OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 19 — Sep. 13, 2010
  • pp: 20059–20071
« Show journal navigation

Simultaneous multi-laser, multi-species trace- level sensing of gas mixtures by rapidly swept continuous-wave cavity-ringdown spectroscopy

Yabai He, Ruifeng Kan, Florian V. Englich, Wenqing Liu, and Brian J. Orr  »View Author Affiliations


Optics Express, Vol. 18, Issue 19, pp. 20059-20071 (2010)
http://dx.doi.org/10.1364/OE.18.020059


View Full Text Article

Acrobat PDF (1637 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

The greenhouse-gas molecules CO2, CH4, and H2O are detected in air within a few ms by a novel cavity-ringdown laser-absorption spectroscopy technique using a rapidly swept optical cavity and multi-wavelength coherent radiation from a set of pre-tuned near-infrared diode lasers. The performance of various types of tunable diode laser, on which this technique depends, is evaluated. Our instrument is both sensitive and compact, as needed for reliable environmental monitoring with high absolute accuracy to detect trace concentrations of greenhouse gases in outdoor air.

© 2010 OSA

1. Introduction

It is both scientifically and technologically challenging to devise instrumental techniques with adequate absolute quantitative accuracy and reliability for trace-level detection of greenhouse-gas components at ambient concentrations in our atmospheric environment. A capability to make measurements of this type is relevant to environmental issues that are of great public concern [1

1. Inventory of U. S. Greenhouse Gas Emissions and Sinks, 1990–2008 (U.S. Environmental Protection Agency #430-R-10–006; April 2010); www.epa.gov/climatechange/emissions/usinventoryreport.html

]. In this regard, near-infrared (IR) continuous-wave (cw) cavity-ringdown (CRD) absorption spectroscopy is a particularly promising technique for such measurements.

CRD spectroscopy [2

2. K. W. Busch, and M. A. Busch, eds., Cavity-Ringdown Spectroscopy: an Ultratrace-Absorption Measurement Technique, Vol. 720 of ACS Symposium Series (Am. Chem. Soc., 1999).

4

4. G. Berden, and R. Engeln, eds., Cavity Ring-Down Spectroscopy: Techniques and Applications (Wiley, 2009).

] is well established as a highly sensitive technique for absolute measurement of optical absorption. It depends on determining the decay rate of coherent light in a high-finesse optical cavity containing a sample (usually gas-phase) of interest. The intensity of this intra-cavity light typically decays as a simple exponential function, with the decay rate (or inverse time constant τ–1) depending on cavity mirror reflectivity and additional intra-cavity optical losses (due to processes such as absorption and scattering). Optical absorption causes the decay rate τ–1 to increase relative to that of an absorber-free cavity or to that at non-absorbing wavelengths, thereby providing a direct measure of optical absorption by components of the sample medium inside the cavity. The inherent sensitivity of the CRD technique stems both from the extremely long effective path lengths of optical interaction (up to tens of kilometers) that are attained by using highly reflective, low-loss cavity reflectors and from the insensitivity of the decay rate to fluctuations in laser intensity.

Here we focus on our special variant of cw-CRD spectroscopy [5

5. Y. He and B. J. Orr, “Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases,” Appl. Phys. B 75(2-3), 267–280 (2002). [CrossRef]

8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

], in which ringdown decay is established and subsequently measured simply either by rapidly sweeping the cavity length (and hence its optical resonance frequencies) while fixing or slowly scanning the laser wavelength [5

5. Y. He and B. J. Orr, “Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases,” Appl. Phys. B 75(2-3), 267–280 (2002). [CrossRef]

,8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

] or by rapidly sweeping the laser wavelength (with fixed or progressively stepped cavity length) [6

6. Y. He and B. J. Orr, “Rapid measurement of cavity ringdown absorption spectra with a swept-frequency laser,” Appl. Phys. B 79(8), 941–945 (2004). [CrossRef]

,7

7. Y. He and B. J. Orr, “Continuous-wave cavity ringdown absorption spectroscopy with a swept-frequency laser: rapid spectral sensing of gas-phase molecules,” Appl. Opt. 44(31), 6752–6761 (2005). [CrossRef] [PubMed]

]. Coherent interaction between the laser light and the optical cavity then facilitates build-up and subsequent decay of laser radiation in the cavity at its optical resonance frequencies, without needing to incorporate a fast electro-optic or acousto-optic switch to interrupt or modulate the cw input radiation. Moreover, the laser frequency does not need to be locked to resonate with the ringdown cavity and is required to be stable only during the optical build-up time of the cavity. A further advantage of these rapidly swept approaches to cw-CRD absorption spectroscopy is that the appropriate instrumentation can be significantly simpler, more compact, and more rugged than other typical cw-CRD spectrometers. In addition, they facilitate a single-ended, fiber-coupled transmitter-receiver configuration in a retro-reflected, optical-heterodyne-detected (OHD) mode [5

5. Y. He and B. J. Orr, “Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases,” Appl. Phys. B 75(2-3), 267–280 (2002). [CrossRef]

,8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

].

1.1 Background to our research on multi-species spectroscopic sensing

We are concerned in this paper with quantitative detection of greenhouse-gas molecules such as carbon dioxide (CO2), methane (CH4), and water vapor (H2O) in natural isotopic abundance at ambient concentrations in air. Our rapidly swept cw-CRD approach is readily amenable to such multi-species detection schemes. Moreover, this method enables all three molecular species to be detected simultaneously, within the period (a few ms) of the cavity sweep cycle.

A conventional (but time-consuming) cw-CRD spectroscopic approach is to scan the wavelength of the monochromatic laser radiation through adjacent spectroscopic features of different chemical species. As a laborious prelude to implementing our simultaneous multi-laser cw-CRD spectroscopic detection scheme, Fig. 1
Fig. 1 Top trace: three superimposed survey CRD absorption spectra of outdoor air measured by a swept-cavity near-IR cw-CRD spectrometer with a single tunable diode laser. Lower traces: stick spectra for the three greenhouse-gas components (CO2, CH4, and H2O vapor), based on the HITRAN spectroscopic database [9].
(top trace, with τ–1 plotted on the CRDS ordinate) presents a superimposed set of three survey absorption spectra (plotted in green, red, and blue) of outdoor air with pressure P ≈750 Torr and T ≈300 K, sampled from the environment outside our laboratory building and flowed continuously at a rate of ~50 L h–1 through the ringdown cell. Each spectrum was measured by means of a swept-cavity cw-CRD spectrometer using a single widely tunable 3-mW external-cavity diode laser (ECDL; Photonetics, model Tunics Plus), scanned very slowly with high spectroscopic resolution through the wide wavelength range of 1540–1640 nm (spanning 100 nm in steps of 0.005 nm at 1 data point per second). Note that the higher baseline level and noise around 1620–1640 nm are attributable to reduced mirror reflectivity and laser stability in that region.

The dominant features of these survey spectra are attributable to weak absorption by CO2, CH4, and H2O in outdoor air, as indicated by their stick spectra based on the HITRAN spectroscopic database [9

9. L. S. Rothman, I. E. Gordon, A. Barbe, D. C. Benner, P. F. Bernath, M. Birk, V. Boudon, L. R. Brown, A. Campargue, and J.-P. Champion, “The HITRAN 2008 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110(9-10), 533–572 (2009) (see also earlier editions of HITRAN.). [CrossRef]

]. Since all three of these greenhouse-gas molecules have absorption features in the wavelength region used for fiber-optical telecommunications, our CRD spectrometer is able to take advantage of low-cost, monolithic, miniature tunable diode lasers, as well as fiber-optical components and networks for remote, distributed-cavity CRD-based sensing systems [5

5. Y. He and B. J. Orr, “Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases,” Appl. Phys. B 75(2-3), 267–280 (2002). [CrossRef]

,8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

].

