Experiments using sub-wavelength diameter tapered optical fibers in Rubidium vapor

Abstract

In this work, we describe experimental research on a relatively new nonlinear optics system comprised of a sub-wavelength diameter Tapered Optical Fiber (TOF) suspended in atomic Rubidium (Rb) vapor. The compression of the evanescent optical mode propagating along the TOF enables a dramatic increase in the nonlinear interactions between the fields and the surrounding Rb atoms, thereby allowing the observation of a variety of nonlinear optical effects with very low-power fields. Specifically, we report on the observation of saturated absorption with nW power levels and, more significantly, the observation of two-photon absorption using power-levels corresponding to only 10's to 100's of photons interacting with the Rb atoms at a given time. One significant drawback to this TOF in Rb system is that at the relatively high atomic densities needed for many of these experiments, Rb atoms accumulating on the TOF surface can cause a significant loss of overall transmission through the fiber. Here we report direct measurements of the time-scale associated with this transmission degradation for various Rb density conditions. We find that transmission is affected almost immediately after the introduction of Rb vapor into the system, and declines rapidly as the density is increased. More significantly, we show how a heating element designed to raise the TOF temperature can be used to reduce this transmission loss and dramatically extend the effective TOF transmission lifetime. Our results indicate that it is possible to achieve relatively high TOF transmission, even in the presence of the relatively high Rb vapor densities that would be needed for many low-power nonlinear optics applications. This study represents a significant step in moving the basic TOF in Rb system from a laboratory setting towards a practical ultra-low-power nonlinear optics device.