Long Range Trace Detection in Aqueous Aerosol using Remote Filament-Induced Breakdown Spectroscopy

(R-FIBS)

J.-F. Daigle1, G. Méjean1, W. Liu1, F. Théberge1, H.L. Xu1, Y. Kamali1, J. Bernhardt1, A. Azarm1, Q. Sun1, P. Mathieu2, G. Roy2, J.‐R Simard2 and S.L. Chin1

1Centre d’Optique, Photonique et Laser (COPL) et le Département de Physique, de Génie Physique et d’Optique, Université Laval,

Québec, Québec G1K 7P4, Canada 2The Sensing (Air & Surface) & Optronics Section of the Defense Research and Development Center-Valcartier, QC, G3J 1X5, Canada

E-mail: jfdaigle2@yahoo.ca

Abstract: R-FIBS is used for probing salt water aerosol. We demonstrate experimentally that it can be used to sense ppm level concentrations up to 70 m away and shows potential for kilometer range applications. ©2007 Jean-François Daigle OCIS codes: 010.1100, 010.1280, 010.3640, 140.7090

Remote, real-time detection of atmospheric aerosols has become a critical issue for both environmental and security/defense purposes. In the former case, the development of global climate models through total characterization of atmospheric clouds cannot be achieved without determining the composition inside the water microdroplets and other aerosols.

At short distances, nanosecond Laser-Induced Breakdown Spectroscopy (ns-LIBS) has proven its feasibility on aqueous aerosols [1]. However, because of diffraction, it seems very difficult to extend this technique for remote sensing purposes. This limitation can be circumvented by the use of femtosecond terawatt laser pulses. The propagation of such laser pulses in air is dominated by the formation of filaments. The intensity inside a filament is clamped down to approximately 5×1013 W/cm2 in air [2] which is high enough to ionize molecules. Remote Filament-Induced Breakdown Spectroscopy (R-FIBS) has already demonstrated its efficiency on metallic [3] and solid biological [4] samples. It thus represents an attractive candidate for remote sensing of atmospheric aerosols’ constituents.

Recently, Fujii et al. demonstrated that sodium atomic lines could be observed from salt water aerosols using R-FIBS [5]. The authors focused 160 mJ pulses/50 fs laser pulses centered at 800nm with a 20 m concave mirror onto a thick aerosol target. They were able to detect sodium fluorescence from aerosols at a salt concentration of 300 g/L from 16 m away.

In the present work, using a similar technique, but with a special focusing telescope that allows the control of filaments at long distances [6], we demonstrated a 4 orders of magnitude improvement in the detection of sodium fluorescence over the results of Fujii et al [5]; i.e. sodium can be observed down to the concentration of 30 mg/L (corresponding to 30 ‘ppm’) from 50m away. Moreover, for the first time, the four main atomic hydrogen spectral lines of the Balmer series were also detected.

Intense femtosecond 10 Hz laser pulses, centered at 800 nm from a Ti:Sapphire laser system were used to generate the filaments in air. They were focused, using a variable focal length telescope [6] specially designed to reduce white light generation and create strong short filaments. Similar to Fujii et al [5], aerosol was generated by using a commercial ultrasonic humidifier (Sunbeam, Health at home) and injected into a mobile aerosol chamber. The salt solution was prepared by dissolving appropriate quantities of salt in de-ionized water. A 75 cm long open-ended cylindrical plastic pipe (diameter 4 cm) is placed inside the chamber as a concentrator to increase the droplet density (thick cloud condition). Later, to demonstrate the feasibility for real atmospheric applications, the droplet concentration is decreased by removing the plastic pipe. The latter case corresponds roughly to a droplet concentration of 1000 cm-3 (thin cloud condition) which is approximately 10 times the concentration of the thinnest natural clouds. Beside the focusing telescope, a typical fiber coupled LIDAR setup [6] collected the R-FIBS signals, which were delivered to a SpectraPro-500i SP-558 spectrometer equipped with a PIMAX:512 ICCD camera.

