Abstract

A dual-beam detector is used to measure atmospheric trace species by differential absorption spectroscopy with commercial near-infrared InGaAs laser diodes. It is implemented on the Spectromètre à Diodes Laser Accordables, a balloonborne tunable diode laser spectrometer devoted to the in situ monitoring of CH4 and H2O. The dual-beam detector is made of simple analogical subtractor circuits combined with InGaAs photodiodes. The detection strategy consists in taking the balanced analogical difference between the reference and the sample signals detected at the input and the output of an open optical multipass cell to apply the full dynamic range of the measurements (16 digits) to the weak molecular absorption information. The obtained sensitivity approaches the shot-noise limit. With a 56-m optical cell, the detection limit obtained when the spectra is recorded within 8 ms is ∼10-4 (expressed in absorbance units). The design and performances of both a simple substractor and an upgraded feedback substractor circuit are discussed with regard to atmospheric in situ CH4 absorption spectra measured in the 1.653-µm region. Mixing ratios are obtained from the absorption spectra by application of a nonlinear least-squares fit to the full molecular line shape in conjunction with in situ P and T measurements.

© 2000 Optical Society of America

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2000 (1)

1999 (3)

1998 (2)

R. D. May, “Open-path, near-infrared tunable diode laser spectrometer for atmospheric measurements of H2O,” J. Geophys. Res. 103, 19161–19172 (1998).
[CrossRef]

D. M. Sonnenfroh, W. J. Kessler, J. C. Magill, B. L. Upschulte, M. G. Allen, D. J. W. Barrick, “In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer,” Appl. Phys. B 67, 275–282 (1998).
[CrossRef]

1997 (1)

1996 (2)

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

1995 (2)

P. Cancio, C. Corsi, F. S. Pavone, R. U. Martinelli, R. J. Menna, “Sensitive detection of ammonia absorption by using a 1.65 µm distributed feedback InGaAsP diode laser,” Infrared Phys. Technol. 36, 987–993 (1995).
[CrossRef]

M. G. Allen, K. L. Carleton, S. J. Davis, W. J. Kessler, C. E. Otis, D. A. Palombo, D. M. Sonnenfroh, “Ultrasensitive dual-beam absorption and gain spectroscopy: applications for near-infrared and visible diode laser sensors,” Appl. Opt. 29, 3240–3249 (1995).
[CrossRef]

1994 (3)

1992 (2)

C. R. Webster, R. D. May, “In-situ stratospheric measurements of CH4, 13CH4, N2O, and OC18O using the BLISS tunable diode laser spectrometer,” Geophys. Res. Lett. 19, 45–48 (1992).
[CrossRef]

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

1991 (1)

P. C. D. Hobbs, “Reaching the shot noise limit for $10,” Opt. Photon. News 2(4), 17–23 (1991).
[CrossRef]

1990 (2)

H. I. Schiff, D. R. Karecki, G. W. Harris, D. R. Hastie, G. I. Mackay, “A tunable diode laser system for aircraft measurements of trace gases,” J. Geophys. Res. 95, 10147–10153 (1990).
[CrossRef]

C. R. Webster, R. D. May, R. Toumi, J. A. Pyle, “Active nitrogen partitioning and the nighttime formation of N2O5 in the stratosphere: simultaneous in situ measurements of NO, NO2, HNO3, O3 and N2O using the BLISS diode laser spectrometer,” J. Geophys. Res. 95, 13851–13866 (1990).
[CrossRef]

1981 (1)

1965 (1)

1964 (1)

Abbas, M. M.

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

Abrams, M. C.

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

Allen, M. G.

D. M. Sonnenfroh, W. J. Kessler, J. C. Magill, B. L. Upschulte, M. G. Allen, D. J. W. Barrick, “In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer,” Appl. Phys. B 67, 275–282 (1998).
[CrossRef]

M. G. Allen, K. L. Carleton, S. J. Davis, W. J. Kessler, C. E. Otis, D. A. Palombo, D. M. Sonnenfroh, “Ultrasensitive dual-beam absorption and gain spectroscopy: applications for near-infrared and visible diode laser sensors,” Appl. Opt. 29, 3240–3249 (1995).
[CrossRef]

Altmann, J.

Anders, J.

