Vol48/04/2005def 767 ANNALS OF GEOPHYSICS, VOL. 48, N. 4/5, August/October 2005 Key words diode laser – absorption spectroscopy – optical fiber 1. Introduction The analysis of volcanic gases is widely recognised as a powerful tool to validate geolog- ical models relevant to reliable predictions of vol- canic events (Baubron et al., 1991; Notsu et al., 1993). Indeed, the chemical composition of cer- tain gases, in volcanic effluxes, may provide di- rect information on deep magmatic processes and hydrothermal circulation (Chiodini et al., 2003). In particular, the absolute concentration and the isotopic content of some molecular species gives an indication of sources and sinks of volcanic gases, whereas its time variation may be a conse- quence of changes in the status of a volcano. These changes can be due to chemical reactions of magmatic gases with rocks or fluids occurring along their path to the surface. With this in mind, the evolution of volcanic activity can be monitored using sensitive instru- mentation for gas analysis (Chiodini et al., 2003). However, the compactness and rugged- ness of field sensors are critical requirements for monitoring of volcanic areas, where the equipment is often exposed to high humidity, large temperature variations, and fumigation by corrosive gases. Furthermore, high precision and accuracy levels are necessary in order to re- trieve significant information from experimen- tal data. An ideal volcanic sensor would also al- low for continuous, in situ gas concentration determination over long time periods, with un- attended and remote operation. So far, volcanic gases have been usually mon- itored by means of laboratory analysis methods, such as mass spectrometry and gas cromatogra- phy, which require in situ sampling, which is of- ten impractical and hazardous. In addition, the time resolution of sampling-based methods may be not sufficient for some applications. An alternative approach is given by optical techniques for remote-sensing, such as Differen- Novel laser-based techniques for monitoring of volcanoes Paolo De Natale (1), Giuseppe De Natale (2), Gianluca Gagliardi (1), Livio Gianfrani (3) and Alessandra S.D. Rocco (1) (1) Istituto Nazionale di Ottica Applicata, Comprensorio Olivetti, Pozzuoli (NA), Italy (2) Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Napoli, Italy (3) Dipartimento di Scienze Ambientali, Seconda Università di Napoli, and INFM-Gruppo Coordinato Napoli 2, Caserta, Italy Abstract An overview of novel laser techniques suitable for volcanic monitoring, based on different kinds of infrared laser sources, is presented. Their main advantages and drawbacks are discussed focusing on the achievable sensitivi- ty and precision levels in analysis of gaseous species. Some of the most recent experimental results obtained in laboratory development as well as in field tests of home-built laser spectrometers are reported. New perspectives in optical devices aimed at geochemical and geophysical applications are also considered. Mailing address: Dr. Gianluca Gagliardi, Istituto Na- zionale di Ottica Applicata, Comprensorio Olivetti, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy; e-mail: ga- gliardi@ino.it 768 Paolo De Natale, Giuseppe De Natale, Gianluca Gagliardi, Livio Gianfrani and Alessandra S.D. Rocco tial Optical Absorption Spectrometers (DOAS), LIDAR (Svanberg, 2002) or open-path Fourier- Transform Infrared Spectrometers (FTIR) (Op- penheimer, 2002). The first two methods provid- ed good results in field volcanic measurements, as in the case of Galle et al. (2003), and Weibring et al. (2002), although some of these apparatus are bulky and expensive, and often require a time-consuming post-acquisition data process- ing. More recently, new instruments, based on UV-correlation and DOAS techniques, have been developed in a more compact and user- friendly fashion (Porter et al., 2002). In the last few decades, the use of near-in- frared room-temperature diode lasers for gas sensing has significantly increased (Werle, 1998). Compared to spectroscopic techniques that rely on incoherent sources as well as on lead-salt and solid-state lasers, semiconductor diode lasers offer the possibility to develop re- liable and compact spectrometers, with reason- able costs. Also, they present several advan- tages in terms of tuneability, spectral selectivi- ty and power consumption. The possible use of these devices in combination with optical fibers is particularly convenient for volcanic monitor- ing applications (Ginfrani et al., 2000). Thanks to the great advances in semiconductor and op- toelectronic technologies, most of these fea- tures have been recently transferred to the mid- infrared, with novel coherent radiation sources such as those based on difference-frequency generation and on quantum-cascade semicon- ductor structures (Tittel et al., 2002). In this spectral region, several atmospherically rele- vant molecules, like H2O, CO2, CH4 and SO2, exhibit strong absorptions (Harvard Smithson- ian Center for Astrophysics, 2003). 