Frequency Comb Vernier Spectroscopy for Broadband, High-Resolution, High-Sensitivity Absorption and Dispersion Spectra

A femtosecond frequency comb provides a vast number of equidistantly spaced narrow band laser modes that can be simultaneously tuned and frequency calibrated with 15 digit accuracy. Our Vernier spectrometer utilizes all of theses modes in a massively parallel manner to rapidly record both absorption and dispersion spectra with a sensitivity that is provided by a high finesse broadband optical resonator and a resolution that is only limited by the frequency comb line width while keeping the required setup simple.

Figure 1

(a) General setup: Fabry-Perot (FP) resonator with a scanning mirror; L1, G, L2,  spectrograph assembly; CCD, two-dimensional imaging detector, scanned synchronously with FP. (b) Frequency comb (dashed lines) transmission function of the resonator (solid line) form the Vernier. Dots mark transmitted modes for two scan positions. (c) Expected pattern on the detector behind the spectrometer. Arrows mark the order of the frequency comb modes.

Figure 2

Raw data image as seen by the CCD; white corresponds to no light. The horizontal axis is the dispersion axis of the grating and the vertical axis is the scan axis. Weak spots caused by absorption lines in the $A$ bands of ${\mathrm{O}}_2$ are clearly observed. The period in the pattern in vertical direction indicates the vernier ratio of $69/68$ (solid boxes mark exemplary one pattern that was identified). The dashed box marks a unique data set. The varying spot spacing and brightness common to all spot columns originates from a nonlinearity of the scan and is corrected for in the data analysis.

Figure 3

(a) Spectrum extracted from the raw data (solid line) and HITRAN [11] database values (dashed red line). The phase shows a global positive second-order dispersion of the resonator and spikes at the resonances. (b) Zoom of the data set, highlighting the excellent agreement between our data (circles) and the HITRAN database (red line).