Abstract

A laser absorption spectrometer is described which uses a tunable diode laser and a 1-m multipass White cell to detect NO2 in air with a sensitivity of better than 100 ppt. The modulation techniques employed to achieve this sensitivity are described in detail, and the noise mechanisms, which currently limit the detectable absorption coefficients to ≳10−7 m−1, are examined.

© 1980 Optical Society of America

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References

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  1. H. Preier, Appl. Phys. 20, 189 (1979).
    [CrossRef]
  2. J. Reid, J. Shewchun, B. K. Garside, E. A. Ballik, Appl. Opt. 17, 300 (1978).
    [CrossRef] [PubMed]
  3. J. Reid, B. K. Garside, J. Shewchun, M. El-Sherbiny, E. A. Ballik, Appl. Opt. 17, 1806 (1978).
    [CrossRef] [PubMed]
  4. M. El-Sherbiny, E. A. Ballik, J. Shewchun, B. K. Garside, J. Reid, Appl. Opt. 18, 1198 (1979).
    [CrossRef]
  5. J. Reid, B. K. Garside, J. Shewchun, Opt. Quantum Electron. 11, 385 (1979).
    [CrossRef]
  6. R. J. Breeding et al., Geophys. Res. 78, 7057 (1973).
    [CrossRef]
  7. H. S. Johnston, Ann. Rev. Phys. Chem. 26, 315 (1975).
    [CrossRef]
  8. L. S. Rothman, S. A. Clough, R. A. McClatchey, L. G. Young, D. E. Snider, A. Goldman, Appl. Opt. 17, 507 (1978).
    [CrossRef] [PubMed]
  9. R. A. McClatchey, W. S. Benedict, S. A. Clough, D. E. Burch, R. F. Calfee, K. Fox, L. S. Rothman, J. S. Garing, “AFCRL Atmospheric Absorption Line Parameters Compilation,” Environmental Research Paper 434, AFCRL-TR-73-0096 (Air Force Cambridge Research Laboratories, Bedford, Mass., Jan.1973).
  10. Laser Analytics, Inc. model LO-3 long path cell.
  11. Laser Analytics, Inc. model LCM laser control module.
  12. In addition to the multipass cell fringes, we occasionally see fringes with spacings of ~200 MHz, corresponding to the distance between the multipass cell and the detector.
  13. Wilks infrared variable long path gas cell.
  14. All estimates of NO2 concentration are made by converting the measured absorption coefficient to equivalent NO2 using the line strengths and linewidths given in the AFGL compilation.8 For the 1604.162-cm−1 doublet, we checked the AFGL data against a calibrated 800-ppm NO2 mixture supplied by Matheson. The experimental absorption was 10% less than that predicted by the AFGL compilation, but this discrepancy may well be due to the finite lifetime of NO2 mixtures.
  15. Infrared Associates three-stage TE-cooled HgCdTe detectors.
  16. R. S. Eng, A. W. Mantz, T. R. Todd, Appl. Opt. 18, 1088 (1979).
    [CrossRef] [PubMed]

1979 (4)

1978 (3)

1975 (1)

H. S. Johnston, Ann. Rev. Phys. Chem. 26, 315 (1975).
[CrossRef]

1973 (1)

R. J. Breeding et al., Geophys. Res. 78, 7057 (1973).
[CrossRef]

Ballik, E. A.

Benedict, W. S.

R. A. McClatchey, W. S. Benedict, S. A. Clough, D. E. Burch, R. F. Calfee, K. Fox, L. S. Rothman, J. S. Garing, “AFCRL Atmospheric Absorption Line Parameters Compilation,” Environmental Research Paper 434, AFCRL-TR-73-0096 (Air Force Cambridge Research Laboratories, Bedford, Mass., Jan.1973).

Breeding, R. J.

R. J. Breeding et al., Geophys. Res. 78, 7057 (1973).
[CrossRef]

Burch, D. E.

R. A. McClatchey, W. S. Benedict, S. A. Clough, D. E. Burch, R. F. Calfee, K. Fox, L. S. Rothman, J. S. Garing, “AFCRL Atmospheric Absorption Line Parameters Compilation,” Environmental Research Paper 434, AFCRL-TR-73-0096 (Air Force Cambridge Research Laboratories, Bedford, Mass., Jan.1973).

