Chirped Fiber Grating Stress Sensor Based On Temperature Insensitivity

Fiber grating is a new kind of all-fiber passive device that results from that the refractive index of optical fiber core is periodicity microperturbation. Because of the excellent properties such as compatible with optical fiber, easy fabrication, little insert loss, small volume and low price, fiber gratings have wide application in optical fiber communication and sensor field. Therefore, more and more attention has been paid to the fabrication and applications of optical fiber grating since the first optical fiber grating was prepared successfully. Chirped fiber grating is a kind of fiber gratings whose optical cycle (the effective refractive index of grating neff is multiplied by the period of perturbation of refractive index Λ) becomes bigger gradually along axial direction. In chirped fiber grating, different wavelength incident lights are reflected in different position along axial direction, so chirped optical fiber grating has wide reflection bandwidth and gradual group delay which are non-existent in other kind of optical fiber gratings. It is these unique properties that make chirped fiber gratings have unique applications both in optical communication and sensor field. For example, chirped fiber gratings can be used as reflector in fiber for fabricating many key devices in optical communication such as fiber bandpass filter in terms of the broad reflection bandwidth. According to gradual group delay, chirped fiber gratings can be used as fiber dispersion compensator for compensating fiber dispersion in fiber communication system and compressing broadened light pulse, accordingly, the distance between two relay stations is extended.In this paper, we explored the operating principle and fabrication and applications in sensor field. The outlines are as follows:In respect of theory, we strictly derived couple mode theory that describes the principle of fiber gratings from Maxwell equation and analysed the properties of chirped fiber grating, obtained the relation between chirped fiber grating's parameters such as change of refractive index, length, chirp value of chirped fiber grating and the properties such as reflectivity, bandwidth and dispersion value of grating, which explains that dispersion value of grating depends on chirp value of grating.We introduced the fabrication method and applications of chirped optical fiber grating in detail. Chirped fiber grating is a extensive used optical fiber grating. Generally, it can be prepared by holographical method, phase mask method, double exposure method, etching method. Its application is extensive in compensation of fiber dispersion, amplification of ultrashort light pulse, Er doped fiber amplifier, fiber filter and fiber sensor . In this paper, we emphasized its applications in sensor field and simple testing method.3. In experiment, we fabricated three different kinds of chirped fiber gratings. The first kind, the front end of optical fiber grating was fixed in quartz plate, the back end was naked. When optical fiber grating was subject to stress or strain along axial direction, corresponding reflected wavelength of front end did not changed, because the front end of optical fiber grating was fixed in quartz plate whose width is far greater than the diameter of fiber, applied strain can be negligible. But corresponding reflected wavelength of back end shifted to long wave, which caused the reflection spectrum to broaden and caused reflected power to increase. When axial stress increased, the middle of reflection spectrum splitted. Splitting width broadened and reflected power reduced with stress or strain increasing. In a given range, total power of reflection spectrum changed linearly with stress or strain changing. So we can obtain the magnitude of stress or strain by measuring reflected power. When the ambient temperature of optical fiber grating varied, temperature sensitivity coefficient of the entire optical fiber grating was same because thermal expansion coefficient of quartz plate is identical with that of optical fiber...