Study On Ionization Process In Diode Pumped Rubidium Vapor Laser Gain Medium

As a new type of diode pumped gaseous laser, diode pumped alkali vapor lasers(DPALs) have both advantages of traditional solid-state and gaseous lasers, and possess the potential for single-aperture power scaling. Although DPAL works in a simple three-level mechanism, alkali atom has many high lying energy levels and low ionization energy, so various ionization channels exist in DPALs. As the diode pump power increased, whether these ionization channels could develop into limiting factors on power scaling become an issue that worth to be considered. In this thesis, we study the ionization channels in diode pumped Rb DPAL gain medium1. Firstly, the physical mechanisms of the main ionization channels in Rb DPAL were introduced including energy pooling, photoexcitation, penning ionization, photoionization, associative ionization and Hornbeck-Molnar ionization process etc., the recombination processes were introduced including radiative recombination and three-body recombination processes, and the diffusion effect of both ions and neutral atoms were described. A multilevel transient rate equation model for DPAL was set up by considering the ionization channels, combination process and diffuse effect. The model was solved by using the Matlab code.2. The time-based evolution of the ionization process was theoretically studied. At typical DPAL operation conditions with buffer gas pressure of ~1atm, pump intensity of ~10k W/cm2, and the temperature of T ~130?, the ionization process experienced ~10-3ms to reach a steady state. It was found that, the diffusion process played an important role on both the time evolution and steady state value of the ionization degree. When the diffusion process was not considered, the ionization process experienced ~10-2ms to reach the steady state, and the steady state ionization degree(~10%) was far beyond the value(~0.05%) when diffusion process was considered. The result implies that a flowing gain medium may solve the ionization issue in a high power DPAL. The influences of different ionization channels on ionization process were also studied.3. By using optogalvanic method, the quantitative measurements of the ionization degree in diode pumped Rb DPAL gain medium were conducted. The influences of important parameters on both the photocurrent and ionization degree were studied and analyzed, including the pump intensity, the buffer gas pressure and the temperature(the Rb concentration). At helium pressure of 500 torr, pump intensity of 1.5k W/cm2, and temperature of 130?, a maximal ionization degree of 5×10-5 was obtained, which was a weak plasma.4. In order to study the photoexcitation effect that induced by the far wing excitation(5P?5D, 776nm) of the pump light(780.2nm), a 776 nm linewidth narrowed diode laser was realized by using a Littrow configuration. The linewidth was narrowed from 2.5nm to 0.13nm(FWHM), the output power was 10 W, and external cavity efficiency was 67%. The Rb gain medium was pumped by a combination of both the 776 nm and 780.2nm diode lasers, an enhanced 420 nm fluorescence was observed, but the ionization degree showed a decrease by compared with the case of single 780.2nm pumping. The reason was analyzed, but need further validation.