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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 3 — Feb. 6, 2006
  • pp: 1304–1313
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Combining a difference-frequency source with an off-axis high-finesse cavity for trace-gas monitoring around 3 μm

P. Malara, P. Maddaloni, G. Gagliardi, and P. De Natale  »View Author Affiliations


Optics Express, Vol. 14, Issue 3, pp. 1304-1313 (2006)
http://dx.doi.org/10.1364/OE.14.001304


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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

1. Introduction

A scheme for high-sensitivity gas detection that overcomes most of these drawbacks is off-axis integrated-cavity-output spectroscopy (OA-ICOS). It combines the detection principle of CEAS with a very simple set-up, using a multipass-like geometry for the high finesse optical cavity. This so called “off-axis” alignment results in the excitation of an extremely dense mode spectrum and may ideally lead to a continuous transmission. With this in mind, the interaction between the laser and the cavity can be considered “always-resonant”, thus eliminating the need for frequency locking. Furthermore, as the excitation of any transverse TEMmn mode contributes to the detection of the intracavity absorber, the off-axis set-up is almost insensitive to vibrations and small misalignments. The OA-ICOS scheme was first proposed and demonstrated in the near IR spectral region, providing very low minimum detectable absorption coefficients thanks to the pathlength increase, though only few works performed a quantitative concentration analysis on gas samples [19–23

19. J. B. Paul, L. Lapson, and J. Anderson, “Ultrasensitive absorption spectroscopy with a high-finesse optical cavity and off-axis alignment,” Appl. Opt. 40, 4904–4910 (2001). [CrossRef]

]. The mid-IR is particularly attractive for trace gas detection as the strongest ro-vibrational transitions of several molecular species occur in this spectral region, and some examples of highly-sensitive spectroscopic detection based on OA-ICOS have been already reported [24

24. Y. A. Bakhirkin, A. A. Kosterev, C. Roller, R. F. Curl, and F. K. Tittel, “Mid-infrared quantum cascade laser based off-axis integrated output spectroscopy of biogenic nitric oxide detection,” Appl. Opt. 43, 2257–2266 (2004). [CrossRef] [PubMed]

, 25

25. 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]

]. However, the spectral coverage of available laser sources in the mid IR is still discontinuous. In particular, the window between 2.5 and 3.5 μm, where the C-H, N-H and O-H bonds exhibit their characteristic vibrations, is not accessible to single laser devices (except for HeNe and CO overtone lasers, which have discrete line-tunability though). Thanks to wide tunability, low-noise and narrow linewidth, optical parametric oscillators (OPOs) and difference-frequency generators (DFGs), proved to be the most effective tools for high resolution and high sensitivity spectroscopy in this wavelength interval [26

26. M. Ebrahimzadeh: in Solid-State Mid-Infrared Laser Sources, Topics in Appl. Phys.89, I. T. Sorokina and K. L. Vodopyanov, eds. (Spriger-Verlag, Berlin2003) p. 179.

, 27

27. 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, Berlin2003) p. 445.

]. However, OPO-based spectrometers need special devices in order to be used in cw, mode-hop-free operation, thus making the set-up cumbersome and sophisticated [12

12. G. von Basum, D. Halmer, P. Hering, M Mürtz, S. Schiller, F. Müller, A. Popp, and F. Kühnemann, “Parts per trillion sensitivity for ethane in air with an optical parametric oscillator cavity leak-out spectrometer,” Opt. Lett. 29, 797–799 (2004). [CrossRef] [PubMed]

, 28

28. E. V. Kovalchuk, D. Dekorsy, A. I. Lvovsky, C. Braxmaier, J. Mlynek, A. Peters, and S. Schiller, “High-resolution doppler-free molecular spectroscopy with a continuous-wave optical parametric oscillator,” Opt. Lett. 26, 1430–1433 (2001). [CrossRef]

]. On the other hand, the output power of DFG sources, typically limited to a few hundreds μW, has so far prevented their use in conjunction with off-axis high-finesse cavities, since the most relevant drawback of OA-ICOS technique is the significantly reduced transmission through the cavity. Recent improvements in non-linear optical crystals as well as in rare-earth-doped fiber amplifiers have allowed the development of more powerful DFG sources, now competitive with OPOs and commercially available semiconductor lasers for spectroscopic use [29–31

29. 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]

].

