| Optical fiber sensor can realize the monitoring of temperature,strain,pressure,vibration and other external physical quantities.In practical applications,optical fiber sensors are unavoidable to be affected by multiple physical quantities at the same time.Thus,the issue of cross sensitivity between different parameters will occur,which has impacts on the accuracy of the fiber sensors in measurements.In this paper,an optical fiber sensor sensitive to the single parameter is designed,which solves the problem of cross sensitivity between temperature and strain to a certain extent.Secondly,on the basis of solving the issue of cross sensitivity,a dual-parameter optical fiber sensor that can simultaneously measure two parameters is designed.Finally,a three-parameter optical fiber sensor is designed to measure temperature,strain,and refractive index simultaneously.The main research work and innovations of this paper are summarized as follows:1.A temperature-insensitive strain fiber sensor based on the few-mode fiber(FMF)is experimentally fabricated.The sensor has an FMF-FMF-FMF cascade structure and is fabricated by core-offset splicing.The results show that the strain sensitivity of the proposed fiber sensor is-3.13 pm/με when the strain ranges from 0 με to 700 με,and the linear correlation coefficient is 0.994.Within the temperature range of 20-40 ℃,the maximum wavelength shift is 5 pm/℃;while in the temperature range of 40-60 ℃,there is no wavelength shift that can solve the problem of cross sensitivity at the ambient temperature range of 40-60 ℃.The proposed strain fiber sensor can not only improve the strain measurement accuracy under different ambient temperatures,but also has the advantages of compact structure and easy laying.2.A novel strain-insensitive temperature sensor composed of the few-mode fiber and photonic crystal fiber(PCF)is designed and experimentally demonstrated.The proposed sensor has an FMF-PCF-FMF hybrid optical fiber structure that can be achieved by the automatic splicing procedure of the fiber fusion splicer,which greatly improves the robustness of the proposed sensor.In addition,the symmetrical structure at both ends makes the sensor head not limited by the incident direction of the signal light,which greatly improves the flexibility of the sensor.The experimental results indicate that the temperature sensitivity of the proposed optical fiber sensor is as high as 38.6 pm/ ℃ in the temperature range of 20-80 ℃.Meanwhile,the strain sensitivity of the sensor can be as low as-0.45 pm/με when the strain ranges from 0 με and 3000 με,which indicates that the sensor is insensitive to strain.The designed temperature fiber sensor shows advantages of compact structure,convenient manufacture and good robustness,and presents both high temperature sensitivity and ultra-low strain sensitivity.Thus,it provides a reference for the design of dual-parameter optical fiber sensor.3.A novel dual-parameter fiber sensor based on the combination of few-mode fiber and fiber Bragg grating(FBG)is developed,which can simultaneously measure the two physical parameters of strain and temperature.The fiber sensor has a hybrid fiber structure of SMF-FMF-FBG,and the signal light of the sensor is incident from the singlemode fiber(SMF)and output from FBG.Experimental results suggest that there is a resonance peak with extinction ratio as high as 23 d B in the transmission spectrum of the sensor.The strain sensitivity of the resonance peak is-2 pm/με in strain from 0 με to 450με,and the linear correlation coefficient is 0.998.In the temperature range from-10 ℃to 30 ℃,the temperature sensitivity the of the resonance peak is-47 pm/ ℃,and the linear correlation coefficient is 0.995.The proposed dual-parameter fiber sensor has the advantages of simple manufacturing,low manufacturing cost,high temperature and strain sensitivity,good linearity and so on.It shows great application value in the field of tunnel vault settlement and environmental temperature monitoring.4.A high sensitivity dual-parameter fiber sensor composed of non-core fiber(NCF)and two-core fiber(TCF)is designed,which can measure strain and temperature simultaneously.The fabricated sensor has a hybrid symmetrical structure of NCF-TCFNCF-TCF-NCF.The strain sensitivity of resonance peak A of the sensor is-5.46 pm/μεwhen strain increases from 0 με to 600 με,and the linear correlation coefficient is 0.985.Meanwhile,the temperature sensitivity of resonance peak A of the sensor is 48.6 pm/ ℃when the temperature varies from 35 ℃ to 60 ℃,and the linear correlation coefficient is0.958.The hybrid symmetrical structure of NCF-TCF-NCF-TCF-NCF can further improve the temperature sensitivity and strain sensitivity of the dual-parameter fiber sensor when compared to the SMF-FMF-FBG sensor structure,showing great application potential in the field of high-accuracy strain and temperature dual-parameter monitoring.5.A multi-parameter fiber sensor based on NCF-FMF-NCF symmetrical structure is designed and experimentally fabricated,which can simultaneously measure the physical parameters of strain,temperature,and liquid refractive index(RI).There are three resonance peaks in the experimental transmission spectrum of the proposed fiber sensor.According to the different sensitivity responses of the three resonance peaks to strain,temperature and RI,the three-parameter sensing measurement can be realized.When the strain ranges from 0 με to 1400 με,the strain sensitivity of the three resonant peaks are-3.5 pm/με,-1.9 pm/με,and-4.1 pm/με,respectively.When the temperature increases from 35 ℃ to 55 ℃,the temperature sensitivities of the three resonance peaks are 162 pm/℃,194 pm/℃,and 162 pm/℃,respectively.When the RI of glycerol solution increased from 1.3707 to 1.39809,the RI sensitivities of the three resonance peaks are22.9 pm/RIU,24.6 pm/RIU,and 97 pm/RIU,respectively.Based on the measurement principle of multi-parameter fiber sensor and the wavelength resolution of the sensor is50 pm,it can be concluded that the measurement accuracy of strain,temperature,and refractive index is 50.155 με,0.685 °C,and 1.755 RIU,respectively.The three-parameter fiber sensor has the advantages of compact structure,simple configuration,and high sensitivity,presenting great application potential in the field of strain,temperature,and refractive index three-parameter sensing detection. |