2. Near-IR spectroscopy using DFB diode lasers Near-infrared diode laser sources, initially developed for telecom applications, are current- ly based on III-V semiconductor alloys. They operate in continuous mode, at room tempera- ture, exhibiting single-mode emission in the 1- 2 µm range, with linewidths in the order of 10 MHz and output power of several mW. A sup- plementary DFB (distributed-feedback) struc- ture also ensures very high spectral purity and mode-hop-free frequency tuning. Diode-laser spectrometers rely on resonant absorption of coherent radiation by a gas sample, contained in an optical cell, whose transmission can be monitored by a pre-amplified room-tem- perature photodiode. According to Lambert-Beer Law, the transmitted power is related to the gas concentration N (in molecules/cm3) and to the cell length L (in cm), via the equation (2.1) where P0 is the incident power, S(T) the transi- tion linestrength (in cm/molecule), ν the laser frequency, ν0 the line-centre frequency and g(ν−ν0) the normalized line shape function. In the last decade, several experimental schemes, mostly based on modulation of laser frequency, such as Wavelength- (WMS) or Fre- quency-Modulation Spectroscopy (FMS), have been developed in an attempt to increase detec- tion sensitivity. Using these techniques, the re- trieval of quantitative information from the ab- sorption spectrum of a gas sample can be achieved by comparison with that of a reference cell, containing a known mixture. In this case, the chemical composition and thermodynamic conditions of the reference cell must be exactly the same as those of the sample under investi- gation. Any such difference may induce a sys- tematic error in the measurement process, thus affecting the achievable accuracy, in spite of the high sensitivity. This is of particular importance in the case of gases from volcanoes, where large temperature and pressure variations are possible. Different approaches, based on a pri- ori calibration of WMS or FMS signals (Gold- stain and Adler-Golden, 1993), exhibit limited precision levels and are difficult to implement, and are thus ill-suited to environmental applica- tions. Recently, we demonstrated that measure- ments of absolute gas concentrations with high precision can be efficiently performed using a very simple method, based on direct absorption, which does not require any frequency modula- tion technique or calibration cell (Gagliardi et al., 2001). We employed an InGaAsP/InP DFB diode ( ) ( ) ( )expP P S T g NL0 0= - -o o o6 @ 769 Novel laser-based techniques for monitoring of volcanoes laser, emitting at a wavelength of 2-µm and able to probe strong ro-vibrational lines of CO2 and H2O. A Herriott-type multiple-reflection cell, with a path-length of 50 m, was used as a gas cell. Absorption spectra were recorded for both species, and their concentration values were re- trieved, through eq. (2.1), by measuring the inte- grated absorbances of the two absorption lines. This was possible after the product between the absorption pathlength and the linestrength had been measured using a certified gas mixture. Laboratory tests showed that a short-term repro- ducibility of 2‰ and an overall accuracy below 1% can be achieved in measurements in ambient CO2, within a wide range of concentration values (Gagliardi et al., 2002a). Simultaneous measure- ments of CO2 and H2O concentrations at similar precision levels were also demonstrated, a line pair of the two species being observed within one laser frequency scan. One of the main advantages of such a near- IR laser spectrometer is its compatibility with telecom optical fibers, suitable for remote and in situ gas detection. Also, the principle of op- eration is completely independent of chemical composition and pressure of the gas sample. A portable version of the 2-µm spectrometer was subsequently developed and tested to monitor gaseous emissions in volcanic areas, during a field campaign at Vulcano Island and Solfatara volcano, in Italy, during July and November, 2002 (Rocco et al., 2004). The experimental set- up is schematically represented in fig. 1. The spectrometer is split into two parts, mounted on different breadboards, and connected by a tele- com, single-mode, 30-m-long optical fiber. The laser source and collimating optics are mounted on one breadboard while the gas detection sys- tem on a another one, with a large hole