Characteristics Analysis Of Fiber Grating Devices By V-I Transmission Matrix Method And Research On Acousto-Optic Modulating And Cascaded Sensing

ABSTRACT:The Fiber Bragg Grating(FBG) based optical devices play an important role in the high-speed fiber optic communication system and the applications in Web of Things. Fast and efficient access to spectra performances of single or cascaded fiber gratings is the fundation to ensure optimizing the design of fiber laser, filter, amplifier, other fiber optic devices and complex distributed sensing network. In this thesis, the Ⅴ-Ⅰ transmission matrix method (VITMM) was presented to deeply research the characteristics of FBG based optical devices with theoretical analysis and experiment demonstration. The all fiber acousto-optic grating modulator was theoretically analyzed and experimentally optimized. The design philosophy and applications of two kinds of cascaded fiber grating testing system, quasi distributed FBG and superimposed chirped fiber grating, were introduced. The main achievements of this thesis are listed as follows.1-. The Ⅴ-Ⅰ transmission matrix model was firstly established to analyze the spectra characteristics of fiber grating Fabry-Perot(F-P) cavity. Compared with four common theoretical analysis methods(quasi transmission matrix method, parallel-plate interference superimposing method, recurrence method and multi-layer method), the VITMM can improve the computing efficiency under the premise of ensuring analysis accuracy when it was applied to simulate the reflection and transmission spectra of fiber grating F-P cavity.2、The VITMM was applied to analyze the influences of resonator’s parameters on the mode selection and laser output of Distributed Bragg Reflector(DBR) fiber laser in three aspects:the interval and number of spectral lines, the effect of grating reflectivity on the depth of spectra lines and the condition of single longitudinal mode output and threshold cavity length. Then the transmission spectra analysis of fiber grating laser with FBG based single-cavity resonator and double-cavity resonator was performed, respectively. The experiment setup of DBR fiber laser whose resonator was formed by a uniform fiber Bragg grating(UFBG) and a chirped fiber Bragg grating(CFBG) was carried out. The spectra of laser output in experiment was consistent with the simulation result.3、For the ultrasonic detection by FBG, the VITMM was firstly presented to analyze the refractive index distribution and the period change of uniform FBG caused by the geometric and strain-optic effect of ultrasonic wave. The relation of the shift of FBG reflected spectra with the ratio of the wavelength of ultrasonic wave to the FBG length was obtained, and the system was able to detect ultrasonic wave ranging from15to1380KHz.4、The theoretical model of single-mode fiber based acousto-optic grating was established by the three-layer uniform circular waveguide structure, and the strain performance was demonstrated experimentally. Fourier mode coupling(FMC) method was applied to achieve the reflected spectra of FBG based acousto-optic grating under various ultrasonic frequencies and acoustically induced strains. The FMC method can improve the computing efficiency effectively compared to tranditional computing method. A new way for generating longitudinal acoustic wave by shear-mode PZT, which has high acousto-optic coupling efficiency, was presented5、The temperature test was carried out by using a sensing system composed by1000cascaded FBGs with low reflectivites. The temperature ranged from40to150℃, and the sensitivity of FBG was10.7pm/℃. As the input optical power keeping the same, the relation between the returning optic power and the first-order crosstalk power of any FBG with FBG numbers, at various FBG reflectivities, was given. Two ways for optimizing the signal-noise-ratio of system were proposed:adjusting the working status of electro-optical modulator or adding tunable filter. After the optimization, the signal-noise-ratio of system was raised from5dB to15dB. The superimposed chirped fiber grating was fabricated by moving phase mask and adjusting stress, respectively. The free spectra range of that grating can be adjusted by changing the displacement of phase mask or the stress along the fiber axis. The superimposed chirped fiber grating can be used as a sensor employed in the measurement of tempreture, stress, strain and so on.