In this paper we present a novel DFG-based spectrometer, emitting between 2.9 and 3.5 μm with a power of several milliwatts, in which the radiation is coupled for the first time to an off-axis high-finesse optical resonator for high-sensitivity and quantitative spectroscopy of methane. An analytical model, based on the off-axis cavity response function, enabled us to exploit the relationship existing between the integrated absorbance and the gas concentration to extract its value directly from the area under the recorded absorption profile. Using this approach, CH4 abundance in ambient air was directly measured at different total pressures, and a minimum detectable concentration level below 1 ppbv∙Hz-1/2 was extrapolated.

2. Experimental setup

In our experiment, the DFG beam was coupled to a 90-cm long high-finesse optical cavity. The resonator was made by a vacuum-tight stainless-steel tube, with identical mirrors, each equipped with three independent tilting screws and a piezoelectric actuator for fine adjustment and length modulation. The tube was connected through proper vacuum fittings to a diaphragm-molecular drag pump that ensured high purity conditions inside. Gas samples at controlled pressures could be injected into the cavity either from a 99.99-% CH4 gas cylinder or from laboratory air thanks to precision valves. The pressure was monitored by a pair of capacitive manometers with 1 and 1000-Torr full-scale pressure ranges. Cavity mirrors were spherical with radius of curvature r=6 m, diameter d=½” and nominal reflectivity R=99.97%. The off-axis alignment was achieved starting from on-axis position (TEM00 alignment), then horizontally shifting the beam out of the cavity axis, and slightly tilting it in the vertical direction. The light emerging from the output cavity mirror was finally focused onto a 3-stage thermoelectrically-cooled InAs detector (Judson, mod. J12TE3-66D-R01M).

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.

As demonstrated by Herriott et al. [32

32. D. Herriott, H. Kogelnik, and R. Kompfner, “Off-axis paths in spherical mirror interferometers,” Appl. Opt. 3, 523–526 (1964). [CrossRef]

], the off-axis geometry leads to a multiple-reflection radiation path in the cavity, resulting in a series of m spots in elliptical pattern. In this way, the beam does not overlap itself until m round-trips, corresponding to the so-called “re-entrant condition”. In the ideal case, with the spots lying on a circumference concentric to the cavity mirror, the angle θ between two successive spots is cos(θ/2)=1-L/r, where L is the cavity length, so that the beam fulfills the re-entrant condition after having traced n circles, namely p spots on each mirror (=2nπ). The number of round trips p only depends on L and r, and so the cavity effective free-spectral-range (FSR) equals c/2pL. In practice, the density of the excited modes indicates how close the alignment is to the optimal one. In our setup, starting from an on-axis FSR of 166 MHz, a 15-MHz mode spacing was measured, yielding a number of round trips p=11 with n=1 (see Fig. 2). The upper limit to the p value is actually set by the finite beam spot size and mirror diameter, both affecting the maximum number of separated spots that can be accommodated on the mirror surface, superposition resulting in hard-removable fringes. The actual sensitivity enhancement of the OA-ICOS method basically relies on the ability to smooth out the cavity mode structure. This can be virtually accomplished in the case of an off-axis FSR well below the laser linewidth. However, because of the limitations discussed above, this is not our case: large peak-to-peak intensity fluctuations were observed, as can be seen from the transmission spectrum shown in Fig. 2. As we will discuss later on, in order to flatten the cavity frequency response more effectively, both laser-frequency and cavity-length modulations were introduced and combined to time integration. The residual frequency dependence was further reduced by narrowing the scan interval (down to 4 ms), so that the crossing through resonance of each mode was not long enough to complete cavity energy build-up [33

33. J. Morville, D. Romanini, M. Chenevier, and A. Kachanov, “Effects of laser phase noise on the injection of a high-finesse cavity,” Appl. Opt. 41, 6980–6990 (2002). [CrossRef] [PubMed]

, 34

34. 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]

].

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.

3. Experimental results and discussion

In order to express in analytical form the behaviour of an off-axis cavity injected by a cw laser, a constant source term is added to the standard differential equation used to describe the change of the intracavity power [19

19. J. B. Paul, L. Lapson, and J. Anderson, “Ultrasensitive absorption spectroscopy with a high-finesse optical cavity and off-axis alignment,” Appl. Opt. 40, 4904–4910 (2001). [CrossRef]

], which yields

dIdt=c2L[I0MT2I(1R)]
(1)

where I0 is the incident intensity, M is a factor between 0 and 1 describing the amount of incident radiation coupled to the resonator and T the mirror transmittivity. Therefore, the steady-state transmitted intensity is given by It=I0MT2/[2(1-R)], and, even for M=1, is reduced by a factor T/2 compared with the expected transmission of an on-axis resonantly-coupled cavity. Consequently, relatively powerful sources and sensitive detectors are desirable to achieve a high sensitivity with an off-axis geometry. As an example, in our case, for a 1 mW incident power and a detector-preamplifier gain of 1 V/μW, a maximum transmitted signal of about 50 mV was measured.

