Nonlinear optical filter with ultranarrow bandwidth approaching the natural linewidth.

We demonstrate a nonlinear optical filter with ultranarrow bandwidth in cesium vapor. The optical filter operates on the 6S(1/2)→7P(3/2) transition at 455 nm. The single peak transmission at the 6S(1/2), F=4→7P(31/2), F'=5 transition is 9.7% with a bandwidth of 6.2 MHz, whereas at the 6S(1/2), F=3→7P(3/2), F'=2, 3 (cross-over) transition is 6.1% with a bandwidth of 3.9 MHz. The bandwidth approaching the natural linewidth is improved at least two orders of magnitude compared with conventional Faraday anomalous dispersion optical filters. This technique can also be applied to other alkali atoms.