Research On High-power Narrow-linewidth Yb-Raman Fiber Laser System

The power scaling of high-power single-mode narrow-linewidth fiber laser at long wavelength(1.1-1.2 μm)is limited by the gain competition,nonlinear effects,etc.Recently,the Yb-Raman fiber laser that utilizes both Yb ion gain and Raman gain has been applied for the generation of high-power fiber lasers at long wavelength,which could combine the power scaling capability of Yb-doped fiber laser and the wavelength flexibility of Raman fiber laser.This thesis mainly focuses on the experimental and theoretical research on high-power narrow-linewidth fiber laser system.The specific research content and results are as follows:1.The effect of seed lasers on the output characteristics of the Yb-Raman fiber amplifier is studied experimentally.Firstly,four types of fiber optical sources,i.e.fiber oscillator,random distributed feedback Raman fiber laser,superfluorescent fiber source,and phase-modulated single-frequency fiber laser,are used as the Raman-pumped seeds separately.The experimental results show that the output spectra are almost not broadened and the second-order Raman threshold is the highest when employing the phase-modulated single-frequency fiber laser in the Yb-Raman fiber amplifier.When the cost and system integration are considered,the employment of superfluorescent fiber source is also an effective solution to suppress the spectral broadening and the second-order Raman Stokes light.Secondly,a comparative study is conducted by using an oscillator or a phase-modulated single-frequency fiber laser as the Raman signal seed.The results indicate that the spectral linewidth could be easier to maintain and the second-order Raman threshold could be enhanced when the temporal property of the Raman signal seed is more stable.In addition,the effect of seed filtering in the YbRaman fiber laser is also studied.The results show that by filtering out the background spectral noise could help reduce the amplified spontaneous emission noise in the output laser effectively and alleviate the spectral broadening phenomenon,however the sideband spectra may be partially broadened.2.The power scaling of temporally stable Yb-doped fiber laser is investigated to provide high-power Raman-pumped seeds for narrow-linewidth Yb-Raman fiber amplifiers.Firstly,a phase-modulated single-frequency fiber laser modulated by a white noise source serves as a seed laser to suppress the spectral broadening and stimulated Raman scattering effect,and,as a result,2 k W narrow-linewidth Yb-doped fiber laser is obtained.Secondly,a phase-modulated dual-frequency fiber laser modulated by a sinusoidal signal is used as the seed laser and 2 k W temporally stable Yb-doped fiber laser is achieved,which provides a Raman-pumped source with lower cost and more compact structure for high-power narrow-linewidth Yb-Raman fiber amplifiers.Moreover,a backward-pumped Yb-doped fiber laser is built to increase the thresholds of transverse mode instability and stimulated Raman scattering effect,and 3.3 k W narrow-linewidth fiber laser is achieved.Finally,a homemade double-tapered Yb-doped fiber is used and the bending loss mechanism is also utilized to further improve the threshold of stimulated Raman scattering effect and transverse mode instability.Consequently,3630 W output laser with 0.21 nm linewidth and a maximum output power of 4180 W with 0.59 nm linewidth are obtained.3.The spectral evolution of the high-power Yb-Raman fiber laser is analyzed.Firstly,the spectral evolution model of the high-power Yb-Raman fiber laser is established by solving the nonlinear propagation equations and rate equations simultaneously.A fiber oscillator is used as the Raman-pumped seed in the experiment to verify the accuracy of the model in describing the output characteristics of Raman signal power,power of second-order Raman Stokes light,spectral shape,spectral linewidth,etc.Meanwhile,the advantages of using the superfluorescent fiber source as the Raman-pumped seed in suppressing spectral broadening and increasing the secondorder Raman threshold are demonstrated.Accordingly,a kilowatt-level Yb-Raman fiber laser is achieved experimentally.In addition,the physical mechanism of the effect of the Raman-pumped seed on the output properties of the amplifier is studied theoretically.The results indicate that the effect of the Raman-pumped seed mainly origins from the difference in the intensity noise transmitted from the Raman pump light to the Raman signal light.4.The high-power narrow-linewidth Yb-Raman fiber laser system is experimentally studied.Firstly,a forward pumped Yb-Raman fiber amplifier at 1120 nm is demonstrated using phase-modulated single-frequency fiber lasers as the Ramanpumped seed and Raman signal seed.The spectral broadening phenomenon and the generation of second-order Raman Stokes light are effectively suppressed.As a result,a maximum output power of 2 k W is obtained,which is the highest power ever reported in narrow-linewidth Yb-Raman fiber lasers.Secondly,the phase-modulated dualfrequency laser is used as the Raman-pumped seed,and the feasibility of realizing highpower narrow-linewidth Yb-Raman fiber laser is verified.This work provides a lowcost and compact Raman-pumped seed for the high-power narrow-linewidth Yb-Raman fiber laser system.Moreover,a narrow-linewidth tandem-pumped Yb-Raman fiber amplifier at 1120 nm is demonstrated with the maximum output power of 0.98 k W.Further power scaling is limited by the generation of second order Stokes light and the spectral sideband broadening.This work could provide a reference for the power scaling of narrow-linewidth tandem-pumped Yb-Raman fiber laser.5.The temporally stable high-power pump source is applied in the high-power random distributed feedback Raman fiber laser.Firstly,a hundred-watt level proof-ofconcept platform is built to compare the output characteristics of the random distributed feedback Raman fiber lasers pumped by the fiber oscillator and phase-modulated singlefrequency fiber laser.The results show that using temporally stable pump source(phase-modulated single-frequency fiber laser in this work)has advantages in power conversion,signal power ratio,the suppression of spectral broadening,the suppression of second-order Raman Stokes light,etc.Furthermore,the phase-modulated singlefrequency fiber laser is used as the pump source in a random distributed feedback Raman fiber laser,and 1.5 k W output power is obtained,which is the record power of currently reported random distributed feedback Raman fiber lasers.