High-precision frequency measurements of the ν1 + ν3 combination band of 12C2H2 in the 1.5 μm region

A pair of 1.5 μm semiconductor laser frequency standards have been developed for optical telecommunications use, stabilised to transitions of 12C2H2 and 13C2H2, using cavity-enhanced Doppler-free saturation absorption spectroscopy. The absolute frequencies of 41 lines of the ν1 + ν3 band of 12C2H2, covering the spectral region 1520-1545 nm, have been measured by use of a passive optical frequency comb generator, referenced to 13C2H2 transitions of known frequency. The mean experimental uncertainties (coverage factor k = 1) of the frequency values are 3.0 kHz (type A) and 10 kHz (type B). Improved values of the band origin ν0, rotational constants B′ and B″, and centrifugal distortion coefficients D′, D″, H′, and H″ are presented.