Research On Fiber Grating Sensing And Key Technology Of Sagnac Loop

Since the 1970s, the fiber Bragg grating has been extensively used in the fields of optical communication and optical sensing. Long Period Fiber Bragg Grating has the grating period from dozens to hundreds microns. Compared with other passive components, it is mostly concerned these years because of its simple structure, low back-reflection, little insertion loss, anti-electromagnetic interference and polarization insensitive. Due to the special cross-section of the polarization maintaining fiber, the fiber Bragg grating carved in this kind of fiber is equivalent to carve the fiber Bragg grating in the fast and slow axes respectively. When it is used for sensing measurement, the physical changes will affect the optical properties of polarization maintaining fiber and fiber Bragg grating. The resonant wavelengths of fast and slow of Polarization Maintaining Fiber Bragg Grating shift. Therefore it can achieve the purpose of sensing. Fiber Sagnac loop which has the features of making simply and independent of the polarization state of input light is an important device in the field of fiber.This article focuses on the sensing characteristics of Long Period Fiber Bragg Grating and Polarization Maintaining Fiber Bragg Grating, while using fiber Sagnac loop filter to increase the measurement sensitivity. Specific work as follows:1) The various output characteristics of Sagnac loop with different medium is firstly analyzed. The theories of Sagnac loop with uniform fiber Bragg grating and high-birefringence fiber are studied using Jones matrix. The MATLAB software is utilized to simulate numerically. Then, the transmission characteristics of Sagnac loop with one-section and two-section high-birefringence fiber are introduced in detail. On the basis of numerical simulate of Sagnac loop with 0ne-section high-birefringence fiber and two-section high-birefringence fiber with the length ratios of 1:2(2:1) directly in series, the experimental study is carried out. The optical spectrum from the experiment is in good agreement with the simulated spectrum.2) The coupled mode theory of Long Period Fiber Bragg Grating is discussed. The response characteristics of temperature, axial strain, transverse load and environment refractive index are studied based on these theories. The liquid level sensor is produced using the response characteristics of refractive index of Long Period Fiber Bragg Grating. This sensor has simple structure, low cost, high sensitivity and other features. When the liquid-level are changing between 0 and 17.5mm, the transmission power of LPFG changes, and the change amount is up to 6.8719dBm and 13.4360dBm, and its sensitivity are 0.3896dBm/mm and 0.7678dBm/mm respectively.3) The transmission characteristics of Polarization Maintaining Fiber Bragg Grating are presented and the sensing mechanism of its temperature and strain is studied. Based on these principles, the sensing structure is designed to measure temperature and strain simultaneously. In this setup, Polarization Maintaining Fiber Bragg Grating as a sensing head, fiber Sagnac loop as the edge filter is used for changing the wavelength varies of fast and slow axes to power varies. The temperature and axial strain can be measured simultaneously by measuring the wavelength difference and power difference of the resonant wavelength of fast axes and slow axes. The experimental results show that the axial strain and temperature sensitivities reach 0 pm /με, -0.00361 dB /με, -0.672pm/℃and -0.0236dB/℃respectively.