The spectrum presented in figure 1 is the result of 100 shots accumulation with the aerosol chamber positioned at 5 m from the LIDAR system under the thick cloud condition. The corresponding laser pulse duration was negatively chirped to 80 fs at 70 mJ/pulse. In addition to the strong sodium fluorescence, the

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presence of the four main atomic hydrogen fluorescence lines of the Balmer series indicates that the laser filaments had induced breakdown of the water droplets.

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Under the thick cloud condition, from 5 m away, the detection limit was measured to be inferior to 3 mg/L (3 ‘ppm’). For longer distance measurement, we performed the experiment in the corridor next to the laboratory. In order to avoid early filamentation before the pulse arrived at the corridor, negatively chirped 10 ps/72 mJ pulses were launched. Sodium fluorescence could be observed 50 m away from the LIDAR at down to a concentration of ~ 30mg/L (30 ‘ppm’) under thick cloud condition. This should be contrasted to Fujii et al’s result [5] at a concentration of 300g/L from 16 m away using twice as much pulse energy. Our result represents an improvement of more than 4 orders of magnitude.

Figure 1: Typical spectrum

Sodium fluorescence under the thin cloud condition was also measured with the chamber positioned at 50 m away from the LIDAR under the following conditions: salt concentration, 1.25 g/L; Optical thickness, 0.15; filament length, 1.5 m and 1000 laser shots accumulation. It has been reported that ‘filamenting’ pulses can propagate almost unaffected through a cloud of optical thickness of 1.2 (35 cm long cloud) [7]. In our experiment, the optical thickness was roughly 0.15 (120 cm long cloud). Thus, in our case, filaments can penetrate through the whole cloud and excite the fluorescence.

The above measured thin cloud signal is used to make an extrapolation. Assuming a detection limit of three times the standard deviation of the signal integrated over 10000 shots, similar filaments penetrating through a cloud thickness L = 20 m and a salt concentration of 50 mg/L (50 ‘ppm’), the detection range is estimated to reach 1.25 km using the relation Signal~L/R2 where R is the distance between the cloud and the LIDAR. Since the closest clouds, Cumulus humilis, are located around 500-1000 m over sea level, this result opens a way for efficient ‘ppm’ level remote sensing of atmospheric aerosols in air.

In conclusion, we experimentally demonstrated the feasibility of remote sensing the composition of salt water aerosols at the ‘ppm’ level in air by R-FIBS. This technique has the potential for real application to remote sensing of atmospheric cloud.

This work was supported in part by NSERC, DRDC-Valcartier, Canada Research Chairs, Canada Foundation for Innovation and FQRNT. [1] B.C Windom, P.K. Diwakar, D.W. Hahn, Spectrochimica Acta Part B 61, 788–796 (2006) [2] A. Becker, N. Aközbek, K. Vijayalakshmi, E. Oral, C.M. Bowden, S.L. Chin, Appl. Phys. B 73, 287 (2001) [3] K. Stelmaszczyk, P. Rohwetter, G. Méjean, J. Yu, E. Salmon, J. Kasparian, R. Ackermann, J.-P. Wolf, and L. Wöste, Appl. Phys.

Lett. 85, 3977 (2004) [4] H.L. Xu, W. Liu, S. L. Chin, Optics Letters, 31, 1540-1542 (2006 ) [5] Takashi Fujii, Naohiko Goto, Megumu Miki, Takuya Nayuki, and Koshichi Nemoto, Optics Letters, Accepted, Online version. [6] W. Liu, F. Theberge, J.-F. Daigle, P.T. Simard, S.M. Sarifi, Y. Kamali, H.L. Xu, S.L. Chin , Applied Physics B , 85 , 55 (2006) [7] François Courvoisier, Véronique Boutou, Jérôme Kasparian, Estelle Salmon, Guillaume Méjean, Jin Yu, and Jean-Pierre Wolf, Appl.

Phys. Lett. 83, 2 (2003) [8] M. Rodriguez, R. Bourayou, G. Méjean, J. Kasparian, J. Yu, E. Salmon, A. Scholz, B. Stecklum, J. Eislöffel, U. Laux, A.P. Hatzes,

R. Sauerbrey, L. Wöste, J.-P.Wolf, Phys. Rev. E 69, 036 607 (2004)

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