【作者】 黄琳；

【导师】 刘永智；

【作者基本信息】 电子科技大学 ， 光学工程， 2009， 博士

【摘要】 纳秒（ns）量级的短脉冲激光在激光雷达、激光测距、光时域反射（OTDR）和激光加工方面有广泛的应用。尤其是波长1．06μm的短脉冲很容易被金属、塑料及陶瓷材料吸收，因此更适宜应用在激光加工方面。本文针对产生ns量级短脉冲激光的调Q掺Yb³⁺光纤激光器进行了深入的研究。结合实验室现有条件进行了如下工作：（1）概述调Q光纤激光器理论，对实现高功率调Q激光器的若干种方法进行了比较分析。讨论了几个重要因素：放大的自发辐射光（ASE）、Q开关速度和效率、泵浦吸收、光纤结构对主动调Q光纤激光器性能的影响。阐述了用于进行被动调Q的光纤中受激布里渊散射（SBS）的原理。（2）对基于光纤中SBS相位共轭效应的自调Q掺Yb³⁺双包层光纤激光器进行了理论和实验研究。从相位共轭的原理出发，对描述光纤中信号波与声波互相作用产生后向斯托克斯（Stokes）波的纳维-斯托克斯方程及能量传输方程数值求解得到光纤中不同信号光功率下的SBS反射率，并由此通过对描述掺Yb³⁺双包层光纤激光器的速率方程数值求解分析了不同单模光纤长度下，脉冲重复频率和脉冲能量与泵浦功率的关系。搭建掺Yb³⁺双包层光纤作为增益介质、多个长度单模光纤作为SBS池的调Q光纤激光器进行实验研究，在作为SBS介质的单模光纤长度为2km、泵浦功率为742mW时输出较稳定激光脉冲，脉冲宽度约8．2ns，重复频率6．3MHz，平均功率207mW。（3）从干涉环的滤波原理出发，对带有光纤干涉环的掺Yb³⁺双包层自调Q光纤激光器进行了理论和实验研究。通过结合SBS相位共轭原理和干涉环原理对掺Yb³⁺双包层光纤激光器速率方程数值求解，分析了构建光纤干涉环的耦合器的耦合率及泵浦功率对输出激光脉冲重复频率的影响；根据Patrick Even关于掺Yb³⁺双包层光纤（YDDCF）激光器中光子数守恒的半数值模型分析了耦合器的耦合率及泵浦功率对输出脉冲平均功率和脉冲能量的影响。实验测试了不同泵浦功率下脉冲重复频率和平均功率，在泵浦功率为1．21W时，激发了波长比光纤布拉格光栅（FBG）中心反射波长红移0．12nm的二阶Stokes脉冲，输出激光脉冲的平均功率为374mW，脉冲宽度为15．7ns，重复频率约9．6kHz。（4）从高双折射（Hi-Bi）光纤萨格纳克（Sagnac）环形滤波器原理出发讨论了利用受压电陶瓷（PZT）周期性调制的Hi-Bi光纤Sagnac环形滤波器和窄线宽FBG对光纤激光器实现主动调Q的原理，并设计全光纤结构的调Q掺Yb³⁺双包层光纤激光器进行了实验研究。实验中PZT驱动方波电压信号的频率为10kHz、高电平为8V。在泵浦功率3W时获得了脉冲宽度1．13μs，输出平均功率935mW的激光脉冲，激光光谱中心波长1064．18nm。实验验证了这种调Q方法是可行的。（5）从Sagnac干涉环原理出发，阐述了非线性放大环镜（NALM）的非线性传输特性和开关阈值功率，并利用表现为对光强依赖性的NALM开关特性设计了基于NALM的全光纤调Q掺Yb³⁺双包层光纤激光器，并对该激光器进行了实验研究。实验在NALM放大倍数为13dB、泵浦功率1．1W时，得到稳定的激光脉冲，该脉冲3dB宽度为39．7ns，重复频率17．24kHz，平均输出功率325mW，输出激光光谱中心波长为1064．219nm。实验验证了这种调Q方法是可行的。（6）在不同初始边界条件下数值求解描述全光纤声光（AOM）调Q掺Yb³⁺双包层光纤激光器的速率方程，讨论了泵浦方式和输出耦合镜反射率对激光器输出脉冲特性及光纤中储能的影响。根据仿真结果从谐振腔内腔损耗和放大自发辐射光（ASE）光的建立时间及强度对激光器输出信号光平均功率、脉冲宽度、脉冲能量、光纤中储能的影响两个角度进行分析讨论。（7）从描述主动调Q激光器中光子数和上能级粒子数的速率方程出发，分别导出了以获得最大脉冲能量为出发点或以获得最大脉冲峰值功率为出发点来对增益光纤长度进行优化得到的数值解；分析了一定泵浦功率和腔内往返被动损耗下，对于不同输出耦合镜反射率，激光峰值功率和脉冲能量等激光参量的特性；据此分析得到不同输出耦合镜反射率下，优化增益光纤长度的不同方法。更多还原

