Study On Power Scaling And Control Technology Of Random Fiber Laser

Conventional high power Yb-doped fiber amplifiers(YDFAs)usually use a typical Yb-doped fiber oscillator as the seed source,which suffers self-pulsation effect with temporal instability.The temporally instable seed source produces high peak power and causes strong nonlinear effects,which restrict the power scalability of the fiber laser system.Random fiber laser is a new type of fiber laser with temporally stable output based on simple configuration.It can be a practical solution to overcome the temporal instability of conventional seed source.This thesis investigates on the approaches and properties of random fiber laser especially in high power,high efficiency operation,control technology and other practical applications.The random fiber laser system utilizes Raman gain and random distributed feedback from long passive fibers.It has become a new attracting topic for scientific research,and contains advantages such as simple structure,high conversion efficiency,smooth spectral shape,temporal stability,etc.In this thesis,the theoretical model of random fiber laser is established based on the propagation equations with consideration on the Rayleigh scattering effect and appropriate boundary conditions.Firstly,the variation of random lasing thresholds is numerically calculated with regard to the fiber length and end reflections based on the theoretical model.The maximum output power and maximum optical conversion efficiency of 1st order random laser are also analyzed by predicting the thresholds for higher order Raman Stokes waves.Secondly,the half-opened cavity model is set up by changing the boundary condition based on the conventional random fiber laser with open cavity structure.The new random laser thresholds and the altered longitudinal power distributions by introducing the half-opened structural design are also studied.The calculation result shows that the half-opened cavity effectively reduces the lasing threshold,and both the forward and backward propagating Stokes waves are significantly gathered to ensure high power,high efficiency output from one single end.Emphasizing on the numerical calculations of the random fiber laser with short cavity-length,we discover that the short cavity-length model is quite sensitive to the weak parasitic reflections,and the power scalability of random fiber laser can be further advanced by simply cutting down the passive fiber length.Overall,the theoretical analyses and the numerical calculations reveal that the short cavity-length,half-opened cavity design is the key to realizing high power,high efficiency random fiber laser operation.The experimental research on realizing high power random fiber laser output is carried out.In this part,a short cavity-length random fiber laser is experimentally established based on the numerical calculations to realize high power operation.The laser system uses 120-meter-long passive fiber with half-opened cavity design.It achieves near 200 watt record power output at 1173 nm with an ultrahigh high optical conversion efficiency of 89%.Both the power level and the optical conversion efficiency are the highest values ever reported for random fiber lasers.Secondly,the random fiber laser working at 1070 nm wavelength is experimentally achieved for the first time.The temporal and frequency features of the laser system are carefully investigated,displaying the temporal stability and the noise filtering process.The 1070 nm working wavelength of random fiber laser has successfully matched with the gain spectrum of high power MOPA system for the first time.Finally,the temporally stable random fiber laser is applied as the seed source in a high power Yb-doped fiber amplifier with MOPA configuration.The highest output power reaches 1030 W with 1 nm FWHM,and the slope efficiency is around 79.7%.The nonlinear spectral-broadening effect is well restrained during the whole power amplification process.Such spectral-broadening-free property in high power fiber amplification has offerred significant reference for spectral beam combination and other applications that require narrow-linewidth high power lasing.Further studies on the control technology of spectral and mode properties are progressed in random fiber laser experiments.We establish random fiber laser systems with tunable and multi-wavelength spectral features based on a tunable filter and a polarization maintaining fiber loop mirror respectively.The achieved tunable or multi-wavelength spectra possess tens of nanometers wide spectral range with high signal-to-noise ratios.Another experimental setup based on few-mode fiber Bragg grating and random distributed feedback is built with efficient excitation of higher-order modes by the offset splice method.The system demonstrates an all-fiber laser capable of realizing switchable LP01 and LP11 spatial modes output.What's more,the linearly polarized random fiber laser is reported for the first time by using the fiber coiling method for polarization mode selection in an all polarization maintaining fiber laser configuration.