Recording extensive survey absorption spectra as in Fig. 1 is not a practical way to pursue active environmental sensing. However, data thus collected help us either to identify more localized spectral regions in which two or more molecular species of interest have adjacent characteristic features or to identify interference-free isolated features in absorption spectra, using less elaborate diode lasers with limited tuning range. For instance, we have adopted such a method at near-IR laser wavelengths around ~1.5765 μm to examine dilute gas-phase mixtures of carbon monoxide (CO) and CO2 via their overlapping absorption spectra [10

10. R. A. Shorten, Y. He, and B. J. Orr, “Swept-cavity ringdown absorption spectroscopy: put your laser light in and shake it all about,” Aust. J. Chem. 56(3), 219–231 (2003). [CrossRef]

]. We note that gas-phase CO:CO2 concentration ratios are a key control measure of combustion efficiency in industrial processes such as smelting. Likewise, we have used a similar approach to monitor acetylene (C2H2) diluted in moist N2 gas at ~1.5201 μm by scanning part of the (ν1 + ν3) band of C2H2 and the overlapping spectrum of H2O vapor [8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

].

However, a more efficient method for instantaneous multi-species cw-CRD spectroscopy is to use several fixed-wavelength tunable diode lasers, each with its output pre-tuned to probe a characteristic absorption feature of a particular gas-phase molecular species of interest. We have previously demonstrated this multi-laser, multi-species cw-CRD spectroscopic concept with separate diode lasers in the case of on- and off-resonance wavelengths for CO2 gas at ~1.54 μm [10

10. R. A. Shorten, Y. He, and B. J. Orr, “Swept-cavity ringdown absorption spectroscopy: put your laser light in and shake it all about,” Aust. J. Chem. 56(3), 219–231 (2003). [CrossRef]

]. In later (unpublished) work, we achieved simultaneous gas-phase detection of CO and CO2 at ~1.56 and ~1.57 μm, respectively. We have now extended this approach using tailored multi-wavelength coherent radiation from a set of pre-tuned near-IR diode lasers and have reported preliminary results [11

11. Y. He, R. Kan, F. V. Englich, W. Liu, and B. J. Orr, “Multi-wavelength sensing of greenhouse gases by rapidly swept continuous-wave cavity ringdown spectroscopy” in Conference on Lasers and Electro-Optics / International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CMDD3; www.opticsinfobase.org/abstract.cfm?URI= CLEO-2009-CMDD3

] on greenhouse-gas molecules (CO2, CH4, H2O) in outdoor air. This paper reports details and outcomes of that research.

1.2 Background to other cw-CRD approaches to multi-species spectroscopic sensing

There are various other examples of multi-species cw-CRD spectroscopy, including an early multiplexed near-IR study [14

14. G. Totschnig, D. S. Baer, J. Wang, F. Winter, H. Hofbauer, and R. K. Hanson, “Multiplexed continuous-wave diode-laser cavity ringdown measurements of multiple species,” Appl. Opt. 39(12), 2009–2016 (2000). [CrossRef]

], in which acousto-optic modulators (AOMs) were used to toggle between two separate 1.4-μm distributed feedback (DFB) diode lasers, each pre-tuned to absorption features of vapor-phase species such as methanol (CH3OH) and isopropanol ((CH3)2CHOH) in the presence of residual H2O vapor. A similar approach has been applied for rapid fire detection via the gas-phase combustion products CO, CO2, C2H2, and hydrogen cyanide (HCN) [15

15. E. A. Fallows, T. G. Cleary, and J. H. Miller, “Development of a multiple gas analyzer using cavity ringdown spectroscopy for use in advanced fire detection,” Appl. Opt. 48(4), 695–703 (2009). [CrossRef] [PubMed]

], with an AOM used to toggle between two DFB diode lasers operating at ~1.57 μm (to monitor CO and CO2) and ~1.53 μm (to monitor C2H2 and HCN). More recently, a multi-species near-IR cw-CRD spectrometer, using two multiplexed DFB diode lasers, has been used to monitor 12CO2 and 13CO2 (at ~1.57 μm) and CH4 (at ~1.65 μm) [16

16. C. Wang, N. Srivastava, B. A. Jones, and R. B. Reese, “A novel multiple species ringdown spectrometer for in situ measurements of methane, carbon dioxide, and carbon isotope,” Appl. Phys. B 92(2), 259–270 (2008). [CrossRef]

]. Likewise, another advanced near-IR cw-CRD spectrometer employed two multiplexed DFB diode lasers for multi-species detection of CH4 and H2O (at ~1.651 μm) and of CO2 (at 1.603 μm) [17

17. E. R. Crosson, “A cavity ring-down analyzer for measuring atmospheric levels of methane, carbon dioxide, and water vapor,” Appl. Phys. B 92(3), 403–408 (2008). [CrossRef]

]. Addition of a chromatographic separation stage to a continuous-flow near-IR cw-CRD spectrometer has facilitated isotopic 13C/12C analysis of gas-phase mixtures of the hydrocarbons CH4, ethane (C2H6), and propane (C3H8), with 13CO2 and 12CO2 detected at ~1.603 μm after catalytic conversion of the hydrocarbons [18

18. A. A. Kachanov, E. R. Crosson, and B. A. Paldus, “Tunable diode lasers: expanding the horizon for laser absorption spectroscopy,” Opt. Photonics News 16(7), 44–50 (2005). [CrossRef]

].

Progress in multi-species near-IR cw-CRD spectroscopy has also been made possible by the availability of broadly tunable telecommunication lasers that can cover a much wider range of characteristic wavelengths than is possible with lasers that operate at a single discrete wavelength (which may be tuned relatively slowly). Using DFB diode-laser arrays [19

19. R. N. Zare, D. S. Kuramoto, C. Haase, S. M. Tan, E. R. Crosson, and N. M. R. Saad, “High-precision optical measurements of 13C/12C isotope ratios in organic compounds at natural abundance,” Proc. Natl. Acad. Sci. U.S.A. 106(27), 10928–10932 (2009). [CrossRef] [PubMed]

], molecular species have been detected at ambient concentrations in air with high sensitivity. Alternatively, miniature swept-frequency lasers based on microelectromechanical structure (MEMS) technology have facilitated rapid acquisition of wide-ranging near-IR spectra in the telecommunications C-band and have been used to detect CO2 and H2O vapor in N2 [6

6. Y. He and B. J. Orr, “Rapid measurement of cavity ringdown absorption spectra with a swept-frequency laser,” Appl. Phys. B 79(8), 941–945 (2004). [CrossRef]

,7

7. Y. He and B. J. Orr, “Continuous-wave cavity ringdown absorption spectroscopy with a swept-frequency laser: rapid spectral sensing of gas-phase molecules,” Appl. Opt. 44(31), 6752–6761 (2005). [CrossRef] [PubMed]

].

