Fbg Sensor Demodulation Method Based On Optical Rotatory Dispersion Effect Study

Fiber Bragg grating sensor is currently one of the fastest growing optical fibersensors, which uses the way of wavelength encoding to measure, and it can be used onthe absolute measurement of the outside parameters, and its application range is veryextensive. Fiber Bragg grating sensing signal demodulation, that is to detect the shift ofsensitive fiber Bragg grating center wavelength, is the most key technical link of fibergrating sensing system. As to the sensor detecting system realized by a single fiberBragg grating, such as fiber Bragg grating acceleration sensor, flow sensor etc., thereported demodulation method is usually according to the fiber Bragg grating centerwavelength using in the sensing systems to demodulate, so the correspondingdemodulation devices cannot be used in common. For distributed optical fiber gratingsensor network, the F-P scanning filtering method is the most widely used, but the F-Pscanning filter must be real-time calibrated because of the hysteresis, nonlinear and driftexistence between scanning wavelength and voltage. In guarantee calibration precision,how to expand working range of calibration equipment to meet the reuse more sensitivefiber Bragg grating demand is also the problem need to be solved. This paper developedthe universal single fiber Bragg grating sensor demodulation device applied thewavelength detection method based on the optical rotatory dispersion effect; developedscanning F-P filter calibration device with the scope of work to more than100nm. Inorder to adapt to the development of future demand, we developed the wavelengthdetection device which can be applied to1310nm band and1550nm band. In particular,we do the research work on practical of the device with the actual conditions of use. Thepaper mainly did research work as follows:We used quartz crystal as rotatory dispersion medium to build the wavelengthdetection device base on the optical rotatory dispersion. In order to eliminate the influence of the detection precision caused by the instability of the incident light sourcepower, we used double optical path polarization detection system and designed thecorresponding mechanical device to ensure the accuracy and mechanical stability of theoptical path adjustment. Use thermal expansion coefficient and thermo-optic coefficientto estimate, and the error of wavelength detection is less than±0.4pm caused by thetemperature change when the environment change of quartz crystal is controlled in±0.1℃. We made a400x400x200mm3small thermostat using cheap refrigeration sliceand heater to achieve the temperature control demand. The thermostat has large inertia,delay, nonlinear characteristics, so we chose the principle of generalized predictivecontrol which is valid for this controlled plant and designed generalized predictivecontrol recursive algorithm to make the control accuracy of the thermostat reached±0.1℃. At the same time, we used the system composed of light source, couplers,photoelectric switches, A/D converter and signal processing unit to measure thesymmetry of circuit system.We developed the calibration device of fiber F-P tunable filter based on the opticalrotatory dispersion effect. Used quartz crystal as rotatory dispersion medium, andadopted double optical path polarization detection system to develop the correspondingcalibration device. Used the optical rotatory dispersion effect of quartz crystal inC-Band to obtain one to one correspondence between the rotation angle and wavelengthin the polarized plane of the linearly polarized light. We used the fittingwavelength-rotation angle relation curve as wavelength reference standard to realize thescanning F-P calibration, and wavelength detection deviation is less than±6pmcompared to measurement value of spectrometer. Also we focused on how to improvethe calibration equipment repeatability, long-term stability and reliability problems.We developed the wavemeter which can be suitable for both1310nm band and1550nm band. By properly chose the length of quartz crystal, we ensured the doubleoptical path polarization detection system to satisfy the orthogonal offset condition in 1310nm band and1550nm band at the same time. Avoid the measuring error caused bythe readjustment of polarizer azimuth angle used in different wave bands detection.Through the experiment, the wavemeter's wavelength detection deviation we developedis less than±6pm compared to measurement value of spectrometer on the1310nmand1550nm band.We used the wavemeter based on the optical rotatory dispersion effect, which canbe used on both1310nm band and1550nm band to design the fiber Bragg gratingdemodulation device to do temperature sensing demodulation of the fiber Bragg gratingon the1550nm band. Moreover, in order to make the wavemeter suitable for thedifferent power levels of light source, the fiber collimator and the fiber in light sourcesturn to the polarization controller, the light power into the wavemeter can be from10%to80through the adjustment of the angle between the polarization controller and thepolarizer. So photoelectric detector can keep on an appropriate, fixed light intensitywhen detect the lasers of different light power levels. The first order temperaturesensitivity coefficient of optical fiber Bragg grating, measured by this device, is8.04×106/℃, compared with the theoretical value, the first order temperaturesensitivity coefficient error of this device is0.76×10-6/℃. The thermo-opticcoefficient and thermal expansion coefficient of fiber Bragg grating obtained by opticalfiber Bragg grating demodulation devices we developed, is very close to that actualmeasured, consistent with the theoretical value, has the same order of magnitude, so ourexperiment device is feasible.