Ruitao Zheng

Electrically tunable dispersion compensator with fixed center wavelength using fiber Bragg grating

We present the design and development of a novel tunable dispersion compensator with fixed center wavelength that is based on the electrical adjustment of the chirp of a fiber Bragg grating (FBG). Both temperature gradient and strain gradient are employed to adjust the chirp of the FBG jointly. The electrical current flowing through the taper on-fiber thin-film heater will introduce a temperature gradient on the FBG. The shrinkage of a negative thermal expansion coefficient (NTEC) ceramic due to the temperature rise will compress the tapered FBG mounted inside it, and this will introduce a strain gradient on the FBG. The center wavelength of the FBG will be kept fixed because the effect of temperature rise on the FBG and the effect of compression of the FBG will offset each other. Applying an electrical power of less than 0.68 W, we demonstrate a linearly chirped FBG whose dispersion can be continuously adjusted from -178 ps/nm to -302 ps/nm with a central wavelength shift of as small as 0.16 nm.

Optimization technique for simple reconstruction of the index modulation profile of symmetric fiber Bragg gratings from their reflective spectrum

We propose a new and simple method to reconstruct the refractive-index modulation of a symmetric fiber Bragg grating (FBG) from its magnitude reflection spectrum (i.e., phase information is not required). The reconstruction method uses an FBG model based on some known parameters of the fabricated FBG (grating length, number of subgratings, and grating period) and an optimization technique, namely, the Quasi-Newton method, to find the local parameter values of the FBG. To our knowledge, this is the first time that the reconstruction of FBG is experimentally realized by an optimization technique. Compared with other reconstruction methods, the proposed method is so simple that it requires only the reflection spectrum of the fabricated FBG.

Hybrid Tabu Algorithm for the Synthesis and Fabrication of Fiber Bragg Gratings

Fiber Bragg gratings (FBGs) are widely employed as optical filters for performing various functions such as add/drop multiplexers, dispersion compensators and multiplexers/demultiplexers for use in optical communication systems and optical sensors because of a number of advantages that include low insertion loss, low polarization sensitivity, all-fiber geometry, compactness, easy fabrication and low cost (Kashyap, 1999; Othonos & Kalli, 1999). In addition, the technology of ultraviolet (UV) photoinduced FBGs is quite mature to allow the fabrication of a wide variety of FBGs with complex characteristics. To meet the increasing demand for large capacity of the next generation of optical communication systems (i.e., wavelength division multiplexing (WDM) networks) and optical sensors, there is an important need for a powerful design tool that can be used for the synthesis or design of FBGs from the specified frequency responses that can be practically realized. This synthesis or inverse problem of determining a FBG structure from a given frequency response (i.e., magnitude and phase responses) is common in many application areas. The design tool must be efficient and reliable to enable the synthesis of FBG-based filters with prescribed frequency responses, depending on the application requirements. That is, the design tool must be able to determine the index modulation profile and hence the structure of an FBG from a given frequency response. In addition, the design tool must be powerful enough for use in the diagnosis or characterization during and after fabrication of an FBG. Although several synthesis methods such as those based on the layer-peeling algorithm have been proposed for the synthesis of FBGs from the specified frequency responses, the index modulation profiles of the synthesized FBGs are not optimized and are often complex, making practical realization difficult (Feced et al., 1999; Poladian, 2000; Skaar et al., 2001; Rosenthal & Horowitz, 2003). To overcome this problem, optimization methods have been demonstrated as an attractive approach because it allows weighting mechanisms to be incorporated into the desired frequency response of an FBGbased filter to be synthesized, resulting in an optimum and practically realizable index modulation profile of the FBG structure. Furthermore, the optimization algorithms also allow additional constraints to be included in the weighting mechanisms of the specified frequency response to suit certain condition(s) or constraint(s) of a particular fabrication system. In optimization, the FBG synthesis problems are formulated as nonlinear objective

Tabu search algorithm to optimize the design of fiber Bragg gratings

A Tabu Search (TS) algorithm, to our knowledge, for the first time, is introduced into the optimized design of Fiber Bragg gratings (FBGs). By combining the Transfer Matrix Method (TMM) for calculating the reflection spectrum and the TS algorithm, we obtained a new method for synthesis the FBGs with advanced characteristics. A new appodization profile is proposed as an example to demonstrate the effectiveness of the method that is general to be useful for inverse problems in FBGs application.