Fabrication And Sensing Characteristics Of Interferometric Fiber-polymer Microstructures

Microstructures fabricated by two-photon polymerization(TPP)technique are widely applied in micro mechanical,micro optical,and biological and medical fields owing to their advantages of high resolution,high precision and biological compatibility.In recent years,the microstructures on the optical fiber end-faces fabricated by using TPP technique have been reported to modulate the output light field.These structures are widely applied in integrated devices as the complex photoelectric conversion system is effectively avoided.Optical fiber sensors also have important applications in the integrated micro-device owing to their small sizes,light weights and anti-electromagnetic interference.However,their sensitivities in temperature,strain and acoustic detection are difficult to be further improved due to the low thermal expansion coefficient and high young's modulus of fiber material.In this paper,we proposed novel fiber sensing structures by integrating the microstructure on the fiber end using TPP technique.Their sensing properties were investigated theoretically and experimentally.The research work has been summarized in details as follows.A transmission Mach-Zehnder(MZ)fiber temperature sensing structure with high sensitivity was fabricated by using TPP technique.A polymer waveguide with length of 300 ?m was fabricated inside the fiber micro-cavity by using femtosecond laser TPP technique,which forms a MZ interferometer.The high quality transmission spectrum was obtained by precisely controlling the length of polymer microstructure and its position inside the micro-cavity.Both the theory and experiment investigating result show that,as the polymer waveguide is with higher coefficient of thermal expansion,the temperature sensitivity of proposed MZ fiber sensing structure reaches-205 pm/°C.A reflection liquid cavity Fabry–Pérot(FP)fiber sensing structure with high temperature sensitivity was fabricated by using TPP technique.An ultracompact rectangular liquid polymer cavity was fabricated on the end face of the single-mode fiber by using TPP technique.The light was reflected at the end face of optical fiber and two interfaces of solid polymer wall,which forms three waves FP interference.Compared with the thermal expansion coefficient of solid polymer(52 ppm/°C),the one of liquid polymer reaches 4.5×105 ppm/°C.Therefore,through sealing the liquid polymer inside the FP cavity,the temperature sensitivity of the proposed sensing structure reaches up to 877 pm/°C.Compared with the same size of all solid state polymer fiber temperature sensing structure,its sensitivity is increased by 3 times.An ultracompact open cavity FP with high refractive index(RI)sensitivity was fabricated by using TPP techniqu.A triumphal arch polymer microstructure was fabricated on the end face of fiber by using TPP technique,which combined with the fiber forms a three waves FP open cavity.As the measured liquid can be filled into the FP open cavity,the optical difference was changed directly.The RI sensitivity of proposed sensing structure can reach up to 1539 nm/RIU.In practice,the RI of liquid is affected by the ambient temperature,using the sensitivity matrix method,the liquid RI and temperature can be simultaneously determined by measuring the temperature RI sensitivities of two peaks,the measurement accuracy of liquid RI was improved.An ultracompact suspended thin film FP fiber sensing structure with high acoustic sensitivity was fabricated using TPP technique.The theory of FP fiber acoustic sensing structure shows that the acoustic sensitivity is related to the laser work wavelength.By precise controlling the interference length of FP,the largest sensitivity of FP just was fixed at the laser wavelength of 1550 nm.A suspended thin film FP fiber acoustic sensing structure with cavity length of 16 ?m and film thickness of 1.5 ?m was fabricated by using TPP technique,it's the largest slope of reflection spectrum was just at 1550 nm.When the frequency is 1 k Hz,the acoustic sensitivity of suspended thin film FP reaches up to 1 V/Pa,and the linearity reaches up to 94.814%.