Barrick, D. J. W.

D. M. Sonnenfroh, W. J. Kessler, J. C. Magill, B. L. Upschulte, M. G. Allen, D. J. W. Barrick, “In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer,” Appl. Phys. B 67, 275–282 (1998).
[CrossRef]

Baumgart, R.

Bechara, J.

H. I. Schiff, G. I. Mackay, J. Bechara, “The use of tunable diode laser absorption spectroscopy for atmospheric measurements,” in Air Monitoring by Spectroscopic Techniques, M. W. Sigrist, ed., Vol. 127 of Chemical Analysis Series (Wiley, New York, 1994), pp. 239–318.

Benner, D. Chris

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Brown, L. R.

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Camy-Peyret, C.

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Cancio, P.

P. Cancio, C. Corsi, F. S. Pavone, R. U. Martinelli, R. J. Menna, “Sensitive detection of ammonia absorption by using a 1.65 µm distributed feedback InGaAsP diode laser,” Infrared Phys. Technol. 36, 987–993 (1995).
[CrossRef]

Carleton, K. L.

Chang, A. Y.

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

Chave, R. G.

Corsi, C.

P. Cancio, C. Corsi, F. S. Pavone, R. U. Martinelli, R. J. Menna, “Sensitive detection of ammonia absorption by using a 1.65 µm distributed feedback InGaAsP diode laser,” Infrared Phys. Technol. 36, 987–993 (1995).
[CrossRef]

Davis, S. J.

Dingler, F.

Durry, G.

Flaud, J.-M.

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Flesh, G. J.

Gamache, R. R.

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Garmire, E.

Goldman, A.

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Gunson, M. R.

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

Harris, G. W.

H. I. Schiff, D. R. Karecki, G. W. Harris, D. R. Hastie, G. I. Mackay, “A tunable diode laser system for aircraft measurements of trace gases,” J. Geophys. Res. 95, 10147–10153 (1990).
[CrossRef]

Hastie, D. R.

H. I. Schiff, D. R. Karecki, G. W. Harris, D. R. Hastie, G. I. Mackay, “A tunable diode laser system for aircraft measurements of trace gases,” J. Geophys. Res. 95, 10147–10153 (1990).
[CrossRef]

Herman, R. L.

Herriott, D. R.

Hobbs, P. C. D.

P. C. D. Hobbs, “Ultrasensitive laser measurements without tears,” Appl. Opt. 36, 903–920 (1997).
[CrossRef] [PubMed]

P. C. D. Hobbs, “Reaching the shot noise limit for $10,” Opt. Photon. News 2(4), 17–23 (1991).
[CrossRef]

P. C. D. Hobbs, “Shot noise limited optical measurements at baseband with noisy lasers,” in Laser Noise, R. Roy, ed., Proc. SPIE1376, 216–221 (1991).
[CrossRef]

Houser, G. D.

Hovde, D. C.

J. A. Silver, D. C. Hovde, “Near-infrared diode laser airborne hygrometer,” Rev. Sci. Instrum. 65, 1691–1694 (1994).
[CrossRef]

Irion, F. W.

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

Karecki, D. R.

H. I. Schiff, D. R. Karecki, G. W. Harris, D. R. Hastie, G. I. Mackay, “A tunable diode laser system for aircraft measurements of trace gases,” J. Geophys. Res. 95, 10147–10153 (1990).
[CrossRef]

Kendall, J.

Kessler, W. J.

D. M. Sonnenfroh, W. J. Kessler, J. C. Magill, B. L. Upschulte, M. G. Allen, D. J. W. Barrick, “In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer,” Appl. Phys. B 67, 275–282 (1998).
[CrossRef]

M. G. Allen, K. L. Carleton, S. J. Davis, W. J. Kessler, C. E. Otis, D. A. Palombo, D. M. Sonnenfroh, “Ultrasensitive dual-beam absorption and gain spectroscopy: applications for near-infrared and visible diode laser sensors,” Appl. Opt. 29, 3240–3249 (1995).
[CrossRef]

Kogelnik, H.

Kompfer, R.

Lübken, F.-J.

Mackay, G. I.

