Study On Polarization Control And Sensing Characteristics Of Narrow Linewidth Fiber Grating Laser

In recent years,narrow linewidth fiber grating lasers have been widely used in modern information society,such as in the fields of fiber communication system,fiber sensor,coherent laser radar,microwave photon system and terahertz due to their single frequency,narrow linewidth,ultra-low noise,strong anti-interference,ultra-high monochromaticity and ultra-high coherence.In this work,we have systematically studied a kind of narrow linewidth fiber laser based on active phase-shifting fiber grating and its application in sensing field.In this work,the theoretical simulation and preparation of the phase-shifting fiber grating,the optimization of the resonator structure and linewidth measurement of the fiber laser,the birefringence control and polarization characteristics of the laser,and the sensing application of the high birefringence narrow linewidth fiber laser are studied.The main contents of this work are listed as followed:1.The fabrication of fiber grating by UV laser phase mask scanning has been systematically studied.We have successfully designed and built an experimental device for fabricating fiber grating by UV laser phase mask method.We have systematically studied the fabrication process of fiber grating.By using static phase mask method and dynamic phase mask method,we have successfully written high-quality uniform fiber Bragg grating(FBG)on hydrogen-loaded fiber.The reflectivity is more than 99%.We have studied the grating bandwidth with the different grating length and reflectivity.In addition,high-quality phase-shifting fiber grating(PS-FBG)was fabricated by UV laser shielded phase mask scanning method.The PS-FBGs with different phase shifted values were fabricated by different scanning speed and scanning length.The 3 d B bandwidth of the phase shifted peak was less than 4 pm.Based on the coupling mode theory and the transmission matrix theory of the fiber grating,the spectral characteristics of FBGs and PS-FBGs are simulated.The effects of fiber grating length and core refractive index modulation on the spectra of FBG and PS-FBG are analyzed.The preparation parameters of FBG and PS-FBG are summarized through the combination of theoretical analysis and experimental research.2.A narrow linewidth fiber laser based on symmetric step-apodized phase-shifted fiber grating is proposed and demonstrated.The performance of narrow linewidth fiber grating laser is systematically studied.A high-performance distributed feedback(DFB)fiber laser based on PS-FBG inscribed in an active erbium-doped fiber using a UV laser phase mask scanning method has been obtained.The output performance of DFB fiber laser is analyzed and tested,including pump threshold power,slope efficiency,stability and linewidth characteristics.In order to improve the slope efficiency of DFB fiber laser,the structure of PS-FBG is optimized.The symmetric step-apodized PS-FBG is proposed.The refractive index of the core on both sides of the phase shifted region is symmetrical step-apodized distribution,which could effectively improve the output power of DFB fiber laser.The slope efficiency of the symmetric step-apodized DFB fiber laser is higher than that of the uniform DFB fiber laser.The DFB fiber laser has a narrow linewidth(2.6 k Hz)and high stability(after 24 hours of long-term operation,the fluctuations of the lasing wavelength and output power are less than 12 pm and 0.13 d B,respectively).3.Two polarization control techniques for narrow linewidth fiber grating lasers are proposed and demonstrated.The fiber side polishing technique and femtosecond laser direct writing stressor technique are successfully used to realize the polarization control of the fiber grating laser.Firstly,the fiber side polishing system is established.The birefringence of the PS-FBG could be changed when the resonator of the fiber grating laser,i.e.,the active PS-FBG,was side polishing,resluting in the beat frequency could be tuned.The influences of the polishing thickness,the polishing roughness and the polishing direction on the polarization beat frequency of the fiber grating laser are systematically studied.The beat frequency signal of the fiber grating laser could be adjusted from 475.5 MHz to 2080.4 MHz.Moreover,a preparation method of single polarization DFB fiber laser is proposed.The threshold power of one polarization state of the DFB fiber laser is changed by fiber side polishing technology to ensure that only one polarization state can be normally excited,resulting in obtaining the single polarization operation of the fiber laser.In addition,a setup of femtosecond laser direct writing stressor system is built.An 800 nm femtosecond laser is used to write stressor directly in the cladding of active fiber to realize polarization regulation of fiber grating laser.The birefringence of the fiber grating can reach a level(3.0×10-4)of commercial polarization maintaining fiber such as Panda fiber by introducing straight and sawtooth stressor in fiber cladding.The beat frequency signal of the DFB fiber laser could be adjusted 121.1 MHz to 3393.4 MHz using the femtosecond laser direct writing stressor technique.4.The sensing characteristics of the dual-polarization fiber grating laser are studied.Firstly,the high birefringence fiber grating based on the sawtooth stressor is used as the sensor head to realize the simultaneous strain and temperature measurement.The Bragg wavelength of fiber grating and the wavelength difference induced by the dual-polarization mode are used to demodulate.The experimental results show that the strain sensitivity is 1.24 pm/??,2.14×10-2 pm/??,and the temperature sensitivity of 9.52 pm/°C,0.13 pm/°C,respectively.Moreover,a kind of highly birefringent DFB fiber laser sensor based on femtosecond laser direct writing double stressors structure is studied.By monitoring the beat signal and the lasing wavelength of the fiber grating laser,the strain and temperature could be measured simultaneously.The strain sensitivity is 34.5 k Hz/??,1.25 pm/??,and the temperature sensitivity is 684.6 k Hz/°C,11.5 pm/°C,respectively.In addition,the DFB fiber laser transverse pressure sensor is also systematically studied.The transverse pressure is measured by monitoring the beat signal of the DFB fiber laser.The experimental results show that the response of the DFB fiber laser to transverse pressure exihibits obvious directionality,and the maximum response sensitivity to the transverse pressure is 132.89 MHz/(N/mm).In this work,the design,construction,optimization and application of the narrow linewidth fiber grating laser are systematically studied from the aspects of the theoretical simulation,system construction,device processing,and performance test,etc.The narrow linewidth fiber grating lasers have significant academic significance in the optical communication field used as an ideal coherent light source and great application value in fiber laser sensing field.