Research Of Tunable Optical Filter Based On Micro-cavity

Tunable optical filter is one of the core optical devices, and it is widely used in wavelength division multiplexing system. It can select the frequency that we expect from a broadband optical signal. For this kind of filter, we still more concerned about the static filtering characteristics, tunable filter characteristic and the response of the device.Here, the design of all the devices is based on SOI substrate, thus the device is fully compatible with mature semiconductor process, and can obtain a good performance of integration. Further, for the structure of the device, in either case species of tuning mode, we are using a Fabry-Perot resonant microcavity model for the design and analysis. Such planar waveguide-type microcavity structure can be provided ideal FSR, FWHM, Finesse factor, response time etc. to the device. Besides, it can guarantee the excellent characteristics of frequency-selective, fast response, and large range of tuning.For this microcavity structure based filter, there are two common ways to achieve the mode of tuning, to change the length of the microcavity or to change the refractive index of the cavity. Starting from these two points, respectively, for different tuning modeling analysis, theoretical calculations, optical and thermal field simulation are given. Both theoretically devices complete design process and the simulation results are presented. Wherein the method for changing the refractive index of the cavity is mainly rely on the thermo-optical effect of the material, this method is simple and mature; the method of changing the length of cavity is achieved by a bias voltage, which drives the movable comb electrodes connecting with movable DBRs.First of all, the design for a "silicon/air" gap, silicon F-P cavity of a DBR based thermo-optical tunable optical filter is presented. Here, we use the single mode condition of large-sectional area ridge waveguide to determine the size of the ridge type; the confirmation for the optical simulation;" λ/4" model to determine the size of the device; transfer matrix to calculate the static filtering characteristics of the device. For the device, FSR=66.2nm, FWHM=0.05nm. Finally, by theoretical calculation and ANSYS simulation,8nm tunable filter is achieved by the heat rate of2130μW/μm3. The response time is100μs, and the process tolerance of theoretical analysis is also obtained.Secondly, we use the large thermo-optic coefficient of PUR to replace silicon as the microcavity material to obtain a greater range of thermal optical tuning. Due to the refractive index of the polymer is larger than Si, so the size of the device is not the same. The proposed two polymer microcavity structure models are respectively described by theory analysis, calculation and simulation. Wherein for the "silicon/polymer" gap DBR, polymer F-P cavity model, FSR=35nm, FWHM=0.1nm, which gets a filtering range of30nm. The response time is2.03ms. For model of the "silicon/air" gap DBR, polymer F-P cavity, FSR=36.8nm, FWHM=0.065nm realized1530nm-1565nm entire C-band range of tuning. The response time is about2.1ms, and the production process is also included.Finally, we use electrostatically driven comb electrodes to get a wider tuning by changing the length of F-P cavity. The ridge-type size is H=7μm, h=4μm, W=6μm. For the above design, FSR=197nm, FWHM=0.016nm. When the applied voltage is17.2V,1530nm～1610nm filtering performance can be achieved by design the electrostatic comb and micro spring size and structure.