Appl. Phys. B 82, 655–658 (2006)
Applied Physics B
DOI: 10.1007/s00340-005-2123-8
Lasers and Optics
h.l. xuu
j.f. daigle
q. luo
s.l. chin
Femtosecond laser-induced
nonlinear spectroscopy
for remote sensing of methane
Center d’Optique, Photonique et Laser (COPL) et Département de Physique, de Génie Physique
et d’Optique, Université Laval, Québec, Québec G1K 7P4, Canada
Received: 25 August 2005/Revised version: 4 December 2005
Published online: 17 January 2006 • © Springer-Verlag 2005
ABSTRACT Femtosecond laser-induced nonlinear molecular
spectroscopy is applied for sensing atmospheric greenhouse gas
methane (CH4 ). The high intensity inside the Ti-sapphire femtosecond laser filaments can dissociate the CH4 molecules into
small fragments which emit characteristic fluorescence. Backward CH radical fluorescence is used to quantitatively analyze
the pollutant concentration and its remote detection limit.
PACS 42.68.Wt;
42.65.Jx; 95.75.Fg
The concentration of atmospheric trace species is a subject
of interest for understanding the evolution of global warming and stratospheric ozone depletion [1–3]. Various laserbased techniques for monitoring the concentration of atmospheric trace species, such as differential optical absorption
spectroscopy (DOAS) and tunable diode laser infrared spectroscopy (TDLAS), have been put forward [4, 5]. These techniques when combined with the light detection and ranging
(LIDAR) method can remotely provide real-time online sensing of pollutants in the atmosphere. Good analytical performance (parts-per-million (ppm) to parts-per-billion (ppb) detection limits) can be generally obtained. Even the detection
limits can reach to the parts-per-trillion (ppt) level for some
species, such as Hg. However, with these techniques the laser
usually may only be optimized one pollutant at a time [4].
Recently, femtosecond laser pulses have been successfully applied to remote sensing of pollutants with the idea
of producing a remote “white lamp” in the atmosphere [6].
When a powerful femtosecond laser pulse propagates in air,
self-phase modulation and self-steepening occur, leading to
a strong spectral broadening of the femtosecond pulse from
the ultraviolet (UV) to the infrared (IR). The final pulse is
a white light laser pulse (supercontinuum) [7–10]. Therefore,
based on white light laser linear absorption spectroscopy of
the backscattered light, range-resolved detection and identification of various atmospheric pollutant species can be
achieved.
In this paper, we present an alternative method for remote sensing of atmospheric contaminants using intense femu Fax: 1-418-656 2623, E-mail: huailiang.xu.1@ulaval.ca
tosecond laser pulses. Compared to the scheme of white light
absorption, this technique is based upon femtosecond laserinduced nonlinear fluorescence spectroscopy [11]. It is already well known that, because of the dynamic balance between Kerr self-focusing and defocusing of plasma produced
by multiphoton/tunnel ionization of the air molecules, the
peak intensity inside the filament is limited to ∼ 1014 W/cm2
(intensity clamping) [12, 13]. This intensity is high enough
to dissociate gas molecules resulting in a clean characteristic
fluorescence [11, 14–16]. By probing the characteristic fluorescence lines from the backward direction, the pollutant
molecules can be remotely localized and identified. In principle, one laser is sufficient to induce characteristic fluorescence
from many molecular species.
Methane (CH4 ) is, like carbon dioxide (CO2 ), a greenhouse gas. It is a colorless, odorless, non-toxic gas, which can
be continuously produced in manure pits and released into
air at a steady rate. If, in a facility such as a poorly ventilated deep mine shaft, the concentration of methane reaches
50 000 ppm (5%) [17], fiery explosion can easily be ignited
from a spark. Although non-toxic to humans and livestock,
methane can cause asphyxiation. In the present work, methane
is selected as an example for atmospheric gas analysis using
femtosecond laser pulses.
