Investigation On All-fiber Sensors Based On Femtosecond Laser Direct Writing

Optical fiber sensors are suitable for parameter measurement in a variety of extreme environments due to their light weight,compact structure and anti-electromagnetic interference,making them an indispensable and important part in the field of sensing.With the continuous development of laser processing technology,femtosecond laser direct writing technology has been widely used in the fabrication of various optical fiber devices because of its advantages of high processing rate,low thermal effect and high precision 3D processing.Compared with the traditional preparation and processing methods,femtosecond laser direct writing processing can meet the current requirements for miniaturization,high mechanical strength and all-fiber of sensor devices.Therefore,the all-fiber sensor based on femtosecond laser direct writing technology not only has important research value,but also has broad application prospects.In this thesis,two kinds of all-fiber sensors are designed based on femtosecond laser direct writing technology,and the corresponding theoretical analysis,device preparation and sensing measurement are carried out,respectively.The main contents of this thesis are as follows:1.An all-fiber sensor with Mach-Zehnder interferometer(MZI)and fiber Bragg grating(FBG)written in parallel in the fiber core was designed and fabricated.Based on the femtosecond laser direct writing technology,in the single-mode fiber(SMF)core,the effects of the transmission spectrum by different writing sizes of MZI,different writing positions of FBG,and the spatial distance between the MZI and FBG are investigated.The strain and temperature were measured simultaneously.The sensor has ultra-compact structure and good resistance to ambient humidity change,which is a promising high stability strain sensor.2.A fiber optic directional coupler was designed and demonstrated for multi-directional bending measurement.Based on femtosecond laser direct writing technology,two direct waveguides were written in parallel in a no-core fiber.One straight waveguide penetrates into the SMF at both ends,serving as a guide for input and output light,while the other straight waveguide was close to the first one,coupling and constructing a fiber optic directional coupler.The width and spacing of the two waveguides were optimized by simulation.Multi-directional bending measurement were carried out,and the experimental results indicate that the sensor can not only measure the bending size,but also distinguish the bending angle.In addition,the sensor has good anti-interference ability to environmental temperature and humidity changes,and combined with its inherent flexibility in preparation,it can become a potential candidate device in high-precision,vector bending measurement systems.