  We present a study of the spectral linewidth of collimated blue light (CBL)
that results from wave mixing of low-power cw laser radiation at 780 nm and 776
nm and an internally-generated mid-IR field at 5.23 um in Rb vapour. Using a
high-finesse Fabry-Perot interferometer the spectral width of the CBL is found
to be less than 1.3 MHz for a wide range of experimental conditions. We
demonstrate that the CBL linewidth is mainly limited by the temporal coherence
of the applied laser fields rather than the atom-light interaction itself.
Results obtained with frequency modulated laser light allow an upper limit of
several hundred kHz to be set for the linewidth of the collimated mid-IR
radiation at 5.23 um, which has not been directly detected.
