High Sensitivity And Polarization Modulation Optical Fiber Sensor Measurement Technology Application Research Based On Dislocation And No Core Fiber

Optical fiber sensing technology is a new detection technology sensing signal,which is based on light wave as carrier,and optical fiber as medium.As the carrier of the measured signal and the medium of communication,the optical fiber has the ability to resist electromagnetic interference,easy to be received by the variety of photoelectric detector,expedient to facilitate the photoelectric conversion,suitable for remote monitoring and so on.The development trend of optical fiber sensing technology is aiming for high sensitivity,high precision,multi-parameter,synchronous online,function integration and network,etc..In this paper,the working principle of fiber optic sensors,which include four kinds of new structures,comprising interference sensor,laser sensor,cavity ring down sensor and polarization sensor are studied systematically.On the basis of analyzing the theory of these sensors,the experimental measurement of temperature,concentration and rotation angle of the sensors are inquired in detail.The main research contents and innovative achievements include the following aspects.?1?Based on the principle of mode propagation according to the characteristics of the fiber,electric field distribution of the Mach-Zehnder interference and Michelson interference mixed structure is analysized.Mixed structure corresponding to the mode interference optical fiber sensor structure is composed of dislocation optical fiber,no core fiber and Faraday rotation mirror.The theoretical model of dislocation at different positions is building,and the interference signal transmission mode data is further to fit.The method of the using Faraday rotation mirror to eliminate the polarization state changes,to compensate polarization caused by fiber signal attenuation and to eliminate the effect of external disturbance is in-depth compared and analysized.The study for hybrid fiber interferometric sensing technology applied to the temperature and concentration measurement is significant and indispensable.?2?The experimental study on the mixed structure of mode interference technology is applied to the temperature and concentration measurement.1)Temperature sensing system based on the mixed structure interferometer with enhanced sensitivity.With the application of using Faraday rotation mirror,the optical fiber sensor has been possessed a wide measurement range and characteristic to effectively eliminate the influence of random polarization state changes.Finally,the sensitivity and the stability are improved.2)based on the multimode interference effect theory of the no core fiber,a temperature self compensating sensing system including temperature and concentration measurement is realized.The measurement of temperature and liquid concentration by detecting the wavelength shift of the reflected cladding mode is realized by using the single-mode/no-core/Single-mode/dislocation sensing part,and the stability of the system is further improved.At the same time,the linear feature of the concentration sensing about the three kinds of heavy metal solution is verified by the experimental results,which providing an effective means to detect the concentration of heavy metal solution in a certain range.The temperature measurement method based on the dislocation optical fiber between laser sensing and FRM interferometer are analyzed and compared.The main difference between the two methods are that the dislocation fiber is equal to the filter in the laser temperature measurement system.The position of the laser output is only selected at the wave crest of the interference spectrum.Therefore,the center wavelength of the laser will shift with the change of temperature.The experimental results show that the sensitivity and the stability of the FRM interference sensing method are better than that of the laser temperature sensing method.The dates of spectral analysis and algorithm optimization measured by the dislocation optical fiber temperature sensing system and the no core dislocation fiber concentration sensing system are achieved.By using the improved BP neural network and the improved ELMAN neural network,the expected and predicted results of the sensing system are realized.An improved neural network architecture is presented.The process of network training and the selection of parameters are analyzed in detail.Finally,the forecast value and error of temperature and concentration data are compared and analyzed.At the same time,the feasibility of the algorithm is verified in the optical fiber temperature and concentration sensing.It is also provides a new idea for the accurate measurement of the pressure,curvature and other physical quantities of the misplaced optical fiber.?3?The temperature sensing system based on the fiber cavity ring down spectroscopy technology is proposed.According to the Lambert-Beer Law theory,the ring cavity loss and the relation between the ring down time and the dislocation length of sensing part varied with temperature have been analyzed.The variation of the displacement and the change of the offset length caused by the temperature can be detected by measuring the ring-down time.The method of spectrum demodulation is improved and the process of signal acquisition and processing is also simplified.The experimental results show that when choosing different displacement,the relationship between the ring-down time and offset D is obtained by curve fitting.Meanwhile,the waveform of ring-down output is affected by the offset.If the input optical power is large enough,the total loss of the cavity is increased,and the ring-down time of the cavity is reduced obviously.?4?In view of the problems of the low precision in measurement and the difficulty to demodulation existing in interferometric concentration sensing system,a new type of two-stage modulation of the optical rotation angle and the concentration of the polarization measurement sensor system is proposed.This technology combines the traditional DC-modulation magneto-optic detection technology with the AC-modulation magneto-optic detection technology.The error brought by the change of the magnetic rotation angle is caused by the method of pure AC modulation.The AC/DC modulation signal source with pulse width modulation control is designed and implemented.It is experimental verified that the terbium-doped(Tb³⁺)optical fiber can be used for AC/DC modulation concentration measurement system.The system adopts an all optical fiber link including the alignment device,which makes the sensing equipment more portable and compact.