Abstract

When used for trace-gas detection, laser absorption spectroscopy is usually limited by false absorption signals that are traceable to interferometric effects induced by windows and other pairs of optical surfaces. Here we introduce a new technique that can selectively reject these étalon fringes while preserving the true absorption signal over a wide range of étalon free spectral range to absorption linewidth ratios. We present a theoretical analysis and experimental verification by using a tunable lead salt diode laser.

© 1992 Optical Society of America

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References

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  1. J. Wormhoudt, A. C. Stanton, A. D. Richards, H. H. Sawin, “Atomic chlorine concentration and gas temperature measurements in a plasma etching reactor,” J. Appl. Phys. 61, 142–148 (1987).
    [CrossRef]
  2. D. E. Cooper, C. B. Carlisle, “High-sensitivity FM spectroscopy with a lead-salt diode laser,” Opt. Lett. 13, 719–721 (1988).
    [CrossRef] [PubMed]
  3. C. B. Carlisle, D. E. Cooper, H. Preier, “Quantum noise-limited FM spectroscopy with a lead-salt diode laser,” Appl. Opt. 28, 2567–2576 (1989).
    [CrossRef] [PubMed]
  4. C. R. Webster, “Brewster-plate spoiler: a novel method for reducing the amplitude of interference fringes that limit tunable-laser absorption,” J. Opt. Soc. Am. B 2, 1464–1470 (1985).
    [CrossRef]
  5. J. A. Silver, A. C. Stanton, “Optical interference fringe reduction in laser absorption experiments,” Appl. Opt. 27, 1914–1916 (1988).
    [CrossRef] [PubMed]
  6. J. Reid, M. El-Sherbiny, B. K. Garside, E. A. Ballik, “Sensitivity limits of a tunable diode laser spectrometer, with application to the detection of NO2 at the 100-ppt level,” Appl. Opt. 19, 3349–3354 (1980).
    [CrossRef] [PubMed]
  7. D. T. Cassidy, J. Reid, “Harmonic detection with tunable diode lasers—two tone modulation,” Appl. Phys. B 29, 279–285 (1982).
    [CrossRef]
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  9. M. Gehrtz, G. C. Bjorklund, E. A. Whittaker, “Quantum-limited laser frequency-modulation spectroscopy,” J. Opt. Soc. Am. B 2, 1510–1525 (1985).
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  10. E. A. Whittaker, M. Gehrtz, G. C. Bjorklund, “Residue amplitude modulation in laser electro-optic phase modulation,” J. Opt. Soc. Am. B 2, 1320–1326 (1985).
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  11. M. Gehrtz, W. Lenth, A. Y. Young, H. S. Johnston, “High-frequency-modulation spectroscopy with a lead-salt diode laser,” Opt. Lett. 11, 132–134 (1986).
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  12. H. C. Sun, E. A. Whittaker, “Diode laser spectroscopy for plasma processing diagnostics” in LEOS ’90 Conference Proceedings, J. Andrews, ed. (Institute of Electrical and Electronics Engineers, New York, 1990), pp. 577–578.
  13. G. Guelachvili, K. N. Rao, Handbook of Infrared Standards With Spectral Maps and Transition Between 3 and 2600 μm (Academic, Orlando, Fla., 1986), pp. 280–281.
  14. E. A. Whittaker, C. M. Shum, H. Grebel, H. Lotem, “Reduction of residual amplitude modulation in frequency-modulation spectroscopy by using harmonic frequency modulation,” J. Opt. Soc. Am. B 5, 1253–1256 (1988).
    [CrossRef]

1989

1988

1987

J. Wormhoudt, A. C. Stanton, A. D. Richards, H. H. Sawin, “Atomic chlorine concentration and gas temperature measurements in a plasma etching reactor,” J. Appl. Phys. 61, 142–148 (1987).
[CrossRef]

1986

1985

1982

D. T. Cassidy, J. Reid, “Harmonic detection with tunable diode lasers—two tone modulation,” Appl. Phys. B 29, 279–285 (1982).
[CrossRef]

1980

Ballik, E. A.

Bjorklund, G. C.

Carlisle, C. B.

Cassidy, D. T.

D. T. Cassidy, J. Reid, “Harmonic detection with tunable diode lasers—two tone modulation,” Appl. Phys. B 29, 279–285 (1982).
[CrossRef]

Cooper, D. E.

El-Sherbiny, M.

Garside, B. K.

Gehrtz, M.

Grebel, H.

Guelachvili, G.

G. Guelachvili, K. N. Rao, Handbook of Infrared Standards With Spectral Maps and Transition Between 3 and 2600 μm (Academic, Orlando, Fla., 1986), pp. 280–281.

Johnston, H. S.

Lenth, W.

Lotem, H.

Preier, H.

Rao, K. N.

G. Guelachvili, K. N. Rao, Handbook of Infrared Standards With Spectral Maps and Transition Between 3 and 2600 μm (Academic, Orlando, Fla., 1986), pp. 280–281.

Reid, J.

Richards, A. D.

J. Wormhoudt, A. C. Stanton, A. D. Richards, H. H. Sawin, “Atomic chlorine concentration and gas temperature measurements in a plasma etching reactor,” J. Appl. Phys. 61, 142–148 (1987).
[CrossRef]

Sawin, H. H.

J. Wormhoudt, A. C. Stanton, A. D. Richards, H. H. Sawin, “Atomic chlorine concentration and gas temperature measurements in a plasma etching reactor,” J. Appl. Phys. 61, 142–148 (1987).
[CrossRef]

Shum, C. M.

