Abstract

High-sensitivity measurements of infrared absorptions due to isolated lines broadened by air at atmospheric pressure are reported. Tunable diode lasers and harmonic techniques were employed to detect absorptions as small as 0.01% over path lengths up to 250 m through the open air. The limiting noise source is caused by a base line signal which varies with the optical alignment. The reported sensitivity is not a fundamental limit. Techniques for further increasing sensitivity are discussed.

© 1982 Optical Society of America

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References

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    [CrossRef]

1981

1980

J. Reid, M. El-Sherbiny, B. K. Garside, E. A. Ballik, Appl. Opt. 19, 3349 (1980).
[CrossRef] [PubMed]

P. D. Goldan, W. C. Kuster, D. L. Albritton, A. L. Schmeltekopf, J. Geophys. Res. 85, 413 (1980).
[CrossRef]

1979

1978

1975

1965

R. Arndt, J. Appl. Phys. 36, 2522 (1965).
[CrossRef]

Albritton, D. L.

P. D. Goldan, W. C. Kuster, D. L. Albritton, A. L. Schmeltekopf, J. Geophys. Res. 85, 413 (1980).
[CrossRef]

Arndt, R.

R. Arndt, J. Appl. Phys. 36, 2522 (1965).
[CrossRef]

Ballik, E. A.

Chaney, L. W.

El-Sherbiny, M.

Eng, R. S.

Garside, B. K.

Goldan, P. D.

P. D. Goldan, W. C. Kuster, D. L. Albritton, A. L. Schmeltekopf, J. Geophys. Res. 85, 413 (1980).
[CrossRef]

Harward, C. N.

Hinkley, E. D.

Hoell, J. M.

Killinger, D. K.

Ku, R. T.

Kuster, W. C.

P. D. Goldan, W. C. Kuster, D. L. Albritton, A. L. Schmeltekopf, J. Geophys. Res. 85, 413 (1980).
[CrossRef]

Labrie, D.

J. Reid, D. Labrie, Appl. Phys. B26, 203 (1981).

Mantz, A. W.

McClenny, W. A.

Menyuk, M.

Reid, J.

Rickel, D. G.

Rothman, L. S.

Russwurm, G. M.

Sample, J. O.

Schmeltekopf, A. L.

P. D. Goldan, W. C. Kuster, D. L. Albritton, A. L. Schmeltekopf, J. Geophys. Res. 85, 413 (1980).
[CrossRef]

Shewchun, J.

Todd, T. R.

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Figures (8)

Fig. 1
Fig. 1

Schematic diagram of the apparatus used for long-path open-air monitoring-.

Fig. 2
Fig. 2

First and second harmonic background and absorption signals. The upper series of traces were recorded with a reference cell containing an absorbing gas in the beam. The detector output is shown for a 1-kHz modulation sweeping symmetrically about line center. The other traces display the lock-in amplifier output as the tunable diode laser is swept slowly over the position of the line.

Fig. 3
Fig. 3

Effect of alignment changes on background signals. The upper traces show a typical fourth harmonic background and a fourth harmonic absorption signal scaled to correspond to an atmospheric line having an absorption of 1% at line center. The lower traces demonstrate the reproducibility of the background for successive scans taken with and without a change in alignment.

Fig. 4
Fig. 4

Tuning characteristics of a specific diode laser as a function of the modulation frequency. The injection current of the laser was modulated with a sinusoid of constant amplitude 10-mA rms. Modulation at frequencies of 150 Hz or less is most efficient in producing the wavelength excursions necessary for harmonic spectroscopy. The half-response point is diode and temperature specific.

Fig. 5
Fig. 5

Effect of feedback on laser linewidth. Trace A is taken in the absence of feedback. For Trace B <0.3% of the laser power is directed back into the laser cavity from a retroreflector 600 m distant In each case the laser tunes over the same NO doublet. The absorbing gas was contained in a low-pressure reference cell.

Fig. 6
Fig. 6

Measurement of an atmospheric absorption line at 1142.17 cm−1 due to an N2O line and a weaker overlapping H2O line. The upper trace shows a sixth harmonic signal observed at 800 Hz by placing an atmospheric pressure calibration cell in the beam. The middle trace shows the same region observed over a round-trip open-air path of 60 m. For the lower trace a mirror was placed immediately in front of the telescope in order to observe the background. Each trace took 4 min to complete.

Fig. 7
Fig. 7

Detection of an atmospheric line over a 240-m round trip. Sixth harmonic detection at 800 Hz was used. The distortion to the right is caused by a mode hop. Scan time was 4 min.

Fig. 8
Fig. 8

Results obtained in a multipass cell. For the lower trace the laser wavelength is set to line center of an N2O line at 1145.331 cm−1. As the path length is changed from 40 to 240 m the sixth harmonic signal increases, corresponding to an incremental absorption of 0.25%. The inset is a scan of the demodulated sixth harmonic signal as a function of wavelength taken for a path length of 240 m in pure N2. Optical fringes are clearly visible in this trace.

Tables (1)

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Table I Minimum Detectable Concentration for Several Molecules of Atmospheric Interest; Concentrations are Quoted for Detection at Atmospheric Pressure and a Sensitivity of 10−6 m−1

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