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Frequency-comb infrared spectrometer for rapid, remote chemical sensing

Albert Schliesser, Markus Brehm, Fritz Keilmann, and Daniel W. van der Weide

Open Access Open Access

Abstract

We demonstrate real-time recording of chemical vapor fluctuations from 22m away with a fast Fourier-transform infrared (FTIR) spectrometer that uses a laser-like infrared probing beam generated from two 10-fs Ti:sapphire lasers. The FTIR’s broad 9–12μm spectrum in the “molecular fingerprint” region is dispersed by fast heterodyne self-scanning, enabling spectra at 2cm-1 resolution to be recorded in 70μs snapshots. We achieve continuous acquisition at a rate of 950 IR spectra per second by actively manipulating the repetition rate of one laser. Potential applications include video-rate chemical imaging and transient spectroscopy of e.g. gas plumes, flames and plasmas, and generally non-repetitive phenomena such as those found in protein folding dynamics and pulsed magnetic fields research.

©2005 Optical Society of America

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Figures (6)
Fig. 1.
Fig. 1. Principle of quasi-continuous, broadband spectroscopy using multi-heterodyne detection. The evenly spaced narrow lines of the infrared spectrum to be measured (red) constitute a frequency comb. Pairwise interference with a slightly detuned reference comb (green) generates a comb spectrum of much lower “beat” frequencies (blue) that carries amplitude and phase information of the infrared spectrum.

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Fig. 2.
Fig. 2. Comb-FTIR spectrometer consisting of two independent, mode-locked Ti:sapphire lasers (green beams) to produce mid-IR comb spectra (red beams) in GaSe, a beam combiner to launch the probing dual beam, and a HgCdTe detector.

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Fig. 3.
Fig. 3. (A) Single interferograms with 70μs acquisition time window each (16μs displayed, t = 0 arbitrarily chosen at window center), without (background) and with a gas absorption cell (NH3); (B) IR spectra calculated from (A) by Fourier transformation, vs. both radio frequency and IR frequency scales; background spectrum with maximum normalized to 1 (dotted), NH3 cell transmittance (full black). For comparison we give the transmittance of conventional FTIR obtained at 2cm-1 resolution and 60s acquisition time (32 spectra averaged, red trace).

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Fig. 4.
Fig. 4. c-FTIR raw beam spectra with color coding for beam intensity, monitoring a suddenly applied NH3 puff (at about 1.8s) at 44 spectra/s—a direct demonstration of real-time sensing applications.

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Fig. 5.
Fig. 5. Pulse coincidence between two pulse trains with repetition rates f r and f r + Δ, respectively. (A) Free running pulse trains have a constant Δ (upper graph) and exhibit self-scanned pulse coincidences (red bands in middle graph) at a period of 1/Δ, which results in the observation of repeated interferograms (lower graph). Note the long waiting time between interferograms. (B) Triggered increase of Δ-e.g. by shortening one laser cavity—shortens the time between coincidences, but resets Δ just before the coincidences, and thus, before the occurrence of interferograms. (C) Triggered flipping of the sign of Δ provides continuous coincidence scans with alternating directions.

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Fig. 6.
Fig. 6. Rapid remote c-FTIR recording of 3000 IR spectra in a total time of 3.15s, over a path of 44m length (left panel). Near a retroreflector outside the laboratory an open NH4OH bottle had been placed just below the infrared beam path, so small emanating NH3 clouds are sensed in real time. The right panels display, for two instances, seven consecutive spectra separated by 1ms.

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Equations (3)

Equations on this page are rendered with MathJax. Learn more.

E ( t ) = ∑ n = M N E n cos ⁡ ( 2 πn f r t + ϕ n )
E ′ ( t ) = ∑ n = M N E ′ n cos ⁡ ( 2 πn ( f r + Δ ) t + ϕ ′ n ) .
∑ n = M N E n E ′ n cos ⁡ ( 2 πn Δ t + ϕ ′ n − ϕ n ) ,
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