Calfee, R. F.

R. A. McClatchey, W. S. Benedict, S. A. Clough, D. E. Burch, R. F. Calfee, K. Fox, L. S. Rothman, J. S. Garing, “AFCRL Atmospheric Absorption Line Parameters Compilation,” Environmental Research Paper 434, AFCRL-TR-73-0096 (Air Force Cambridge Research Laboratories, Bedford, Mass., Jan.1973).

Clough, S. A.

L. S. Rothman, S. A. Clough, R. A. McClatchey, L. G. Young, D. E. Snider, A. Goldman, Appl. Opt. 17, 507 (1978).
[CrossRef] [PubMed]

R. A. McClatchey, W. S. Benedict, S. A. Clough, D. E. Burch, R. F. Calfee, K. Fox, L. S. Rothman, J. S. Garing, “AFCRL Atmospheric Absorption Line Parameters Compilation,” Environmental Research Paper 434, AFCRL-TR-73-0096 (Air Force Cambridge Research Laboratories, Bedford, Mass., Jan.1973).

El-Sherbiny, M.

Eng, R. S.

Fox, K.

R. A. McClatchey, W. S. Benedict, S. A. Clough, D. E. Burch, R. F. Calfee, K. Fox, L. S. Rothman, J. S. Garing, “AFCRL Atmospheric Absorption Line Parameters Compilation,” Environmental Research Paper 434, AFCRL-TR-73-0096 (Air Force Cambridge Research Laboratories, Bedford, Mass., Jan.1973).

Garing, J. S.

R. A. McClatchey, W. S. Benedict, S. A. Clough, D. E. Burch, R. F. Calfee, K. Fox, L. S. Rothman, J. S. Garing, “AFCRL Atmospheric Absorption Line Parameters Compilation,” Environmental Research Paper 434, AFCRL-TR-73-0096 (Air Force Cambridge Research Laboratories, Bedford, Mass., Jan.1973).

Garside, B. K.

Goldman, A.

Johnston, H. S.

H. S. Johnston, Ann. Rev. Phys. Chem. 26, 315 (1975).
[CrossRef]

Mantz, A. W.

McClatchey, R. A.

L. S. Rothman, S. A. Clough, R. A. McClatchey, L. G. Young, D. E. Snider, A. Goldman, Appl. Opt. 17, 507 (1978).
[CrossRef] [PubMed]

R. A. McClatchey, W. S. Benedict, S. A. Clough, D. E. Burch, R. F. Calfee, K. Fox, L. S. Rothman, J. S. Garing, “AFCRL Atmospheric Absorption Line Parameters Compilation,” Environmental Research Paper 434, AFCRL-TR-73-0096 (Air Force Cambridge Research Laboratories, Bedford, Mass., Jan.1973).

Preier, H.

H. Preier, Appl. Phys. 20, 189 (1979).
[CrossRef]

Reid, J.

Rothman, L. S.

L. S. Rothman, S. A. Clough, R. A. McClatchey, L. G. Young, D. E. Snider, A. Goldman, Appl. Opt. 17, 507 (1978).
[CrossRef] [PubMed]

R. A. McClatchey, W. S. Benedict, S. A. Clough, D. E. Burch, R. F. Calfee, K. Fox, L. S. Rothman, J. S. Garing, “AFCRL Atmospheric Absorption Line Parameters Compilation,” Environmental Research Paper 434, AFCRL-TR-73-0096 (Air Force Cambridge Research Laboratories, Bedford, Mass., Jan.1973).

Shewchun, J.

Snider, D. E.

Todd, T. R.

Young, L. G.

