Abstract

We report on metrological-grade spectroscopic system based on a distributed-feedback quantum cascade laser (QCL), tunable in the wavelength range from 8.56 to 8.63 μm, phase-locked to a tooth of a difference frequency generation (DFG) mid-IR frequency comb [1]. The beat note is directly generated in the mid-IR region without any conversion scheme resulting in a compact system that combines the broad-band parallel detection capabilities of a mW-level mid-IR comb to the high-precision performances of a mid-IR narrow linewidth source. The obtained ∼30 dB signal-to-noise ratio of the detected beat-note together with the achieved closed-loop locking bandwidth of ∼500 kHz allows for a residual integrated phase noise of 0.78 rad (1 Hz - 5 MHz), for an ultimate resolution of ∼21 kHz, limited by the measured linewidth of the mid-IR comb. By tuning the comb repetition rate, the QCL frequency is scanned with an ultimate accuracy of a GPS-disciplined Rb frequency standard. This system is a powerful tool for high-resolution molecular spectroscopy in the mid-IR spectral region. In particular, it will be used for the absolute measurement of the line center frequency of a two-photon transition in a molecular beam of CHF₃ allowing for a determination of the possible variation of the electron to proton mass ratio at level of 10⁻¹⁵yr⁻¹ [2].

© 2015 IEEE