Fig. 1. Blue light energy versus the Rb atom density and the corresponding temperature of the Rb cell, measured with a fixed pumping energy. The inset is a spectrum of the blue light. The pumping-pulse energy used here is about 0.85 mJ. The blue light is separated from the residual pump light with a grating for measuring the pulse energy; the collecting efficiency for light of 420 nm is not optimized.

Fig. 2. Pulse energy and conversion efficiency of blue light versus the pumping pulse energy. Noting that the pump and output pulse energies are raw data, the transmission loss of the uncoated window (8%) and the diffraction efficiency of the grating (40%) are considered when calculating the efficiency.

Fig. 3. Pulse energy of the blue light versus pumping wavelength (recorded by energy meter). Pumping energy is stable when scanning the wavelength.

Fig. 4. Relative intensities of 420.3 and 421.7 nm blue lights versus pumping wavelength. Spectra were collected with a fiber-coupled spectrometer. The fiber cannot collect the whole blue spot like the pyroelectric detector used in Fig. 3, so larger fluctuations appear on curves in Fig. 4 than in Fig. 3.

Fig. 5. Typical two-photon absorption profile of Rb vapor. The pumping energy is stable when scanning the wavelength and the curve shows a decreased energy of output pulses when absorption happens.