| Mode-locked laser pulse is widely applied in various areas, including laserprocessing, biomedical field, laser ranging, laser spectroscopy, optical communication,etc., for its short pulse duration, high peak power and other advantages. In recentyears, the mature development of the SESAM mode-locking technique has enabledsubstantial practicality of passive mode-locking technique. With the development ofthe high-power mode-locked laser technology, the direct output power ofmode-locked laser oscillator has grown increasingly high, and already been able tomeet usage requirements in some areas. The repetition rate of mode-locked laser pulseis determined by the resonant cavity length. In consideration of implementationcomplexity, a solid-state mode-locked laser more often used is around1m-3m inlength, with a corresponding pulse repletion rate of50-150MHz. Currently, in manyactual applications, the pulse frequency required is mainly at the MHz or even lowerlevel, while the demand for pulse energy keeps growing. A capacity-dumped laseroutputs pulse energy between the laser oscillator stage and the amplifier. Amode-locked laser incorporating the cavity-dumping technique can dump high-powerultra-short pulse out of cavity without changing the length of the resonant cavity, withthe repetition rate being adjustable by the cavity dumper. Meanwhile, as compared tothe master oscillator power amplifier (MOPA) laser, it has a simple structure and iseconomically efficient. Therefore, the cavity-dumped mode-locked laser is a type oflaser of great research value. This paper combines the SESAM and in-band pumpingtechniques, and by use of GM (gain in meddle) resonant cavity and throughoptimization of parameters, obtains high-average-power (18W) picosecond laseroutput without post amplifier. With the cavity dumping technique being applied to ahigh-power mode-locked laser, picosecond laser output with high single pulse energy(1.4μJ) and at high repetition rate (1MHz) is obtained.1. This paper introduces the research value and application prospects ofmode-locked lasers, and summarizes the technical evolution as well as methods ofimplementation of major high-power mode-locker lasers and cavity-dumpedmode-locked lasers at present.2. The effective absorption coefficients of the Nd:YVO4crystal for808nm and888nm pumped laser were measured and compared. Based on the measurementresults, a simulation analysis of crystal thermal stress was conducted with theLASCAD software, and it is pointed out in the paper that the888nm pumping mode can provide higher injected power under a thermal stress condition bearable to thecrystal. The output power of a short-cavity laser in the two pumping modes wasmeasured, and it is pointed out in the paper that the two modes of pumping obtainapproximating optical efficiency of the short-cavity laser. The thermal focal lengths oftwo Nd:YVO4crystals with respective dimensions of4×4×15mm3and4×4×30mm3in the888nm dumping mode were measured, and based on the measurement results,the design method for the resonant cavity of a large-mode-volume end-pumpedall-solid-state-mode-locked laser is discussed. A research was conducted to compareand analyze the characteristics of GE-cavity (gain in end) and GM-cavity modelocked lasers, and it is pointed out in the paper that the GM resonant cavity is moresuitable for a large-mode-volume telescope-type mode-locked laser.4×4×15mm3and4×4×30mm3Nd:YVO4crystals with respective doping concentration of0.5at.%and1at.%were used to set up a SESAM mode-locked laser, respectively, generatingCW-mode-locked pulse output with the repetition rate of40MHz at the maximumaverage power of13.5W and CW-mode-locked pulse output with the repetition rateof30MHz at the maximum average power of18W, respectively. The variationrelationship between the duration and line width of pulse output by the laser and thepump power at a fixed OC rate was measured, and also explained by referring to theGM cavity characteristics.3. According to the rate equation, simulative computation of the dynamicprocess of pulse output of the regenerative amplifier and the cavity-dumpedmode-locked laser was conducted by the Runge-Kutta method. An analysis is made ofthe influence of such parameters as pulse output frequency, small-signal gaincoefficient, loss coefficient and resonant cavity length on the dynamic characteristicof pulse of the regenerative amplifier. Under the condition of completely identicalparameters, the three crystals, Nd:YAG、Nd:YVO4and Nd:YLF, were compared interms of dynamic characteristic of pulse, when they were used as regenerativeamplifier gain medium. The differences between simulation models of dynamiccharacteristic of the cavity-dumped laser and the regenerative amplifier are comparedand analyzed, and a method is proposed for computing the boundary conditions of acavity-dumped mode-locked laser. The influence of small signal gain coefficient, losscoefficient, dumping ratio and resonant cavity length on the dynamic characteristic ofpulse of the cavity-dumped laser is compared and analyzed, and according to theresults concluded form the simulative computation, a method is proposed for preventing multiple periods of pulse energy output by the cavity-dumped laser.4. Experiments on a high-repetition-rate SESAM mode-locked cavity-dumpedlaser were conducted, generating single-energy-value cavity-dumped mode-lockedpulse output at the repetition rate of100KHz-1MHz. At the repetition rate of1MHz, mode-locked pulse with pulse duration of30ps and with maximum pulseenergy of1.4μJ was obtained. A quarter-wave plate was inserted into the cavity tointroduce loss, and by a comparison between the experiments and the simulationresults, the validity of the simulation of multiple periods of pulse by means ofcavity-dumped mode-locked dynamic model was verified. |
PDF Full Text Request