H. I. Schiff, D. R. Karecki, G. W. Harris, D. R. Hastie, G. I. Mackay, “A tunable diode laser system for aircraft measurements of trace gases,” J. Geophys. Res. 95, 10147–10153 (1990).
[CrossRef]

H. I. Schiff, G. I. Mackay, J. Bechara, “The use of tunable diode laser absorption spectroscopy for atmospheric measurements,” in Air Monitoring by Spectroscopic Techniques, M. W. Sigrist, ed., Vol. 127 of Chemical Analysis Series (Wiley, New York, 1994), pp. 239–318.

Magill, J. C.

D. M. Sonnenfroh, W. J. Kessler, J. C. Magill, B. L. Upschulte, M. G. Allen, D. J. W. Barrick, “In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer,” Appl. Phys. B 67, 275–282 (1998).
[CrossRef]

Malathy Devi, V.

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Manney, G. L.

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

Martinelli, R. U.

P. Cancio, C. Corsi, F. S. Pavone, R. U. Martinelli, R. J. Menna, “Sensitive detection of ammonia absorption by using a 1.65 µm distributed feedback InGaAsP diode laser,” Infrared Phys. Technol. 36, 987–993 (1995).
[CrossRef]

Massie, S. T.

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

May, R. D.

D. C. Scott, R. L. Herman, C. R. Webster, R. D. May, G. J. Flesh, E. J. Moyer, “Airborne Laser Infrared Absorption Spectrometer (ALIAS-II) for in situ atmospheric measurements of N2O, CH4, CO, HCl and NO2 from balloon or remotely piloted aircraft platforms,” Appl. Opt. 38, 4609–4622 (1999).
[CrossRef]

R. D. May, “Open-path, near-infrared tunable diode laser spectrometer for atmospheric measurements of H2O,” J. Geophys. Res. 103, 19161–19172 (1998).
[CrossRef]

C. R. Webster, R. D. May, C. A. Trimble, R. G. Chave, J. Kendall, “Aircraft (ER-2) laser infrared absorption spectrometer (ALIAS) for in situ stratospheric measurements of HCl, N2O, CH4 and HNO3,” Appl. Opt. 33, 454–472 (1994).
[CrossRef] [PubMed]

C. R. Webster, R. D. May, “In-situ stratospheric measurements of CH4, 13CH4, N2O, and OC18O using the BLISS tunable diode laser spectrometer,” Geophys. Res. Lett. 19, 45–48 (1992).
[CrossRef]

C. R. Webster, R. D. May, R. Toumi, J. A. Pyle, “Active nitrogen partitioning and the nighttime formation of N2O5 in the stratosphere: simultaneous in situ measurements of NO, NO2, HNO3, O3 and N2O using the BLISS diode laser spectrometer,” J. Geophys. Res. 95, 13851–13866 (1990).
[CrossRef]

Megie, G.

Menna, R. J.

P. Cancio, C. Corsi, F. S. Pavone, R. U. Martinelli, R. J. Menna, “Sensitive detection of ammonia absorption by using a 1.65 µm distributed feedback InGaAsP diode laser,” Infrared Phys. Technol. 36, 987–993 (1995).
[CrossRef]

Michelsen, H. A.

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

Moyer, E. J.

D. C. Scott, R. L. Herman, C. R. Webster, R. D. May, G. J. Flesh, E. J. Moyer, “Airborne Laser Infrared Absorption Spectrometer (ALIAS-II) for in situ atmospheric measurements of N2O, CH4, CO, HCl and NO2 from balloon or remotely piloted aircraft platforms,” Appl. Opt. 38, 4609–4622 (1999).
[CrossRef]

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

Nagaraju, R.

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

Newchurch, M. J.

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

Otis, C. E.

Palombo, D. A.

Pavone, F. S.

P. Cancio, C. Corsi, F. S. Pavone, R. U. Martinelli, R. J. Menna, “Sensitive detection of ammonia absorption by using a 1.65 µm distributed feedback InGaAsP diode laser,” Infrared Phys. Technol. 36, 987–993 (1995).
[CrossRef]

Perrin, A.

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Pyle, J. A.

C. R. Webster, R. D. May, R. Toumi, J. A. Pyle, “Active nitrogen partitioning and the nighttime formation of N2O5 in the stratosphere: simultaneous in situ measurements of NO, NO2, HNO3, O3 and N2O using the BLISS diode laser spectrometer,” J. Geophys. Res. 95, 13851–13866 (1990).
[CrossRef]

Riedel, W. J.

