Fast online rubidium DPAL atomic concentration measurement by 420  nm probe laser

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 10¹³-10¹⁴cm^-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 5²S_1/2 to 6²P_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.