Research On The Influence Of Energy Transfer Upconversion On Pulsed Neodymium–doped Quasi-three-level Lasers

The quasi-three-level transition in trivalent neodymium(Nd3+) ion could generate lasers around 900 nm, which could be employed in water vapor detecting and differential absorption lidar(DIAL) for ozone measurements. By using frequency doubling technology, blue lasers could be generate, which have plenty applications such as underwater communications, laser reprographics, and spectroscopy detection. According to the investigation on the research development of Nd3+-doped quasi-three-level lasers with different structure crystals, we analyzed the factors restricting the performance improvement and considered energy transfer upconversion(ETU) as a crucial factor. In view of this condition and background, this dissertation intends to investigate ETU process and its influence on Nd3+-doped quasi-three-level pulsed lasers.Based on interaction theory, the ETU micro processes are studied and the population time-dependence character during ETU process is obtained. The detailed research on energy levels in Nd3+-ion gives us the possible ETU processes. With the help of a simplified two-level rate equation, the influence of ETU on the laser parameters are studied, including small signal gain, effective lifetime and thermal loading. It is found that the absorption coefficient shows a dependence on the ETU coefficient in the condition of pump saturation. Thus we could obtain the ETU coefficient by a z-scan technique, which provides the absorption coefficient under different pump density. The peak absorption across section of Nd:YAG around 808 nm is measured to be 6.8±0.2×10-20 cm2 at room temperature and shows a decreasing trend with rising temperature. The cross relaxation coefficient of 3×10-18 cm3/s for 1.0at.% Nd3+-doped Nd:YAG is obtained and it remains nearly contant within the temperature range between 300 K and 450 K. Utilizing the information gathered about Nd:YAG, we are able to employ the z-scan technique to determine the temperature dependence of ETU coefficient, from the upper 4F3/2 laser level. As such WETU was determined to be 4.7±0.5×10-17 cm3/s at 300 K decreasing in a nearly linear fashion at a rate of ~0.4 %/K. The ETU coefficient is estimated to increase from 2.7×10-17 cm3/s to 7.5×10-16 cm3/s when the Nd3+ concentration increases from 0.3at.% to 1.1at.%. The ETU parameters in various neodymium-doped laser crystals(Nd:YAG, Nd:YVO4, Nd:Gd VO4, and Nd:YLF) are studied and compared, providing a database for further research on the influence of ETU effects.To investigate the output characters in Q-switched Nd3+-doped quasi-three-level lasers, we consider ETU effects in the population accumulation process, and build Q-switched quasi-three-level rate equation involving pump distribution, ground state absorption(GSA), intracavity photon and inversion population distribution. We analyzed the influence of GSA on the intracavity photon characters under different pumping distribution. The experimental data in actively Q-switched Nd:YAG 946 nm laser match the simulated results well, verifying the accuracy of our model. Besides the output characters of passively Q-switched Nd3+-doped quasi-three-level laser are simulated and discussed to estimate the effects induced by GSA and the initial transmission of saturated absorber(SA). By solving the population accumulation equation, the estimation of pulse rate frequency(PRF) is conducted with considering ETU effects. A laser diode(LD) end-pumped passively Q-switched Nd:YAG 946 nm laser is built and optimized, generating a peak power of 31.4 k W with a pulse width of 8.3 ns. By optimizing the laser parameters, the passively Q-switched Cr4+:YAG/Nd:YVO4 914 nm laser is also achieved with a highest PRF of ~2 MHz and a average output power of 3.8W.To further improve the output performance of pulsed 900 nm lasers, we study the quasi-three-level laser performance with new-type Nd:Lu VO4 crystal. The parameters including rod length, pump beam waist size and output mirror transmission in continuous-wave(cw) Nd:Lu VO4 916 nm laser are optimized, generating a 1.7 times higher efficiency compared with previous results. With a Cr4+:YAG SA, we research on passively Q-switched Nd:Lu VO4 916 nm laser. A PRF of 730 k Hz and an average output power of 0.85 W are achieved with a pulse width of 30.8 ns in pulsed 916 nm laser. An unstable cavity is employed to compensate the thermal lens effects in the laser rod and an average ouput power of 0.86 W at 20 k Hz is achieved in actively Q-switched 916 nm laser. Then pulsed 458 nm laser is obtained for the first time by using extracavity frequency doubling technology with Bi BO crystal. At 20 k Hz, the average power of pulsed 458 nm laser reaches 144 m W with a frequency doubling efficiency of 16.7% while a highest peak power of 927 W is obtained with a frequency doubling efficiency of 28.3% at 5k Hz.