| High-average-power all-solid-state pulsed sum frequency generation sodium beacon lasers have many advantages,such as high average power,tunable linewidth,high repetition rate,long pulse,compact structure and so orn.Wlien they interact with the sodium atoms in the atmosphere,there will be high saturation threshold,strong backwards resonance scattering,and high signal-to-noise in the generated beacon.Therefore they can be used to solve the problem of all-seasons and all-hours application of artificial laser guide stars.Study on the high-performance sodium beacon lasers can supply important scientific tools for human astronomy detecting and imaging,it can also be beneficial for the development of the advanced all-solid-state lasers.Based on the principle of sodium beacon,the research contents in the thesis are focused on the extra-cavity sum frequency generation physical course,the generation and amplification of the fundamental lasers,and the spectral properties of the modulated longitudinal modes.The main researches and the specific contents are shown as follows:Aiming at the large signal sum frequency generation course with high-rank Gauss beam focused spot,arbitrary power ratio of the injection lasers,presence of phase mismatching due to thermal effect in the actual high-average-power laser system,a high-average-power,Gauss beam,high-efficiency SFG theoretic model is established.The method of conbing finite unit decomposition and Runge-Kutta calculation is utilized to solve the model.The influence of the nonlinear crystal length,absorption coefficient,laser waist radius,total incident power,beam quality to the SFG efficiency is calculated.Based on the model,a novel sheme with a strong 1064 nm laser and a weak 1319 nm laser incident into lithium triborate(LBO)crystal is proposed to get high average power 589 nm laser.When the beam quality(M2 factor)of the 1064 nm and 1319 r1n laser is 1.8,3 respectively,with the total incident power of 250W?300W,the LBO's absorption coefficient of 30 ppm,and the crystal lenth of 50mm or 60mm,by optimizing the two fundamental lasers5 mode matching,up to 40%SFG efficiency is obtained,i.e.more than 100W average power sodium beacon laser will be generated.A new type of sodium beacon laser spectral structure,modulated longitudinal mode,and its realization method are proposed.An ultra-narrow linewidth continuous-wave(CW)single longitudinal mode laser is used as the 1064 nm laser initial seed source,and then the technology of white noise electro-optic phase modulating is adopted to broaden the linewidth.The 1319 nm laser is designed as a multi-longitudinal-mode laser by insertion two combined etalons into the resonator.Then the modulated-longitudinal-mode sodium beacon laser is generated by the two infrared lasers' frequency mixing.It is revealed by the experimental results that when the 1064nm single-longitudinal-mode initial linewidth is less than 100 kHz,it can be broadened to 0.43 GHz-0.58 GHz by phase modulating.The linewidth of 1319 nnm multi-longitudinal-mode is about 0.30 GHz.Finally,the 589 nm laser^s linewidth is broadened to 0.31 GHz?0.74 GHz tunably.It is quite in accrod with the theoretical results in some error range after comparison.It is the first reported tunable linewidth in sodium beacon lasers all over the world.An ultra-high gain,high beam quality,smooth long-pulse 1064 nm laser is accomplished by combined amplification of an fiber laser and two bulk solid-state lasers.Firstly,the CW single frequency fiber amplifier is intensity-modulated to obtain 150mW,M2?1.15,high-repetition-rate,long-pulse seed laser.And then it is power-amplified by two stages of solid-state amplifiers.A non-stable-state amplification theoretical model is established to calculate the amplifiers' output laser properties.It is found that the error between the experimental results and theoretical calculation of the pre-amplifier is-12.4%?7.32%,whereas the error of the main amplifier is-10.8%to-4.3%.Finally,the output power of the 1064nm laser is 214W(with the effective linear polarized power of 209W incident to LBO crystal),the total gain to the initial CW single frequency laser is about 5.7×105,with the beam quality M2 of about 1.82 and the pulse width of 150?s.The generation and the amplification of repetition-rate long-pulse 1319 nm multi-longitudinal-mode laser are researched.The laser is in the structure of master oscillator and power amplifying(MOPA),the master oscillator uses a high beam quality Fabry-Perot cavity to generate 4.65W,160?s fundamental-mode laser,and a second harmonic generation crystal is inserted into the cavity to suppress the relaxation oscillating spikes.The amplifiers contained three groups of Nd:YAG laser modules to amplify the 1319 nm laser by single pass.It is shown that the amplified power is 16.3W,5 LOW,143W respectively,which is less than 19.7%,7.9%,7.1%of the theoretical calculations.The main reason of the much larger error of the first-stage amplifier is that the peak pumping power density is as high as 15.92kW/cm2,which is 3.34 and 8.08 times of the following two amplifiers.Consequently,the depleting of the inversion populations due to the amplified spontaneous emission(ASE)is much larger the others.Finally,the effective linear polarized power incident to LBO crystal is 120W with the beam quality M2 of about 3.04.A single-beam average power of 106W,near-diffraction-limit 589nm sum frequency generation solid state laser is developed for the first time in the world.The temperature phase matching properties of LBO crystal is measured.And it is found that the maximum accepted temperature is 8.0K·cm with the minimum of 6.9 K·cm,it is quite accord with the theoretical value 7.18K·cm.The influence of the power matching ratio of 0.11?9.0 to the SFG efficiency is experimentally researched.When the total incident laser power exceeds 100W,the deviation of the SFG efficiency at 1?1.5 power matching ratio is basically less than 5%,quite accord with the theoretical calculation.Furthermore,the experimental SFG efficiency of two types of LBO crystal with the size of 3mm×12 mm×50mm and 4 mm×12 mm×60mm is studied at different total incident fundamental laser power.It is foxrnd that the actual SFG efficiency is 0.87?1.16 and 0.74?0.84 times of the theoretical values.When the total laser power of 319 W is incident into the LBO crystal of 4 mm×12 mm×60mm,106W 589 nm laser is generated.The SFG efficiency is about 33.2%with the beam quality M2 of about 1.29 and the pulse width of?150?s.Two similar-properties,individual lasers,which output D23-line(589.1591nm)laser and D2b-line(589.1571nm)laser respectively,are polarization-combing in the same aperture.Then the dual-frequency re-pumping sodium beacon laser is obtained without the energy loss.And the ratio of the D2b-line laser to the whole laser can be up to 50%.The beam quality ? is?1.42.The dual-frequency laser is applied to detect sodium beacon and adjust the atmosphere wavefront aberration by adaptive optics in the sky test.It is found that the return flux is 1.93 times of that pumped only by D2a-line laser at the same average-power.The return flux can reach more than 56800 ph/s/cm2 during the pulse length.It is estimated that the saturation intensity is determined to be about 1400 W/m2,which is about 22 times of that of single frequency circular polarized laser.As we can see,the saturation effect of the SGS laser is greatly restrained and this modulated spectral structure is efficient and promising in the future higher average-power sodium beacon lasers. |
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