A New Optical Fiber Sensor Based On F-P Cavity Sensitization

As environmental parameters such as temperature,humidity,strain and refractive index have gradually become important indicators in daily life and industrial production,the detection of environmental parameters has become a widely concerned issue.Compared with semiconductor sensors and photoelectric sensors,optical fiber sensors have the characteristics of high safety,simple structure,good repeatability,high sensitivity and high resolution,and are widely used in environmental monitoring fields.Among various optical fiber sensors,fiber Fabry-Perot interferometer(FPI)sensors have the characteristics of fast response,high sensitivity,wide measurement range and high resolution,and have been widely concerned and studied by researchers.In recent years,researchers have widely used the Fabry-Perot interference principle in the design and fabrication of fiber optic sensors.A large number of optical fiber FPI sensors are used in the engineering industry to achieve environmental monitoring,and how to further improve the sensitivity of optical fiber sensors has become an urgent problem for scholars.In this paper,three high-sensitivity fiber optic sensors based on FPI are designed by using two sensitization mechanisms,sensitive materials and vernier effects,to improve the sensitivity of optical fiber sensing structures.This article examines in detail the physical mechanisms that improve sensor sensitivity,with the following highlights:(1)Briefly describe the research background and research significance of optical fiber sensors,focus on the research progress of recent scholars on the sensitization method of optical fiber sensors,and focus on the sensing mechanism of interferometric optical fiber sensors: fiber FPI and fiber optic Sagnac interferometer(FSI).Starting from the principle of signal superposition,the mechanism and spectral characteristics of optical vernier effect are discussed,the sensitivity amplification formula of the wiper effect sensor is derived,and the relationship with the sensitivity of a single FPI is found,which provides a theoretical basis for the subsequent design of sensitized fiber optic sensor.(2)The sensing principle of the optical fiber FPI sensor based on the sensitive material(ultraviolet glue)was studied,the FPI structure of the optical fiber end face coated with the sensitive material of ultraviolet glue was made,and the experimental device for temperature and air pressure was built.The temperature and pressure experimental results show that the trough wavelength drift of FPI has a good linear relationship with the environmental variables.At 55 ℃ to 90 ℃,the maximum temperature sensitivity of the fiber FPI reaches 1.209 nm/℃,and the linearity is:98.93%.At 0 MPa to 0.3 MPa,the maximum barometric pressure sensitivity reaches4.715 nm/MPa.Through long-term stability experiments,it is found that the maximum error of measuring air pressure and temperature of the sensor is 0.068 MPa and 0.19 ℃,respectively.(3)Based on the principle of vernier effect,a highly sensitive temperature and barometric fiber optic sensor was prepared.The sensor consists of a Fabry-Perot instrument(FPI)and a Sagnac interferometer(SI).FPI consists of a section of quartz capillary inserted into the femtosecond fiber and microchannels processed in the quartz capillary using a femtosecond laser to circulate air inside and outside the Fabry-Perot(F-P)cavity.SI and FPI in the sensor structure are sensitive to temperature and pressure selectivity.In the temperature experiment,SI is used as the sensing unit,FPI is used as the reference unit,and in the pressure experiment,FPI is the sensing unit and SI is the reference unit.The sensor is highly sensitive to both temperature and pressure.The experimental results show that the temperature sensitivity of the sensor is increased by ～13.4 times,up to 23.14 nm/°C,and the barometric pressure sensitivity is increased by about 12.2 times,up to 49.2 nm/MPa.(4)Combining vernier effect,sensitive materials and Fabry-Perot interferometer(FPI),a sensor for measuring relative humidity and temperature with high sensitivity was proposed and experimentally verified.Two segments of single-mode fiber(SMF)are fixed on a precision displacement platform,aligned and leave small gaps,and then filled with polydimethylsiloxane(PDMS)and polyimide(PI),respectively,to form two Fabry-Perot(F-P)cavities.In order to produce a vernier effect when the two F-P cavities are connected in parallel,the length of the two F-P cavities is controlled to ensure that the free spectral range(FSR)of the two F-P cavities is similar.The results show that the F-P cavity filled with PI is sensitive to relative humidity and temperature,while the F-P cavity filled with PDMS is only sensitive to temperature.For the different characteristics of the two F-P cavities,the sensitivity measurement matrix is constructed to realize simultaneous measurement of relative humidity and temperature.They produce a vernier effect when connected in parallel,resulting in relative humidity and temperature sensitivity of-11.388 nm/%RH and 18.436 nm/°C,respectively.