Other recent developments in the area of multi-species near-IR cw-CRD spectroscopy include advanced instrumentation based on a broadband optical frequency comb emitted by a mode-locked laser source [20

20. M. J. Thorpe, K. D. Moll, R. J. Jones, B. Safdi, and J. Ye, “Broadband cavity ringdown spectroscopy for sensitive and rapid molecular detection,” Science 311(5767), 1595–1599 (2006). [CrossRef] [PubMed]

,21

21. M. J. Thorpe, D. D. Hudson, K. D. Moll, J. Lasri, and J. Ye, “Cavity-ringdown molecular spectroscopy based on an optical frequency comb at 1.45-1.65 microm,” Opt. Lett. 32(3), 307–309 (2007). [CrossRef] [PubMed]

]. Such measurements of oxygen (O2), ammonia (NH3) and H2O at 0.76–0.85 μm [20

20. M. J. Thorpe, K. D. Moll, R. J. Jones, B. Safdi, and J. Ye, “Broadband cavity ringdown spectroscopy for sensitive and rapid molecular detection,” Science 311(5767), 1595–1599 (2006). [CrossRef] [PubMed]

] and of NH3, C2H2 and CO at 1.46–1.60 μm [21

21. M. J. Thorpe, D. D. Hudson, K. D. Moll, J. Lasri, and J. Ye, “Cavity-ringdown molecular spectroscopy based on an optical frequency comb at 1.45-1.65 microm,” Opt. Lett. 32(3), 307–309 (2007). [CrossRef] [PubMed]

] have demonstrated a huge multi-species tuning range relative to conventional CRD spectroscopy with discrete-wavelength near-IR cw laser sources, traded off against limited sensitivity, spectral resolution and increased instrumental complexity.

1.3 Specific objectives of this paper

2. Instrumentation

The layout of our rapidly swept IR cw-CRD spectrometer [5

5. Y. He and B. J. Orr, “Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases,” Appl. Phys. B 75(2-3), 267–280 (2002). [CrossRef]

,8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

,10

10. R. A. Shorten, Y. He, and B. J. Orr, “Swept-cavity ringdown absorption spectroscopy: put your laser light in and shake it all about,” Aust. J. Chem. 56(3), 219–231 (2003). [CrossRef]

,11

11. Y. He, R. Kan, F. V. Englich, W. Liu, and B. J. Orr, “Multi-wavelength sensing of greenhouse gases by rapidly swept continuous-wave cavity ringdown spectroscopy” in Conference on Lasers and Electro-Optics / International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CMDD3; www.opticsinfobase.org/abstract.cfm?URI= CLEO-2009-CMDD3

] is illustrated in Fig. 2
Fig. 2 Layout of an optical fiber-coupled, multi-laser near-IR cw-CRD spectrometer. The length of the ringdown cavity is rapidly varied by a piezoelectric translator (PZT). Radiation from several fixed-wavelength diode lasers (DL1, DL2, …) is coupled to the cavity to allow simultaneous measurement of multiple gas species.
. It includes several fixed-wavelength, near-IR cw diode lasers (DL1, DL2, …) and two photodetectors (PD1, PD2). The high-finesse ringdown cavity and its accompanying gas cell have been described in detail elsewhere [8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

]; its two concave mirrors (with ~99.993% reflectivity at ~1.55 μm and a 1-m radius of curvature) have a separation of ~0.53 m which can be rapidly varied by a piezoelectric translator (PZT). With appropriate control electronics, this instrument can operate in either a conventional, forward-propagating mode (using photodetector PD2) or in a backward-propagating OHD mode (using photodetector PD1). In the latter case, the backward-propagating light beam is coupled into the same optical fiber that delivered it from the laser, with a fiber-optical three-port circulator (AOC Technologies model OCIR-3-155, isolation, directivity and return loss greater than 50 dB [8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

]) used to divert the backward-propagating reflected and ringdown radiation to photodetector PD1 for OHD operation [5

5. Y. He and B. J. Orr, “Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases,” Appl. Phys. B 75(2-3), 267–280 (2002). [CrossRef]

,8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

,10

10. R. A. Shorten, Y. He, and B. J. Orr, “Swept-cavity ringdown absorption spectroscopy: put your laser light in and shake it all about,” Aust. J. Chem. 56(3), 219–231 (2003). [CrossRef]

]. The laser source(s) and ringdown light detection can therefore be integrated into a single transmitter-receiver module (left-hand side of Fig. 2). One or more ringdown cavity modules (right-hand side of Fig. 2) can then be located far away from their transmitter-receiver module, for remote-sensing applications by means of fiber-optical networks. Analog circuits allow real-time processing of resulting ringdown signals [5

5. Y. He and B. J. Orr, “Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases,” Appl. Phys. B 75(2-3), 267–280 (2002). [CrossRef]

,8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

], generating a voltage output that is proportional to the ringdown time τ. This yields a direct measure of absorption and concentration of molecules in the optical cavity, without any digital computing process. The instrument can operate in three different configurations: (i) with a single tunable laser [5

5. Y. He and B. J. Orr, “Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases,” Appl. Phys. B 75(2-3), 267–280 (2002). [CrossRef]

,7

7. Y. He and B. J. Orr, “Continuous-wave cavity ringdown absorption spectroscopy with a swept-frequency laser: rapid spectral sensing of gas-phase molecules,” Appl. Opt. 44(31), 6752–6761 (2005). [CrossRef] [PubMed]

,10

10. R. A. Shorten, Y. He, and B. J. Orr, “Swept-cavity ringdown absorption spectroscopy: put your laser light in and shake it all about,” Aust. J. Chem. 56(3), 219–231 (2003). [CrossRef]

], (ii) with a swept-frequency laser [6

6. Y. He and B. J. Orr, “Rapid measurement of cavity ringdown absorption spectra with a swept-frequency laser,” Appl. Phys. B 79(8), 941–945 (2004). [CrossRef]

,7

7. Y. He and B. J. Orr, “Continuous-wave cavity ringdown absorption spectroscopy with a swept-frequency laser: rapid spectral sensing of gas-phase molecules,” Appl. Opt. 44(31), 6752–6761 (2005). [CrossRef] [PubMed]

], and (iii) with multiple lasers set at selected on- and off-resonance wavelengths for molecules of specific interest [6

6. Y. He and B. J. Orr, “Rapid measurement of cavity ringdown absorption spectra with a swept-frequency laser,” Appl. Phys. B 79(8), 941–945 (2004). [CrossRef]

,10

10. R. A. Shorten, Y. He, and B. J. Orr, “Swept-cavity ringdown absorption spectroscopy: put your laser light in and shake it all about,” Aust. J. Chem. 56(3), 219–231 (2003). [CrossRef]

,11

11. Y. He, R. Kan, F. V. Englich, W. Liu, and B. J. Orr, “Multi-wavelength sensing of greenhouse gases by rapidly swept continuous-wave cavity ringdown spectroscopy” in Conference on Lasers and Electro-Optics / International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CMDD3; www.opticsinfobase.org/abstract.cfm?URI= CLEO-2009-CMDD3

]. Configuration (iii), as portrayed in Fig. 2, is the multi-laser variant of rapidly swept cw-CRD spectroscopy that is of central interest in this paper.

Several fixed-wavelength near-IR cw diode lasers (DL1, DL2, …) are each pre-tuned to a characteristic on- or off-resonance feature for the gas-phase molecules of interest, with their outputs combined into a single-mode optical fiber. Each wavelength-component of the diode-laser light is propagated to the ringdown cavity, where it attains CRD resonance at a distinct portion of the cavity-sweep cycle. The N × 1 fiber-optical combiner or optical switch provides flexible access to multiple laser outputs, as indicated schematically in Fig. 2.

