Research On The Characteristics Of DWDM Fiber Grating Filter With Quasi-periodic Structure

DWDM optical filter is one of the most important devices in opticalcommunication systems. With the increasing bit rates in the opticaltransmission network, the tolerance of ONSR, thermal stability anddispersion of DWDM filter is becoming more and more strictly. The supernarrowband DWDM filter with high quality thereby attracted a lot ofattention in recent years.Since Kohmotot designed the Fibonacci dielectric multi-layers anddemonstrated the six cycled and self similarity properties of thedevice, the optical characteristics of Fibonacci quasi-periodicstructure has been intensively studied. It can be seen from the opticalspectrum of the device that there exists a “pseudo band gap” whichis similar to the energy band gap of optical crystals. It has beenobserved that the time delay near the edge of the “pseudo band gap”was3times greater than that of optical crystals.In this paper, the superstructure fiber grating with quasi-periodicstructures was proposed. The sub Bragg gratings and the phase shiftsof the fiber grating were arranged according to the general Fibonacciclass sequences and the optical spectra and the chromatic dispersionof the proposed device were investigated. The main contents of the thesis are as follows:1. The FC3（n）、FC4（n） and FC5（n） quasi-periodic superstructure fibergratings were proposed. First, the2×2transmission matrices forsuperstructure fiber gratings were deduced by using the coupled modetheory with the proper boundary conditions. Then the fiber gratingswith quasi-periodic structure were designed according to thegeneration rules of generalized Fibonacci class sequences: B�'Bn-1A，A�'Bn-1AB by setting①sub fiber grating as A, π/2phase shift as B;②π/2phase shift as A, sub fiber grating as B.2. The reflective spectra of the proposed FCj（n） quasi-periodic fibergratings with different j and n were simulated by using transfer matrixmethod. The results show that the specific FC3（n）、FC4（n） superstructurefiber gratings had a comb like reflective spectra and could be usedas DWDM filters. The relation between the number j, n and the channelspace, the number of channel and the flatness of the channel top wasalso analyzed.3. The time delay and the chromatic dispersion of25GHz FC₄（42） quasiperiodic fiber grating with39channels were calculated and the resultswere compared with those of Bragg fiber grating and sampled fiber Bragggrating. It was demonstrated that the time delay and dispersioncoefficient of the designed device were2-3orders higher than thatof Bragg fiber gratings and sampled fiber Bragg grating and can be widely used for the compensation of chromatic dispersion.