Laser frequency combs enable precise molecular sensing advance
The researchers, led by Dr. Nathalie Picqué, and including collaboration partners at the Laboratoire Interdisciplinaire Carnot de Bourgogne (France) and the Institut des Sciences Moléculaires d’Orsay (France) are using modulators and a nonlinear optical fibre to produce two frequency combs, each with more than a thousand evenly spaced infrared spectral lines with a flat intensity distribution. Line spacing and spectral position can be selected quickly and freely by simply dialing a knob. Such frequency-agile optical combs offer freedom when interrogating a molecular spectrum via a technique called multiplexed dual-comb spectroscopy. Two mutually coherent combs are combined in an interferometer. Refresh rates (80 kHz) and tuning speeds (10 nm s-1) at high signal-to-noise ratio are achieved. The novel combination holds promise for trace gas sensing, a domain relevant to physics, biology, chemistry, industry or atmospheric sciences.
Molecules absorb light at well-defined specific colours or optical frequencies. Usually such characteristic frequencies are located in the infrared region of the electromagnetic spectrum. Precisely measuring a set of such absorption dips identifies the molecules and quantifies their abundance in the probed environment. Because detection of molecules by optical absorption spectroscopy is sensitive and nonintrusive, it finds an increasing number of applications, from biomedical diagnostics to atmospheric sensing. In the gas phase, the absorption lines are narrow so that high spectral resolution is needed to distinguish the different lines. Although many powerful spectroscopic techniques have been developed, rapid and precise high-resolution sensing is still a challenge.
Time-domain interference signal of the beats between the two frequency-agile laser combs and part of the resulting spectrum of the CO2 molecule. (Graphic: MPQ, Laser Spectroscopy Division)
The research study has been described in a letter published in Nature Photonics, 21 December 2015.
Dr. Ming Yan, a post-doc working at the experiment, said: “The frequency-agile frequency comb generator might also become an enabling tool for applications beyond spectroscopy, like for arbitrary waveform generation, radio-frequency photonics, optical coherence tomography or microscopy.”
Reference
G. Millot, S. Pitois, M. Yan, T. Hovhannisyan, A. Bendahmane, T.W. Hänsch, N. Picqué
Frequency-agile dual-comb spectroscopy
Nature Photonics, Advance online publication, https://dx.doi.org/10.1038/nphoton.2015.250 (December 21st, 2015)
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