Rinsland, C. P.

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Rothman, L. S.

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Salawitch, R. J.

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

Schiff, H. I.

H. I. Schiff, D. R. Karecki, G. W. Harris, D. R. Hastie, G. I. Mackay, “A tunable diode laser system for aircraft measurements of trace gases,” J. Geophys. Res. 95, 10147–10153 (1990).
[CrossRef]

H. I. Schiff, G. I. Mackay, J. Bechara, “The use of tunable diode laser absorption spectroscopy for atmospheric measurements,” in Air Monitoring by Spectroscopic Techniques, M. W. Sigrist, ed., Vol. 127 of Chemical Analysis Series (Wiley, New York, 1994), pp. 239–318.

Schulte, H. J.

Scott, D. C.

Silver, J. A.

J. A. Silver, D. C. Hovde, “Near-infrared diode laser airborne hygrometer,” Rev. Sci. Instrum. 65, 1691–1694 (1994).
[CrossRef]

Smith, M. A. H.

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Sonnenfroh, D. M.

D. M. Sonnenfroh, W. J. Kessler, J. C. Magill, B. L. Upschulte, M. G. Allen, D. J. W. Barrick, “In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer,” Appl. Phys. B 67, 275–282 (1998).
[CrossRef]

M. G. Allen, K. L. Carleton, S. J. Davis, W. J. Kessler, C. E. Otis, D. A. Palombo, D. M. Sonnenfroh, “Ultrasensitive dual-beam absorption and gain spectroscopy: applications for near-infrared and visible diode laser sensors,” Appl. Opt. 29, 3240–3249 (1995).
[CrossRef]

Stiller, G. P.

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

Tipping, R. H.

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Toth, R. A.

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Toumi, R.

C. R. Webster, R. D. May, R. Toumi, J. A. Pyle, “Active nitrogen partitioning and the nighttime formation of N2O5 in the stratosphere: simultaneous in situ measurements of NO, NO2, HNO3, O3 and N2O using the BLISS diode laser spectrometer,” J. Geophys. Res. 95, 13851–13866 (1990).
[CrossRef]

Trimble, C. A.

Upschulte, B. L.

D. M. Sonnenfroh, W. J. Kessler, J. C. Magill, B. L. Upschulte, M. G. Allen, D. J. W. Barrick, “In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer,” Appl. Phys. B 67, 275–282 (1998).
[CrossRef]

von Lucke, H.

Webster, C. R.

D. C. Scott, R. L. Herman, C. R. Webster, R. D. May, G. J. Flesh, E. J. Moyer, “Airborne Laser Infrared Absorption Spectrometer (ALIAS-II) for in situ atmospheric measurements of N2O, CH4, CO, HCl and NO2 from balloon or remotely piloted aircraft platforms,” Appl. Opt. 38, 4609–4622 (1999).
[CrossRef]

C. R. Webster, R. D. May, C. A. Trimble, R. G. Chave, J. Kendall, “Aircraft (ER-2) laser infrared absorption spectrometer (ALIAS) for in situ stratospheric measurements of HCl, N2O, CH4 and HNO3,” Appl. Opt. 33, 454–472 (1994).
[CrossRef] [PubMed]

C. R. Webster, R. D. May, “In-situ stratospheric measurements of CH4, 13CH4, N2O, and OC18O using the BLISS tunable diode laser spectrometer,” Geophys. Res. Lett. 19, 45–48 (1992).
[CrossRef]

C. R. Webster, R. D. May, R. Toumi, J. A. Pyle, “Active nitrogen partitioning and the nighttime formation of N2O5 in the stratosphere: simultaneous in situ measurements of NO, NO2, HNO3, O3 and N2O using the BLISS diode laser spectrometer,” J. Geophys. Res. 95, 13851–13866 (1990).
[CrossRef]

Weitkamp, D. C.

Wolf, H.

Zander, R.

