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Author(s):Lee, Kevin
Contributor(s):Maslowski, Piotr; Kowzan, Grzegorz; Schibli, Thomas R; Lee, Chien-Chung; Fermann, Martin; Jiang, Jie; Mohr, Christian
Subject(s):Mini-symposium: High-Precision Spectroscopy
Abstract:Optical cavities enhance sensitivity in absorption spectroscopy. While this is commonly done with single wavelengths, broad bandwidths can be coupled into the cavity using frequency combs. The combination of cavity enhancement and broad bandwidth allows simultaneous measurement of tens of transitions with high signal-to-noise for even weak near-infrared transitions. This removes the need for time-consuming sequencing acquisition or long-term averaging, so any systematic errors from long-term drifts of the experimental setup or slow changes of sample composition are minimized. Resolving comb lines provides a high accuracy, absolute frequency axis. This is of great importance for gas metrology and data acquisition for future molecular lines databases, and can be applied to simultaneous trace-gas detection of gas mixtures. Coupling of a frequency comb into a cavity can be complex, so we introduce and demonstrate a simplification. The Pound-Drever-Hall method for locking a cavity and a frequency comb together requires a phase modulation of the laser output. We use the graphene modulator that is already in the Tm fiber laser cavity for controlling the carrier envelope offset of the frequency comb, rather than adding a lossy external modulator. The graphene modulator can operate at frequencies of over 1~ MHz, which is sufficient for controlling the laser cavity length actuator which operates below 100~kHz. We match the laser cavity length to fast variations of the enhancement cavity length. Slow variations are stabilized by comparison of the pulse repetition rate to a GPS reference. The carrier envelope offset is locked to a constant value chosen to optimize the transmitted spectrum. The transmitted pulse train is a stable frequency comb suitable for long measurements, including the acquisition of comb-resolved Fourier transform spectra with a minimum absorption coefficient of about $2times10^{-7}$ wn. For our 38 cm long enhancement cavity, the comb spacing is 394~MHz. With our 300 wn bandwidth at 2 $mu$m, we simultaneously measure the full comb line resolved chem{CO_2} vibrational manifold at 4850 wn. Other spectral ranges can be accessed by using graphene with different gain fibers or nonlinear frequency conversion.
Issue Date:24-Jun-15
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:ACS
Date Available in IDEALS:2016-01-05

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