The presence of weakly absorbing species in the cavity can be taken into account by replacing the reflectivity with R’=Re -α(ω)PLR(1-α(ω)PL) and the output signal can be rewritten as

It(ω)=I0MT22[(1R)+(ω)PL]
(2)

with α(ω) the absorption coefficient of the selected transition and P the absorber pressure. When the per-pass fractional absorption (αPL) is small compared to the intrinsic cavity losses, the fractional change in the transmitted intensity can be simply expressed in terms of a cavity equivalent absorption pathlength L eq=LR/(1-R). In the following, we will deal with strong ro-vibrational transitions which do not satisfy the latter approximation and, therefore, Eq (2) will be used.

To record an ICOS spectrum the following procedure was adopted. The DFG source was scanned over the molecular transition of interest at a repetition rate of 125 Hz (scan interval=4 ms), by sweeping one of the pumping lasers. By current sine modulation, the same laser was used to introduce a fast frequency dithering of the DFG beam (depth Am=25 MHz, frequency fm=10 kHz). In addition, a slow cavity-length modulation (Am=4.5 GHz, fm=410 Hz) was introduced by a piezo element on the cavity input mirror. The signal coming from the detector was low-pass filtered (f 6dB/oct low =3 kHz) and then averaged for 4 s (500 samples), resulting in an effective detection bandwidth of 6 Hz. These values were accurately chosen during a preliminary experiment and corresponded to the highest signal-to-noise ratio, preserving the absorption lineshape without any distorsion as well. In these experimental conditions a number of spectra were acquired. An example is given in Fig. 3, showing simultaneously two lines around 2961 cm-1, belonging respectively to CH4 and CH3D in a 100-mTorr sample of pure methane in natural isotopic abundance. The inset also shows the background baseline for the empty cavity, used to extract the noise level. The pressure-dependent absorption coefficients of both lines were previously measured in a 50-cm-long reference cell, in a higher pressure range, obtaining α=(3.45±0.03)∙10-5 cm-1torr-1 for the less abundant isotopologue and α=(8.51±0.02)∙10-5 cm-1torr-1 for the dominant one. Therefore, from the CH3D line (S/N=600 Hz1/2), we calculated a noise-equivalent absorption coefficient given by

σmin=αpSN=5.7109cm1Hz.
(3)

The measured absorption coefficient and Eq (2) also provided a realistic estimate of the equivalent pathlength and thus of the mirror reflectivity. Since a relative absorption of 45.0±0.2 % was measured from the spectrum of Fig. 3, we obtained Leq=1.80±0.02 km corresponding to R≅99.95 %.

In the following, we demonstrate that these features can be successfully exploited to provide direct gas concentration from the absorption spectra in the cavity. From Eq. (2), it is straightforward to calculate the integrated absorbance, that is

IAIt,α=0It(ω)It,α=0=α(ω)α(ω)+1RPLR
(4)

where P = cPtot is now the partial pressure, proportional to the CH4 concentration c, and I t,α=0 is the empty-cavity transmitted intensity. The left-hand term of Eq. (4) could be retrieved from the experimental spectra according to the following procedure. For each sample pressure the baseline level I t,α=0 and the line profile It(ω) were acquired. The normalized signals were then analyzed by a nonlinear least-squares routine based on Levenberg-Marquardt algorithm using different line models for low and high pressure regimes. In particular, for Ptot=120, 190 and 250 Torr a Voigt function was assumed for the absorption coefficient, while at higher pressures a Lorentzian profile was adopted. Also, the amplitude modulation due to the DFG source scan was taken into account by a quadratic baseline in the fit procedure. Finally, the area under the fit curves was numerically measured for each pressure. The full-width half-maximum (FWHM) derived from these fits exhibited a very good linear dependence on gas pressure with intercept consistent with the expected Doppler width. That enabled us to measure the pressure broadening coefficient Cp, which was found to be 0.144±0.002 cm-1 atm-1.

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).