Ann. Rev. Phys. Chem. (1)

H. S. Johnston, Ann. Rev. Phys. Chem. 26, 315 (1975).
[CrossRef]

Appl. Opt. (5)

Appl. Phys. (1)

H. Preier, Appl. Phys. 20, 189 (1979).
[CrossRef]

Geophys. Res. (1)

R. J. Breeding et al., Geophys. Res. 78, 7057 (1973).
[CrossRef]

Opt. Quantum Electron. (1)

J. Reid, B. K. Garside, J. Shewchun, Opt. Quantum Electron. 11, 385 (1979).
[CrossRef]

Other (7)

R. A. McClatchey, W. S. Benedict, S. A. Clough, D. E. Burch, R. F. Calfee, K. Fox, L. S. Rothman, J. S. Garing, “AFCRL Atmospheric Absorption Line Parameters Compilation,” Environmental Research Paper 434, AFCRL-TR-73-0096 (Air Force Cambridge Research Laboratories, Bedford, Mass., Jan.1973).

Laser Analytics, Inc. model LO-3 long path cell.

Laser Analytics, Inc. model LCM laser control module.

In addition to the multipass cell fringes, we occasionally see fringes with spacings of ~200 MHz, corresponding to the distance between the multipass cell and the detector.

Wilks infrared variable long path gas cell.

All estimates of NO2 concentration are made by converting the measured absorption coefficient to equivalent NO2 using the line strengths and linewidths given in the AFGL compilation.8 For the 1604.162-cm−1 doublet, we checked the AFGL data against a calibrated 800-ppm NO2 mixture supplied by Matheson. The experimental absorption was 10% less than that predicted by the AFGL compilation, but this discrepancy may well be due to the finite lifetime of NO2 mixtures.

Infrared Associates three-stage TE-cooled HgCdTe detectors.

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Figures (6)

Fig. 1
Fig. 1

Schematic diagram of the laser absorption spectrometer (LAS).

Fig. 2
Fig. 2

Spectra of pure NO2 and NO2 in air. For the upper trace a 10-cm cell containing NO2 at low pressure was placed in the laser beam, and conventional amplitude was employed. The lower trace was taken using second harmonic detection with the laser beam traversing a path length of 40 m in air at 30 Torr and a small calibration cell giving the equivalent of 80-ppb NO2. The predicted HDO and NO2 spectra are shown at the bottom of the figure.8,9

Fig. 3
Fig. 3

High sensitivity scan of the multipass cell set for 40 m. Fringes are caused by interference between the main optical beam making 40 passes through the cell and the much weaker beams which make 36 (or 44) passes. A second harmonic line shape corresponding to ~5-ppb NO2 (or 10−5-m−1 absorption coefficient) is also shown to indicate sensitivity.

Fig. 4
Fig. 4

Effect of additional jitter modulation. The LAS is set for second harmonic detection at 6 kHz, but a small jitter modulation at 370 Hz was added to the main 3-kHz sine wave. The upper traces show an expanded view of the fringes as the peak-to-peak jitter amplitude was varied from 0 to 2 fringe spacings. The lower trace is a repeat of Fig. 3 but with jitter amplitude set at 1 fringe spacing. Note the significant improvement in sensitivity as the fringes are electronically washed out.

Fig. 5
Fig. 5

Lifetime measurement of NO2/air mixtures in a stainless steel multipass cell. The LAS was set to scan repetitively over the 1604.162-cm−1 doublet of NO2, and the cell was filled with air plus a trace of NO2 at ~30-Torr total pressure. The time constant used was only 0.1 sec, but the noise level is <1 ppb. Note the rapid decay of the NO2 in the sealed cell.

Fig. 6
Fig. 6

Lower trace repeats the measurement of Fig. 5 but with the diode laser wavelength held at the center of the NO2 absorption line. A mixture of a few ppb NO2 in air was introduced into the cell, and the concentration monitored with the LAS. The upper trace shows the noise levels which limit sensitivity. Trace AB is an expanded view of noise under monitoring conditions taken with a 10-sec time constant. Sensitivity is 75-ppt NO2 or 1.6 × 10−7 m−1. At time B, all modulation signals were removed from the laser current, but second harmonic detection at 6 kHz continues. Hence trace BC records random amplitude noise on the laser beam. The final section of trace C → D is taken with the laser switched off and corresponds to detector noise at a frequency of 6 kHz.

Tables (1)

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Table I Random Amplitude Noise (or Beam Noise) Characteristics of Several Diode Lasers

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