Research On Fiber Curvature Sensors Based On Femtosecond Laser Direct Writing

Fiber sensors have raised much attention since they were first proposed in 1970s,which contributes to their unique advantages,such as compactness,stability,high sensitivity and immunity to electromagnetic interference.As a result of the development of fiber sensor technology and the growing demand for sensor devices in this information age,more and more fiber sensors are adopted in various fields such as construction health,machinery manufacturing,medical treatment,energy exploration,aviation,environmental monitoring and so on.Meanwhile,femtosecond laser micromachining with good precision in three-dimensional space machining and high flexibility in the choice of materials was proposed in 1990s and applied in the fabrication of optical devices.This technology is maturing at an extraordinary pace,which offers a new method for the manufacture of fiber sensor.This paper focuses on the technology of femtosecond laser micromachining and later applies it to the realization of new fiber curvature sensors.Both simulation and experimental measurement are conducted to give a research into the properties of the sensors.The main work and conclusions are summarized as follows:?1?The physical mechanisms in the femtosecond laser micromachining are described,which mainly includes nonlinear absorption and avalanche ionization.We analyze the influence of the main experimental parameters,and based on that,we realize different structures with our femtosecond laser micromachining system.These structures include fiber bragg gratings?FBGs?,different kinds of waveguides in SiO₂ glass and micro-hole in hollow fiber.?2?A novel fiber curvature sensor based on in-fiber Mach-Zehnder interferometer?MZI?is proposed.It consists of a section of no-core fiber and the waveguide which is made by femtosecond direct writing in it.Through three-dementional simulation with Beam Propagation Method?BPM?,we find the relationship between the interference spectra and the geometrical structure.The sensitivity depends on the bending direction with the maximum of 14.98 nm/m^-1.The interference of temperature can be ignored.The sensor shows simplicity,mechanical robustness and high sensitivity.?3?A novel fiber sensor for simultaneous vector curvature sensing and temperature sensing using an integrated structure including an in-fiber MZI and a FBG in fiber core.The whole structure is made by femtosecond direct writing in the same section of SMF.The properties of the sensor are investigated through a three-dimensional model with BPM.The wavelength of the interference dip exhibits a large linear response ranging from 0 to 10 m^-1for curvature measurement with the maximum sensitivity of 1.09 nm/m^-1.The temperature sensitivities of FBG and MZI are 10 pm/?and-11 pm/?,respectively.