used in modern communication system for the merits of low transition loss and wide band. Since optical networks have huge capacity, good wavelength transparency, compatibility and expansibility, it has become the best choice for high speed communication system. As key parts of optical networks, optical switches are widely used in wavelength-division multiplexing (WDM) system. Polymer could be applied to the fabrications of arrayed waveguide gate, optical splitter, variable optical attenuator, modulator, micro-ring resonance, and other photonics devices. Especially in the applications of thermal optical switches and high-speed electro-optic (EO) switches, polymers show unique advantages and have become the research focus of many researchers.In the middle of 90's last century, great progress has been made for poled polymers devices in optical performances and structures. Recently, the developing directions of polymer optical switches are high-speed, low loss, low switching voltage and low cross talk. In 2008, a 2×2 high-speed electro-optic switch based on multimode interference coupler was demonstrated by Fuji Xerox Inc. of Japan, switching time of which is smaller than 6ns. Domestic research in this area started late in 1980's, research institutions, such as, Zhejiang University, Tsinghua University, Shanghai Jiaotong University and other research institutes began to carry out research in this field and has made some progress. However, there is still a wide gap when compared with foreign countries. Taking into account the good application prospects of polymer-optical switches in high-speed optical communication, we did related research work with the aid of National Key Basic Research Program. We synthesized organic/inorganic hybrid electro-optic material by use of sol-gel method. According to the device structures, side-chain non-linear materials of DR1/PMMA and hybrid materials DR1/TiO 2 /SiO2 are used in the electro-optical switch fabrication of strip-loading and ridge waveguide. A detailed study of the material synthesis, characterization of polarization, switching waveguides and electrodes design, devices preparation process and parametric test are introduced in this thesis. With the development of communication system, there are urgent needs for simple structure, high-performance, low-power, low-cost solutions in automatic protection in optical network switching, IP routing, optical fiber communications device testing, trunk and optical cross connect(OXC) and other less demanding applications on the speed. Polymer thermo-optic (TO) switches just could meet the above requirements. Much work has been carried out by NTT of Japan, HHI of Germany, the Akzo of Netherland, IBM of the United States, many other world famous companies and high-tech research institutes. A variety of interference type and numerical structure thermo-optic switches and switch arrays has developed. Therefore, we studied on thermo-optic switches based on epoxy polymer PMMA material. Firstly, the software Rsoft is used to design waveguide switches in simulation, and then Si substrate with thermal oxidation growth SiO2 film is chosen as the lower cladding, methyl methacrylate-glycidyl with epoxy resin is selected for the waveguide core layer, using photolithography, plasma etching and other semiconductor process, a 2×2 multimode interference thermo-optic switch is fabricated. The switching power, crosstalk, extinction ratio and other parameters were tested. The main content and innovation points are as follows:In Chapter I, it briefly describes the important applications of optical switch in the optical fiber communication network, which illustrates the needs to carry out the research work. Materials and structures of optical switches based on different working principles are discussed in detail. Comparing the characteristics of polymers and inorganic materials, it points out the advantages of polymer optical switch. Development and research status of polymer EO/TO switches are reviewed.In Chapter II, it describes the working principle of the electro-optic switch, analyzes the characteristics of organic/inorganic material DR1/TiO2 /SiO2 . Strip-loading and quasi-rectangular waveguide based on the above EO material and ridge waveguide based on DR1/PMMA material are analyzed according to planar optical waveguide theory, respectively. Through software Rsoft, the output lights of waveguides are simulated.In Chapter III, on the basis of non-linear mechanism of electro-optic materials, sol-gel method is adopted in the preparation of organic/inorganic hybrid material DR1/TiO2 /SiO2 . A detailed analysis of the reaction affected by the composite change of ethanol, hydrochloric acid, water and temperature is carried out. Refractive index of composite materials is found to be changed in the range 1.48~1.66 measured by ellipsometer, which facilitate the design of the waveguide structure. By using of atomic force microscopy, the film-forming properties are characterized. Nonlinearity generated by contact poling and corona poling is studied, too. Electro-optic coefficientsγ3 3 of DR1/TiO2 / SiO2 , DR1/PMMA films are measured by use of reflection method. It is about 20pm/V at the wavelength of 1310nm and has a good uniformity and stability of polarization for DR1/TiO2 /SiO2 . According to the characteristics of different electro-optic materials, rectangle, strip-loading and quasi-rectangular waveguides are designed and fabricated. Through the fiber-coupling test system, the output light of the waveguide is observed. Optical and electrical properties of directional coupler and multimode interference 2×2 electro-optic switches are tested, and switching time, insertion loss, crosstalk and other related parameters are obtained. According to microwave theory, the coplanar waveguide (CPW) and micro-strip line (MSL) switching electrodes are designed. Coupling between the electric field and optical field generated by different electrodes is compared. Microwave transmission characteristics of metal electrodes are characterized with vector network analyzer, results of which show that the 3dB bandwidth of electrodes on Si substrate is greater than 1.5GHz. It fully meets the design requirements.In Chapter IV, related theory on the silicon-based polymer thermo-optic switch is introduced, in particular the part of multi-mode interference coupler design. Because poly-methyl methacrylate (PMMA) has merits of good adaptability for fabrication and low-cost, it is adopted in the design of 2×2 multimode interference coupler switch, which selects Si as substrates and SiO2 as lower cladding. Through Rsoft thermal analysis software modules to simulate the waveguide thermo-optic effect, the crosstalk in the design at cross state and bar state is found to be -24.2dB and -21.9dB, respectively. Extinction ratio at cross state and bar state is 29.8dB and 23dB, respectively. The switching power is about 3.6mW.In Chapter V, thermo-optic characteristics of the PMMA material are described. It explains the reason why selecting PMMA material to fabricate polymer thermo-optic switches from the point of view of thermo-optic coefficient stability. A 1×1 M-Z thermo-optic switch with PMMA as the main material is fabricated by planar microelectronics techniques of spin-coating, photolithography, plasma etching and evaporation, which substrate is Si. The 2×2 multimode interference coupler thermo-optic switch with SiO2 as the lower cladding is fabricated too. Characteristics of the device, such as switching power, switching speed, insertion loss, extinction ratio, crosstalk, are obtained. Test results show that the rise time and fall time of M-Z thermo-optic switch are both about 0.4ms. The switching power is about 15.1mW and the extinction ratio is 16.5dB. Rise time and fall time of the 2×2 multimode interference thermo-optic switch are 0.5ms, respectively. Fiber to fiber insertion loss is about 17dB, switching power is about 45mW. Crosstalk in the cross state and bar state is -19dB and -21dB, respectively, and the extinction ratio is 27dB and 19dB at each state. The further improvements of the device indicators are proposed too. The above work built a solid foundation for the realization of planar optical waveguide devices integration.The innovation points are:(1) Organic/inorganic EO material of DR1/TiO2/SiO2 is synthesized by sol-gel method, analyzed effects of different reaction conditions to the material properties. According to material characteristics, a strip-loading waveguide electro-optic switch with PMMA as the guide layer is fabricated by standard semiconductor planar fabrication techniques. Electrical and optical properties of the device are tested through the system built by ourselves.(2) According to the advantage of possessing large thermo-optic coefficient, a 2×2 multimode interference thermo-optic switch with SiO2 lower cladding and P(MMA-GMA) waveguide material is designed and fabricated. The structure is simple and switching performance is stable.