Artful Fingerprinting

Precise molecular line shapes recorded with laser frequency combs

February 14, 2019

A team of the Laser Spectroscopy Division of the Max Planck Institute of Quantum Optics, around Dr. Nathalie Picqué, has recorded exquisite spectroscopic “fingerprints” of molecular gases such as acetylene or ethylene in the important mid-infrared region with mode-locked femtosecond lasers.

Figure 1: Experimental dual-comb spectrum with showing 82000 resolved comb lines recorded in half an hour (a).The molecular fingerprints, here of ethylene, are well sampled (b) by the comb, while the individual comb lines show a shape (c) exactly following the theoretical expectation.

Dual-comb spectroscopy with two lasers of slightly different pulse repetition rates is emerging as a powerful tool for accurate, fast and sensitive broadband spectroscopy of molecules. Like common Fourier-transform spectroscopy, it needs only a single fast photo-detector, but it is free from the limitations on recording speed and resolving power imposed by mechanically moving parts.

There remains the challenge, however, that the two frequency comb sources have to maintain mutual optical phase coherence during data taking. Any instability makes it impossible to average the “interferograms” in the detector signal over time, in order to improve the signal-to-noise ratio, or to obtain useable spectra even under light-starved conditions. In practice, this goal is difficult to reach. Mutual coherence times reaching 1 second have previously been demonstrated with elaborate laser stabilization schemes.

Now, Zaijun Chen, doctoral student, has achieved mutual coherence times exceeding 30 minutes: he corrects phase fluctuations of the laser pulses after they have left the laser by an acousto-optic feed-forward servo control scheme. This feed-forward approach has first been perfected in the near-infrared spectral region with mode-locked erbium-doped femtosecond lasers, as published in Nature Communications [2].

In more recent experiments (Fig. 1) just reported in the Proceedings of the National Academy of Sciences of the United States of America [1], the team has used nonlinear difference frequency generation to produce frequency combs of long mutual coherence times in the more important mid-infrared 3-µm spectral region, where most molecules have strong and characteristic absorption spectra. “The new instrument without moving parts measures mid-infrared high-resolution spectra, over a broad span. All the spectral elements are simultaneously measured, leading to spectra with precise molecular line shapes and direct calibration of the frequency scale to an atomic clock”, explains Zaijun Chen. Intriguing opportunities for precision molecular spectroscopy over broad spectral bandwidths are thus open up. “The 3-µm spectral region is ideally suited for the sensitive detection of hydrocarbons as well as oxygen- or nitrogen-containing organic compounds. Therefore many questions relevant to fundamental and applied spectroscopy may be addressed,“ concludes Zaijun Chen.

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