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

Appl. Opt. (11)

D. R. Herriott, H. Kogelnik, R. Kompfer, “Off-axis paths in spherical mirror interferometers,” Appl. Opt. 3, 523 (1964).
[CrossRef]

D. R. Herriott, H. J. Schulte, “Folded optical delay lines,” Appl. Opt. 4, 883 (1965).
[CrossRef]

J. Altmann, R. Baumgart, D. C. Weitkamp, “Two-mirror multipass absorption cell,” Appl. Opt. 20, 995–999 (1981).
[CrossRef] [PubMed]

M. G. Allen, K. L. Carleton, S. J. Davis, W. J. Kessler, C. E. Otis, D. A. Palombo, D. M. Sonnenfroh, “Ultrasensitive dual-beam absorption and gain spectroscopy: applications for near-infrared and visible diode laser sensors,” Appl. Opt. 29, 3240–3249 (1995).
[CrossRef]

C. R. Webster, R. D. May, C. A. Trimble, R. G. Chave, J. Kendall, “Aircraft (ER-2) laser infrared absorption spectrometer (ALIAS) for in situ stratospheric measurements of HCl, N2O, CH4 and HNO3,” Appl. Opt. 33, 454–472 (1994).
[CrossRef] [PubMed]

G. D. Houser, E. Garmire, “Balanced detection technique to measure small changes in transmission,” Appl. Opt. 33, 1059–1062 (1994).
[CrossRef] [PubMed]

P. C. D. Hobbs, “Ultrasensitive laser measurements without tears,” Appl. Opt. 36, 903–920 (1997).
[CrossRef] [PubMed]

D. C. Scott, R. L. Herman, C. R. Webster, R. D. May, G. J. Flesh, E. J. Moyer, “Airborne Laser Infrared Absorption Spectrometer (ALIAS-II) for in situ atmospheric measurements of N2O, CH4, CO, HCl and NO2 from balloon or remotely piloted aircraft platforms,” Appl. Opt. 38, 4609–4622 (1999).
[CrossRef]

G. Durry, G. Megie, “Atmospheric CH4 and H2O monitoring with near-infrared InGaAs laser diodes by the SDLA, a balloonborne spectrometer for tropospheric and stratospheric in situ measurements,” Appl. Opt. 38, 7342–7354 (1999).
[CrossRef]

F.-J. Lübken, F. Dingler, H. von Lucke, J. Anders, W. J. Riedel, H. Wolf, “MASERATI: a rocketborne tunable diode laser absorption spectrometer,” Appl. Opt. 38, 5338–5349 (1999).
[CrossRef]

G. Durry, G. Megie, “In situ measurements of H2O from a stratospheric balloon by diode laser direct-differential absorption spectroscopy at 1.39 µm,” Appl. Opt. 39, 5601–5608 (2000).
[CrossRef]

Appl. Phys. B (1)

D. M. Sonnenfroh, W. J. Kessler, J. C. Magill, B. L. Upschulte, M. G. Allen, D. J. W. Barrick, “In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer,” Appl. Phys. B 67, 275–282 (1998).
[CrossRef]

Geophys. Res. Lett. (3)

C. P. Rinsland, M. R. Gunson, R. J. Salawitch, M. J. Newchurch, R. Zander, M. M. Abbas, M. C. Abrams, G. L. Manney, H. A. Michelsen, A. Y. Chang, A. Goldman, “ATMOS measurements of H2O + 2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: dehydration and denitrification in the vortex,” Geophys. Res. Lett. 23, 2397–2400 (1996).
[CrossRef]

M. M. Abbas, H. A. Michelsen, M. R. Gunson, M. C. Abrams, M. J. Newchurch, R. J. Salawitch, A. Y. Chang, A. Goldman, F. W. Irion, G. L. Manney, E. J. Moyer, R. Nagaraju, C. P. Rinsland, G. P. Stiller, R. Zander, “Seasonal variations of water vapor in the lower stratosphere inferred from ATMOS/ATLAS-3 measurements of H2O and CH4,” Geophys. Res. Lett. 23, 2401–2404 (1996).
[CrossRef]

C. R. Webster, R. D. May, “In-situ stratospheric measurements of CH4, 13CH4, N2O, and OC18O using the BLISS tunable diode laser spectrometer,” Geophys. Res. Lett. 19, 45–48 (1992).
[CrossRef]

Infrared Phys. Technol. (1)

