Study On The Applications In The Sensors And Fiber Amplifiers And Fabrication Of Bragg Fiber Gratings

After low loss fiber in the wavelength of 1550nm was successfully developed, optical fiber communication is the most active technology of communication field. Optical fiber communication, satellite communication and mobile communication are the three main orientations of modern communication technology. At present, plenty of communication business continuously emerges, especially fast development of Internet induces rapid increase of communication capacity. Thus, it is necessary to establish large-capacity and high-speed optical fiber communication system. Fiber gratings provide possibilities for the further development of large-capacity and high-speed optical fiber communication system. 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. Thus, since the successful fabrication of first fiber grating, the fabrication and application of fiber gratings is central issue of interest all along. Bragg fiber grating is one type of reflection gratings whose reflective wavelength depends on the product of grating's effective refractive index and grating's period, and grating's reflectivity and bandwidth depend on the change in refractive index along grating and grating's length. Then, designing grating's period and modulating the change in index can fabricate Bragg fiber grating with different reflective wavelength and reflectivity; in addition, Bragg fiber grating has especially sensitivity to the temperature and strain. It is these unique properties that make Bragg fiber gratings have unique applications both in optical communication and sensor field, such as filters and sensors. The work of this thesis is deeply studying the principle, fabrication properties and applications of Bragg fiber gratings. The main achievements of this thesis are shown as following: 1. We strictly deduced couple mode theory that describes the principle of fiber gratings from Maxwell equation. Through grating's apodization, one can deduce grating's sibemobe and the ripple of group delay. The principle of multimode fiber Bragg gratings is studied elementarily. Firstly,in the theory the Maximum number of propagating modes was detruded and introduce the concept of principal mode. Secondly, according to the phase-matching condition of Bragg fiber gratings and the propagation constant of the principal mode, the relationship curve with the resonance wavelength of grating and this propagation constant is plotted,analyzing the resonance wavelength and its propagation constant of the couple between different principal modes;Lastly, to analyze the mode coupling of slated grating in multimode fiber, and the resonance wavelength is the result of the coupling between low order mode and higher order mode, there several pair of modes coupling can occur in the same wavelength, the resonance wavelength between the resonance wavelengths induced by the neighboring principal modes is caused by different modes, there is mode coupling between the same modes. 2. We have fabricated high quality Bragg fiber gratings by using the method of phase mask. Firstly, there high change of refractive index(～3×10-3)Bragg fiber gratings is fabricated in B-Ge codoped fibers with unform phase mask, through UV-trimming to increase the center wavelength of gratings and side mode suppression ratio; Secondly, in the single fiber we have fabricated high quality Bragg fiber gratings (with reflectivity of 99%, sidemode suppression ratio of ～29dB,3-dB bandwidth of 0.4nm,30-dB bandwidth of0.8nm, flattened level in the top of reflective spectrum lower then 0.01dB)by used the method of precondition exposure and apodisation phase mask, this grating's level is in the international standard; Lastly, uniform and slated multimode fiber Bragg gratings is fabrication in the graded refractive index multimode fiber, analyzing and testing multimode fiber graing's temperature and strain properties. Experiment data show that there the same properties about temperature strain are in the slated gratings with the slated angle lower than 20; We firstly discover that the band can couple the energies from lower order mode to higher order mode, to carry out the exchange from lower order mode to higher order mode . 3. In order to make the properties not vary with temperature, we packed the Bragg fiber gratings in negative thermal-expansion coefficient material whose length is almost the same as grating, basing on the theory analysis and experiment test about gratings' temperature and strain properties. In the range of -2 0～60oC, the central wavelength's temperature coefficient of packed Bragg fiber grating fabricated by using phase mask is 0. 0005nm /oC, the reflective bandwidth and reflectivity of packed Bragg fiber grating almost doesn't vary with temperature, the packed Bragg fiber grating's temperature property is improved greatly,the technology level is in the lead. 4. We first propped bend sensor with an embedded etched Bragg fiber grating. Bend sensor is flatly to embed the gratings into flectional beam that is bended when, the grating is bended. For uniform Bragg fiber gratings, when beam is bended , the center wavelength of grating linearly varies with curvature,the grating's reflectivity and bandwidth almost doesn't vary with curvature, we have experimentally demonstrated this property ; For linearly etched Bragg fiber grating with high reflectivity, when the beam is bended, the grating's reflective power or bandwidth linearly vary with curvature and is insensitive to temperature. The reflective power can be detected by OPM. In experiment, we have obtain the resolution of sensor is about 0.0054 m-1(The resolution of OPM is 0.01 mW). The grating's reflection power is independent of temperature in the range from 0 to 50℃. The sensitivity of sensor can be changed by controlling the taper of the grating. A main advantage of this scheme is simple and its cost is lower than other bending sensors with OSA or wavelength meter.