Doppler-free saturation spectroscopy of acetylene inside a photonic band-gap fiber

We are developing portable frequency references based on saturated absorption of acetylene ($^{12}$C$_{2}$H$_{2})$ in the near IR spectral region with sub-MHz accuracy inside hollow core photonic band-gap fibers. Acetylene is a commonly employed reference gas in the telecommunications band. The light from the tunable diode laser at $\sim $ 1531 nm (P11) is amplified by an erbium-doped fiber amplifier (EDFA) and split into a strong pump beam and a weak probe beam which counter propagate inside the gas-filled fiber. The sub-Doppler profile appears as a narrow absorption feature, about 20-40 MHz wide, even at the low pump power of $\sim $18 mW. We also observe narrower sub-Doppler features in the 20 $\mu $m-core fiber than in the 10 $\mu $m --core fiber, consistent with the limitation of transit-time broadening.