P. Cancio, C. Corsi, F. S. Pavone, R. U. Martinelli, R. J. Menna, “Sensitive detection of ammonia absorption by using a 1.65 µm distributed feedback InGaAsP diode laser,” Infrared Phys. Technol. 36, 987–993 (1995).
[CrossRef]

J. Geophys. Res. (3)

H. I. Schiff, D. R. Karecki, G. W. Harris, D. R. Hastie, G. I. Mackay, “A tunable diode laser system for aircraft measurements of trace gases,” J. Geophys. Res. 95, 10147–10153 (1990).
[CrossRef]

C. R. Webster, R. D. May, R. Toumi, J. A. Pyle, “Active nitrogen partitioning and the nighttime formation of N2O5 in the stratosphere: simultaneous in situ measurements of NO, NO2, HNO3, O3 and N2O using the BLISS diode laser spectrometer,” J. Geophys. Res. 95, 13851–13866 (1990).
[CrossRef]

R. D. May, “Open-path, near-infrared tunable diode laser spectrometer for atmospheric measurements of H2O,” J. Geophys. Res. 103, 19161–19172 (1998).
[CrossRef]

J. Quant. Spectrosc. Radiat. Transfer (1)

L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, R. A. Toth, “The HITRAN molecular database: editions of 1991 and 1992,” J. Quant. Spectrosc. Radiat. Transfer 48, 469–507 (1992).
[CrossRef]

Opt. Photon. News (1)

P. C. D. Hobbs, “Reaching the shot noise limit for $10,” Opt. Photon. News 2(4), 17–23 (1991).
[CrossRef]

Rev. Sci. Instrum. (1)

J. A. Silver, D. C. Hovde, “Near-infrared diode laser airborne hygrometer,” Rev. Sci. Instrum. 65, 1691–1694 (1994).
[CrossRef]

Other (2)

H. I. Schiff, G. I. Mackay, J. Bechara, “The use of tunable diode laser absorption spectroscopy for atmospheric measurements,” in Air Monitoring by Spectroscopic Techniques, M. W. Sigrist, ed., Vol. 127 of Chemical Analysis Series (Wiley, New York, 1994), pp. 239–318.

P. C. D. Hobbs, “Shot noise limited optical measurements at baseband with noisy lasers,” in Laser Noise, R. Roy, ed., Proc. SPIE1376, 216–221 (1991).
[CrossRef]

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Figures (12)

Fig. 1
Fig. 1

Schematic of the SDLA balloonborne spectrometer. The instrument is based on a Herriott-type optical multipass cell operated open to the atmosphere. It provides a 56-m absorption path length. A near-infrared InGaAs laser diode is connected to the optical cell by a monomode silica optical fiber. The dual-beam detector combines the signals recorded at the input (B 0) and the output [A 0 T(σ)] of the optical cell to yield the balanced differential signal S Δ = G[ A 0 T(σ) - B 0]. A 0 is the laser flux at the output of the cell in the absence of the absorber. T(σ) is the molecular transmission that is due to ambient CH4. A CH4 reference spectrum (S ref) is also recorded at the output of a closed 8-cm cell to perform the wavelength calibration of the atmospheric spectra.

Fig. 2
Fig. 2

Timing diagram of the SDLA spectrometer. (a) A 64-kHz oscillator is used as a timing reference. (b) The absorption spectra are recorded within 8 ms. The sampling frequency is 64 kHz, and 512 sample points are taken simultaneously over each of the four spectra, S Δ, A 0 T(σ), B 0, and S ref (see Fig. 1), by means of a 16-bit digitizer (Alphi Inc.). When operated under the stratospheric balloon, the SDLA works in average mode: 40 successive 8-ms laser scans are coadded to improve the signal-to-noise ratio. (c) The laser driving current is modulated at 50 Hz; half the modulation period is devoted to recording absorption spectra. The remaining time in one period is used by the onboard processors to undertake different tasks, such as handling the instrument housekeeping signals, carrying out telemetry–telecommand operations or data preprocessing. At the beginning of a laser scan, 2 ms are used to reset the modulation. Then a ramping of the driving current provokes the scanning of the laser frequency over the molecular line shape within 8 ms.