Figure 3
Fig. 3 Signal voltages for a set of 0–3.5-ms rapidly swept scans of the optical cavity of a near-IR cw-CRD spectrometer. Trace (a): piezoelectric control voltage, spanning more than two free spectral ranges of the cavity. Trace (b): CRD signal events recorded with a single laser wavelength (λ1); these occur repetitively whenever the PZT-controlled cavity length is changed by a half-wavelength, so that the cavity round-trip optical pathlength is an integer multiple of the laser wavelength. Traces (c and d): corresponding CRD signal events with two and three laser wavelengths (λ1, λ2) and (λ1, λ2, λ3), respectively; the wavelengths have been finely adjusted to ensure that each signal event is cleanly separated in time within the cavity-sweep cycle of period ~1.1 ms.
shows how swept-cavity CRD spectroscopy with several fixed-wavelength near-IR cw diode lasers enables rapid specific multi-wavelength detection, with one, two and three lasers and with the ringdown cell statically filled with transparent pure nitrogen (N2) gas. With a single monochromatic laser, CRD signal events occur repetitively each time the fixed-wavelength laser radiation passes through optical resonance with the rapidly swept ringdown cavity, its length controlled by the PZT voltage ramp depicted in Fig. 3(a). Figure 3(b) shows that such CRD signal events occur repetitively (period ~1.1 ms) whenever the PZT-controlled cavity length is changed by a half-wavelength, so that the cavity round-trip optical pathlength is an integer multiple of the laser wavelength. Figures 3(c) and 3(d) display corresponding CRD signal events with two- and three-wavelength irradiation. For each laser wavelength (λ1, λ2, λ3), the CRD signal events can be cleanly separated in time by finely adjusting the laser wavelengths and, therefore, their resonance points. Such adjustments correspond to only a fraction of the cavity-mode separation, and are small (several percent) in comparison to the spectral widths of particular absorption features at atmospheric pressure. Incidentally, it may be noted that the spacings between CRD features in Figs. 3(c) and 3(d) appear to be slightly irregular; this is attributable to the fact that, although the PZT voltage ramp in Fig. 3(a) is linear, the linearity of the PZT-driven mirror motion is imperfect. Actual laser wavelengths used to record Figs. 3(b)3(d) were: λ1 ≈1573 nm and λ2 ≈1586 nm (each generated by Santec TSL-210 ECDLs); λ3 ≈1635 nm (generated by our 3-mW Photonetics Tunics Plus ECDL). As will be established in Sec. 3.2 below, these three specific laser wavelengths λ1, λ2, λ3 are suitable for interference-free trace-level detection of CO2, H2O vapor, and CH4, respectively. Each laser runs continuously (which helps us to operate it at a fixed wavelength and on a single longitudinal mode), with the CRD timing sequence spontaneously determined by the setting of the ringdown cavity length and its resonance with each laser wavelength (λ1, λ2 or λ3). We have developed LabVIEW software to control each laser, to log ringdown curves via an analog-to-digital board (National Instruments, model PCI-5122, 100 MHz, 14 bit, 32 MByte in each of two channels), and to analyze ringdown decays. We usually select one laser (the most stable one) as master, and actively control frequencies of the other lasers (via their operating temperature and current) by stabilizing the temporal separation of their ringdown events to that of the master laser within a cavity sweep cycle.

3. Results

The significant experimental results obtained in this project (apart from those depicted already in Figs. 1 and 3 to explain the methodology) are as follows. In Sec. 3.1, we have investigated the key performance characteristics of assorted diode lasers that are available for use. In Sec. 3.2, we have identified well-isolated molecular absorption lines for possible co-existing gas species that are of interest, to optimize detection sensitivity and to minimize interferences that tend to impair unambiguous quantitative analysis. Finally, in Sec. 3.3, we report the crucial outcomes of simultaneous multi-laser, multi-species measurements by rapidly swept cw-CRD spectroscopy on the greenhouse-gas molecules CO2, CH4, and H2O in air.

3.1 Performance of assorted near-IR diode lasers

Operation of cw-CRD spectroscopic systems in this near-IR wavelength region is greatly facilitated by the wide availability of compact, low-cost optical fiber components and cw tunable diode lasers operating around 1.55 μm. However, selection of suitable components requires considerable care, particularly because data sheets and other specifications are often aimed at the telecommunications market, providing characteristics of typical performance that may be too general or lacking in detail for our particular purposes. We have therefore tested several diode lasers thoroughly, with our specific CRD-spectroscopic applications in mind.

Figure 4
Fig. 4 Comparison of cw-CRD spectroscopic sensitivity (NEA) using assorted diode-laser sources. The performance of three ECDL-type light sources is depicted in the left-hand block (red traces) whereas that of three DFB-type diode lasers is depicted in the right-hand block (blue).
displays a set of fixed-wavelength baseline sensitivity traces recorded as noise-equivalent absorbance (NEA) by using our swept-cavity cw-CRD spectrometer (as described in Sec. 2 above) to test six different diode-laser light sources and an absorber-free ringdown cell. Three of the six (left-hand block of Fig. 4, with their performance traced in red) are ECDL-type light sources: New Focus, model 2630 with 6200 controller (tunable over ~1.51–1.59 μm with ~5 mW output power); Photonetics, model Tunics Plus (tunable over ~1.50–1.64 μm at ~3 mW, also used to record Figs. 1, 3, 5
Fig. 5 Observed cw-CRD (a) and HITRAN-simulated (b) absorption spectra of gas mixtures of CO2, CO, CH4, and H2O diluted in N2 at 1 atm in a region where CO2 (red) predominates over other components CO (green) and H2O (blue). The interference-free CO2 features marked <1> and <2> are preferred for CO2 detection by multi-laser cw-CRD spectroscopy.
, and 6
Fig. 6 Observed cw-CRD (a) and HITRAN-simulated (b) absorption spectra of gas mixtures of CO2, CO, CH4, and H2O diluted in N2 at 1 atm in a region where H2O (blue) predominates over other components CO (green) and CO2 (red). The interference-free H2O features marked <1> and <2> are preferred for H2O detection by multi-laser cw-CRD spectroscopy.
); Santec, model TSL-210 (tunable over ~1.52–1.62 μm at ~10 mW, also used to record Fig. 3). The other three of the six light sources (right-hand block of Fig. 4, with their performance traced in blue) are DFB-type diode lasers, each: Eblana Photonics, model EP1550-NLW-B01-400-FA (~1.57 μm); Fitel, model FOL15CDCWD (~1.54 μm); NTT Electronics, model NEL NLK1U5FAAA (~1.64 μm). Each measurement was conducted under similar conditions, with ~2 mW of optical power delivered at the input to the ringdown cell. Each data point is derived from a numerical fit of the exponential decay rate for an average of 256 cavity-ringdown curves. We note that these measurements apply only to the particular laser units used in our tests. Nevertheless, they provide useful information that may be more widely applicable to this representative range of models, despite the fact that manufacturers are often improving and releasing new models.

It should be noted that the ordinate (NEA) scale for the red (ECDL) traces in Fig. 4 is finer by a factor of 10 than that for the blue (DFB laser) traces in Fig. 4. These measurements therefore confirm that, as a general rule, ECDL-type lasers are significantly more stable than DFB-type lasers in terms of their optical frequency (at least on sub-ms time scales) and that the resulting ringdown decay curves are closer to single-exponential with less-pronounced deviations/fluctuations. However, DFB-type lasers are much more compact, robust and cheap. The frequency stability of the light source is a key factor that limits the detection sensitivity of a CRD-spectroscopic system. In these measurements, the standard deviations σ for NEA sensitivities recorded by the ECDL lasers approach ~2 × 10−9 cm−1; the corresponding NEA, normalized to the data rate of ~1 Hz, is ~2 × 10−9 cm−1 Hz-1/2. By contrast, the DFB lasers yield sensitivities and NEAs that are generally inferior by an order of magnitude.

