| Since 1980 s,optical fiber sensing technology has been developing vigorously with the development of optical fiber communication,and now it has become an important ruler to measure the degree of informatization of a country.Optical fiber sensors have many advantages compared to the traditional sensor,since it has received extensive attention and research,but in some special places,such as: natural disaster early warning and other fields,the resolution of traditional optical fiber sensor cannot meet the monitoring requirements.Therefore,the improvement of sensitivity has been one of the key research directions of optical fiber sensing technology,and it is of great significance to study the sensing mechanism of optical fiber sensing.The main content of this paper is focused on high sensitivity sensing.The interferometric fiber optic sensors and the fiber optic sensor based on optical vernier effect are studied.The temperature sensing experiments are carried out to verify the mechanism.The main research contents and experimental results are as follows:(1)A compact and high sensitivity sensor with a fiber-tip structure is proposed and demonstrated.The device is fabricated by splicing a tiny segment of capillary tube between single-mode fiber(SMF)to form an extrinsic cascaded Fiber Fabry-Perot interferometer(FFPI)with a glass cavity cascaded to an intrinsic FFPI with a narrow air-cavity.The theoretical and experimental results demonstrate that the ambient liquid RI and temperature can be simultaneously determined by the intensity and shift of the resonant wavelength in the reflection spectrum.Our proposed device has the highest RI sensitivity of ~252.37 dB/RIU at the RI value of 1.30-1.31 and a high temperature sensitivity of ~10.85 pm/ ?.(2)A device with dual cascaded FFPIs has been proposed and demonstrated,which works on vernier effect and exhibits a much higher temperature sensitivity.The device is fabricated by splicing a short segment of large mode area(LMA)fiber to a short segment of capillary tube fused with a section of SMF to form an extrinsic Fabry-Perot interferometer with a glass cavity cascaded to an intrinsic FFPI with a narrow air cavity.Firstly,the principle of temperature sensing based on vernier effect of this device is analyzed and simulated theoretically,and it is found that the temp erature sensitivity can be improved significantly by using vernier effect compared to that of a single FFPI with an air-cavity or glass cavity by directly tracing resonant dips/peaks.Enhancement mechanism of optical fiber sensing based on optical vernier effect is introduced based on interferometric sensor,and gives the formula of specific amplification;Additionally,summarizes the sensitization to achieve conditions and signal demodulation,and provides theoretical support for sensing scheme.(3)The temperature responses of the FFPI with single air-cavity and dual cascaded cavities were measured,respectively.Experimental results match well with the theoretical analysis carried out.The temperature sensitivity of the proposed sensor was ~18 times higher than the above-mentioned cascade FFPI sensor.Simultaneously,it has been improved greatly from 0.71 pm/? for a single FFPI sensor with an air-cavity to 179.30 pm/? by employing the vernier effect.Additionally,the sensor exhibits good repeatability within the temperature range of 100-500?.The proposed sensor has the advantages of compact size(<1 mm in dimension)and high sensitivity,which makes it promising for temperature sensing in a variety of industries,such as food inspection,pharmacy,oil/gas exploration,environment,and high-voltage power systems. |
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