Abstract

High-sensitivity spectroscopy of methane around 3 μm was carried out by means of a 5.5-mW cw difference-frequency generator in conjunction with a high finesse cavity in off-axis alignment. By cavity-output integration a minimum detectable absorption coefficient of 5.7∙10-9 cm-1Hz-1/2 was achieved, which compares well with results already reported in the literature. Detection of methane in natural abundance was also performed in ambient air, for different values of total pressure, allowing direct concentration measurements via evaluation of the integrated absorbance of the spectra. In particular, at atmospheric pressure, a minimum detectable concentration of 850 parts per trillion by volume (pptv)∙Hz-1/2 was demonstrated.

© 2006 Optical Society of America

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Appl. Opt. (5)

Appl. Phys. (1)

E. C. Richard, K. K. Kelly, R. H. Winkler, R. Wilson, T .L. Thompson, R. J. Mclaughlin, A. L. Schmeltekopf, and A. F. Tuck, “A fast-response near-infrared tunable diode laser absorption spectrometer for in situ measurements of CH4 in the upper troposphere and lower stratosphere,” Appl. Phys. B 75, 183-194 (2002).
[CrossRef]

Appl. Phys. B (15)

H. Dahnke, D. Kleine, W. Urban, P. Hering, and M. Mürtz, “Isotopic ratio measurement of methane in ambient air using mid-infrared cavity leak-out spectroscopy,” Appl. Phys. B 72, 121-125 (2001).
[CrossRef]

L. Menzel, A. A. Kosterev, R. F. Curl, F. K. Tittel, C. Gmachl, F. Capasso, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, A.Y. Cho, and W. Urban, “Spectroscopic detection of biological NO with a quantum cascade laser,” Appl. Phys. B 72, 1-5 (2001).
[CrossRef]

H. Dahnke, D. Kleine, P. Hering, and M. Mürtz, “Real-time monitoring of ethane in human breath using mid-infrared cavity leak-out spectroscopy,” Appl. Phys. B 72, 971-975 (2001).
[CrossRef]

S. Stry, P. Hering, and M. Mürtz, “Portable difference-frequency laser-based cavity leak-out spectrometer for trace-gas analysis,” Appl. Phys. B 75, 297-303 (2002).
[CrossRef]

R. M. Mihalcea, M. E. Webber, D. S. Baer, R. K. Hanson, G. S. Feller, and W. B. Chapman, “Diode-laser absorption measurements of CO2, H2O, N2O, and NH3 near 2.0 µm,” Appl. Phys. B 67, 283-288 (1998).
[CrossRef]

D. S. Baer, J. B. Paul, M. Gupta, and A. O’Keefe, “Sensitive absorption measurements in the near-infrared region using off-axis integrated-cavity-output spectroscopy,” Appl. Phys. B 75, 261-265 (2002).
[CrossRef]

B. Bakowski, L. Corner, G. Hancock, R. Kotchie, R. Peverall, G. A. D. Ritchie, “Cavity-enhanced absorption spectroscopy with a rapidly swept diode laser,” Appl. Phys. B 75, 745-750 (2002).
[CrossRef]

V. L. Kasyutich, C. E. Canosa-Mas, C. Pfrang, S. Vaughan, and R. P. Wayne, “Off-axis continuous-wave cavity-enhanced absorption spectroscopy of narrow-band and broadband absorbers using red diode lasers,” Appl. Phys. B 75, 755-761 (2002).
[CrossRef]

M. L. Silva, D. M. Sonnenfroh, D. I. Rosen, M. G. Allen, and A. O’Keefe, “Integrated cavity output spectroscopy measurements of nitric oxide in breath with a pulsed room-temperature quantum cascade laser,” Appl. Phys. B 81, 705-710 (2005).
[CrossRef]

D. Richter, A. Fried, B. P. Wert, J. G. Walega, and F. K. Tittel, “Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection,” Appl. Phys. B 75, 281-288 (2003).
[CrossRef] [PubMed]

S. Borri, P. Cancio, P. De Natale, G. Giusfredi, D. Mazzotti, and F. Tamassia, “Power-boosted difference-frequency source for high-resolution infrared spectroscopy,” Appl. Phys. B 76, 473-477 (2003).
[CrossRef]

P. Maddaloni, G. Gagliardi, P. Malara, and P. De Natale, “A 3.5-mW continuous-wave difference-frequency source around 3 µm for sub-Doppler molecular spectroscopy,” Appl. Phys. B 80, 141-145 (2005).
[CrossRef]

R. Peeters, G. Berden, A. Apituley, and G. Meijer, “Open-path trace gas detection of ammonia based on cavity-enhanced absorption spectroscopy,” Appl. Phys. B 71, 231-236 (2000).
[CrossRef]

B. Bakowski, L. Corner, G. Hancock, R. Kotchie, R. Peverall, and G. A. D. Ritchie, “Cavity-enhanced absorption spectroscopy with a rapidly swept diode laser,” Appl. Phys. B 75, 745-750 (2002).
[CrossRef]

