The spontaneous emission in a cw rubidium diode dumped alkali laser (DPAL) system was analyzed. The fluorescence from higher lying states decreases with additional buffer gas. The intermediate states (7S, 6P, 5D) decay more slowly with buffer gas and scale super-linearly with alkali density. A detailed kinetic model has been constructed, where the dominant mechanisms are energy pooling and single photon ionization. It also includes pumping into the non-Lorentzian wings of absorption profiles, fine structure mixing, collisional de-excitation, and Penning ionization. Effects of ionization in a high powered CW rubidium DPAL were assessed.
The spontaneous emission in a cw rubidium diode dumped alkali laser (DPAL) system was analyzed. The fluorescence from higher lying states decreases with additional buffer gas. The intermediate states (7S, 6P, 5D) decay more slowly with buffer gas and scale super-linearly with alkali density. A detailed kinetic model has been constructed, where the dominant mechanisms are energy pooling and single photon ionization. It also includes pumping into the non-Lorentzian wings of absorption profiles, fine structure mixing, collisional de-excitation, and Penning ionization. Effects of ionization in a high powered CW rubidium DPAL were assessed.
Diode-pumped alkali laser (DPAL) technology offers a means of achieving high-energy gas laser output through optical pumping of the D-lines of Cs, Rb, and K. The exciplex effect, based on weak attractive forces between alkali atoms and polarizable rare gas atoms (Ar, Kr, Xe), provides an alternative approach via broadband excitation of exciplex precursors (XPAL). In XPAL configurations, we have observed multi-quantum excitation within the alkali manifolds which result in infrared emission lines between 1 and 4 μm. The observed excited states include the 42FJ states of both Cs and Rb, which are well above the two-photon energy of the excitation laser in each case. We have observed fluorescence from multi-quantum states for excitation wavelengths throughout the exciplex absorption bands of Cs-Ar, Cs-Kr, and Cs-Xe. The intensity scaling is roughly first-order or less in both pump power and alkali concentration, suggesting a collisional energy pooling excitation mechanism. Collisional up-pumping appears to present a parasitic loss term for optically pumped atomic systems at high intensities, however there may also be excitation of other lasing transitions at infrared wavelengths.
The diode-pumped alkali laser (DPAL) is a new type of high-powered laser sources which has been paid much attention in recent years. The fluorescent spectra can be used to investigate how the collisions between atomic rubidium and various buffer gases are affected when a sealed rubidium vapor cell is pumped by a LD. In this study, the cross sections between the fine-structure levels of atomic rubidium in a vapor cell were first theoretically deduced by using a gas kinetic procedure. And then, the sensitized fluorescence was experimentally obtained by means of a series of spectral measurements. Finally, the influence of the temperature on the cross section between the fine -structure levels of atomic rubidium was studied with the systematical analyses. The results are thought to be helpful for deeply understanding the theoretical characteristics of a DPAL at the atomic physics level.
Alternative wavelengths for optically pumped alkali vapor lasers have been developed using single photon excitation of higher lying P-states, stimulated Raman processes, two-photon excitation of S and D states, and electric quadruple excitation on S-D transitions. Two photon excitation of Cs 72D leads to competing and cascade lasing producing red and infrared lasers operating on the D-P transitions, followed by ultraviolet, blue, the standard near infrared DPAL transitions operating on P-S transitions. The S-D pump transitions are fully bleached at pump intensities exceeding 1 MW/cm2, allowing for lasing transitions that terminate on the ground state. The kinetics of these systems are complex due to competition for population inversion among the many optical transitions. An optically pumped mid-infrared rubidium pulsed, mirrorless laser has also been demonstrated in a heat pipe along both the 62P3/2 - 62S1/2 transition at 2.73 μm and the 62P1/2 - 62S1/2 transition at 2.79 μm with a maximum energy of ~100 nJ. Performance improves dramatically as the rubidium vapor density is increased, in direct contradiction with the prior work. No scaling limitations associated with energy pooling or ionization kinetics have been observed. Practical application for infrared counter measures depends on the further development of blue diode pump sources. Finally, stimulated electronic Raman scattering and hyper-Raman processes in potassium vapor near the D1 and D2 lines have been observed using a stable resonator and pulsed laser excitation. First and second order Stokes and anti-Stokes lines were observed simultaneously and independently for a pump laser tuning range exceeding 70 cm-1. When the pump is tuned between the K D1 and D2 lines, an efficient hyper-Raman process dominates with a slope efficiency that exceeds 10%. Raman shifted laser may be useful as a target illuminator or atmospheric compensation beacon for a high power diode pumped alkali laser.