The right-hand term of Eq. (4) was evaluated assuming a Lorentzian shape for the absorption coefficient, leading to

IA=Ptotπ(BCP+B2)1
(5)

where B=(1R)πcLRNLS, NL is the Loschmidt number, and S is the transition linestrength. The Lorentzian profile assumed to calculate the integral in the right-hand term of Eq. (4) has been chosen essentially to provide an analytical relationship between the experimental area and the gas concentration. In principle, a full numerical calculation could be implemented for any pressure assuming a Voigt profile for α(ω) in Eq. (4). However, the Lorentzian approximation is well satisfied above 500 Torr, and therefore does not affect the procedure at pressures close to the atmospheric one. From Eq. (5) the IA values directly yield the c parameter, provided the product SLeq is known. In our experiment, S was taken from HITRAN04 database [35

35. Harvard Smithsonian Center for Astrophysics: The Hitran Database 2004http://www.hitran.com

] and for Leq the value previously measured was used.

To demonstrate gas concentration measurements from the acquired spectra, the CH4 abundance in ambient air was estimated. For this purpose we moved to a stronger transition belonging to the ν3 band (ν=2948.107924 cm-1, S=8.4×10-20 cm/molec) and recorded the absorption line profile on the cavity output, using ambient air samples at different total pressures. The resulting spectra are shown in Fig. 4(a), while the corresponding fit lineshapes are plotted in Fig. 4(b).

The main frame of Fig. 4 shows the experimental IA values obtained varying the total pressure, with the error bars given by the root-mean-square fit residuals. Hence, by fitting Eq. (5) to the IA values, a concentration c=1.06±0.01 ppm was extracted. A fully consistent value was obtained from the IA at 760 Torr. Although not far from the expected ambient level (about 1.8 ppm), the obtained value is not strictly consistent with it. This discrepancy can be likely attributed to the value adopted for the SLeq product, which was not derived by a rigorous calibration procedure in a certified gas mixture. However, the method accuracy will probably be the issue of a future work. It is worth noting the very good agreement of the experimental points with the predicted linear trend, even for pressures below 500 Torr. As a proof of the model appropiateness, we remark that the line intercept is consistent with zero within 3σ. Finally, inserting the value obtained for c into Eq. (3) and scaling by the measured signal-to-noise ratio (S/N=1150 Hz1/2), a minimum detectable concentration of 850 ppt/Hz1/2 was found at atmospheric pressure.

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.

4. Conclusions

We have reported on an off-axis cavity-enhanced DFG-based spectrometer working between 2.9 and 3.5 μm. Using ICOS a minimum detectable absorption coefficient of 5.7∙10-9 cm-1Hz-1/2 was obtained, which is comparable with other cavity enhanced spectroscopic techniques. Moreover, the possibility of direct concentration measurements up to atmospheric pressure was demostrated. The developed procedure enabled us to retrieve the gas concentration directly from the spectrum area, accounting for the nonlinearity in the absorption introduced by the cavity. For methane this resulted in a minumum detectable concentration of several hundreds ppt∙Hz-1/2 at atmospheric pressure. One can expect similar sensitivity levels for a number of gas species such as C2H4, NH3 and N2O, absorbing in the operation range of our DFG source. The demonstrated performances can be further improved using higher-reflectivity and/or larger-diameter mirrors. On the other hand, the proposed method may be similarly applied to retrieve the linestrength of very weak ro-vibrational transitions using accurate standard gases. This is particularly relevant in molecular spectroscopy studies, most databases still lacking experimental data in this spectral region.

Finally, the wide source tunability combined to the insensitivity to misalignment and the simplicity of the apparatus, makes our DFG OA-ICOS set-up well suited for in-situ operation. Indeed, work is currently in progress to make the spectrometer set-up more compact and robust using all-fiber optical components. This will lead to the development of a portable analyzer for field, real-time gas-concentration measurements, analogous to a recently-reported near-IR spectrometer, devoted to monitoring of gas effluxes in volcanic areas [36

36. 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]

].

Acknowledgments

This work has profited significantly from discussions with D. Romanini and F. K. Tittel. The experimental activity was funded by the Italian Ministry for Education, University and Research in the framework of “Progetto P.O.N.-S.I.MON.A.”

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34.

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]

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Harvard Smithsonian Center for Astrophysics: The Hitran Database 2004http://www.hitran.com

36.

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]

OCIS Codes
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators
(190.2620) Nonlinear optics : Harmonic generation and mixing
(230.5750) Optical devices : Resonators
(300.6390) Spectroscopy : Spectroscopy, molecular

ToC Category:
Spectroscopy

History
Original Manuscript: October 24, 2005
Revised Manuscript: January 16, 2006
Manuscript Accepted: January 19, 2006
Published: February 6, 2006

Citation
P. Malara, P. Maddaloni, G. Gagliardi, and P. De Natale, "Combining a difference-frequency source with an off-axis high-finesse cavity for trace-gas monitoring around 3 µm," Opt. Express 14, 1304-1313 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-3-1304


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References

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