Resonant four-wave mixing with co-propagating scheme in rubidium vapor cell

We have observed four-wave mixing (FWM) of weak pumping in a resonance double-Λ system for the 5S_1/2 - 5P_1/2 transition of ⁸⁷Rb atoms. When three beams (CPT1, CPT2 and pump) were co-propagated in the double-Λ configuration composed of the common excited state 5P_1/2(F_e=2) and the two ground state 5S_1/2(F_g=1 and 2), we directly measured the generated FWM signal filtering the three beams using polarizer and etalon filters. The spectral width of FWM signal was measured to be ~10 kHz under the condition of coherent population trapping (CPT). Dependence of FWM signals on the intensities of the two beams related CPT and pump beam was investigated in detail.