Demonstration of a low-pressure Rb amplifier with an FADOF-coupled ultra-narrowed diode pumping source

Diode-pumped alkali lasers (DPALs) are promising candidates for high-energy lasers. Traditionally, the volume Bragg grating coupled diode lasers were used as pumping sources with a spectral linewidth of ∼100 pm (FWHM). To match the pump spectrum, buffer gases such as helium at atmospheric pressure are typically employed to broaden the alkali atomic absorption line. In this study, we conducted an ultra-narrow diode laser with pm-level linewidth and used it as the pumping source to demonstrate a low-pressure rubidium vapor amplifier. By inserting an Rb Faraday anomalous dispersion optical filter (FADOF) into the external cavity, the wavelength of the diode laser was strictly locked to the Rb D₂ line (∼780.25 nm), and the linewidth was narrowed from 1.7 nm to 3.9 pm (1.9 GHz). Using this pumping source, effective amplification was achieved even at a CH₄ pressure as low as 50 Torr. At 200 Torr CH₄ pressure, the system attained an amplification factor of ∼344 with a large gain coefficient of 2.9 cm^-1, comparable to the amplification factor of 348 obtained at 400 Torr CH₄. The low-pressure DPALs pumped by pm-level ultra-narrow diode lasers offer advantages such as low pump threshold, high optical conversion efficiency, and good beam quality, which has potential for power scaling applications.