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Study on Active Fiber Grating Sensing Technology

Author: LuoJianHua
Tutor: YuanShuZhong;LiuBo
School: Nankai University
Course: Optics
Keywords: Distributed Bragg reflector Fiber Bragg Grating Sensing technology Active sensing Passive sensing Beat detection Orthogonal polarization Three-dimensional sensing Cylinder Lateral pressure sensing Pure bending sensor Millimeter-wave Wavelength division frequency division Multiplexing Sensor array Dual - bus architecture M × N networking
CLC: TP212
Type: PhD thesis
Year: 2008
Downloads: 417
Quote: 2
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Abstract


With the development of fiber grating technology, study on the application of fiber Bragg grating (FBG) sensor has got more recognition. Real-time detection and monitoring to stress (or strain) of structural systems have been put into effect. Martial researches in fiber grating hydrophone have also been developed. With the increasing of the sensing system, sensing network has developed by degrees.Fiber Bragg grating active sensing was developed based on passive sensing. Using fiber laser as sensing element, active sensing system was constructed and studied on experiment. Active sensing system has many instinctive advantages, such as high Signal-to-Noise, high light power. Compared with generic Broadband Source (BBS), it increased the Signal-to-Noise of sensing signal and has the advantage of easy check-up the wavelength.With the subjects supported by the National High Technology Research and Development Program of China (Grant No. 2006AA01Z217), the National Natural Science Fund projects (Grant No. 60736039) and the Key Laboratory Fund of Optoelectronic Information Technical Science, Ministry of Education of China, the main contents of this paper are as following:1. Introduced systemically the active and passive sensing technology of the fiber Bragg gratings (FBGs). Based on distributed Bragg reflector (DBR) fiber laser, we studied the beat frequency demodulation sensing technology. Compared with generic FBG sensor, it has many instinctive advantages, such as high Signal-to-Noise, high light power, and easy check-up the wavelength.2. The basis theory and analysis method of fiber Bragg grating and fiber laser were presented. The output characteristic of F-P linearity cavity based on FBG was simulated and analyzed. The influence of reflectivity and cavity length on longitudinal mode width and longitudinal mode intervals were also discussed. We drew the conclusion: reduce the cavity length is necessary to achieve single longitudinal mode in fiber laser. 3. Based on fiber Bragg grating, we studied on temperature, strain, wheel-type structure pressure sensor and achieved all-right experimental result. Using cylinder structure, 3-dimensional fiber Bragg grating (FBG) strain sensor was proposed. Three FBGs were used as sensing elements and fixed in the outer surface of the cylinder at the same level according to 120o angle intervals. In principle, the three FBGs had some fixed relationship when we put a strain F on the cylinder. Based on the physical and mathematics principle, theoretical derivations and experiment setup were also showed. According to our experimental results, this sensor could measure the size and angles of the strain or displacement accurately. Combined the wireless sensing network technology, a multi- parameter, multi- dimension wireless sensing network system was studied.4. Based on single longitudinal mode distributed Bragg reflector (DBR) fiber laser, lateral pressure and pure bending sensor was designed. Using single longitudinal mode DBR fiber laser as sensing element, the sensor was based on beat frequency demodulation and has high repeatability and directivity. Based on single longitudinal mode DBR fiber laser, the beat frequency and wavelength two parameters demodulation technology were studied. When the temperature changed, the beat frequency and wavelength were both changed. By detecting the changes of beat frequency and wavelength, we could sense the change of temperature. Theoretical derivations and experiment setup were also showed that two parameters demodulation technology has high sensitivity.5. Based on single longitudinal mode distributed Bragg reflector (DBR) fiber laser, Wavelength-Division-Multiplex (WDM) and Frequency-Division-Multiplex (FDM) techniques were studied. Several sensors with different wavelength and different beat frequency were connected in series to form sensing array. We designed Double Bus Topology Structure which separated the signal and pump light. Using coupler we pumped every sensing element separately which used pump power farthest. Using Wavelength-Division-Multiplex and Frequency-Division-Multiplex to form sensing array, the large-scale network didn’t come true. We designed M×N network by using Space-Division-Multiplex (SDM) and WDM, FDM techniques.

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