Application Research Of Photonic Crystal In OCDMA System

Photonic crystal is called the semiconductor in photonics. Photonic crystal has a widespread application in optical communication for its photonic band gap (PBG) and unique characteristics of controlling the movements of photons. And it is considered as the most competitive material to implement the integrated optical circuit. Based on the band theory, coupled mode theory and defect mode, photonic crystal supplies with a great research platform for optical information processing technique, such as photonic crystal fiber with no cut-off characteristics, photonic crystal cavity for enhancing or inhibitting spontaneous emission, photonic crystal filter, wavelength division multiplexer, and optical buffer based on slow light effects. One of the reasons Optical Code Division Multiple Access (OCDMA), as a main multiplexing technology, is not yet commercialized is that there is no breakthrough progress in encoder/decoder. Now, a main encoding scheme is a structure with Fiber Bragg Gating (FBG) based on one-dimensional photonic crystal and fiber delay line (FDL), since its wavelength selection and time delay. While in microstructure, it is still complicated, hard to tunable and not suitable for integration. Two-dimensional photonic crystal can meet the needs of encoding and decoding in OCDMA system, as the characteristics of time delay and wavelength selection. So it provides new research thoughts and method for OCDMA system.In the thesis, the main work is the research of the application of photonic crystal in OCDMA system, especially in the key device-encoder/decoder. Based on the band theory of two-dimensional photonic, defect-mode and coupled-mode theory is investigated via the plane wave expansion method (PWE) and the time domain finite difference method (FDTD). Also, simulation for the photonic crystal waveguide separately based on line defect and coupled cavities is performed. Utilization of the characteristics of frequency selection and slow light of photonic crystal waveguide and filter, a new device-encoder/decoder for 2D-OCDMA system with multi-channel filters and waveguide in structure is designed. As well, the characteristics of the encoder/decoder are performed by simulation. In the structure, user signals are cut into more optical pulses by the filtering action of defect-cavities as the signature codes requested, and this is frequency coding. Meanwhile, to encode in time domain, the distance between cavities work as fiber delay line(FDL) because of the characteristics of slow light in photonic crystal waveguide. And the time delay comes to nanometer magnitude via the micro-scale. In the thesis, a signature code,{(6,5),(11,6),(2,8)}, in a two-dimensional optical orthogonal code(OOC), (19×19,3,1,1), designed by our own laboratory, is considered as the encoding scheme. According to the signature code, the two-dimensional encoder/decoder based on photonic crystal waveguide is designed. And then, the encoding and decoding characteristics of the structure designed are performed by simulation that is two user 2.5Gbps OCDMA transmission system. Encoders of two users are photonic crystal waveguide and Fiber Bragg Grating (FBG) separately. The results shows that the characteristics of waveguide encoders perform as well as FBG encoder, and the user signals are well decoded by decoder designed with correlation matching.