Fiber Gratings And Parameters Reconstruction

Fiber gratings are kinds of new photonic devices. Not only fabrication methods but also concerned theories and applications about fiber grating have been made great progress, from successful fabrication of the first fiber grating to now.Fiber gratings, which are one of passive photonic devices written by ultraviolet light into the core of a photosensitive fiber, have several distinguished advantages, such as: low insertion loss, small bulk, potentially low cost, insensitive to polarization, and simply spliced into fiber systems. Desired spectral properties or time-delay characteristics can be achieved by changing physical parameters of fiber gratings. With its improvement in fabricating technology, fiber gratings have become one of the most promising and representative passive optical fiber devices, which have wide applications in optical fiber communications and optical fiber sensing fields. Recently, fiber gratings are used as time-delay devices in optically controlled phased array antennas. Therefore, it is very important to design fiber gratings that meet specific time-delay characteristics from the viewpoint of practical applications.In this paper, we firstly introduce the development, photosensitivity, classifying and fabrication of fiber gratings, and their applications in optical fiber communications, fiber sensor and optically controlled phased array antennas. Then, beginning with the Maxwell equations, we derive the coupled mode equations that describe the interaction between the modes propagating in fibers. Furthermore, we analyze uniform fiber Bragg grating by the coupled mode theory and give the analytic solutions by the coupled mode equations. Transfer matrix theory is also briefly introduced, which is used to analyze non-uniform fiber gratings.Secondly, based on the coupled mode theory and the transfer matrix method, the optical properties of uniform fiber gratings, chirped grating and apodized grating are simulated with Matlab programming. The influences of various parameters of fiber grating such as length, period, chirp, the refractive index modulation and apodization function on its reflectance spectral width, maximum reflectivity and time-delay characteristics are studied. These are very useful to design fiber grating devices.Finally, a new method of parameter reconstruction for the fiber gratings from their reflectance spectrum or time-delay characteristics is presented by the genetic algorithm. The objective function and the individuals of population in this method are reflectance spectrum or time-delay characteristics and the parameters of fiber gratings, respectively. The optimal solution that corresponds to the objective function is obtained after many generations calculation.