Thin Cell Spectroscopy at Telecommunication Wavelengths: Towards Acetylene based Compact Frequency References

Molecules are ideal candidates for frequency referencing applications offering a comb of extremely narrow rovibrations (linewidth essentially proportional to pressure) throughout the mid and near infrared spectrum. The telecommunications window (around 1.5μm) is the most important region for such applications, where references are typically provided by saturated absorption spectroscopy of C2H2 or HCN in voluminous macroscopic cells. Driven by the telecommunications market, fiber components have now reached a high level of technological maturity allowing the interface of molecular gases with photonic devices for the realization of compact transportable standards. In answer to this demand, compact hollow core photonic crystal fibres (HCPCF) were filled with molecular (acetylene) gas [1] and chip based waveguides were ‘cladded’ with molecules [2] for the purpose of frequency referencing. Such compact platforms remain of macroscopic length, while a full interpretation of the spectra is a challenge that still needs to be tackled.