Study On Diode Pumped Rubidium Vapor Lasers

As a hybrid gas phase/solid state laser, diode pumped alkali vapor lasers (DPALs)have great potential in the future high power laser field due to its many advantages, forexample the high quantum efficiency, convenient thermal management by flowing thegaseous medium, high transmittance for near infrared laser spectrum, and electricallydriven compact system. In the past decade, the concept and power scaling ability ofDPALs have been demonstrated, and a high efficient CW kilowatt-class DPAL has beensuccessfully realized. Due to the potential and importance of DPALs, we have madestudies on the diode pumped rubidium vapor lasers. The main contents are presented asfollows:1. For rubidium atom, the optical properties and its interaction with buffer gasesare analyzed, the rate equation based model for longitudinally pumped DPALs isproposed as well as the fast convergent and high accurate numerical algorithm, based onthe model the kinetics of DPALs are studied. The results show that, for alkali lasers, thelasing process will dramatically enhance the pump absorption, these lasers are workingin an “atomic engine” mode rather than extracting energy from the small signal gain.The saturation effect is due to the insufficient fine-structure mixing rate, which can besolved by adjusting the component and pressure of buffer gases or by increasing thealkali concentration, and the ideal functional mode for lasers should be in a quasi-twoenergy state. As a three-level laser, the pump intensity should exceed far beyond thethreshold for effective fluorescence suppression. To balance the pump absorption andfluorescence loss, an optimal operation temperature exists for a highest opticalconversion efficiency. At constant pump intensity, the match of narrowed pumplinewidth and low pressure buffer gases will benefit high optical conversion efficiency,as a contrast, the situation that by using pump sources with broader linewidth and buffergases with higher pressure need more intense pump to obtain the same efficiency, butthe latter case shows better tolerance to the shift of wavelength. In longitudinallypumped configuration, the length of gain medium and operation temperature areequivalent. At last, the influence of inner cavity loss and output coupler are analyzed.The conclusions above will be important for a practical alkali laser system design.2. The linewidth narrowing on high power diode lasers are experimentally studied.By use of Littman configuration external cavity on a broad area single emitter laserdiode (BAL), we realize10W output with linewidth below0.06nm, the tuning range is5nm and efficiency is60%. By use of a compound external cavity on a laser diode array(LDA), we successfully suppress the smile effect and obtain41W output power withlinewidth below0.1nm and efficiency of53%. By coupling the LDA into the volumeBragg grating (VBG) based external cavity, we obtained74W output power with0.1nm linewidth and95%efficiency. Due to the high efficiency and compactness, we decide touse the VBG scheme as pumping source for rubidium laser study.3. The characteristics of a rubidium DPAL are experimentally studied. First, westudy the absorption spectrum, pump absorption and fluorescence characteristics of arubidium gain medium with buffer gases of helium and methane, based on this, we usethe VBG coupled LDA to do the pumping experiment with longitudinal configuration atQCW operation mode. As a result, we obtain1.4W peak power with795nm output, theoptical conversion and slope efficiencies are4.8%and7.5%. The analyses show themain reason for low efficiency are the low pump absorption, low mode overlap factorand high inner cavity losses. The threshold behavior is studied by observing the changeof beam pattern when raising the operation temperature. At a total CW pump power of56W, the lasing process last4.4s and terminate due to the damage of cell windows. Theanalyses show that the reason is the reaction between alkali atoms and ethane gas,maybe also the Pyrex window material, that induced by high local temperature rise, andan efficient thermal management could solve the problem.4. The model for transversely pumped DPALs with flowing medium is set up andvalidated by comparing with other researchers’ published experimental results. Basedon this, the power scaling schemes for DPAL with configuration of mutually orthogonalpump, laser and flow directions are studied. The results show that, at a constant andreasonable pump intensity, the increase of the gain length as well as the pump powercan realize linear power scaling and decouple with thermal management, which will bethe main scheme to realize high power DPALs. The design of width and height shouldcomprehensively consider many factors, such as thermal management, operationtemperature, output beam shape, and pump focusing etc. The comparison of single-side,single-side double-pass, and double-side configurations are made and analyzed. Amega-watt class DPAL is conceptually designed, the result shows that the laser couldrealize optical conversion efficiency over85%with all the other parameters reasonableor could be realized in the near future, which demonstrate the great potential of DPALs.5. The model for DPALs in MOPA configuration is set up, a new method tocalculate the ASE effect in a bulk gain material is proposed, which considers both thelongitudinal and transverse ASE effects and couple them into rate equations, and therange of ASE can be calculated. Based on this, the important influencing factors arestudied, such as operation temperature, seed and pump intensities. The results show thatby use of high seed laser intensity to realize saturated amplification, the ASE effect canbe effectively suppressed and will not become a bottleneck in power scaling of DPALs.The study of power scaled scheme show that the increase of gain length and thecorresponding pump power will be the first choice.