Low Power Consumption Polymer Thermo-Optic Switch With Mach-Zehnder Interferometer

In ancient China, people transferred information using the beacon towers, which was the earliest application of light in the field of optical communications. In the twenty-first century, human society enters the era of knowledge economy and information age. As the amount of information grows, the communication systems develop continuously along the direction of high-speed, large capacity and low-cost. The optical communication systems whose information carrier is light have many functions, such as the collection, transmission, modulation, storage and display of information. The realization of these functions depends on photonic or photoelectron, so it has been a qualitative leap in the aspects of information processing speed, transmission speed and storage capacity.With the development of photoelectron technology, optical waveguide devices play an increasingly important role in communication systems. Precisely in response to this need, the polymer photoelectron devices have become a hot research topic in recent years. As the polymer materials have the advantages of low cost, simple process, low dielectric constant, small loss and low birefringence, the research of polymer photoelectron devices is gradually increasing and many breakthroughs are achieved in polymer light-emitting devices, electro-optical modulation device and thermo-optic devices. In diversified optical waveguide devices, optical switches are the device which realize turn-on and switching-off in optical networks, whose main functions are routing selection, wavelength selection, optical cross-connect and self-healing protection. They will become the core of all-optical network in the future. Polymer thermo-optic switches are concerned in polymer photoelectron devices due to their features, such as simple process, good switching performance and low driving power.This thesis mainly studies the polymer thermo-optic switch with Mach-Zehnder (M-Z) interferometer, the main contents include the following aspects: 1. The significances of the researches on optical switches and their application in optical communication systems and all-optical network are introduced, and the types and the performance parameters of optical switches are described. The development of polymer thermo-optic switches is briefly expatiated.2. The principle of M-Z thermo-optic switch is expatiated. The types of thermo-optic switches are introduced, the structure and working mechanism of M-Z thermo-optic switch are explained, and the thermo-optic effect of polymer materials is expatiated. The temperature change of waveguide material and the corresponding thermal power are deduced with the analysis of thermo-optic modulation when the phase shift isπ. The basic issues of thermal conductivity are discussed, the heat conduction equation and boundary conditions are deduced, which are based upon the Fourier's law, and the thermodynamics model of the thermo-optic switch is established.3. The production of polymer planar waveguide devices is detailedly described, which is the basic of the preparation of polymers thermo-optic switch. The three following aspects are mainly includes:(1) With the analysis on the properties of polymer materials, the requirements of material on the planar waveguide devices are expatiated, and the copolymer of methyl methacrylate and glycidyl methyl acrylic (PMMA-GMA) is selected as the waveguide cladding material, PMMA-GMA doped with bisphenol A epoxy resin is selected as the core material. The materials suitable for the production of polymer planar optical waveguide and the thermo-optic switch are selected. The chemical structures of the major components of polymer materials are given, the refractive indexes of the material of the waveguide cladding and the core are tested using the ellipsometer, and the optical properties of the material are analyzed.(2) A set of process flow of polymer planar waveguide is designed. The specific processes are detailedly described, including the substrate processing, spin-coating, vacuum evaporation, lithography and reactive ion etching, the choice of processes and the issues on the operation are mainly introduced. The vacuum evaporation, lithography and reactive ion etching are emphasized. The vacuum evaporation and magnetron sputtering are compared, and the reasons of choosing the vacuum evaporation are explained. The evaporation current is 25~30A, the time is 2~3 minutes, and a 30nm aluminum film is deposited. The photoresist materials and their photochemical sensitivities are briefly introduced, the lithography processes are detailedly described, and the operation of the various steps and the possible problems are introduced. The characteristics and principles of the reactive ion etching are expatiated. The etching RF power is 40W, while the reflective power is 0. It is confirmed that the etching rate is about 100nm/min under these conditions. The production steps of polymer planar waveguide are briefly introduced.(3) Several familiar polymer planar waveguide are prepared with the process flow, which are straight waveguide, Y branch waveguide, directional coupler, multi-mode interference coupler and so on. Through the microscope and scanning electron microscopy the structure of waveguide is observed. A fiber-coupled testing system is set up, including coupling part of the sample, observation systems, infrared observation system and the light source. The capabilities of the straight waveguide and the Y branch waveguide are tested, and the output near-field spots are obtained. The structures and principles of the directional coupler and the multi-mode interference coupler are introduced, the surface forms are observed through the microscope, the capabilities of the devices are tested by the fiber-coupled testing system, and the performance of the devices are analyzed.4. The fabrication and testing of polymer thermo-optic switch with Mach-Zehnder interferometer are introduced, which is the core of this thesis. The main work is:(1) The structure of M-Z waveguide is described, the fabrication methods of polymer M-Z thermo-optic switch is introduced. The surface of the waveguide after reactive ion etching is observed through scanning electron microscopy, the performances of the M-Z waveguide are tested in the fiber-coupled testing system and the output near-field spots are obtained. The M-Z waveguide has a good performance and could work under a low input optical power. The heating electrode is produced on the waveguide with vacuum evaporation and lithography, and aluminum material is selected as the electrode material for the requirements of the device. The structure of the electrode is observed through the microscope, the edges of the electrode are smooth. The electrode has good conductive properties. A M-Z thermo-optic switch is fabricated through the above methods, which is 3.5cm long, 3cm wide.(2) The main performance parameters of the thermo-optic switch are introduced, and the main parameters of the device are tested. A testing system of thermo-optic switch is set up, including light source, coupling part of the sample, observation systems, detecting signal system and signal source.(3) A square wave signal with a DC bias is applied through the electrode, and then the switching property of the device is tested. The rise time of the thermo-optic switch is 1.2ms, the fall time is 0.8ms, and so the switching time is 2ms.(4) The output spectrum of the device is tested around 1550nm, the output spectrums are compared between the largest and smallest output optical powers. A DC signal is applied directly through the electrode, and the thermo-optic switch has the minimum output light intensity when the voltage is 3.9V and the current flowing through the electrode is 4.2mA, so the driving power is 16mW. It is confirmed that the power consumption of the thermo-optic switch is low. The reasons of lower power consumption are analyzed. The extinction ratio of the thermo-optic switch is 15dB, which is measured using optical power meter.Finally, the work of this thesis is summarized, and the present problems and the future directions are described.