3.2 Selection of characteristic transitions to minimize interference for unique identification

Many gas-phase molecules of interest (e.g., CH4, CO2, H2O, CO, C2H2, NH3) have absorption spectra in the 1.5–1.65-μm wavelength region. Their individual rovibrational absorption features span a broad wavelength range, with lineshapes (and widths) affected by Doppler and pressure broadening. For such molecules at ~300 K, full-width-at-half-maximum (FWHM) Doppler widths are ≤0.6 GHz (≤0.02 cm–1), while pressure-broadened linewidths are typically ~5 GHz (~0.17 cm–1) FWHM in air at atmospheric pressure. For a multi-component gas mixture, an absorption measurement at one particular wavelength can often be complicated by contributions from adjacent absorption lines of several molecules. Careful examination of line positions for possible co-existing gas species is therefore necessary, to identify well-isolated absorption lines for each molecule and thereby minimize any interferences that might otherwise obscure their unambiguous identification and quantitative analysis. Our investigation of interferences between various atmospheric species of interest has identified several relatively strong, well-isolated lines for the purpose of spectroscopic detection.

Figures 5(a) and 6(a) show cw-CRD absorption spectra measured for a gas mixture with partial pressures of CO, CO2, and CH4 in the ratio 700:500:1, diluted 500-fold in N2 at a total pressure P = 1 atm, and with residual H2O vapor. The ringdown cell contained this synthetic gas mixture and spectra were recorded by tuning our 3-mW Photonetics Tunics Plus ECDL.

Absorption features suitable for detection of CO2 and H2O vapor are highlighted in the measured cw-CRD spectra in Figs. 5(a) and 6(a), respectively. Figures 5(b) and 6(b) show corresponding HITRAN-based simulations for a mixture of CO2, CO, and H2O.

In the case of CO2 in Fig. 5, the preferred transitions marked <1> (at 1572.6598 nm) and <2> (at 1573.3317 nm) are well isolated from interference by other gas species. These are quite distinct from assorted characteristic wavelengths in the range 1560–1603 nm that have been used [8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

,11

11. Y. He, R. Kan, F. V. Englich, W. Liu, and B. J. Orr, “Multi-wavelength sensing of greenhouse gases by rapidly swept continuous-wave cavity ringdown spectroscopy” in Conference on Lasers and Electro-Optics / International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CMDD3; www.opticsinfobase.org/abstract.cfm?URI= CLEO-2009-CMDD3

,19

19. R. N. Zare, D. S. Kuramoto, C. Haase, S. M. Tan, E. R. Crosson, and N. M. R. Saad, “High-precision optical measurements of 13C/12C isotope ratios in organic compounds at natural abundance,” Proc. Natl. Acad. Sci. U.S.A. 106(27), 10928–10932 (2009). [CrossRef] [PubMed]

21

21. M. J. Thorpe, D. D. Hudson, K. D. Moll, J. Lasri, and J. Ye, “Cavity-ringdown molecular spectroscopy based on an optical frequency comb at 1.45-1.65 microm,” Opt. Lett. 32(3), 307–309 (2007). [CrossRef] [PubMed]

,23

23. R. Kan, W. Liu, Y. Zhang, J. Liu, M. Wang, D. Chen, J. Chen, and Y. Cui, “Large scale gas leakage monitoring with tunable diode laser absorption spectroscopy,” Chin. Opt. Lett. 4, 116–118 (2006).

,27

27. L. Wang, S. Kassi, A. W. Liu, S. M. Hu, and A. Campargue, “High sensitivity absorption spectroscopy of methane at 80 K in the 1.58 μm transparency window: Temperature dependence and importance of the CH3D contribution,” J. Mol. Spectrosc. 261(1), 41–52 (2010). [CrossRef]

] in other CRD-spectroscopic studies of CO2.

Although near-IR absorption by H2O vapor is much stronger outside the wavelength region of the fiber-optical telecommunication C- and L-bands, it is convenient here to use less prominent lines in regions where CO2 and CO also absorb, so that a single set of optical coatings and other components can be used to detect all three of these molecules. The H2O absorption lines designated <1> (at 1586.288 nm) and <2> (at 1591.686 nm) in Fig. 6 are relatively strong in this wavelength range which overlaps spectra of CO2 and CO; as far as we know, they have not previously been used for near-IR CRD-spectroscopic sensing of H2O. We note that, although H2O vapor is not of direct environmental concern as an anthropogenic additive, it is nevertheless one of the most important greenhouse gases, in view of its widespread IR absorption spectrum, its significant continuum absorption, and the effect of global warming on its natural vapor pressure. Likewise, CO is included in our synthetic gas mixture because it often accompanies greenhouse gases of primary concern and is implicated (together with CO2) in combustion processes [10

10. R. A. Shorten, Y. He, and B. J. Orr, “Swept-cavity ringdown absorption spectroscopy: put your laser light in and shake it all about,” Aust. J. Chem. 56(3), 219–231 (2003). [CrossRef]

,15

15. E. A. Fallows, T. G. Cleary, and J. H. Miller, “Development of a multiple gas analyzer using cavity ringdown spectroscopy for use in advanced fire detection,” Appl. Opt. 48(4), 695–703 (2009). [CrossRef] [PubMed]

,21

21. M. J. Thorpe, D. D. Hudson, K. D. Moll, J. Lasri, and J. Ye, “Cavity-ringdown molecular spectroscopy based on an optical frequency comb at 1.45-1.65 microm,” Opt. Lett. 32(3), 307–309 (2007). [CrossRef] [PubMed]

].

Ambient CH4 is typically very much less abundant than CO2, but it can still contribute significantly to global warming and is also implicated in tropospheric ozone production. Several recent near-IR spectroscopic investigations have focused on CH4 [6

6. Y. He and B. J. Orr, “Rapid measurement of cavity ringdown absorption spectra with a swept-frequency laser,” Appl. Phys. B 79(8), 941–945 (2004). [CrossRef]

8

8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

,14

14. G. Totschnig, D. S. Baer, J. Wang, F. Winter, H. Hofbauer, and R. K. Hanson, “Multiplexed continuous-wave diode-laser cavity ringdown measurements of multiple species,” Appl. Opt. 39(12), 2009–2016 (2000). [CrossRef]

,18

18. A. A. Kachanov, E. R. Crosson, and B. A. Paldus, “Tunable diode lasers: expanding the horizon for laser absorption spectroscopy,” Opt. Photonics News 16(7), 44–50 (2005). [CrossRef]

,22

22. F. V. Englich, Y. He, and B. J. Orr, “Continuous-wave stimulated Raman gain spectroscopy with cavity ringdown detection,” Appl. Phys. B 83(1), 1–5 (2006). [CrossRef]

27

27. L. Wang, S. Kassi, A. W. Liu, S. M. Hu, and A. Campargue, “High sensitivity absorption spectroscopy of methane at 80 K in the 1.58 μm transparency window: Temperature dependence and importance of the CH3D contribution,” J. Mol. Spectrosc. 261(1), 41–52 (2010). [CrossRef]

]; these include our own studies of CRD-detected stimulated Raman-gain spectra [22

22. F. V. Englich, Y. He, and B. J. Orr, “Continuous-wave stimulated Raman gain spectroscopy with cavity ringdown detection,” Appl. Phys. B 83(1), 1–5 (2006). [CrossRef]

,26

26. F. V. Englich, Y. He, and B. J. Orr, “Continuous-wave cavity-ringdown detection of stimulated Raman gain spectra,” Appl. Phys. B 94(1), 1–27 (2009). [CrossRef]

] and monitoring of CH4 in ambient and leakage situations [23

23. R. Kan, W. Liu, Y. Zhang, J. Liu, M. Wang, D. Chen, J. Chen, and Y. Cui, “Large scale gas leakage monitoring with tunable diode laser absorption spectroscopy,” Chin. Opt. Lett. 4, 116–118 (2006).