【Abstract】 The short laser pulses with nanosecond magnitude have great applications forextremely Lidar, range finding, OTDR and laser machining. The short pulses on 1.06μmare proper to laser processing because they are absorbed by metals, plastic and ceramicmaterials easily. On the base of our laboratory environments, this dissertation is focusedon Q-switched yb³⁺-doped fiber lasers which include1. Theory of Q-switched fiber laser was summarized. Comparison and analyseswere fulfilled for methods of realizing high-power Q-switched fiber lasers. A theoreticalanalysis of several characteristics which can be critical to performances of activelyQ-switched fiber lasers was given ,such as amplificated spontaneous emission （ASE）,power and rate of pump, choices of fibers, repetition rate of acousto-optic modulator.The principle of Stimulated Brillouin Scattering （SBS） has been expatiated.2. The generation of pulse laser exploiting SBS phase conjugation was investigatedtheoretically and experimentally. Based on the principle of phase conjugation, theinteractivity of signal wave and acoustic wave was studied by numerical calculatingNavy-Stokes equations and energy transfer equations, thus, SBS reflectivity wasobtained for different signal power. The relationship between pulse repetition rate, pulseenergy and pump power was obtained by numerical calculating rate equations whichdescribe the ytterbium-doped double-clad fiber lasers. To investigate the relationshipbetween the length of SBS medium and the attribute of output laser, an all-fiberyb³⁺-doped Q-switched laser was designed. When the length of SMF was 2km, a trainof pulse laser was generated with pulse duration of 8.2ns, pulse repetition rate of6.3MHz, average output power of 207mW at pump power of 742mW. The results showthat the generation of pulse laser is correlative to the length of SMF which is used asSBS pool.3. Based on the principle of filter action of interference ring, the generation ofpulse laser exploiting interference ring was investigated theoretically andexperimentally. The rate equations of ytterbium-doped double-clad fiber lasers were solved under principles of fiber interference loop and stimulated Brillouin scattering bynumerical simulation, to obtain the relationship between pulse repetition rate, pumppower and couple ratio of the coupler which constructed the fiber interference loop. Therelationships between average output power, pulse energy, pump power and couple ratiowere obtained by exploiting the model based on photon balance. A self-Q-switchedYb³⁺-doped double-clad fiber laser was employed experimentally to test how the pumppower affect average power and pulse repetition rate as well. A train of pulse laser wasgenerated with pulse duration of 15.7ns, pulse repetition rate of 9.6kHz, average outputpower of 374mW at pump power of 1.21W. The results show that the increase of pumppower can increase both average power and pulse repetition rate but pulse energy, theincrease of pulse energy needs to choose appropriate couple ratio and the second-orderStokes pulse will generate if the pump power is higher.4. The principle of active Q-switch exploiting narrow linewidth fiber Bragg grating（FBG） and Sagnac fiber loop filter constructed by Hi-Bi fiber was studied based on theprinciple of Sagnac fiber loop filter. The filter is modulated by piezoelectric ceramic（PZT） periodically. An all-fiber Q-switched Yb³⁺-doped double-clad fiber laser wasemployed experimentally. The modulating voltage and frequency of PZT were 8V and10kHz, respectively. A train of pulse laser was generated with pulse duration of 1.13μs,average output power of 935mW at pump power of 3W. The central wavelength ofspectrum was 1064.18nm. The experiment proved this Q-switching is feasible.5. The principle of passive Q-switch exploiting narrow linewidth fiber Bragggrating （FBG） and nonlinear amplification loop mirror （NALM） was studied based onthe on-off character of NALM. An all-fiber Q-switched Yb³⁺-doped double-clad fiberlaser was employed experimentally. When the magnification times of NALM was13dB, a train of pulse laser was generated with pulse duration of 39.7ns, averageoutput power of 325mW at pump power of 1.1W. The central wavelength of spectrumwas 1064.219nm. The experiment proved this Q-switching is feasible.6. The rate equations of all-fiber acousto-optic Q-switched lasers was solved underdifferent initial boundary conditions by numerical simulation to obtain the distributionof upper-level population density along the doped fiber under forward pumping andbackward pumping, and to understand the relationship between pulse energy, averagepower, pulse width, stored energy and pulse repetition rate, signal transmittance of the pump coupler, pump power, coupling ratio of output coupling mirror respectively. Howthe pumping manner and output coupling mirror affect the attribute of output pulse laserwere studied. The analysis of simulation was performed from viewpoint of theestablishing time and power of amplified spontaneous emission light. The results showthat pumping manners affect performance of all-fiber acousto-optic Q-switched laserssignificantly under different pulse repetition rates, so the lasers should be forwardpumped for better characteristics of output pulses under lower pulse repetition rate, andthey should be backward pumped under higher pulse repetition rate.7. Based on the rate equations of Q-switched fiber lasers, the optimum fiber lengthof Q-switched fiber laser for either maximum pulse energy or maximum pulse peakpower was investigated. The relationship between the optimum fiber length and theoutput coupler reflectivity is got by using the mathematical technique of Lagrangemultipliers and numerical computation. As a result, with the fiber length optimized,output pulse energy and pulse peak power can be expressed as functions of the outputcoupler reflectivity, multiplied by a few simple constants. The results show that, at agiven pump power level and a certain round-trip parasitic loss coefficient, there is anoutput coupler reflectivity of demarcation which is inversely proportional to theround-trip parasitic loss. Fiber length should be optimized to yield maximum pulseenergy when the output mirror’s reflectivity is less than the demarcation point, and itshould be optimized to yield maximum pulse peak power when the output mirror’sreflectivity is more than the demarcation point.更多还原