Fig. 3
Fig. 3

Design of the dual-beam detector used on the SDLA balloonborne spectrometer. A simple subtractor circuit is used: both signals at the input (reference) and the output (sample) of the optical cell are balanced by means of gain G 1, and the analogical difference is taken, amplified, and digitized over 16 digits. The value of gain G 1 is changed to maintain a good balance throughout the complete stratospheric flight by observation in real time of the A, B, and S Δ spectra (Fig. 1). Therefore a telemetry–telecommand capability is required here for downlinking the in situ spectra and adapting the balancing gain. 1-mm-diameter InGaAs photodiodes from Epitaxx Inc. and Sensors Unlimited Inc. are operated in photoconductive mode in combination with low-noise Burr–Brown OPA-627 preamplifiers. See text for performances. DAC, digital-to-analog converter; PGA, programmable gain amplifier.

Fig. 4
Fig. 4

Stratospheric CH4 measurement obtained with the dual-beam detector of Fig. 3 during the flight of the SDLA on 10 May 1999 from Aire sur l’Adour (southern France). The spectra are recorded within 320 ms by the coaddition of 40 successive 8-ms laser scans. (a) A reference spectrum is recorded at the output of a closed 8-cm cell to perform the wavelength calibration of the atmospheric spectra. It features the R(3), 2ν 3 triplet of CH4 at 6046.9 cm-1 (1.653 µm). (b) The spectra at the input (B) and the output (A) of the optical multipass cell are also recorded with the purpose of intensity calibration. (c) The differential spectrum obtained when the analogical difference A - B is taken before digitalization over 16 digits. The sloping background that is due to the laser power modulation is removed, and the full dynamic range of the measurements is applied to the weak absorption information in A; at 17 km, ∼0.25% of the laser beam is absorbed in the cell by stratospheric CH4. (d) The CH4 molecular absorption is extracted from the differential spectrum S Δ. The G δerr misbalancing term is obtained from the differential spectrum S Δ by a 3° polynomial interpolation over zero-absorption regions on both sides of the swept spectral range. The CH4 mixing ratio is then retrieved when a nonlinear least-squares fit is applied to the full molecular line shape in conjunction with the in situ pressure and temperature measurements and the HITRAN molecular database.

Fig. 5
Fig. 5

CH4 vertical concentration profiles in the upper troposphere and the lower stratosphere obtained with the SDLA. The instrument was flown twice from a stratospheric balloon in 1999 at northern latitudes (in Sweden) and midlatitudes (in southern France) within the framework of the THESEO campaign. In situ CH4 spectra were recorded with a 1-s temporal resolution by use of the subtractor circuit of Fig. 3. Throughout the complete flight, the balancing between reference (B) and sample (A) spectra was achieved by observation of the atmospheric spectra in real time. Approximately 800 spectra at northern latitudes and 1000 at midlatitudes over the 10,000 recorded for both flights during the ascent and the descent of the gondola were processed. The CH4 mixing ratios were retrieved from a nonlinear least-squares fit to the full molecular line shape from the HITRAN database and the in situ pressure and temperature measurements. At northern latitudes, lower stratospheric CH4 concentrations were measured because of the diabatic descent of air in the Arctic polar vortex.

Fig. 6
Fig. 6

Schematic diagram of the feedback subtractor. The collector current I C2 is a fraction of the sample photocurrent I A . A spectrum acquisition timing sequence starts with a 2-ms balancing phase during which the feedback loop is closed and forces the current ε to zero, leading to I B = I C2 = αI A . It is followed by an 8-ms scanning phase while the laser sweeps the molecular transition by ramping its driving current. The feedback loop is then open and the balanced differential signal I SΔ = ε = I B - αI A is amplified and digitized over 16 digits to yield the differential spectrum. See text and Fig. 7 for more details.

Fig. 7
Fig. 7

Principle of the feedback subtractor explained in the laser frequency domain. Both the signals recorded at the input (reference) and the output (sample) of the optical multipass cell are balanced within 2 ms to achieve B1) = αA1). The laser frequency is then scanned from σ1 to σ2 over the molecular transition by ramping of its driving current within 8 ms. The differential signal S Δ(σ) = GA(σ) - B(σ)] is used to retrieve the molecular absorption. Indeed, both sample and reference spectra can also be balanced within 2 ms at wavelength σ2 to achieve B2) = αA2); α is then maintained constant and used for the next laser scan coming 10 ms later (Fig. 2). Balancing at σ2 instead of σ1 may be of interest if more laser power is available at that frequency. Wavelength-dependent effects, for instance those that are due to the beam splitter used to construct A and B, cause a slight misbalancing Δδerr during the scanning phase. See text for more details.

