1. Broadband Comb-resolved Cavity Enhanced Spectrometer with Graphene Modulator C.-C. Lee, T. R. Schibli Kevin F. Lee C. Mohr, Jie Jiang, Martin E. Fermann Grzegorz Kowzan P. Masłowski

2. Spectroscopy cavity enhanced absorption ~ kilometer path length useful for small signals weak transitions low pressures trace concentrations near & mid infrared

3. Frequency comb and cavity match comb to resonances graphene in laser cavity modulates loss to control comb offset (CEO)

4. Cavity locking compensate for cavity vibrations Pound-Drever-Hall (PDH) locking modulate laser frequency use graphene error signal from reflected light lock laser to cavity

5. Schematic

6. Modulation graphene modulation of few MHz no external modulator needed reduce etaloning less optical loss lower cost

7. Spectrum bandwidth limited by dispersion resonance spacing is chirped LaserCavity transmission

8. CO 2 cell filled with air ~400 ppm CO 2 fit to finesse 3600 effective path length of 0.87 km 38 cm long cell one pass would absorb ~0.2% in 30 s, at 0.036 cm -1 resolution SNR ~100 α min = 2×10 -7 cm -1 Fit Data

9. Comb-resolved FTIR comb has stable repetition rate and offset measure each comb line match FTIR frequency axis to comb

10. Midinfrared OPO 3-6 µm graphene modulation propagates through OPO have locked OPO to cavity at ~3.3 µm

11. Summary graphene modulator stabilizes CEO modulates for PDH cavity enhanced spectra high sensitivity high frequency accuracy