Linewidth of collimated wavelength-converted emission in Rb vapour

We present a study of the spectral linewidth of collimated blue light (CBL) that results from wave mixing of low-power continuous-wave laser radiation at 780 and 776 nm and an internally generated mid-IR field at 5.23 μm in Rb vapour. Using a high-finesse Fabry–Perot interferometer, the spectral width of the CBL is found to be <1.3 MHz for a wide range of experimental conditions. We demonstrate using frequency-modulated laser light that the CBL linewidth is mainly limited by the temporal coherence of the applied laser fields rather than the atom–light interaction itself. The obtained result allows the same 1.3 MHz upper limit to be set for the linewidth of the collimated mid-IR radiation at 5.23 μm, which has not been directly detected.Alexander Akulshin, Christopher Perrella, Gar-Wing Truong, Andre Luiten, Dmitry Budker, Russell McLea