Fig. 8
Fig. 8

Design of the feedback subtractor circuit. 1-mm-diameter InGaAs photodiodes operated in the photoconductive mode are used to detect the sample (A) and the reference (B) signals. The spectra A and B and the differential spectrum S Δ = GA - B) are simultaneously recorded within 8 ms and digitized over 16 digits. The feedback loop is closed during 2 ms (balancing phase) and open during 8 ms (scanning phase) by means of a sample-and-hold circuit. 1-mm-diameter InGaAs photodiodes from Epitaxx (Model 1000T) are used. See text for performances.

Fig. 9
Fig. 9

Ambient CH4 spectrum at room pressure and temperature obtained with the feedback subtractor. (a) Reference spectrum that features the R(3), 2ν 3 triplet of CH4 at 6046.9 cm-1. (b) Signals recorded at the input (B) and the output (A) of the optical multipass cell. A beam splitter takes ∼10% of the laser energy to construct the signal in channel B before the laser beam is coupled with the optical cell (Fig. 1). Approximately 80% of the laser energy is lost in the cell because of the successive reflections on both cell mirrors. Hence A is roughly equal to twice B. (c) Balanced differential spectrum S Δ yielded by the feedback subtractor. By taking the difference between the reference (B) and a fraction of the sample signals (αA), the strong sloping background that is due to the laser power modulation is removed and the full dynamic range of the measurements (16 digits) is applied to the weak absorption information in A. The CH4 spectrum is recorded within 8 ms in the laboratory at room pressure and temperature, and the obtained absorption depth is ∼0.4% over the 56-m absorption path length. The molecular profile is strongly broadened by collisional effects at room pressure compared with the reference spectrum in (a).

Fig. 10
Fig. 10

CH4 absorption at room pressure and temperature recorded within 8 ms. The molecular absorption Abs(σ) is retrieved from the spectra in Fig. 9. The noise in the spectrum achieved with the feedback subtractor is ∼10-4, expressed in absorbance units. The spectrum is recorded within 8 ms during the scanning phase with 512 samples points taken over the fully swept spectral range by means of a 16-digit digitizer.

Fig. 11
Fig. 11

CH4 absorption at room pressure and temperature recorded within 40 × 8 ms. The ambient CH4 is recorded in the laboratory within 320 ms by the coaddition of 40 successive 8-ms laser scans. The signal-to-noise ratio is improved by a factor of ∼6 compared with that of the spectrum in Fig. 10. The CH4 mixing ratio is retrieved from the spectrum with a nonlinear least-squares fit applied to the full line shape and the HITRAN database.

Fig. 12
Fig. 12

Schematic diagram of the model used to quantify the noise limit achieved with the feedback subtractor in the scanning phase (open loop). I B and I A are the reference and the sample photocurrents, respectively. I B0 and I A0 are the dark currents of the used InGaAs photodiodes. I amp and e amp are the rms noise current and the voltage of amplifier A 1, respectively, and R f is the feedback resistance. Gain G is 400 (see Figs. 6 and 8).

Equations (17)

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SΔ=GA0Tσ-B0,
δerr=A0-B0,
Absσ=Gδerr-SΔGA0.
AbsσρmolNTatm, PatmL×lines kνNTatmΦTatm, Patm, σ,
ε=IB-IC2,
IA=IC1+IC2,
IC2/IC1=expqVb/kT,
IC2=IA αVb.
IC2=αIAt0=IBt0.
ISΔ=εt1=IBt1-IC2t1=IBt1-αIAt1.
SΔ=GαA0Tσ-B0.
δerr=αA0-B0,
Absσ=Gδerr-SΔGαA0.
αA0B0
δαΔδerrGA0.
In2=2qαIA+IA0+2qIB+IB0+eamp2Rf2+Iamp2+4kTRf.
enoise2 2qIB Rf G B.

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