Acetylene Optical Frequency Standards
Stabilised diode laser systems based on saturated absorptions in acetylene have been developed at NPL and a number of other standards laboratories, for example PTB and NRC. Acetylene has a number of molecular rotational and vibrational transitions between 1510 nm and 1540 nm for 12C2H2 and 1520 nm to 1550 nm for 13C2H2 and has importance through the provision of a number of reference frequencies within the telecommunications C-band (1528 nm to 1565 nm). One acetylene transition is also listed in the 'mise en pratique' for the realisation of the metre.
The acetylene system shown above is a saturated absorption system with the cell inside a separate cavity to the laser. The laser is locked to the cavity and the cavity is then locked to the Doppler-free acetylene signal, using a third harmonic lock to remove the Doppler limited background. An example of an acetylene feature obtained in this way is shown below. In acetylene, there is no hyperfine structure and so we only observe one peak within each Doppler broadened transition. The ν1 + ν3 combination band of 13C2H2 has more than 50 strong absorption lines in the 1520 nm to 1550 nm region.
At NPL we have developed and characterised two acetylene-stabilised diode laser frequency standards of this type. The Allan deviation of an individual system (shown below) reaches a minimum of 4 parts in 1014 at a sampling time of 5000 s, and the long-term reproducibility is found to be 0.4 kHz, corresponding to a relative standard uncertainty of 2 parts in 1012.
We have also measured the absolute frequency of the P(16) transition in the ν1 + ν3 combination band of 13C2H2 using a combined frequency chain and femtosecond optical frequency comb. Measurements have also been made in 12C2H2. Once the absolute frequency of one transition is known, the separation between components within the acetylene band can be measured using a passive optical frequency comb. The results from these measurements have been used to determine some of the ground and upper state molecular constants in both 13C2H2 and 12C2H2 to a high accuracy.
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