Abstract

A simple method is described for substantially reducing the amplitude of interference fringes that limit the sensitivities of tunable-laser high-resolution absorption spectrometers. A lead-salt diode laser operating in the 7-μm region is used with a single Brewster-plate spoiler to reduce the fringe amplitude by a factor of 30 and also to allow the detection of absorptances of 10−3% in a single laser scan without subtraction techniques, without complex frequency modulation, and without distortion of the molecular line-shape signals. Application to multipass-cell spectrometers is described.

© 1985 Optical Society of America

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References

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  1. 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 NO2at the 100-ppt level,” Appl. Opt. 19, 3349 (1980).
    [CrossRef] [PubMed]
  2. D. T. Cassidy, J. Reid, “Harmonic detection with tunable diode lasers: two-tone modulation,” Appl. Phys. B 29, 279 (1982).
    [CrossRef]
  3. R. Koga, M. Kosaka, H. Sano, “Improvement of etalon-fringe immunity in diode-laser derivative spectroscopy,” Mem. Sch. Eng. Okayama Univ. 16, 21 (1981).
  4. R. T. Menzies, C. R. Webster, E. D. Hinkley, “Balloon-borne diode laser absorption spectrometer for measurements of stratospheric trace species,” Appl. Opt. 22, 2655 (1983).
    [CrossRef]
  5. This approach, suggested by colleagues J. S. Margolis and M. J. Kavaya, Jet Propulsion Laboratory, has not yet been experimentally applied to TDL Spectroscopy.
  6. F. Schuda, M. Hercher, C. R. Stroud, “Direct optical measurement of sodium hyperfine structure using a cw dye lasers and an atomic beam,” Appl. Phys. Lett. 22, 360 (1973).
    [CrossRef]
  7. T. F. Johnston, W. B. Proffitt, “Vertex-mounted tipping Brewster plate for a ring laser,” U.S. Patent Number 4,268,000 (May1981).

1983 (1)

1982 (1)

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

1981 (1)

R. Koga, M. Kosaka, H. Sano, “Improvement of etalon-fringe immunity in diode-laser derivative spectroscopy,” Mem. Sch. Eng. Okayama Univ. 16, 21 (1981).

1980 (1)

1973 (1)

F. Schuda, M. Hercher, C. R. Stroud, “Direct optical measurement of sodium hyperfine structure using a cw dye lasers and an atomic beam,” Appl. Phys. Lett. 22, 360 (1973).
[CrossRef]

Ballik, E. A.

Cassidy, D. T.

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

El-Sherbiny, M.

Garside, B. K.

Hercher, M.

F. Schuda, M. Hercher, C. R. Stroud, “Direct optical measurement of sodium hyperfine structure using a cw dye lasers and an atomic beam,” Appl. Phys. Lett. 22, 360 (1973).
[CrossRef]

Hinkley, E. D.

Johnston, T. F.

T. F. Johnston, W. B. Proffitt, “Vertex-mounted tipping Brewster plate for a ring laser,” U.S. Patent Number 4,268,000 (May1981).

Koga, R.

R. Koga, M. Kosaka, H. Sano, “Improvement of etalon-fringe immunity in diode-laser derivative spectroscopy,” Mem. Sch. Eng. Okayama Univ. 16, 21 (1981).

Kosaka, M.

R. Koga, M. Kosaka, H. Sano, “Improvement of etalon-fringe immunity in diode-laser derivative spectroscopy,” Mem. Sch. Eng. Okayama Univ. 16, 21 (1981).

Menzies, R. T.

Proffitt, W. B.

T. F. Johnston, W. B. Proffitt, “Vertex-mounted tipping Brewster plate for a ring laser,” U.S. Patent Number 4,268,000 (May1981).

Reid, J.

Sano, H.

R. Koga, M. Kosaka, H. Sano, “Improvement of etalon-fringe immunity in diode-laser derivative spectroscopy,” Mem. Sch. Eng. Okayama Univ. 16, 21 (1981).

Schuda, F.

F. Schuda, M. Hercher, C. R. Stroud, “Direct optical measurement of sodium hyperfine structure using a cw dye lasers and an atomic beam,” Appl. Phys. Lett. 22, 360 (1973).
[CrossRef]

Stroud, C. R.

F. Schuda, M. Hercher, C. R. Stroud, “Direct optical measurement of sodium hyperfine structure using a cw dye lasers and an atomic beam,” Appl. Phys. Lett. 22, 360 (1973).
[CrossRef]

Webster, C. R.

Appl. Opt. (2)

Appl. Phys. B (1)

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

Appl. Phys. Lett. (1)

F. Schuda, M. Hercher, C. R. Stroud, “Direct optical measurement of sodium hyperfine structure using a cw dye lasers and an atomic beam,” Appl. Phys. Lett. 22, 360 (1973).
[CrossRef]

Mem. Sch. Eng. Okayama Univ. (1)

R. Koga, M. Kosaka, H. Sano, “Improvement of etalon-fringe immunity in diode-laser derivative spectroscopy,” Mem. Sch. Eng. Okayama Univ. 16, 21 (1981).

