1564 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 11, NOVEMBER 2003 Dispersion-Order Selectable Chromatic Dispersion Compensator Using Strain-Profile Modification Blocks Jaejoong Kwon and Byoungho Lee, Senior Member, IEEE Abstract—A new method for tuning the dispersion profile of a fiber grating is described. The method involves compressing the fiber grating attached to a structure in which the cross-sectional area varies with its position. Because the cross-sectional area de- fines the compression rate, the local Bragg reflection wavelength that determines the dispersion profile varies along the position. A structure in which the cross-sectional area can be simply altered by inserting–extracting a part of structure was designed. Therefore, the dispersion order of the fiber grating attached to the structure can be selected. Using the method, a linearly–quadratically chirped fiber grating was made and various group delay slopes depending on the applied stress were obtained. Index Terms—Gratings, optical fiber communication, optical fiber devices, optical fiber dispersion. I. INTRODUCTION W HEN progressing from 2.5 to 10 Gb/s systems, chro- matic dispersion is one of the most serious problems that limit the longest transmission distance. While dispersion limits a 2.5-Gb/s channel to roughly 900 km, a 10-Gb/s channel would be limited to approximately 60 km. To operate beyond this distance, a method for dispersion compensation must be em- ployed. Ideally, the dispersion compensator should be tunable because the optical path lengths will vary depending on variable transmission routes and environmental effects such as temper- ature variations. Among the various possible dispersion com- pensators, chirped fiber Bragg gratings (CFBGs) have emerged as a potentially powerful technology because of their poten- tial for low loss, small footprint, low optical nonlinearity, and the simple tuning mechanism involved. Grating tuning mech- anisms are largely based on a strain gradient or a temperature gradient [1]–[4]. In the case of the strain gradient-based method [4], the shape of the deflecting beam defines the strain gradient. Therefore, once the beam shape is selected, although the dis- persion value can be varied, the order of the dispersion slope is fixed. Therefore, if the compensator is designed for linear dis- persion, it is difficult to recover nonlinearly dispersed signals. Temperature gradient-based tuning mechanisms also experience the same problem. We previously introduced a tuning method using a tapered elastic plate that induces linear–nonlinear chirp Manuscript received March 18, 2003; revised June 27, 2003. This work was supported in part by Novera Optics, Palo Alto, CA. The authors are with the School of Electrical Engineering, Seoul National University, Seoul 151-744, Korea (e-mail: byoungho@snu.ac.kr). Digital Object Identifier 10.1109/LPT.2003.818678 Fig. 1. Schematic diagram of the setup used to induce an order-selectable strain gradient in the fiber grating. The designation numbers 1 and 2 mean the “curve 1” and “curve 2” of the structure boundaries, respectively. The parameter means the half-width of the structure, i.e., and mean the half-widths as functions of position for curves 1 and 2, respectively. on a fiber grating [5]. However, even with this method, the dis- persion order is fixed if an elastic plate is chosen. In this letter, we propose an advanced tuning method, which has novel char- acteristics that enable the dispersion order of a CFBG to be se- lected. II. PRINCIPLE OF DESIGN Fig. 1 shows a schematic diagram of the proposed method for inducing a strain gradient (compressive strain) to a fiber grating. Because a fiber Bragg grating (FBG) is attached to the tapered structure, when a strain is applied longitudinally, the fiber grating and the structure are simultaneously compressed. The compression rate is determined by the width of the structure because the thickness of the structure is uniform. The chirp of a fiber grating at each position varies linearly with the applied strain at that position [6]. The merit of the proposed method is that it is possible to select the structure width function by simply inserting or extracting a pair of small blocks (we refer to these as control blocks). If a strain is applied, in the case where the con- trol blocks are inserted, the local compression rate is determined depending on the outer width of the structure (curve 2 in Fig. 1), and in the case where the control blocks are extracted, the local compression rate is determined by the inner width (curve 1 in Fig. 1). In addition, the magnitude of dispersion can be tuned by adjusting the total strain. Therefore, by designing the spa- tial variation in the inner–outer width of the structure appropri- ately, we can switch the dispersion order and tune the dispersion magnitude. 1041-1135/03$17.00 © 2003 IEEE