Research On Fiber Sensing Techniques Based On Femtosecond Laser Direct Writing Optical Waveguides

Optical fiber sensors have outstanding advantages such as light weight,compact structure,good robustness,anti-electromagnetic interference,corrosion resistance,etc.They can also realize long-distance sensing,and build sensing networks.Therefore,optical fiber sensors are widely used in various aspects of civil and military fields.The development of femtosecond laser direct writing technology has provided a new way for the preparation of novel fiber sensors.Compared with other fabrication processes,femtosecond laser direct writing technology has the advantages of high processing accuracy,flexible parameter adjustment,and no need for fiber sensitization treatment.Femtosecond laser direct writing technology can flexibly write complex waveguides in fibers,thus easily accomplishing the fabrication of various new fiber devices.In the last decade,fiber sensors based on optical waveguides prepared by femtosecond laser direct writing have been studied one after another.There is a large gap in this field,which needs to be further explored.In this dissertation,the femtosecond laser direct writing optical waveguide technology has been used to prepare a variety of new fiber sensors with better performance,lower cost and more compact structure,and their working principles and sensing applications have been systematically investigated.The main work of this dissertation is as follows:（1）Femtosecond laser is used to directly write a straight waveguide in the fiber core to excite higher-order modes in a few-mode fiber.A novel in-fiber Mach-Zehnder interferometer（MZI）is prepared based on this method.The operating principle and spectral characteristics of this in-fiber MZI are investigated.The curvature response and temperature crosstalk of the device are tested.The device has a wide range of curvature measurement(0～10 m^-1)and low temperature crosstalk(about 1.2×10^-3 m^-1/°C).（2）A cascaded in-fiber MZI and fiber Bragg grating（FBG）structure is inscribed in a single-mode fiber（SMF）using femtosecond laser direct writing technology through the coating of the fiber.The combination structure can be used to measure curvature and temperature simultaneously.The MZI interference dip wavelength and the FBG resonance wavelength are only sensitive to curvature and temperature,respectively.Compared with other reported curvature and temperature sensors,the device has the advantages of simple preparation,no assembly,good mechanical properties,and compact structure（4 mm）.（3）A novel in-fiber MZI with strong polarization dependence is fabricated by using femtosecond laser direct writing technology.The device enables a highly sensitive intensity demodulation-based torsion sensor.By setting the appropriate initial polarization state,direction-identifiable torsion sensing can be achieved.Compared with the reported torsion sensors based on intensity demodulation,the device has the advantages of compact structure（5 mm）,high torsion sensitivity（5763.96 d B/（rad/mm））,low temperature/strain crosstalk,simple preparation,and good mechanical properties.（4）A high-order long-period fiber grating（HO-LPG）is fabricated by using femtosecond laser direct writing technology.The effect of the size of the diaphragm in front of the femtosecond laser focusing objective on the aspect ratio of the induced refractive index modulation region has been investigated to optimize the fabrication of the HO-LPG.A series of high-order Bragg resonance peaks can be observed in the reflection spectrum of the HO-LPG,which can be used for real-time temperature monitoring.The resonance peak of long-period fiber grating in transmission spectrum can be used for two-dimensional vector curvature sensing.Compared with other intensity-demodulated curvature sensors based on grating structure,the device has obvious advantages in curvature sensitivity(22.463 d B/m^-1),temperature crosstalk（about 0）,and structure size（2.1 mm）.（5）Two refractive index modulation forms induced by femtosecond laser in one step are combined to construct an ultra-compact dual-effect line-by-line inscribed fiber Bragg grating（LBL-FBG）.The dual-effect LBL-FBG has the characteristics of both FBG and MZI.Unlike conventional LBL-FBG,the femtosecond laser induces both positive refractive index modulation（PRIM）and negative refractive index modulation（NRIM）in one step when fabricating the dual-effect LBL-FBG.A series of line-by-line inscribed PRIM regions overlap to form an equivalent straight waveguide,which is the key to the MZI effect.The NRIM regions are independent of each other and do not overlap,so a series of NRIM lines with fixed periods form a conventional LBL-FBG structure.The construction of the dual-effect LBL-FBG further broadens the fabrication approach of femtosecond laser direct-writing optical fiber devices.