A slope-assisted Brillouin optical time domain reflctometry system based on grade index multi-mode fiber (GI-MMF) was presented. The coherent detection was applied in this system and the Brillouin gain spectrum (BGS) was obtained by scanning work frequency. The BGS was inhomogeneously broadened by adjusting the lateral offset between single mode fiber (SMF) and GI-MMF. The bandwidth of BGS with different lateral offset was analyzed and the BGS with bandwidth of 111MHz was achieved at lateral offset of 8 μm. For realizing slope-assisted technology, a bandpass electrical filter was added behind the balanced photo-detector to realize the function of frequency selection. The strain intensity responses with different work frequency were analyzed for maintaining significant linear relationship between strain and signal intensity. The system realized the maximum strain dynamic measurement of 3000 με with the spatial resolution of 5 m along ~1 km GI-MMF at vibrational frequency of 7.83 and 15.47 Hz. The measured error of vibrational frequency was less than 0.2 and 1.5 Hz, respectively. The obtained strain intensity responses were 0.00296 and 0.00292 mV/με, respectively. The measured strain range of this system was more than three times that of traditional systems based on SMF and could be achieved at relatively low cost. The proposed scheme has potential application prospects in large dynamic strain diagnosis.
A high sensitivity and low cost all-fiber temperature sensor combined a single-mode-polarization maintaining-single-mode optical fiber (SPS) structure and a Sagnac loop is proposed and experimentally demonstrated. In this sensor, the SPS structure is reeled into a circle is inserted into a Sagnac loop. Experimental results show that the radius of the circle has significant influence on temperature sensitivity. The temperature sensitivity can reach up to 1.678nm/°C for the radius of 3cm.
The hybrid fiber optic interferometers are proposed and experimentally demonstrated. In our schemes, the hybrid fiber optic interferometers are constructed by single mode-multimode-polarization maintaining-single mode optical fiber (SMPS) structure and a Sagnac loop. The temperature and strain characteristics of the hybrid interferometers are studied in experiment, and the sensitivities depending on the length of polarization maintaining optical fiber (PMF) and multimode optical fiber (MMF) are detailedly investigated in experiment. The experimental results have demonstrated that the PMF and MMF lengths have low affect on the strain sensitivity but has great influence on the temperature sensitivity. The achieved strain sensitivity is 37.2pm/με for 10cm PMF and 12cm MMF. The achieved strain sensitivity is 38.0pm/με for 12cm PMF when the length of MMF is fixed at 15cm, and is 37.2 pm/με for 12cm MMF when the length of PMF is fixed at 10cm. The obtained temperature sensitivities is 1.723nm/°C when the length of MPF is 8cm with the fixed length of 15cm MMF, and the obtained temperature sensitivities reach 1.848nm/℃when the length of MMF is 12cm with the fixed length of 10cm PMF.
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