,24

24. R. Kan, W. Liu, Y. Zhang, J. Liu, M. Wang, D. Chen, J. Chen, and Y. Cui, “A high sensitivity spectrometer with tunable diode laser for ambient methane monitoring,” Chin. Opt. Lett. 5, 54–57 (2007).

].

To guide our experiments here, frequencies and line strengths from the HITRAN spectroscopic database [9

9. L. S. Rothman, I. E. Gordon, A. Barbe, D. C. Benner, P. F. Bernath, M. Birk, V. Boudon, L. R. Brown, A. Campargue, and J.-P. Champion, “The HITRAN 2008 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110(9-10), 533–572 (2009) (see also earlier editions of HITRAN.). [CrossRef]

] have been used to simulate absorption spectra for CH4 and H2O in the 1635–1655-nm region, slightly outside the C- and L-band optical-fiber telecommunication wavelength ranges (1530–1565 nm and 1565–1625 nm, respectively), but still accessible to some diode lasers (in particular, DFB-type diode lasers). Rovibrational structure in spectra of CH4 occurs in closely-spaced clusters of lines, which are merged by Doppler and pressure broadening; such spectra of trace CH4 in air at 1 atm and 300 K have a FWHM Doppler width of 0.565 GHz (0.0188 cm–1) and a pressure-broadened profile with a FWHM of ~3.9 GHz (~0.13 cm–1), so that its net Voigt convolution is effectively Lorentzian. The measurements of CH4 that we report in Sec. 3.3 were performed at 1635.414 nm, because other more prominent features revealed by our HITRAN simulations (e.g., at 1650.956 nm and 1653.725 nm) are beyond the 1640-nm operating limit of our available tunable diode lasers and cavity reflectors.

3.3 Simultaneous multi-laser multi-species measurements

Our conclusive simultaneous multi-species experimental measurements were performed with up to three diode lasers, as proposed in Fig. 3. Guided by Sec. 3.2, three distinct diode-laser wavelengths (1572.660 nm, 1635.414 nm, and 1586.288 nm) were set to monitor the greenhouse-gas molecules CO2, CH4, and H2O in air, respectively. As in Figs. 3(b) and 3(d), the two shorter wavelengths (detecting CO2 and H2O vapor) were generated by a pair of 10-mW Santec TSL-210 lasers, while our 3-mW Photonetics Tunics Plus ECDL was used to generate the longest wavelength (detecting CH4).

Representative results are shown in Fig. 7
Fig. 7 Use of swept-cavity cw-CRD spectroscopy to monitor greenhouse gases CO2 (red), CH4 (orange) and H2O (blue) in outdoor air at fixed diode-laser wavelengths of 1572.660 nm, 1635.414 nm, and 1586.288 nm, respectively. The decay rate of the absorber-free ringdown cavity at ~1635 nm is relatively high because of reduced mirror reflectivity, as indicated by measurements during the first 100 s of each trace when the cavity is flushed by pure N2 gas.
. During the first 100 s of the measurements depicted, an absorber-free baseline CRD decay rate was recorded for each simultaneously monitored wavelength (with red, orange, and blue traces indicating CO2, CH4, and H2O vapor in air, respectively), after flushing the ringdown cell with pure dry N2 gas at atmospheric pressure. A two-way valve was then used to switch the gas flow in the ringdown cell (where P ≈750 Torr and T ≈300 K) from pure N2 to air sampled from outside the laboratory building via a 5-m length of 6.35-mm-diameter nylon pressure tubing at a flow rate of ~50 L h–1.

It should be noted that the baseline of the CRD decay rate for the absorber-free cell at ~1635 nm (orange trace, for CH4) is ~20% higher than for the other two wavelengths (~1573 nm and ~1586 nm, for CO2 and H2O vapor, respectively). This is attributed to inferior reflectivity of the ringdown cavity mirrors beyond ~1620 nm (as is already evident in the survey spectrum of Fig. 1). Note also that the blue trace for H2O vapor takes at least 1 minute to settle, owing presumably to equilibration by adsorption of H2O on the ringdown cell walls, etc. after the gas flow is switched from pure N2 to outdoor air.

The wavelength range that has been used, as described above for simultaneous CRD-spectroscopic trace-level monitoring of CO2, CH4, and H2O vapor in air, is amenable to multi-species detection of various other gases (e.g., NH3, CO, and C2H2) which have absorption features in the same wavelength range. These could be detected by incorporating additional laser sources or by operating existing lasers at different wavelengths.

For high-precision cw-CRD sensing applications, dual-wavelength operation is straightforward and effective, because it enables one monochromatic laser wavelength to be set at a characteristic absorption feature for a particular species of interest, with a second monochromatic laser wavelength set off resonance to record a simultaneous baseline reference. This approach [5

5. Y. He and B. J. Orr, “Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases,” Appl. Phys. B 75(2-3), 267–280 (2002). [CrossRef]

] helps us to correct any drift of the baseline decay rate in real-world applications, where operating conditions might change substantially and frequently.

In order to monitor more than three distinct species with high sensitivity and precision, it would be best to combine our novel simultaneous multi-laser approach to cw-CRD spectroscopy with well-proven temporally multiplexed techniques (as surveyed in Sec. 1.2). The number of distinct species that can then be monitored is limited primarily by the number of suitable single-wavelength tunable lasers that are available.

4. Conclusions

Our multi-laser, multi-species approach to near-IR cw-CRD spectroscopic sensing employs communications-band fiber-optical and photonics components to devise a compact, robust, field-deployable, economical instrument that is well suited to highly sensitive detection of trace gas mixtures in various industrial, medical, agricultural, or environmental settings. In the context of this paper, this technique has been adapted to achieve accurate, quantitative monitoring of greenhouse-gas molecules at trace levels in the atmosphere.

We have shown in this paper that active scanning of laser wavelength(s) is not necessarily required to perform reliable trace-level monitoring of particular gas-phase molecules for which interference-free spectral characteristics have been established. This reduces the required wavelength-tuning performance of the laser sources that can be employed. The multi-laser variant of our distinctive swept-cavity near-IR cw-CRD technique uses several fixed-wavelength tunable diode lasers, each pre-tuned to a characteristic on- or off-resonance feature of a gas molecule of interest, with their outputs combined into a single-mode optical fiber and directed simultaneously to the ringdown cavity. Each component of the combined laser radiation attains CRD resonance at a separate part of the cavity-sweep cycle. This provides an attractive alternative to traditional multi-species spectroscopic approaches, in which different characteristic wavelengths are spatially separated via dispersive optics or wavelength-selective filters and monitored by multiple photodetectors or a CCD array.

