Abstract

A new fiber-optic technique to eliminate residual amplitude modulation in tunable diode laser wavelength modulation spectroscopy is presented. The modulated laser output is split to pass in parallel through the gas measurement cell and an optical fiber delay line, with the modulation frequency / delay chosen to introduce a relative phase shift of π between them. The two signals are balanced using a variable attenuator and recombined through a fiber coupler. In the absence of gas, the direct laser intensity modulation cancels, thereby eliminating the high background. The presence of gas induces a concentration-dependent imbalance at the coupler’s output from which the absolute absorption profile is directly recovered with high accuracy using 1f detection.

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References

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2003

1999

1997

1995

1993

1990

C. B. Carlisle and D. E. Cooper, “Tunable diode laser frequency modulation spectroscopy through an optical fiber: High sensitivity detection of water vapour,” Appl. Phys. Lett. 56(9), 805–807 (1990).
[CrossRef]

1982

1978

Axner, O.

Ballik, E. A.

Carlisle, C. B.

C. B. Carlisle and D. E. Cooper, “Tunable diode laser frequency modulation spectroscopy through an optical fiber: High sensitivity detection of water vapour,” Appl. Phys. Lett. 56(9), 805–807 (1990).
[CrossRef]

Cassidy, D. T.

Cooper, D. E.

C. B. Carlisle and D. E. Cooper, “Tunable diode laser frequency modulation spectroscopy through an optical fiber: High sensitivity detection of water vapour,” Appl. Phys. Lett. 56(9), 805–807 (1990).
[CrossRef]

Duffin, K.

Garside, B. K.

Hanson, R. K.

Jeffries, J. B.

Johnstone, W.

Kluczynski, P.

Liu, J. T. C.

McGettrick, A. J.

Moodie, D. G.

Philippe, L. C.

Reid, J.

Robert, P.

Schilt, S.

Shewchun, J.

Stewart, G.

Thévenaz, L.

Zhu, X.

Appl. Opt.

Appl. Phys. Lett.

C. B. Carlisle and D. E. Cooper, “Tunable diode laser frequency modulation spectroscopy through an optical fiber: High sensitivity detection of water vapour,” Appl. Phys. Lett. 56(9), 805–807 (1990).
[CrossRef]

J. Lightwave Technol.

J. Opt. Soc. Am. B

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Figures (3)

Fig. 1
Fig. 1

Experimental setup.

Fig. 2
Fig. 2

LIA signals for 10%, 1% and 0.1% methane in nitrogen for (a) RAM-nulled case, and (b) Non-nulled case, showing the large background level that obscures the absorption signal.

Fig. 3
Fig. 3

Relative transmission for methane for (a) concentration 10.13%, pressure 1.067 bar temperature 22.6°C, and (b) concentration 1.02%, pressure 1.082 bar and temperature 23.6°C.

Equations (4)

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Iω(λc)=ΔI(λc)cosωtΔI(λc)α(λc)ClcosωtI(λc)dα(λ)dλ|λcδλ(λc)Clcos(ωtψ)
OP1no gas=ΔI1(λc)cosωt+ΔI2(λc)cos(ωt+π)=[ΔI1(λc)ΔI2(λc)]cosωt
OP1gas=ΔI1(λc)eeα(λc)Cl=1cosωtΔI2(λc)cosωt
It(λc)I0(λc)=eα(λc)Cl=1+(OP1gasOP1nogas)ΔI1(λc)cosωt

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