Research On High Sensitive Gas Pressure Sensing Technology Of Photonic Crystal Fiber

Optical fiber pressure sensors have been widely used in the fields of environmental monitoring and industry due to their compact structure,simple fabrication,high sensitivity,and immune electromagnetic interference.With the development of photonic crystal fibers,photonic crystal fibers based sensors have become hotspots for research owing to their excellent mode field area,wide single-mode transmission range,and high birefringence effects.In this paper,we propose three types of photonic crystal fibers based pressure sensor,and experimentally investigate the sensitivities of the proposed sensors with respect to gas pressure,temperature and stress.The main contents of this paper are as follows:1.Several common pressure sensing mechanisms based on photonic crystal fiber interference structures were studied,including Fabry-Pérot interference,Sagnac interference,Lyot filter,and Mach-Zehnder interference.Based on the Fabry-Pérot interference structure,the closed or open Fabry-Pérot cavity made of optical fiber is used to sense the gas pressure.The gas pressure causes the change of the cavity length to realize the sensing;Based on the Sagnac interference,the Lyot filter structure uses the birefringence of the polarization-maintaining fiber to perform pressure sensing.The change in gas pressure causes the birefringence of the polarization-maintaining fiber to change to achieve pressure sensing;Based on the Mach-Zehnder interference structure,a Mach-Zehnder interferometer made of optical fiber,the gas pressure changes the refractive index of one of the sensing arms,so that the optical path difference changes,thereby realizing the air pressure sensing.2.A type of transmissive Lyot filter gas pressure sensor based on polarization-maintaining photonic crystal fiber is designed and prepared.The sensor is a section of polarization-maintaining photonic crystal fiber connected between two optical fiber linear polarizers.The polarization-maintaining photonic crystal fiber is used as a sensing unit.When the gas pressure changes,it will cause the birefringence of the polarization-maintaining photonic crystal fiber to change,and the wavelength drift of the interference peaks will be monitored by the optical fiber spectrum analysis instrument to perform demodulation.The experimental results show that the sensitivity of this simple structure pressure sensor is 5.05nm/MPa,and the sensitivity of the gas pressure is independent of the length of the photonic crysral fiber.Furthermore,experimental tests were performed on its temperature and stress characteristics,and the device was found to be insensitive temperature and stress,the temperature and stress crosstalk sensitivity were 1.9 k Pa/? and 0.06 k Pa/??,respectively,so it had good stability.3.Through the optimization of the process,the transmissive Lyot filter sensor is improved into a reflective Lyot filter sensor.The device need only a optical fiber linear polarizer to be spliced with a section of polarization-maintaining photonic crystal fiber to form a probe-type gas pressure sensor.We theoretically analyze the feasibility of this solution,and experimentally prove that the pressure sensitivity of this sensor can also reach 3.94 nm/MPa,which is equivalent to that of the transmission type,but the process and cost are reduced by half.At the same time,experiments show that the sensor is also insensitive to temperature,and the temperature crosstalk sensitivity is 2.28 k Pa/?.4.In order to improve the sensitivity,we have designed a near-balanced Mach-Zehnder interference type gas pressure sensor.The device is simply construed by fusion splicing two sections of HC-PBFs,which are of slightly different lengths,between two 2×2 fiber couplers.The two output ends of each coupler are approximately equal in length.A femtosecond laser is employed to drill a micro-channel in one of the two HC-PBF arms,so that the gas can enter the core.When applied to air pressure test,when the external air pressure changes,the refractive index of the fiber core will be affected,which will cause the optical path difference to change.The experiment result shows that the proposed MZI based gas pressure sensor achieves an ultrahigh sensitivity,up to 2.39 nm/k Pa,which is almost the same as the theoretical calculation,and it is two orders of magnitude higher than that of the previously reported MZI-based gas pressure sensors.Additionally,we have found that the gas pressure sensitivity of the device increases with the absolute length of the hollow-core photonic band-gap fiber increasing,and decreases with the relative length of the hollow-core photon band-gap fiber increasing.The wavelength drift direction of the spectrum is related to the position of the sensing arm where the airflow microchannel is located.