in the centre which allows the gas to flow through the cell. In this way, remote measurements are pos- sible at different sites across a volcanic area. In addition, a custom-built, open-path multiple-re- flection cell, with a 20-m pathlength, was used for direct analysis of fumarolic effluxes without requiring any sampling procedure. Figure 2 shows an example of absorption spectrum. It corresponds to the R(34) rotational line of the ν1+2ν2+ν3 CO2 combination band, and to the 150.15 →14 0.14 line of the ν2+ν3 H2O band, around 5000 cm−1. The absorption signal was recorded at the Solfatara crater, directly on the flux of a small fumarole, scanning the laser fre- quency around a CO2-H2O line pair, while the Fig. 1. Sketch of the experimental arrangement of the portable 2-µm spectrometer. OI – Optical Isolator; SMF – Single Mode Fiber; FP – Fiber Port; MRC – Multiple-Reflection Cell; PD – photodiode; BB1 – breadboard 1 (60 × 60 cm2); BB2 – breadboard 2 (70 × 40 cm2). 770 Paolo De Natale, Giuseppe De Natale, Gianluca Gagliardi, Livio Gianfrani and Alessandra S.D. Rocco experimental spectra were continuously trans- ferred to a laptop computer and analysed by a LABVIEW program. Furthermore, CO2 and H2O concentrations could be monitored in ambient air for several minutes, with a typical uncertain- ty of a few %, thus demonstrating real-time an- alytical capabilities of our spectrometer in hos- tile environmental conditions (Rocco et al., 2004). Thanks to its low power consumption and ruggedness, the apparatus was transported almost everywhere in the selected volcanic sites. Indeed, substitution of optical or mechan- ical components was never necessary during operation in the whole campaign. The main drawback of our diode-laser based spectrome- ter is the influence of temperature fluctuations on measured concentration values. Due to the molecular linestrength dependence on Boltz- mann distribution of population among energy levels, variations in the order of 3%/°C for wa- ter vapor, and less than 1‰/°C for carbon diox- ide are expected. Nevertheless, the gas cell tem- perature was continuously monitored and ac- quired and, when necessary, its values were used to correct the corresponding small changes in the integrated absorbances. 3. Mid-IR spectroscopy with quantum- cascade lasers and difference-frequency based spectrometers The advantages of direct absorption detection can be better exploited when one moves towards longer wavelengths. Indeed, most atmospheric molecules present stronger absorption bands, due to their fundamental vibrations, above 3 µm. That allows for sensitive detection of low abundance gases, like CH4, SO2 and N2O, at their ambient concentration levels. For this reason, much effort has been directed towards the development of new coherent radiation sources emitting in the mid-IR spectral region. Among them are Quan- tum-Cascade Lasers (QCL), covering the interval from 3.5 to 24 µm, rare-earth-doped DFB fiber lasers, in the 1 µm to 1.5 µm range, and Optical Parametric Oscillators (OPO) or Difference-Fre- quency Generation (DFG) in periodically-poled crystals, between 2 and 5 µm. Quantum cascade lasers were invented in 1994, at Lucent Technologies (Murray Hill, NJ) and are essentially based on a multiple mini- band structure resulting from deposition of ultra- thin alternating layers of semiconductor materi- Fig. 2. Direct absorption measurement of CO2 and H2O performed for a small fumarole at the Solfatara vol- cano (Italy). The continuous line represents the Lorentian fit curve. 3 4 771 Novel laser-based techniques for monitoring of volcanoes als. Electron transitions occur between different quantum well levels, in each mini-band, with many-photon emission (Faist et al., 1994). The major limitation of such lasers is the need for cryogenic cooling in order to have continuous- wave (cw) operation. Alternatively, they can be used at room-temperature in pulsed mode (Faist et al., 1998). Nevertheless, QCLs usually exhib- it output power of 50-100 mW and intrinsic emission linewidth well below 1 MHz, with the possibility of a DFB design for high spectral pu- rity and continuous wavelength tuning. Further- more, cw operation of a quantum-cascade de- vice, up to 246 K on a Peltier cooler, has recent- ly been achieved (Hofstetter et al., 2001). After the advent of quantum cascade lasers, several QCL-based spectrometers were success- fully used, either in pulsed or continuous mode, for absorption spectroscopy as well as isotopic detection of species whose transitions are not ac- cessible to conventional room-temperature diode lasers (Webster et al., 2001; Kosterev and Tittel, 2002). Our group recently developed a novel spectrometer relying on a cw DFB QCL at a