Other (2)

This approach, suggested by colleagues J. S. Margolis and M. J. Kavaya, Jet Propulsion Laboratory, has not yet been experimentally applied to TDL Spectroscopy.

T. F. Johnston, W. B. Proffitt, “Vertex-mounted tipping Brewster plate for a ring laser,” U.S. Patent Number 4,268,000 (May1981).

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Figures (6)

Fig. 1
Fig. 1

Schematic diagram showing the inclusion of a plate of thickness d into a passive plane cavity.

Fig. 2
Fig. 2

Plot for various materials of the minimum plate thickness for fringe averaging using an angular oscillation over Δθ = 1° about the appropriate Brewster angle for each material. The plots include the variation in refractive index with wavelength. The dashed line is at 7.65 μm, the TDL wavelength used for demonstration of the technique.

Fig. 3
Fig. 3

Schematic diagram of the short-path absorption experiment.

Fig. 4
Fig. 4

Experimental results (a) using triangular-wave oscillation and (b) comparing three oscillation functions with the spoiler on. The peak-to-peak amplitude of the fringes observed in (a) using second-harmonic detection is a few percent. In (b) the gain for all three channels is increased over (a) by a factor of 2.

Fig. 5
Fig. 5

Experimental results comparing a portion of the second-harmonic N2O spectrum near 1308 cm−1 (b) without, and (c) with, the spoiler on. In (a) the direct absorption spectrum is shown for line identification.

Fig. 6
Fig. 6

Experimental measurements of various signal contributions in the absence of the absorbing molecular gas. (c) Represents the optical beam noise, and (d) the signal channel noise. The arrow lengths to the right represent the peak second-harmonic signal size that a N2O feature of the peak absorptance given would have if simultaneously recorded.

Equations (28)

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T = 1 / [ 1 + F sin 2 ( 2 π L η ν ) ] ,
w λ = 2 η L ,
FSR = c / 2 [ 2 L ( η 2 - sin 2 θ i ) 1 / 2 ] ,
FSR = c / 2 L η .
F * = ( π F 1 / 2 ) / 2 = ( π R 1 / 2 ) / ( 1 - R ) ,
T ( ν ) = 1 - F / 2 [ 1 - cos ( 4 π L η ν ) ] .
F * π R 1 / 2 ,
ν ( t ) = ν TDL + a cos ω t ,
ν TDL = ν 0 + ν 1
ν ( t ) = ν 0 + ν 1 + a cos ω t .
T ( ν , t ) = 1 - F / 2 { 1 - cos [ 4 π L η ( ν 0 + ν 1 + a cos ω t ) ] } .
T ( ν ) = - F 2 4 π L η sin ( 4 π L η ν )
T ( ν ) = - F 2 ( 4 π L η ) 2 cos ( 4 π L η ν ) .
T ( ν 0 + ν 1 + a cos ω t ) = n = 0 H n ( ν 0 + ν 1 ) cos n ω t ,
H n ( ν , t ) = 2 π 0 π T ( ν , a ) cos n θ d θ = 2 π 0 π ( 1 - F 2 { 1 - cos [ 4 π L η ( ν 0 + ν 1 + a cos θ ) ] } ) cos n θ d θ = F π 0 π cos [ 4 π L η ( ν 0 + ν 1 ) + a cos θ ] cos n θ d θ .
H 2 ( ν , t ) = F π 0 π cos [ 4 π L η ( ν 0 + ν 1 + a cos θ ) ] cos 2 θ d θ = F π cos 4 π L η ( ν 0 + ν 1 ) × 0 π cos ( 4 π L η a cos θ ) cos 2 θ d θ ,
J n ( x ) = 1 π 0 π cos ( n θ - x sin θ ) d θ ,
J 2 ( x ) = 1 π 0 π cos ( x sin θ ) cos 2 θ d θ = - 1 π 0 π cos ( x cos θ ) cos 2 θ d θ ,
H 2 ( ν , t ) = - F cos [ 4 π L η ( ν 0 + ν 1 ) ] J 2 ( 4 π L η a ) .
m = a / γ
L = η 0 ( l 0 - l 2 ) + d η ,
d = d / cos θ r
l 2 = d cos ( θ i - θ r ) .
L = η 0 l 0 - d [ η 0 cos ( θ i - θ r ) + η ) ] ,
Δ L = d 1 [ cos ( θ i 1 - θ r 1 ) + η ] - d 2 [ cos ( θ i 2 - θ r 2 ) + η ] .
Δ L L = - Δ ν ν .
Δ L min = λ / 4.
d l = d sin ( θ i - θ r ) / cos θ r .

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