A. Rocco, G. De Natale, P. De Natale, G. Gagliardi, and L. Gianfrani, “A diode-laser-based spectrometer for in-situ measurements of volcanic gases,” Appl. Phys. B 78, 235-240 (2004).
[CrossRef]

Chem. Phys. Lett. (1)

D. Romanini, A. A. Kachanov, N. Sadeghi, and F. Stockel, “CW cavity ring down spectroscopy,” Chem. Phys. Lett. 264, 316-322 (1997).
[CrossRef]

Int. Rev. Phys. Chem (1)

G. Berden, R. Peeters, and G. Meijer, “Cavity ring-down spectroscopy: Experimental schemes and applications,” Int. Rev. Phys. Chem 19, 565-607 (2000).
[CrossRef]

J. Opt. Soc. Am B (1)

J. Ye, Long-Sheng Ma, and J. L. Hall, “Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy,” J. Opt. Soc. Am B 15, 6-15 (1998).
[CrossRef]

Opt. Commun. (1)

K. Nakagawa, T. Katsuda, A. S. Shelkovnikov, M. de Labachelerie, and M. Ohtsu, “Highly sensitive detection of molecular absorption using a high finesse optical cavity,” Opt. Commun. 107, 369-372 (1994).
[CrossRef]

Opt. Lasers. Eng. (1)

G. Gagliardi, and L. Gianfrani, “Trace-gas anlysis using diode lasers in the near-IR and long-path techniques,” Opt. Lasers. Eng. 37, 509-520 (2002).
[CrossRef]

Opt. Lett. (4)

Rev. Sci. Instrum. (2)

J. T. Hodges, and R. Ciurylo, “Automated high-resolution frequency-stabilized cavity ring-down absorption spectrometer,” Rev. Sci. Instrum. 76, 023112 (2005).
[CrossRef]

G. Gagliardi, R. Restieri, G. De Biasio, P. De Natale, F. Cotrufo, and L. Gianfrani, “Quantitative diode laser absorption spectroscopy near 2 µm with high precision measurements of CO2 concentration,” Rev. Sci. Instrum. 72, 4228-4233 (2001).
[CrossRef]

Science (1)

G. J. German, and D. J. Rokestraw, “Multiplex spectroscopy: determining the transition moments and absolute concentrations of molecular species,” Science 264, 1750-1753 (1994).
[CrossRef]

Other (3)

M. Ebrahimzadeh: in Solid-State Mid-Infrared Laser Sources, Topics in Appl. Phys. 89, I. T. Sorokina, and K. L. Vodopyanov, eds. (Spriger-Verlag, Berlin 2003) p. 179.

F. K. Tittel, D. Richter, and A Fried: in Solid-State Mid-Infrared Laser Sources, Topics in Appl. Phys. 89, I. T. Sorokina and K. L. Vodopyanov, eds. (Spriger-Verlag, Berlin 2003) p. 445.

Harvard Smithsonian Center for Astrophysics: The Hitran Database 2004 <a href="http://www.hitran.com">http://www.hitran.com</a>

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Figures (4)

Fig. 1.
Fig. 1.

Experimental set-up. OI: optical isolator, FP: fiber port, C: collimating lens, HWP/QWP: half/quarter wave plate, L1/L2: lenses for spatial mode-matching, DM: dichroic mirror, AL: achromatic lens, PPLN: periodically-poled lithium-niobate non-linear crystal, Ge-F: Germanium filter. The function generator on the external cavity diode laser provides current modulation, while the one connected to the cavity piezo element is used for cavity-lenght modulation.

Fig. 2.
Fig. 2.

Effective free-spectral-range of the off-axis cavity over a 6 GHz scan (recorded on a timescale of 400 ms). Inset: mode spacing (15 MHz) shown on an expanded horizontal scale.

Fig. 3.
Fig. 3.

Absorption line profiles from the cavity, corresponding to ro-vibrational transitions of the CH3D ν4 and CH4 ν24 bands, respectively at 2960.617586 cm-1 and 2960.65530 cm-1. The cavity was filled with pure methane in natural isotopic abundance at 100 mTorr pressure. The inset shows the background baseline (recorded in absence of gas) used to extract the noise level (S/N=600 Hz1/2).

Fig. 4.
Fig. 4.

Integrated absorbance (experimental points) as a function of the air-sample total pressure for the CH4 transition at 2948.107924 cm-1. The IA values and the error bars were obtained from the fit lineshapes according to the procedure described in the text. A weighted linear fit was performed on these points in order to extract the gas concentration c from the slope Eq. (5). Inset a) shows the spectra recorded at increasing pressure values, while the corresponding fit lineshapes are plotted in inset b). At atmospheric pressure a signal-to-noise ratio S/N=1150 Hz1/2 was measured.

Equations (5)

Equations on this page are rendered with MathJax. Learn more.

dI dt = c 2 L [ I 0 MT 2 I ( 1 R ) ]
I t ( ω ) = I 0 M T 2 2 [ ( 1 R ) + ( ω ) PL ]
σ min = α p S N = 5.7 10 9 cm 1 Hz .
IA I t , α = 0 I t ( ω ) I t , α = 0 = α ( ω ) α ( ω ) + 1 R PLR
IA = P tot π ( B C P + B 2 ) 1

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