Study On Characteristics Of Photonic Crystal Fibers And Its Application In Optical Filter

In this dissertation, We use a full-vector finite-element method to theoretically study birefringence and chromatic dispersion characteristic of photonic crystal fiber, and analyse the change of birefringence and chromatic dispersion with the hole pitch and hole shape. With this, we design a PCF with birefringence approximatively square to the operation wavelength. And then build up a equal channel spacing Sagnac filter based on this PCF.The details are described as follows:First, a kind of Hi-Bi PCF with in-cladding elliptical hole is proposed. The phase birefringence, group birefringence and chromatic dispersion characteristic in such a PCF are studied using a full-vector finite-element method. Numerical result shows that its phase birefringence with magnitude of order of 10-3 and the negligible group birefringence near 1550nm can be realized. Moreover, a high phase birefringence and low group birefringence PCF with more than 400nm flattened dispersion bandwidth in the near 1550nm communications window is numerically demonstrated.Second, phase birefringence and chromatic dispersion of high birefringent photonic crystal fiber with introducing four small air holes near the core into the cladding is investigated by using a full vector finite-element method. Numerical result shows that its phase birefringence can be of magnitude of order of 10-3 at long wavelength span. Base on which, we fit the curves of different structural parameters PCF's birefringence as a function of wavelength using quadratic polynomial, in order to find the best structural parameters in quadratic polynomial. We finally aquire the PCF, the absolute value of first-order term of quadratic polynomial is smaller than the second-order term by a seven-order of magnitude, which indicates that designed PCF with birefringence approximatively square to the wavelength; Moreover, By optimizing its structural parameters, a PCF with near zero and broad-band flat dispersion over 780 nm wavelength range can be designed, of which the absolute value of the dispersion coefficient change|△＜|±D2.5ps/km/nm.Third, phase birefringence and group birefringence of elliptical-hole cladding PCF with introducing four small air holes near the core is investigated by using a full vector finite-element method. Numerical result shows that the birefringence of this PCF is induced by factors of elliptical air hole cladding and introducing four small air holes near the core together; Following, we move the location of the innermost cladding air holes to the inside and outside by 5%∧, and make a numerical simulation of the model after location fluctuation of the air hole.Based on the simulation results, we can deduce that the the inevitable location fluctuation of the air hole in the the production process will not affect the characteristics of the fiber birefringence.Fourth, the paper presents a method to design a optical filter based on Sagnac interferometer with equal channel spacing.The filter is achieved by designning a photonic crystal fiber which phase birefringence is square to wavelength. We have made a numerical simulation of the Sagnac filter. the channel spacing of the proposed filter only changes by 0.03nm, about one order of magnitude less than those constructed with conventional high birefringent fibers (HBFs),basically realized the equal channel spacing Sagnac filter.