Saturated absorption spectroscopy reveals the narrowest features so far in molecular gas-filled hollow-core photonic crystal fiber. The 48-68 mu m core diameter of the kagome-structured fiber used here allows for 8 MHz full-width half-maximum sub-Doppler features, and its wavelength-insensitive transmission is suitable for high-accuracy frequency measurements. A fiber laser is locked to the (C2H2)-C-12 nu(1) + nu(3) P(13) transition inside kagome fiber, and compared with frequency combs based on both a carbon nanotube fiber laser and a Cr:forsterite laser, each of which are referenced to a GPS-disciplined Rb oscillator. The absolute frequency of the measured line center agrees with those measured in power build-up cavities to within 9.3 kHz (1 sigma error), and the fractional frequency instability is less than 1.2 x 10(-11) at 1 s averaging time.
Knabe, K., Wu, S., Lim, J. K., Tillman, K. A., Light, P. S., Couny, F., Wheeler, N., Thapa, R., Jones, A. M., Nicholson, J. W., Washburn, B. R., Benabid, F., & Corwin, K. L. (2009). 10 kHz accuracy of an optical frequency reference based on (C2H2)-C-12-filled large-core kagome photonic crystal fibers. Optics Express, 17(18), 16017-16026. https://doi.org/10.1364/OE.17.016017