| As a new type of optically pumped gas lasers,diode-pumped metastable rare gas lasers?DPRGLs?have earned a lot of attention for its potential in high-power output with good beam quality.Till now,10-W-level continuous-wave?CW?laser output has been realized and1000-W-level power amplification is in progress.This article takes Ar/He gas mixture as the specific object in both experimental and theoretical studies.An external cavity diode laser?ECDL?with a volume Bragg grating?VBG?is used for linewidth narrowing of a single-emitter diode laser.Using a VBG with a diffraction efficiency of 70%and a bandwidth of 0.05 nm,the linewidth is narrowed to be only 1.2%?10 GHz?of that in free running mode,close to the absorption linewidth of metastable Ar atoms?Ar*?at an atmospheric pressure of He?8 GHz?.The power efficiency is approximately 65%and the maximum power output of 6.5 W is obtained.This ECDL pump source can be used for Ar*pumping.Capacitively coupled parallel-plate RF discharge is suitable for production of large-volume plasma.13.56 MHz RF discharge in a sealed chamber is used for excitation of Ar/He gas mixture.Static long-term glow discharge?more than 3 hours?can be realized at 1.3atmospheres?Ar/He=1:49?.The size of plasma is 4 cm3 and the current RF Ar*number density,measured by means of tunable diode laser absorption spectroscopy,is in the range of10100 to 1011 cm-3.Optical emission spectra analysis is employed for plasma diagnosis of gas temperature,electron excitation temperature and electron density.The impact of gas pressure,discharge state?glow or arc?and electrode types?bare metal electrodes or dielectric isolated ones?on the plasma parameters above are analyzed,as well as the influence of water vapor and nitrogen content on Ar*population.Simulations for longitudinally diode-pumped Ar*lasers provide guidence for subsequent experimental study.Factors influencing the optical-optical efficiency and the Ar*density are analyzed and discussed in the double-pass longitudinal pumping structure Ar*laser.When the number density of Ar*is on the order of 1013 cm-3,the small signal gain coefficient of Ar*lasers can be as high as1 cm-1,and the optical conversion efficiency will reach 55%.The influence of the reduced field intensity?E/N?on the number density of Ar*is analyzed,and it shows that the E/N required is at least 10 Townsend(1 Townsend=10-17V·cm2)to achieve efficient excitation of the 1s5 energy level.A dual-wavelength pumping scheme is proposed for the‘bottleneck'problem caused by the low thermal relaxation coefficients between the 1s4 and 1s5 energy levels at room temperature.Numerical calculation results show that the problem can be eased efficiently by adding a second pump light?assisted,965.7 nm,close to the wavelength of one kind of high-power laser diodes?with weak pump intensity?1%of the main pump intensity at 811.53 nm?along the 1s4-2p10 transition,without increasing the number density or heating the working gases.Experimental study of diode-pumped Ar*lasers is conducted.Continuous wave laser output of diode-pumped Ar*lasers is realized operating in the static mode,and the beam profile indicates a high-quality mode(close to TEM00 mode).The absorbed pump power is estimated to be1.5 W?pump intensity of 90 W/cm2?through the 4 cm3 plasma.The results indicate that the gas pressures have direct impact on the spatial mode and power of output laser as well as the electrode types.With the goal of power amplification,a transverse pumping double-pass model for DPRGLs is established and theoretically simulated,in which some intra-cavity information can be obtained including the number density of energy levels,the spatial distribution of pump light,waste heat and fluorescence,and the spectral distribution of the pump light.The plasma volume required for different output power levels is simulated,and the results show that for a megawatt-class output,the required discharge volume is at least a few liters.The thermal management problem of DPRGL under the megawatt output level is striking,but can be solved through the existing high-power gas laser technology.Theoretical studies and experimental explorations of DPRGL in this paper have achieved some meaningful conclusions and will contribute to further development of DPRGL. |
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