wavelength of 8.06 µm, manufactured at Lucent Technologies Labs. In the experimental set-up, described in Gagliardi et al. (2002b), the laser was placed in a liquid-N2 cryostat, and stabilised by an active temperature controller. The laser beam was coupled to a 17-cm long absorption cell, filled with the gas sample, whose transmis- sion was monitored by a liquid-N2 cooled HgCdTe detector. The QCL spectrometer en- abled us to detect the presence of very low CH4 concentration. Indeed, several methane absorp- tion spectra were recorded at sub-Torr pressures, in correspondence of the strong P(10) A2(1)- A1(1) line, belonging to the ν4 fundamental band. Some of these spectra, with increasing ab- sorbance, are shown in fig. 3, for different gas pressures in the cell. A wavelength-modulation spectroscopic technique, with phase-sensitive 1st-harmonic demodulation, was also imple- mented in order to detect CH4 isotopomers. An example of simultaneous recording of 13CH4 and 12CH4, for natural isotopic abundance methane, is given in fig. 4. The spectra were observed within a single QCL frequency scan, amounting to 1 GHz. Furthermore, simultaneous detection of nitrous oxide and its isotopomers 14N15N16O, 15N14N16O, 14N14N18O and 14N14N17O was possible, in the same spectral region, with similar perform- ances (Gagliardi et al., 2002b). An example is re- ported in fig. 5. In these operational conditions, a sensitivity level corresponding to a minimum de- tectable pressure of few mTorr was estimated, for pure methane and nitrous oxide, in a 1-m absorp- tion pathlength. In addition, a signal repro- Fig. 3. Direct absorption spectrum of pure methane, around 1241 cm−1. The gas pressure in the cell was varied from 100 to 400 mTorr. Fig. 4. Simultaneous detection of 13CH4 and 12CH4, in natural isotopic abundance (1.1% and 98.9%, respective- ly), in the presence of 150 mTorr of pure methane. 772 Paolo De Natale, Giuseppe De Natale, Gianluca Gagliardi, Livio Gianfrani and Alessandra S.D. Rocco ducibility better than 1 % was demonstrated in detection of all species. This feature is relevant to a possible use of the 8-µm spectrometer for measurements of the 13C/12C and 15N14N, 18O/16O, 17O/16O isotope ratios in CH4 and N2O, respec- tively. Although the results obtained in our labora- tory tests are encouraging, some drawbacks come from the use of a quantum cascade laser. For instance, the cryogenic system used for laser operation is bulky and expensive, while the high- current supply, usually needed for laser driving, makes its use somewhat difficult. As a conse- quence, considerable effort is still required in or- der to make this kind of spectrometer compact and well suited for field measurements. A novel apparatus, based on difference-fre- quency generation in a periodically-poled lithi- um niobate crystal, is currently being devel- oped in our lab. Mixing an Er-fiber-amplified diode laser, emitting at 1.55 µm, and an extend- ed-cavity 1.05-µm diode laser, externally am- plified by an Yb-fiber, we expect to produce co- herent radiation around a wavelength of 3 µm, with powers up to 10 mW. Such a DFG spec- trometer would satisfy some crucial require- ments for spectroscopy, such as high spectral resolution, high power and wide tuneability. In- deed, its spectral coverage ranges from 2.9 to 3.5 µm. This is of great relevance if we consid- er the current lack of laser sources in the region between 2.2 and 3.5 µm, where strong absorp- tion bands of N2O and CH4 as well as of H2O, NH3 and NO2 take place. It is worth noting that neither QCLs can presently cover this spectral window, due to manufacturing difficulties. 4. Conclusions and future perspectives Several spectroscopic set-ups, based on dif- ferent laser sources, have been reviewed. Some experimental results were presented on monitor- ing of CO2 and H2O emitted by volcanic fu- maroles, using a compact and portable spectrom- eter operating at 2 µm. Using such a spectrome- ter, continuous, fast and in situ detection of these species was possible, measuring their concentra- tions with precision levels of a few %. For future developments, novel near-IR laser sources, such as Tm-doped fiber-based lasers, are also being considered as an alternative to diode lasers, espe- cially in view of their possible integration in an optical fiber network for multiple-point, simulta- neous monitoring of gas concentrations. Using the same fiber, the absorption spectrometer can also be combined with fiber Bragg-grating sen- sors for local measurements of strain and tem- perature (Rao, 1997). Such a network would be particularly relevant to the implementation of re- liable methods for volcanic surveillance. 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