The experiments are conducted using a Ti:sapphire femtosecond laser system. Briefly, the output pulses from a Ti:sapphire oscillator (Spectra Physics Tsunami), were positively
chirped to about 200 ps in a stretcher and amplified in a regenerative amplifier (Spectra Physics Spitfire). The 10 Hz
pulses, extracted out from the regenerative amplifier, were further amplified in a two-pass Ti:sapphire amplifier. A portable
compressor was used to shorten the pulse duration, measured with a positive light single shot autocorrelator (SSA), to
about 45 fs. The energy per pulse, controlled by a half wave
plate and a polarizer, can be varied from 0.5 to 12 mJ. The
pulse spectrum is centered at 800 nm with a 23 nm bandwidth
(FWHM). The beam radius at the output of the compressor
was a ≈ 4.2 mm (1/ e level of intensity).
As shown in Fig. 1, the laser beam at the output of the compressor was focused by a BK7 lens ( f = 1 m, thickness =
2 mm). A dielectric mirror (M1, diameter 76.2 mm), with high
reflectivity at around 800 nm and high transmission for UV
light, is used to reflect the beam in a perpendicular direction
into a 4 m long tube with a CaF2 input window (thickness
656
FIGURE 1
Applied Physics B – Lasers and Optics
Experimental setup
4 mm). The distance between the focusing lens and the input
window of the tube is about 12 cm. The reason for designing
this long vacuum tube is to be able to electronically gate out
the scattered white light induced by the last window of the
tube. The white light is due to self-phase modulation and selfsteepening of multiple filaments generated inside the last window [18, 19]. This allows spectral measurement over a reasonable period of time after the passage of the laser pulse. The
tube contains a mixture of CH4 and air at atmospheric pressure
with variable CH4 concentrations. The fluorescence signal
was observed in the backward direction; it was collected and
focused, using a fused-silica lens ( f = 10 mm, diameter =
50.8 mm), onto the entrance slit of an imaging spectrometer
(Acton Research Corp., SpectraPro-500i). The distance between the collection lens and the input window of the tube
is about 60 cm. The spectral resolution is about 0.4 nm using
a grating of 1200 grooves/mm (blazed wavelength at 500 nm)
with 100 µm entrance slit width. In order to avoid the scattered light from the laser pulse, a dielectric fused silica mirror
(M2, reflectivity > 99.9% for 800 nm ± 50 nm at 0◦ incidence
angle, diameter 76.2 mm) was placed just before the focusing lens. The dispersed fluorescence was detected by a gated
intensified charge coupled device (ICCD, Princeton Instruments Pi-Max 512).
For spectral measurements, the gate width of the ICCD
was set to 18 ns and the data were averaged over 100 shots.
With this configuration, the fluorescence signals recorded
covered the distance from the starting point of the filament up
to the last window of the tube. Figure 2a presents the spectrum of pure methane collected from the backward direction.
The methane pressure is 20 Torr and the energy of a single pulse is 5 mJ. The three spectral bands in the region of
420– 450 nm, 385– 405 nm and 308 – 320 nm are assigned to
the A 2∆ → X 2Π, B 2Σ → X 2Π and C 2Σ + → X 2Π transitions of CH radical, respectively [20]. The dissociation mechanism of CH4 has been discussed elsewhere [21]. Briefly, the
intense laser field, at an intensity over 1014 W/cm2 , weakens the molecular chemical bonds and causes polyatomic
molecules to dissociate into small neutral fragments. The
tetrahedral methane molecule thus undergoes a stepwise disintegration. The hydrogen atoms are individually cleaved
from the CH4 molecule resulting in the production of excited
FIGURE 2
Emission spectra of methane (a) and air (b)
CH radicals, which fluoresce. Figure 2b shows a fluorescence
spectrum of air at atmospheric pressure. The spectral lines are
+
+
2
2
assigned to the first negative band of N+
2 (B Σu → X Σg )
3
3
and the second positive band of N2 (C Πu → B Πg ) [14].
In this case, the nitrogen molecules are ionized and the excited state B 2Σu + of N+
2 are populated through the ejection of
an inner valence electron, leading to radiative decays. Subsequent electron–ion recombination leads to the emission of the
second positive band [22, 23].
