Enhancement of the Doppler-free saturated absorption signal based on ⁸⁷Rb second excited state transition at 420 nm

We report on the characterization of Doppler-free resonances detected by probing the 5S_1/2\to 6P_3/2 transition of the ^87Rb atom at 420 nm. Compared to the commonly studied 5S\to 5P transition, the 5S\to 6P transition offers advantages of higher transition frequency and narrower natural linewidth. We investigate how the pump laser intensity and cell temperature affect the linewidth and amplitude of the saturated absorption signal for the 420-nm second excited state transition 5S_1/2 (F=2) \to 6P_3/2 in ^87Rb atoms. Furthermore, we introduce a 795-nm repump laser to improve the quality of the saturated absorption signal. At a relatively low vapor cell temperature of 43 ^\circC, the amplitude-to-linewidth ratio increased by 110%. These results are highly relevant for the development of stable ultraviolet laser sources and their associated applications.