Modeling And Characteristics Of Diode-pumped Cs Vapor Laser

Recently, alkali vapor lasers are researched extensively because they cover the wavelengths of blue light, near infrared and mid infrared. Among these lasers, the diode-pumped alkali vapor lasers(DPALs) are deeply studied by researchers especially. DPALs are pumped at the D2 line(n2S1/2â†'n2P3/2) and lased at D1 line(n2P1/2â†'n2S1/2) of alkali atom, thus their quantum efficiencies are very high(Cs,95.2%; Rb, 98.1%; K, 99.5%). The wavelengths of DPALs are located in near infrared(Cs, 894.6 nm; Rb, 795.0 nm; K, 770.0 nm). DPALs have the advantages of weak thermal effect, good beam quality, compact structure, small volume and so on, they belong to diode-pumped gas lasers. Due to these advantages, DPALs are highly potential high power lasers and have been operated at kilo Watt class by using Cs.Thus, it is hopeful to apply DPALs in the fields of laser processing, aerospace,medicine, and national defense. Therefore, the Cs DPAL is studied theoretically and experimentally in this thesis. The main contents include:1. Theoretical modeling. In consideration of collisional broadening effect of alkali D2 line and Gaussian spectrum of LD, the model of end-pumped continuous wave(CW) DPAL is established by utilizing the theory of rate equation and introducing the concept of effective pump intensity. The CW model is fast resolved with the use of fixed-point iterate algorithm and is proved validity at a certain event by following experiments. Then the pulse-pumped DPAL model is also established onthe basis of CW model.2. Theoretical analysis on CW DPAL. Firstly Cs DPAL is taken for example, the intensity dependences of population density of each energy-level, gain and absorption characteristics, which are in the states of small-signal gain building, gain saturation and steady oscillating, are analyzed respectively. Results show that, in the process of creating laser, both the effective pump intensity and gain coefficient are decreasing from their small-signal values but the absorption coefficient increasing, and finally they all reach constant values simultaneously. Meanwhile, the threshold problems of Cs DPAL are also deeply studied, including the threshold pump intensity and the critical values of temperature, gain length, stimulated emission cross-section, and fine-structure relaxation rate. Results show that, values of the lowest temperature,shortest gain length, smallest stimulated emission cross-section and slowest fine-structure relaxation rate are restricted by each other. Then, the CW output characteristics of Cs DPAL are simulated, and key parameters of temperature, buffer gas pressure(helium and ethane) and output mirror reflectivity are optimized simultaneously. Finally, the characteristics of hydrocarbon-free Rb DPAL and K DPAL under each optimum state are comparatively studied. Results show that,hydrocarbon-free K DPAL has less fluorescence emission, higher gain coefficient and lower threshold pump power.3. Theoretical analysis on pulsed pumped DPAL. In Cs DPAL, the temporal evolution processes of population density of each energy-level are simulated and the relaxation oscillating are analyzed. Results show that, primary spike pulse with extremely short pulse-width and relatively high peak-power could be formed in the relaxation oscillating of Cs DPAL. By utilizing this characteristic, Cs DPAL could be gain-switched to obtain laser pulse with high peak-power and good temporal resolution, which is named gain-switched Cs DPAL. Then, parameters such as peak-power and pulse-width are calculated for gain-switched Cs DPAL. Results show that, the pulse-width could be shortened and peak-power could be increased by optimizing the output mirror reflectivity and temperature, and shortening the cavitylength as well.4. Experimental research. The experiment platform of Cs DPAL is designed and established. 1) On the condition of CW pumped mode, the output mirror reflectivity and collimating lens focal length are optimized, and the dependences of laser characteristics on the parameters like temperature are analyzed. Finally, a CW 894.57 nm Cs laser with fundamental transverse mode and line-width of 0.032 nm is outputted, and the maximum CW power, slope efficiency and optical to optical efficiency are 1.74 W, 29.15%, and 24% respectively. 2) On the condition of pulse pumped mode, the power and slope efficiency of quasi CW Cs DPAL are studied, the thermal effect is discussed, and the waveform and pulse-width are measured. Finally,a pulse Cs laser with maximum peak-power of 3 W, maximum slope efficiency of24.2% and maximum optical to optical efficiency of 20.53% is obtained.