Research On Polymer/silicon Thermo-optic Switches And Their Reliability

The fiber optical communication technology is developing rapidly, and the communication field is undergoing a lot of great changes. The optic switch with high-capacity, high-speed, low loss and high reliability is more and more important in communication technology. The optic switch is the core element of optical add-drop multiplexing and optical cross connect and play an important role in automatic protection switching, batch testing optical device, on-line monitoring, and so on.The thermo-optic switch is of extreme interest because of its small dimension, flexibility, good stability, etc. Its characteristic parameters(crosstalk, insert ion loss, switching time, etc.) can meet requirements in many application fields and attract a wide spread attention. In this paper, the thermo-optic switches are designed and manufactured, and their characteristics are researched. The main work of this paper is as follows:1. The transmission characteristics of light in the waveguide is researched using the electromagnetic theory. Starting from Maxwell’s equations, the wave equation of light and lateral Helmholtz equation are deduced. The modes of slabs waveguide, rectangular waveguide and rib waveguide are analyzed. The characteristic equations of TE guided mode and TM guided mode are deduced and the effective refractive indexes are obtained. The thermo-optic effect is introduced and the polymer material is considered as the ideal material that is used to make the thermo-optic devices because of its larger thermo-optic coefficient and lower thermal conductivity. The general form of the heat diffuse equation and the stable heat conduction equation are obtained by solving Fourier equation. Three boundary conditions including the surface of the fixed temperature, the surface of the fixed heat flux and convection boundary condition are proposed.The working principle of MZI thermo-optic switches is introduced and the expression of output intensity is elicited.2. The MZI thermo-optic switches are designed and prepared. With the performance advantages of larger thermo-optic coefficient, high transparency, better mechanical property and good thermal stability, the SU-8 photoresist is chosen as the waveguide core layer material. The PMMA material has lower cost and better dielectric property and it is easy to be synthesized independently and form better thin film. At the same time, the thickness of the thin film and the refractive index are easy to be adjusted and its refractive index matches that of SU-8 material, so it is chosen as the waveguide cladding layer material. On the basis of the boundary conditions and the initial conditions, the heat conduction equation is solved. According to the thermal field distribution, the design philosophy is presented and it shows that a narrow heating electrode is in favour of the heating efficiency. The rectangular polymer/silica hybrid waveguide thermo-optic switch is designed and the light field and the thermal field are analyzed. Meanwhile, the best structure parameters are provided to decrease the switching power and pick up response speed and the technology process is introduced. The same work is done to the ridge waveguide thermo-optic switch and it shows that the PMMA under cladding layer is beneficial to the switching power and the SiO2 under cladding layer is beneficial to the response speed.3. According to the contradiction between the switching power and the response speed using the PMMA under cladding layer and the SiO2 under cladding layer, the thermo-optic switch with polymer/silica hybrid and air trench waveguide structures is proposed. The thermo-optic switch is designed and compared with the thermo-optic switch mentioned above, and the results show that the thermo-optic switch with air trench makes up for the heat loss because of the SiO 2 under cladding. The thermo-optic switch is fabricated and measured. The fiber-to- fiber insertion loss is about 9.6d B and the extinction ratio is 29.6dB. The switching power is 3.4mW, which is much lower than that of the switch without air trench structure. Moreover, the rise time is 183.1μs and the fall time is 139.9μs. The surface thermal field distribution is tested and the result shows the switch with air trench structure can prevent heat dissipation effectively.4. The effects of the working temperature to the thermo-optic switch are tested,which shows that the temperature has no effect on the rise time, the extinction ratio and the resistance of Al electrode, and the temperature has effect on the fall time, the switching power and the insertion loss at higher temperature because the heater is right below the sample, which prevents the heat loss. After continuously s witching fifteen million times, the switch can work steadily. No cracks are found and the insertion loss has no change when the temperature varied from 25℃ to 85℃ for ten heating cycles. Moreover, the polarization dependent loss and the long-term stable experiment are tested. It turned out that the thermo-optic switch has better stability and reliability.