Our choice of rubidium atoms as the optically active species becomes now clear. The large elasticity of collisions between the electronically excited states of the rubidium D-lines and noble gas atoms has long been proven [25]. The quasi-molecular potential has a depth of the order of kBT, allowing for a significant part of the atoms’ kinetic energy to be extracted. Finally, the cooling transitions are in the near-infrared spectral regime, making them conveniently optically accessible to commercial laser systems. The cooling power Pcool from these experiments can be estimated in a similar way as for anti-Stokes cooling [26, 27] from the mean fluorescence frequency νfl, the incident power Popt, and the absorption probability of the gas mixture at incident frequency ν, α(ν).P Pcool opt fl= -a n n nn( ) (1.1) The typical energy that can be extracted from the sample per cooling cycle is of order of the thermal energy kBT. In the experiments performed so far, we observe cooling powers Pcool in the order of up to 100 mW [17, 23, 28, 29]. Note that it is important to choose a detuning with a high-enough absorption probability a(ν). We are aware that the collisional broadening plays a crucial role here. A measurement of typical fluorescence data recorded with the dense gas mixture will be shown in Section 1.4.1. Experimentally, it turns out to be crucial that the used buffer gas is of high purity to suppress the influence of concurring heating effects, of which quenching is the most known. In the case of quenching, the excitation energy is transferred to an atom of a third species polluting the dense gas mixture when colliding with the quasi-molecule. Relaxation can then occur via nonradiative decay, and so energy is kept in the system, heating the gas. To reduce possible quenching, an effect known to occur, for example, from an admixture with nitrogen, we use a buffer gas with high purity, namely argon gas preferably with a purity of 99.9999 (argon 6.0). Residual heating can also be caused by energy pooling. In this scenario, during the collision of two excited alkali atoms one of them gets further excited. Its relaxation to the ground state implicates the emission of a fluorescence photon with higher energy. For the case of rubidium atoms, the reaction equation of that process can be written as [30]: Rb Rb Rb Rb* * **+ + + Æ + + +mv mv mv mv12 22 12 22 2 2 2 2 . (1.2) The sum of the energies of the single excited rubidium atoms before the collision is higher than the energy of the twice excited atom. The surplus of the energy is thus kept in the system as kinetic energy of the collisional partners [31]. For rubidium atoms subject to a high-pressure noble gas, fluorescence originating from the 6p Æ 5s transition then occurs, and can be detected as blue light at a wavelength λ ≈ 420 nm in the experiments. At room temperature, the alkali atoms are in the solid phase. To produce a sufficient alkali vapor number density the system is heated to temperatures of up to 680 K, while the buffer gas pressure ranges between 50 bar and 230 bar. Under these conditions, we calculate that the dense gas ensemble consists of approximately 1017 cm-3 alkali atoms and 1021 cm-3 noble gas atoms. Then, we can estimate the mean free path length λfp for a gas of colliding atoms (cross section σ) and number density n l sfp = n (1.3) Let us consider the geometric cross section πd2, where d denotes the diameter of a rare gas atom, which is twice the van der Waals radius (d = 2rvdW = 3.76 × 10−10 m for argon [32]). Then, for argon pressure of 200 bar, the mean free path length is in the order of only a few nanometers, λfp ≈ 2.3 nm. Following the Boltzmann distribution, still considering a temperature T = 680 K, one can also estimate the collisional rate to 2.4 × 1011/s. Comparing this to the natural lifetime of the first excited alkali atom level (27 ns), we calculate that 104collisions occur within this period of time. 1.3 Experimental Setup and MethodsIn the following section, we present our experimental setup in detail, including the techniques used to measure atomic fluorescence and temperature changes inside the gas mixture. We offer insights into the high-pressure cells employed (Section 1.3.1), as well as into the spectroscopic techniques (Section 1.3.2) and the thermal deflection setup (Section 1.3.3).
Rate coefficients for energy-pooling collisions between low-lying excited states of Ba have been measured. The 6s6p(3P1) level in Ba is pumped by a cw diode laser, and radiative decay and collisions with buffer-gas atoms also populate the 5d6s(1D) and 5d6s(3D) metastable levels. The densities of these low-lying excited states are measured by the absorption of lines from a Ba hollow-cathode lamp, and the states populated by energy-pooling collisions are studied by comparing their fluorescence intensity to that of the laser-excited level. The rate coefficients are on the order of gas-kinetic rates, and they are not strongly dependent on energy defect, spin changes, or angular-momentum changes.
The alkali atom concentration plays an important role in the performance of a diode pumped alkali vapor laser (DPAL). At the rubidium DPAL operational region, the alkali concentration is as high as 1 0 13 − 1 0 14 c m − 3 , which is “optically thick,” or opaque, for the 780 nm or 795 nm doublet D lines when the traditional scanning absorption spectrum method is used for concentration measurement. To solve this problem, we propose the use of a probe laser of 420 nm, which corresponds to the 5 2 S 1 / 2 to 6 2 P 3 / 2 transition and has a lower absorption cross section compared to the D-line doublet. Due to the moderate absorption strength at a fixed 420 nm probe wavelength, we realized fast, online measurement of the Rb concentration in a real-world DPAL. By combining it with the quasi-two-level model, we further provided the population distribution in the lower three energy levels. This fast, online diagnostic method could be well applied in DPAL concentration measurement, and could show the dynamics of a laser’s performance.