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 yb^{3+}-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-fiberyb^{3+}-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-switchedYb^{3+}-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 Yb^{3+}-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 Yb^{3+}-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. |