Linear-polarized Tunable Narrow Linewidth Ytterbium Doped Fiber Amplifier

The high power,Yb-doped,linearly-polarized,tunable,and narrow-linewidth fiber amplifiers have many important applications in coherent combining,spectral combining,nonlinear frequency conversion and remote detection.However,the applications are now limited,as the output power and tuning range are limited by the stimulated Brillouin scattering(SBS),amplified spontaneous emission(ASE)and polarization.To achieve the purpose of high power,narrow linewidth,linearly-polarized and tunable fiber amplifiers,we study the processing of SBS and ASE,and corresponding suppression approaches theoretically and experimentally in this dissertation.Firstly,this paper demonstrates the limiting factors,reviews the current research status of high power,linearly-polarized,narrow linewidth and tunable fiber amplifiers at home and broad.It is indicated that only the SBS and ASE are suppressed,can the high power,linearly-polarized,narrow linewidth and tunable fiber amplifiers be developed.The second part is related to phase modulation.The SBS model is modified with the phase modulation being taken into consideration.And researches on parameters such as modulation frequency and modulation depth of three types of common-used signals(sinusoidal signal,white noise signal(WNS)and pseudo-random binary sequence signal(PRBS))are conducted with three-wave-coupled model to obtain feasible methods of linewidth optimization and power enhancement.The result indicates the optimized single-stage sinusoidal modulation frequency is ~100 MHz in the case that the Brillouin gain bandwidth is ~30 MHz.When laser seed is modulated by WNS,the SBS threshold can be raised by increasing modulation depth and modulation frequency.As for PRBS,the SBS mitigation effect is closely related to pattern length.Therefore,the PRBS with larger N is preferred in high modulation frequency case.The third part concerns the limitation factors of wavelength tuning range in narrow linewidth,tunable amplifiers.Based on the rate equations and small signal gain model,approaches for ASE mitigation are discussed.Considering parameters such as seed power,fiber length,doping concentration and core/cladding diameter,the amplifier output characteristics at different wavelengths are compared in detail to propose the guideline for the improvement of wavelength tuning range,output signal to noise ratio(SNR)and output power.It is found that the tuning range can be broadened by increasing seed power,the ratio of core diameter to cladding diameter and reducing fiber length.Besides,the tuning range can be red-shifted by increasing seed power and fiber length and decreasing the ratio of core diameter to cladding diameter.By adopting fiber length and core/cladding ratio matching laser wavelength,the output SNR is optimized.The last part demonstrates the experiments on high power,linearly-polarized and tunable fiber amplifiers.Based on the afore-discussed results on SBS and ASE mitigation,the linearly-polarized,tunable,narrow-linewidth fiber amplifiers are researched experimentally.By optimizing fiber length to suppress ASE and ultilizing high dopant,large mode area fiber to suppress SBS,we obtain single-frequency output that owns >250 W maximal output power,1065-1090 nm tuning range,30 MHz linewidth and >20 dB polarization extinction ratio(PER).After that,high power,broaden tuning range,narrow linewidth,high PER amplifiers with output power >1 kW,tuning range about 1065-1090 nm,linewidth about 0.12 nm,PER over 13 dB are obtained with phase modulation.Through theoretical and experimental studies,this paper demonstrates the SBS and ASE suppression approaches in high power,tunable,narrow-linewidth amplifiers,and obtained kilowatt-level linearly-polarized tunable lasers.Further,the potential of this amplifiers system to broaden tuning range and increase output power is also analyzed,which can be guidelines on high power,linearly-polarized,tunable and narrow-linewidth amplifiers.