Research On Sensitization Characteristics Of Interferometric Fiber Optic Sensor Based On Photonic Crystal Fiber

Fiber optic sensors are widely used in aerospace,biomedical,oil exploration,marine monitoring and other fields because of the advantages of anti-electromagnetic interference,easy integration,low cost,corrosion resistance,etc.The in-line MachZehnder interferometer(MZI)has become an important research direction in fiber optic sensors due to the superiorities of small size,high sensitivity and simple fabrication.In this paper,two in-line MZI sensing structures for measuring temperature and relative humidity are designed and fabricated.Moreover,the research work of increasing the sensor sensitivity is carried out in three aspects: microstructure,changing the coupling method and coating.1.An interferometric fiber optical sensor based on tapered photonic crystal fiber(PCF)for simultaneous measurement of temperature and relative humidity is designed and fabricated.The structure is composed of two sections of single-mode fiber(SMF)fused to a section of PCF.A taper structure is added to the center of PCF in the sensing area.The structure is simulated by Rsoft software for transmission energy distribution,and the feasibility of the structure design is verified.The length of the sensing area PCF is changed during the sensor fabrication process.The optimal solution for the sensor length is obtained by comparing its interference spectrum with the spatial spectrum.The experimental results show that the sensor spectrum has two dips in the observation range.The temperature sensitivities are 0.061 nm/°C and 0.028 nm/°C in the temperature range of 30 to 90 ?,respectively.The relative humidity sensitivities are0.039 nm/%RH and 0.032 nm/%RH in the range of 30 to 70 %RH,respectively.The sensors all have a linear fit of 0.990 and above in the temperature and relative humidity measurements.The center wavelength shifts in stability experiments are less than the unit sensitivity of the sensor.2.A PCF interferometric fiber optic sensor for temperature and relative humidity measurements is designed and fabricated.This sensing structure is composed of two sections of no core fiber(NCF)and a section of PCF to form an NCF-PCF-NCF(NPN)structure.The PCF is the sensing area of the sensor and the NCF is the coupler of the sensor.In order to verify the coupling performance of NCF,the same length of multimode fiber(MMF)is used for comparison experiment,and a PCF temperature and relative humidity sensor based on MMF coupling is fabricated.The experimental results of NPN show that the sensor spectrum has three dips in the observation range.The temperature sensitivities of NPN sensor are 0.059 nm/°C,0.074 nm/°C and 0.130nm/°C,respectively,when the temperature is in the range of 25? to 70 ?.The sensitivities of the NPN sensor are 0.020 nm/%RH,0.027 nm/%RH and 0.036 nm/%RH,respectively,when the relative humidity ranges from 30 %RH to 70 %RH.The temperature sensitivities of the sensor are effectively improved by changing the sensing structure while simplifying the fabrication process.3.A PCF relative humidity sensor coated with graphene quantum dots(GQDs)is designed and fabricated.The sensing structure is made by fusing a section of PCF between two NCF sections.The experimental results show that the sensor spectrum has three dips in the observation range.The relative humidity sensitivities are 0.111nm/%RH,0.089 nm/%RH and 0.063 nm/%RH at a fixed temperature of 25 °C and a relative humidity range of 30?70 %RH,respectively.The relative humidity sensitivities of the sensor are effectively improved by coating GQDs based on the same structure as in scheme 2.The sensor has the advantages of high sensitivity,simple fabrication and good stability.Therefore,it can be widely used in biological,chemical,pharmaceutical and environmental condition monitoring fields,and also provides a new idea for the indepth research of relative humidity sensors.In summary,two sensor structures are designed and fabricated in this paper.The first structure improves the temperature and relative humidity measurement sensitivities of the sensor through the microstructure method and realizes the dual parametric measurement of temperature and relative humidity simultaneously.The second structure is to change the sensor coupling method.The sensor fabrication process is simplified and the temperature sensitivities are twice that of scheme 1.Finally,the relative humidity sensitivities are 3.1 times that of scheme 2 by coating the GQDs.