As in most forms of spectroscopic sensing (including those in which a continuously scanned monochromatic source or a dispersed broadband source is used to record an extensive portion of a spectrum), “false positive” signals due to spectral interferences occasionally arise. However, as demonstrated in Secs 3.1 and 3.2, we have carefully characterized the frequency stability of the diode lasers to be used and critically selected spectral features for the molecular species of interest, thereby ensuring that multi-laser, multi-species cw-CRD signals as in Sec. 3.3 (notably Fig. 7) are virtually free of interferences from any unidentified species that is likely to accompany the targeted greenhouse-gas molecules in ambient air. Nevertheless, in applying our multi-laser, cw-CRD method to real-world environmental sensing, we need to be alert to avoid potential interferences (e.g., from unidentified atmospheric pollutants). Moreover, our approach is likely to be less viable when applied to sensing of trace gas mixtures for which the chemical composition is unknown or difficult to anticipate.

Acknowledgments

We acknowledge financial support from the Australian Research Council and from the Chinese Academy of Sciences (which funded a six-month visit by Ruifeng Kan to Macquarie University in 2008/9).

References and links

1.

Inventory of U. S. Greenhouse Gas Emissions and Sinks, 1990–2008 (U.S. Environmental Protection Agency #430-R-10–006; April 2010); www.epa.gov/climatechange/emissions/usinventoryreport.html

2.

K. W. Busch, and M. A. Busch, eds., Cavity-Ringdown Spectroscopy: an Ultratrace-Absorption Measurement Technique, Vol. 720 of ACS Symposium Series (Am. Chem. Soc., 1999).

3.

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

4.

G. Berden, and R. Engeln, eds., Cavity Ring-Down Spectroscopy: Techniques and Applications (Wiley, 2009).

5.

Y. He and B. J. Orr, “Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases,” Appl. Phys. B 75(2-3), 267–280 (2002). [CrossRef]

6.

Y. He and B. J. Orr, “Rapid measurement of cavity ringdown absorption spectra with a swept-frequency laser,” Appl. Phys. B 79(8), 941–945 (2004). [CrossRef]

7.

Y. He and B. J. Orr, “Continuous-wave cavity ringdown absorption spectroscopy with a swept-frequency laser: rapid spectral sensing of gas-phase molecules,” Appl. Opt. 44(31), 6752–6761 (2005). [CrossRef] [PubMed]

8.

Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]

9.

L. S. Rothman, I. E. Gordon, A. Barbe, D. C. Benner, P. F. Bernath, M. Birk, V. Boudon, L. R. Brown, A. Campargue, and J.-P. Champion, “The HITRAN 2008 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110(9-10), 533–572 (2009) (see also earlier editions of HITRAN.). [CrossRef]

10.

R. A. Shorten, Y. He, and B. J. Orr, “Swept-cavity ringdown absorption spectroscopy: put your laser light in and shake it all about,” Aust. J. Chem. 56(3), 219–231 (2003). [CrossRef]

11.

Y. He, R. Kan, F. V. Englich, W. Liu, and B. J. Orr, “Multi-wavelength sensing of greenhouse gases by rapidly swept continuous-wave cavity ringdown spectroscopy” in Conference on Lasers and Electro-Optics / International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CMDD3; www.opticsinfobase.org/abstract.cfm?URI= CLEO-2009-CMDD3

12.

M. Wang, Y. Zhang, J. Liu, W. Liu, R. Kan, T. Wang, D. Chen, J. Chen, X. Wang, H. Xia, and X. Fang, “Applications of a tunable diode laser absorption spectrometer in monitoring greenhouse gases,” Chin. Opt. Lett. 4, 363–365 (2006).

13.

H. Xia, W. Liu, Y. Z. Zhang, R. K. Kan, M. Wang, Y. He, Y. Cui, J. Ruan, and H. Geng, “An approach of open-path gas sensor based on tunable diode laser absorption spectroscopy,” Chin. Opt. Lett. 6, 437–440 (2008). [CrossRef]

14.

G. Totschnig, D. S. Baer, J. Wang, F. Winter, H. Hofbauer, and R. K. Hanson, “Multiplexed continuous-wave diode-laser cavity ringdown measurements of multiple species,” Appl. Opt. 39(12), 2009–2016 (2000). [CrossRef]

15.

E. A. Fallows, T. G. Cleary, and J. H. Miller, “Development of a multiple gas analyzer using cavity ringdown spectroscopy for use in advanced fire detection,” Appl. Opt. 48(4), 695–703 (2009). [CrossRef] [PubMed]

16.

C. Wang, N. Srivastava, B. A. Jones, and R. B. Reese, “A novel multiple species ringdown spectrometer for in situ measurements of methane, carbon dioxide, and carbon isotope,” Appl. Phys. B 92(2), 259–270 (2008). [CrossRef]

17.

E. R. Crosson, “A cavity ring-down analyzer for measuring atmospheric levels of methane, carbon dioxide, and water vapor,” Appl. Phys. B 92(3), 403–408 (2008). [CrossRef]

18.

A. A. Kachanov, E. R. Crosson, and B. A. Paldus, “Tunable diode lasers: expanding the horizon for laser absorption spectroscopy,” Opt. Photonics News 16(7), 44–50 (2005). [CrossRef]

19.

R. N. Zare, D. S. Kuramoto, C. Haase, S. M. Tan, E. R. Crosson, and N. M. R. Saad, “High-precision optical measurements of 13C/12C isotope ratios in organic compounds at natural abundance,” Proc. Natl. Acad. Sci. U.S.A. 106(27), 10928–10932 (2009). [CrossRef] [PubMed]

20.

M. J. Thorpe, K. D. Moll, R. J. Jones, B. Safdi, and J. Ye, “Broadband cavity ringdown spectroscopy for sensitive and rapid molecular detection,” Science 311(5767), 1595–1599 (2006). [CrossRef] [PubMed]

21.

M. J. Thorpe, D. D. Hudson, K. D. Moll, J. Lasri, and J. Ye, “Cavity-ringdown molecular spectroscopy based on an optical frequency comb at 1.45-1.65 microm,” Opt. Lett. 32(3), 307–309 (2007). [CrossRef] [PubMed]

22.

F. V. Englich, Y. He, and B. J. Orr, “Continuous-wave stimulated Raman gain spectroscopy with cavity ringdown detection,” Appl. Phys. B 83(1), 1–5 (2006). [CrossRef]

23.

R. Kan, W. Liu, Y. Zhang, J. Liu, M. Wang, D. Chen, J. Chen, and Y. Cui, “Large scale gas leakage monitoring with tunable diode laser absorption spectroscopy,” Chin. Opt. Lett. 4, 116–118 (2006).

24.

R. Kan, W. Liu, Y. Zhang, J. Liu, M. Wang, D. Chen, J. Chen, and Y. Cui, “A high sensitivity spectrometer with tunable diode laser for ambient methane monitoring,” Chin. Opt. Lett. 5, 54–57 (2007).

25.

A. W. Liu, S. Kassi, and A. Campargue, “High sensitivity CW-cavity ring down spectroscopy of CH4 in the 1.55 μm transparency window,” Chem. Phys. Lett. 447(1-3), 16–20 (2007). [CrossRef]

26.

F. V. Englich, Y. He, and B. J. Orr, “Continuous-wave cavity-ringdown detection of stimulated Raman gain spectra,” Appl. Phys. B 94(1), 1–27 (2009). [CrossRef]

27.

