High-resolution two-photon spectroscopy of rubidium within a confined geometry

We present two-photon spectroscopy of a thermal rubidium vapor confined to the hollow core of a photonic-crystal fiber. Linewidths as narrow as 10 MHz were observed on the 5S1/2→5D5/2 transition enabling the hyperfine splitting of the excited state to be resolved. Very strong nonlinear absorption ($>90%$) was observed, with substantial absorption maintained over large detunings (9 GHz) from an intermediate state. These attributes make this system ideal for many frequency metrology and quantum optics applications.