Key Techniques Of Forest Resources And Environment Monitoring Based On Optical Fiber Sensing

The health monitoring of the forest resources and enviorment is important to preserve theecological environment and improve the gross national products. However the traditionaldetections are expensive and incomplete. Optical fiber sensing technology has advantages ofimmunity to electromagnetic disturbance, high sensitivity, corrision resisitance, long distancetransimission, capable of working under heavily polluted enviorment, and no pollution to themeasured objects. Combining the superiority of the optical fiber sensing technology, opticalfiber sensors used in the monitoring of the forest resources is proposed in the paper, which hasconsiderable application value. This paper mainly focused on the principles and experimentalstudy of fiber humidity sensor, fiber refractive index sensor and fiber multi-parameters sensor.Several novel kinds of fiber sensors are proposed and fabricated, which are theoriticallyanalyzed and experimentally studied. The main aspects are listed as follows:(1) The refractive index (RI) sensing properties of microfiber Bragg grating (MFBG) istheoretically studied. The MFBG is fabricated through chemical etching. Moreover, the RIand the humidity responses of the MFBG are experimentally studied. The experimentalresults show that the MFBG with8.9μm diameter has a RI sensitivity of1.70nm/RIU in theRI range of1.335-1.389RIU, while a RI sensitivity of10.91nm/RIU in the RI range of1.389-1.403RIU, which agrees with theoretical analysis. The humidity sensitivity of theMFBG is3pm/%RH in the humidity range of55%-80%RH, which expands the MFBGapplication.(2) A novel sensor based on single-mode–multimode fiber core–single-mode (SMS) fiberstructure connected with a Fiber Bragg Grating (FBG) a section of is proposed anddemonstrated. The beam propagation of the SMS fiber structure with different length ofmultimode fiber core (MMFC), diameter of MMFC, mode order and sourrounding refractiveindex (SRI) are theoretically simulated through beam propagation method. Experimentalresults show that the RI sensitivity is193.55dBm/RIU in the RI range of1.335-1.404RIU, thehumidity sensitivity is0.07dBm/%RH in the humidity range of50%-85%RH, and thetemperature sensitivity is0.008nm/℃in the temperature range of20-80℃. This confirms thatthe sensor could realize the measurement of RI and humidity based on power detection, andeliminates the temperature effects.(3) A new method about fabrication of fiber taper is proposed. Three kinds of novelMach-Zehnder interferometers are fabricated based on this fiber taper, which are theoritically and experimentally studied. Experimental results show that the humidity sensor based on twoarc-induced ellipsoid fiber tapers in-fiber Mach-Zehnder interferometer (MZI) has a linearhumidity sensitivity of-0.047nm/%RH in the humidity range of50%-95%RH without coatingfilm. The MZI based on multimode fiber sandwiched in two fiber tapers has a linear humiditysensitivity of0.12dBm/%RH in the humidity range of35%-90%RH and a temperaturesensitivity of0.003nm/℃in the temperature range of20-100℃, which implies that this kindof sensor could simultaneously measure humidity and temperature. The RI sensor based onMZI formed by sandwiching a section of MMFC into two fiber tapers has an enhanced RIsensitivity of430.934nm/RIU in RI range of1.335-1.394RIU.(4) A MZI based on single-mode–thin-core–multimode fiber core–single-mode (STMS) fiberstructure for refractive index (RI) measurement is proposed and experimentally demonstrated.The thin-core fiber length, the multimode fiber length and diameter, and SRI effect on thebeam propagation and output power coupling coefficients are analyzed by beam propagationmethod. The STMS fiber structure samples are fabricated and experimentally studied. Theexperimental results show that the STMS fiber structure has a RI sensitivity of148.27nm/RIUin RI range of1.333-1.403RIU, humidity sensitivity of0.207nm/%RH in humidity range of55%-90%RH, and temperature sensitivity of0.007nm/℃in the temperature range of20-8℃.The sensor has advantages of easy fabrication, high sensitivity, and linear reponse.(5) A novel sensor formed by combining a Long Period Fiber Grating (LPFG) in apolarization-maintaining fiber Sagnac loop is proposed. The transmission spectrum of thefiber Sagnac interferometer is modulated by LPFG. Through measuring the simultaneousmeasurement of temperature, RI, and strain can be realized. The experimental results showthat the temperature sensitivity of the sensor is0.128nm/℃in temperature range of23-93℃,and the RI sensitivity is49.38dBm/RIU in RI range of1.32-1.40RIU.The strain sensitivity ofthe sensor is0.005dBm/με when applied strain on LPFG, while0.015nm/με as applied strainon polarization-maintaining fiber. The sensor can achieve simultaneous measurement oftemperature and strain or RI, eliminate the cross sensitivity of the temperature, anddistinguishes the area of applied strain.The above sensors have advtanges of easy fabrication, inexpensive, high sensitivity, andcompact structure, which can satisfy the needs of the sensors in the forest resources andenviorment monitoring.