L. Wang, S. Kassi, A. W. Liu, S. M. Hu, and A. Campargue, “High sensitivity absorption spectroscopy of methane at 80 K in the 1.58 μm transparency window: Temperature dependence and importance of the CH3D contribution,” J. Mol. Spectrosc. 261(1), 41–52 (2010). [CrossRef]

ToC Category:
Spectroscopy

History
Original Manuscript: July 22, 2010
Revised Manuscript: August 31, 2010
Manuscript Accepted: August 31, 2010
Published: September 3, 2010

Citation
Yabai He, Ruifeng Kan, Florian V. Englich, Wenqing Liu, and Brian J. Orr, "Simultaneous multi-laser, multi-species trace-level sensing of gas mixtures by rapidly swept continuous-wave cavity-ringdown spectroscopy," Opt. Express 18, 20059-20071 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-19-20059


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. Inventory of U. S. Greenhouse Gas Emissions and Sinks, 1990–2008 (U.S. Environmental Protection Agency #430-R-10–006; April 2010); www.epa.gov/climatechange/emissions/usinventoryreport.html
  2. K. W. Busch, and M. A. Busch, eds., Cavity-Ringdown Spectroscopy: an Ultratrace-Absorption Measurement Technique, Vol. 720 of ACS Symposium Series (Am. Chem. Soc., 1999).
  3. G. Berden, R. Peeters, and G. Meijer, “Cavity ring-down spectroscopy: experimental schemes and applications,” Int. Rev. Phys. Chem. 19, 565–607 (2000). [CrossRef]
  4. G. Berden, and R. Engeln, eds., Cavity Ring-Down Spectroscopy: Techniques and Applications (Wiley, 2009).
  5. Y. He and B. J. Orr, “Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases,” Appl. Phys. B 75(2-3), 267–280 (2002). [CrossRef]
  6. Y. He and B. J. Orr, “Rapid measurement of cavity ringdown absorption spectra with a swept-frequency laser,” Appl. Phys. B 79(8), 941–945 (2004). [CrossRef]
  7. Y. He and B. J. Orr, “Continuous-wave cavity ringdown absorption spectroscopy with a swept-frequency laser: rapid spectral sensing of gas-phase molecules,” Appl. Opt. 44(31), 6752–6761 (2005). [CrossRef] [PubMed]
  8. Y. He and B. J. Orr, “Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity,” Appl. Phys. B 85(2-3), 355–364 (2006). [CrossRef]
  9. L. S. Rothman, I. E. Gordon, A. Barbe, D. C. Benner, P. F. Bernath, M. Birk, V. Boudon, L. R. Brown, A. Campargue, and J.-P. Champion, “The HITRAN 2008 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110(9-10), 533–572 (2009) (see also earlier editions of HITRAN.). [CrossRef]
  10. R. A. Shorten, Y. He, and B. J. Orr, “Swept-cavity ringdown absorption spectroscopy: put your laser light in and shake it all about,” Aust. J. Chem. 56(3), 219–231 (2003). [CrossRef]
  11. Y. He, R. Kan, F. V. Englich, W. Liu, and B. J. Orr, “Multi-wavelength sensing of greenhouse gases by rapidly swept continuous-wave cavity ringdown spectroscopy” in Conference on Lasers and Electro-Optics / International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CMDD3; www.opticsinfobase.org/abstract.cfm?URI= CLEO-2009-CMDD3
  12. M. Wang, Y. Zhang, J. Liu, W. Liu, R. Kan, T. Wang, D. Chen, J. Chen, X. Wang, H. Xia, and X. Fang, “Applications of a tunable diode laser absorption spectrometer in monitoring greenhouse gases,” Chin. Opt. Lett. 4, 363–365 (2006).
  13. H. Xia, W. Liu, Y. Z. Zhang, R. K. Kan, M. Wang, Y. He, Y. Cui, J. Ruan, and H. Geng, “An approach of open-path gas sensor based on tunable diode laser absorption spectroscopy,” Chin. Opt. Lett. 6, 437–440 (2008). [CrossRef]
  14. G. Totschnig, D. S. Baer, J. Wang, F. Winter, H. Hofbauer, and R. K. Hanson, “Multiplexed continuous-wave diode-laser cavity ringdown measurements of multiple species,” Appl. Opt. 39(12), 2009–2016 (2000). [CrossRef]
  15. E. A. Fallows, T. G. Cleary, and J. H. Miller, “Development of a multiple gas analyzer using cavity ringdown spectroscopy for use in advanced fire detection,” Appl. Opt. 48(4), 695–703 (2009). [CrossRef] [PubMed]
  16. C. Wang, N. Srivastava, B. A. Jones, and R. B. Reese, “A novel multiple species ringdown spectrometer for in situ measurements of methane, carbon dioxide, and carbon isotope,” Appl. Phys. B 92(2), 259–270 (2008). [CrossRef]
  17. E. R. Crosson, “A cavity ring-down analyzer for measuring atmospheric levels of methane, carbon dioxide, and water vapor,” Appl. Phys. B 92(3), 403–408 (2008). [CrossRef]
  18. A. A. Kachanov, E. R. Crosson, and B. A. Paldus, “Tunable diode lasers: expanding the horizon for laser absorption spectroscopy,” Opt. Photonics News 16(7), 44–50 (2005). [CrossRef]
  19. R. N. Zare, D. S. Kuramoto, C. Haase, S. M. Tan, E. R. Crosson, and N. M. R. Saad, “High-precision optical measurements of 13C/12C isotope ratios in organic compounds at natural abundance,” Proc. Natl. Acad. Sci. U.S.A. 106(27), 10928–10932 (2009). [CrossRef] [PubMed]
  20. M. J. Thorpe, K. D. Moll, R. J. Jones, B. Safdi, and J. Ye, “Broadband cavity ringdown spectroscopy for sensitive and rapid molecular detection,” Science 311(5767), 1595–1599 (2006). [CrossRef] [PubMed]
  21. M. J. Thorpe, D. D. Hudson, K. D. Moll, J. Lasri, and J. Ye, “Cavity-ringdown molecular spectroscopy based on an optical frequency comb at 1.45-1.65 microm,” Opt. Lett. 32(3), 307–309 (2007). [CrossRef] [PubMed]
  22. F. V. Englich, Y. He, and B. J. Orr, “Continuous-wave stimulated Raman gain spectroscopy with cavity ringdown detection,” Appl. Phys. B 83(1), 1–5 (2006). [CrossRef]
  23. R. Kan, W. Liu, Y. Zhang, J. Liu, M. Wang, D. Chen, J. Chen, and Y. Cui, “Large scale gas leakage monitoring with tunable diode laser absorption spectroscopy,” Chin. Opt. Lett. 4, 116–118 (2006).
  24. R. Kan, W. Liu, Y. Zhang, J. Liu, M. Wang, D. Chen, J. Chen, and Y. Cui, “A high sensitivity spectrometer with tunable diode laser for ambient methane monitoring,” Chin. Opt. Lett. 5, 54–57 (2007).
  25. A. W. Liu, S. Kassi, and A. Campargue, “High sensitivity CW-cavity ring down spectroscopy of CH4 in the 1.55 μm transparency window,” Chem. Phys. Lett. 447(1-3), 16–20 (2007). [CrossRef]
  26. F. V. Englich, Y. He, and B. J. Orr, “Continuous-wave cavity-ringdown detection of stimulated Raman gain spectra,” Appl. Phys. B 94(1), 1–27 (2009). [CrossRef]
  27. L. Wang, S. Kassi, A. W. Liu, S. M. Hu, and A. Campargue, “High sensitivity absorption spectroscopy of methane at 80 K in the 1.58 μm transparency window: Temperature dependence and importance of the CH3D contribution,” J. Mol. Spectrosc. 261(1), 41–52 (2010). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited