Gapless High Resolution
We conceived and implemented an effective approach for gapless spectral coverage of the entire comb range of QCL-FCs. In a DCS configuration, synchronized laser current modulation has been used to sweep the spectra of both the interrogating and the local oscillator combs, maintaining the beat notes within the available detection bandwidth, and enabling a reduction of the spectral point spacing from 9.8 GHz (the free spectral range of the lasers) to below 1 MHz. The measurements covered 55 cm−1 (1.65 THz) around 1200 cm−1 (36 THz) without any gaps and with a resolution of 0.001 cm−1 (30 MHz).
Absolute frequency referencing for swept dual-comb spectroscopy with mid-IR QCLs
QCL frequency comb spectrometer in phase-sensitive dual-comb configuration with unbalanced Mach–Zehnder interferometer and down-mixing of intermode beat. This procedure maps the measured beat notes in a swept QCL frequency comb DCS setup to the optical frequencies of the corresponding lines in the interrogating comb by measuring the down-mixed repetition frequency and by employing an unbalanced Mach–Zehnder interferometer to measure the comb shift. The resulting frequency axis is accurate to within 0.001 cm−1 (30 MHz).
Figure 2: Experimental setup and schematic for accurate frequency scale, featuring the distributed feedback (DFB) laser serving as frequency reference, the gapless dual comb spectrometer, and the setups to measure the repetition rate and the optical frequency during an acquisition. MPC: multipass cell, AMP: amplifier, BPF, LPF: bandpass (low-pass) filter, DBM: double-balanced mixer, LOGAMP: logarithmic amplifier, EQ: equalizer, COMB GEN: comb generator, WFG: wave form generator, DAQ: data acquisition unit, DS, DN, DR, DF: photodetector.
Figure 3: Absorption spectrum of low-pressure CH4 acquired in 54 ms by our spectrometer. The top panel is digitally filtered for visibility while the lower panel features two absorption lines at the original resolution of 660 kHz (adopted from Komagata et al., Phys. Rev. Res., 2023).
- Komagata K.N., Wittwer V.J., Südmeyer T., Emmenegger L., and Gianella M., "Absolute frequency referencing for swept dual-comb spectroscopy with midinfrared quantum cascade lasers", Phys. Rev. Res., 5(1), 013047, 2023. Publication Link
- Hillbrand J., Bertrand M., Wittwer V., Ak N.O., Kapsalidis F., Gianella M., Emmenegger L., Schwarz B., Sudmeyer T., Beck M., and Faist J., "Synchronization of frequency combs by optical injection", Opt. Exp., 30(20), 36087–36095, 2022. Publication Link
- Gianella M., Vogel S., Wittwer V.J., Südmeyer T., Faist J., and Emmenegger L., "Frequency axis for swept dual-comb spectroscopy with quantum cascade lasers", Opt. Lett., 47(3), pp. 625–628, 2022. Publication Link
- Komagata K.N., Gianella M., Jouy P., Kapsalidis F., Shahmohammadi M., Beck M., Matthey R., Wittwer V.J., Hugi A., Faist J., Emmenegger L., Südmeyer T., and Schilt S., "Absolute frequency referencing in the long wave infrared using a quantum cascade laser frequency comb", Opt. Exp., 30(8), 12891–12901, 2022. Publication Link
- Komagata K., Shehzad A., Terrasanta G., Brochard P., Matthey R., Gianella M., Jouy P., Kapsalidis F., Shahmohammadi M., Beck M., Wittwer V.J., Faist J., Emmenegger L., Südmeyer T., Hugi A., and Schilt S., "Coherently-averaged dual comb spectrometer at 7.7 µm with master and follower quantum cascade lasers" Opt. Exp., 29(12), 19126–19139, 2021. Publication Link
