Design On Tunable Optical Filters Based On One-dimensional Photonic Crystal

The application of WDM technology have improved the capacity ofoptical communication, it also put forward higher performancerequirements to the optical network monitoring module at the same time.Having low absorption and high efficient narrow-band filteringcharacteristics becomes the inevitable requirement for the filter, the keyparts of optical network monitoring module and is used as the separatingwavelength. Tunable photonic crystal filter can effectively improve thedetection accuracy and reduce the size of monitoring modules and theloss of light to become the best choice in all optical filters.Existingtunable photonic crystal filters is mainly utilizing magneto optical,electro-optic, thermo-optic and photo-refractive effect such as to tunedthe frequency of defect modes in photonic band gap to achieve thepurpose of tuned filter. But there are some problems like small tuning range, poor tuning control, and additional calibration or adjustment layer,which brought trouble on the preparation.Based on the limitations of existing tunable photonic crystal filters,this paper introduced an air cavity in the photonic crystals, by changingthe cavity structure and light incident angle to realize efficient filter.Specific research contents and results are summarized as follows:(1) Based on the plane wave expansion method and transfer matrixmethod, this paper design and simulate the bandgap properties of thestructure (A/B)n/D/(A/B)n. By changing the defect cavity we get anone-dimensional photonic crystal filter with adjustable thickness of airlayer, and the influence of air thickness on the filtering properties iscalculated using MATLAB in the visible and near infrared band. Theorderly increase of air thickness d3from0.2a to0.2a can realize thetuning range of220nm for single transmission peak in the visible light; Abimodal are appeared in forbidden band when d3is1.7a, and the bandgap range is also significantly broaden. That is to say the number ofconduction band and the width of forbidden band can be controlled by airthickness. Under the condition of considering the dispersion, it canachieve continuous tunable filter in all near infrared communication Cband under arbitrary values n when air thickness increased from372.3nmto394.3nm. The relationship between the change in thickness and themove of peak wavelength is linear, meanwhile, FWHM keep in constant in the whole process of the tuning, which show good filteringperformance.(2) Due to different incidence angle causes different equivalentthickness of dielectric layer, we design an one-dimensional photoniccrystal filter with adjustable incident angle and simulates the influence ofangles on filtering properties. As for visible light band, with the increaseof incident angle, the forbidden band of TM mode will not cover, whileTE mode always contain the whole visible light and achieve the widerange filter of212nm. As for near infrared band, the photonic band gapof both two modes of the designed filter can always cover the wholecommunication band when the incident angle less than80°. Comparedwith TM mode, the move of the forbidden band of TE mode is notsensitive for the incident angle. The move of the band gap width for TMmode is strongly influenced by the changes on the angle of incidence, butthe transmission peak is on the contrary slightly, the moving range oftransmission peak is wider for TE mode than TM mode; The relationshipbetween the two modes is also studied under the same incident angle, it isfound that lower number cycle photonic crystal structure and smallerincident angle is needed for ordinary light to achieve angle tunable filter.Angle tunable filter is more suitable for polarization light.(3) Analyzing the relationship between thickness error of(CaF2/Si)n/Air/(CaF2/Si)nand transmission intensity of corresponding defect mode we can know that: as the cycle number n increased from4to6, the distribution of high transmitted intensity is become narrow, whichshow that the greater periodicity of photonic crystal, the more difficult onthe preparation. But in the same period, the thickness error of onematerial can be made up for by adjusting the thickness of another materialthickness to reduce the difficulty of the preparation.