Silver, J. A.

Stanton, A. C.

J. A. Silver, A. C. Stanton, “Optical interference fringe reduction in laser absorption experiments,” Appl. Opt. 27, 1914–1916 (1988).
[CrossRef] [PubMed]

J. Wormhoudt, A. C. Stanton, A. D. Richards, H. H. Sawin, “Atomic chlorine concentration and gas temperature measurements in a plasma etching reactor,” J. Appl. Phys. 61, 142–148 (1987).
[CrossRef]

Sun, H. C.

H. C. Sun, E. A. Whittaker, “Diode laser spectroscopy for plasma processing diagnostics” in LEOS ’90 Conference Proceedings, J. Andrews, ed. (Institute of Electrical and Electronics Engineers, New York, 1990), pp. 577–578.

Webster, C. R.

Whittaker, E. A.

Wormhoudt, J.

J. Wormhoudt, A. C. Stanton, A. D. Richards, H. H. Sawin, “Atomic chlorine concentration and gas temperature measurements in a plasma etching reactor,” J. Appl. Phys. 61, 142–148 (1987).
[CrossRef]

Young, A. Y.

Appl. Opt.

Appl. Phys. B

D. T. Cassidy, J. Reid, “Harmonic detection with tunable diode lasers—two tone modulation,” Appl. Phys. B 29, 279–285 (1982).
[CrossRef]

J. Appl. Phys.

J. Wormhoudt, A. C. Stanton, A. D. Richards, H. H. Sawin, “Atomic chlorine concentration and gas temperature measurements in a plasma etching reactor,” J. Appl. Phys. 61, 142–148 (1987).
[CrossRef]

J. Opt. Soc. Am. B

Opt. Lett.

Other

H. C. Sun, E. A. Whittaker, “Diode laser spectroscopy for plasma processing diagnostics” in LEOS ’90 Conference Proceedings, J. Andrews, ed. (Institute of Electrical and Electronics Engineers, New York, 1990), pp. 577–578.

G. Guelachvili, K. N. Rao, Handbook of Infrared Standards With Spectral Maps and Transition Between 3 and 2600 μm (Academic, Orlando, Fla., 1986), pp. 280–281.

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Figures (5)

Fig. 1
Fig. 1

Theoretical curves of EFMRn(Xe) (solid curves) and ASRRn(XΓ) (dashed curves) plotted for the special case Γ = ve: (a) n = 1, (b) n = 2. Periodic maxima and minima are shown in the EFMRn(Xe) plots. The n = 2 plots show narrower passbands.

Fig. 2
Fig. 2

Measured EFMRn(Xe) for (a) n = 1 and (b) n = 2 with a 1.445-GHz FSR solid Ge étalon. The detection bandwidth of the system was set to 160 Hz (system maximum) during the measurements. Zeros in the n = 2 curve are smaller than those in the n = 1 curve.

Fig. 3
Fig. 3

Replot of Fig. 1 for Γ = 40 MHz and ve = 660 MHz. vs is used for the horizontal axis instead of the dimensionless Xe and XΓ in order to permit immediate choice of vs. (a) n = 1, (b) n = 2.

Fig. 4
Fig. 4

Measured CWFMS spectra of OCS in He mixture. The laser frequency refererence point is 871.6123 cm−1. OCS partial pressure was 2.73 mTorr, and vs = 1 GHz. (a) n = 2 spectrum, 660-MHz FSR étalon fringes are visible. RAM2 is zero; (b) n = 1 spectrum, étalon fringes are nulled. RAM1 is slightly laser frequency dependent. Insets are added to enhance the visibility of the biggest étalon fringe.

Fig. 5
Fig. 5

Measured CWFMS spectra of OCS in He mixture. The laser frequency reference point is 871.6123 cm−1. OCS partial pressure was 84 μTorr and vs = 100 MHz. The signal was measured at the same scale as shown in Fig. 4. (a) n = 2 spectrum, étalon fringes are suppressed to below noise floor. RAM2 is again zero. The inset shows that the biggest fringe was suppressed to be comparable to the noise floor. (b) n = 1 spectrum, étalon fringes are visible but suppressed. RAM1 is still visible.

Equations (9)

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A ( ν ) = V ( ν ) + A e ( ν ) + RAM ( ν ) ,
A e ( ν ) = 4 R sin 2 ( π ν / ν e ) ( 1 - R ) 2 + 4 R sin 2 ( π ν / ν e ) ,
A e ( ν ) = 4 π R ν e sin ( 2 π ν ν e ) .
ν ν c + ν s T ( ω , t ) ,             A ( ν ) A ( ν c , t ) ,
A ( ν c , t ) = n = 0 [ V n ( ν c ) + A e n ( ν c ) + RAM n ( ν c ) ] × cos ( n ω t ) ,
V n ( ν c ) = 2 ν s ν c - ν s / 2 ν c + ν s / 2 V ( ν ) cos { n π [ ν - ( ν c - ν s / 2 ) ν s ] } d ν ,
A e n ( ν c ) = 2 ν s ν c - ν s / 2 ν c + ν s / 2 A e ( ν ) cos { n π [ ν - ( ν c - ν s / 2 ) ν s ] } d ν .
EFMR n ( X e ) = peak - to - peak value of A en peak - to - peak value of A e ,
ASRR n ( X Γ ) = peak value of V n peak value of V ,

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