In order to check the feasibility of this technique to remote sensing of pollutants in the atmosphere, the CH4 gas
was mixed with air. Figure 3a shows the fluorescence spectrum of a mixture of air and methane with a CH4 concentration
of 2.6% (volume/volume) at atmospheric pressure. As shown
in the inset (1) of Fig. 3a, the A 2∆ → X 2Π spectral band of
CH radical is still clearly observed but is contaminated by
Emission spectra of mixture of CH4 and air with a CH4 concentration of 2.6% (volume/volume): (a) the gate width of ICCD is 18 ns
(b) the gate width of ICCD is 14 ns. The inset (1) shows the spectrum in
a higher resolution. The inset (2) illustrates the same spectrum without the
air background
FIGURE 3
XU et al.
Femtosecond laser-induced nonlinear spectroscopy for remote sensing of methane
the nitrogen fluorescence. The inset (2) of Fig. 3a shows the
same spectrum obtained by subtracting the air background.
Therefore, CH radical fluorescence can be utilized to quantitatively characterize the concentration even if the fluorescence
signals are buried in the background signals of air. An alterative method for background signals attenuation is to use
time-dependent measurements [11]. Figure 3b shows the fluorescence spectrum of a mixture (CH4 , 2.6%) measured 4 ns
later than the previous one, with a gate width of 14 ns. Here
we can see that nitrogen signals are much weaker and two
nitrogen spectral lines around the A 2∆ → X 2Π band of CH
disappear. This is because the excited states B 2Σu + of N+
2 and
C 3Πu of N2 have a very short lifetime (about 1 – 2 ns) [24]
and the excited state A 2∆ of CH radical has a lifetime of
about 6 – 8 ns when mixed with air at atmospheric pressure.
The latter can be clearly seen in Fig. 4, where the time resolved measurements have been performed. The rectangular
points represent the normalized CH signals integrated over the
selected wavelength interval between 430 and 432 nm and the
circular points show the N2 signals integrated between 425
and 427 nm. The data were averaged over 600 laser shots. The
inset in Fig. 4 shows the same plot, but in a semi-logarithmic
representation. Therefore the strong nitrogen contaminations
can be gated out while the CH emissions remain rather strong.
To find the detection limit under our experimental conditions, the integrated signal intensities (between 430 and
432 nm) of the backscattered fluorescence from the excited
state A 2∆ of CH radical (see Fig. 3b) have been measured as
a function of the CH4 concentration, as shown in Fig. 5. In
this measurement, due to the large volume of the tube used
for this experiment, it is of crucial importancc to obtain a homogeneous mixture of CH4 and air, in order to measure the
real signal intensity emitted at this concentration. By measuring the signal intensities as a function of the mixing time, we
found that a mixture normally requires about five hours’ diffusion to become homogeneous. As a matter of fact, in Fig. 5,
a clear signal can still be observed for the lowest concentration
of 260 ppm (0.026%). The solid line in Fig. 5 is the calibration
curve, which is linear over the concentration range studied.
We obtained a 3σ detection limit of 40 ppm from the calibra-
Time-dependent measurements. The inset shows the same data
in a semi-logarithmic representation
FIGURE 4
657
tion line (σ is the standard deviation of the background level).
We also checked the signal intensities of the backscattered fluorescence (the excited state A 2∆ ) as a function of the CH4
concentration by subtracting the background produced by air
(see the inset (2) of Fig. 3b). This gives a similar value (about
55 ppm) as that obtained by time-dependent measurements.
As mentioned in the introduction section, some laser remote
sensing techniques could measure the trace atmospheric constituents with detection sensitivities in the ppb, even in the ppt
concentration range. Therefore, additional efforts are needed
to improve the detection sensitivity of our technique. However, the purpose of this paper is only to show the principle
that femtosecond laser-induced nonlinear spectroscopy could
be applied to remote sensing of pollutants in the atmosphere.
The improvement of the detection sensitivity of this technique
remains a subject of interest experimentally.
In the measurement, the distance between the filament and
the spectrometer is about 1.5 meters. It is difficult to perform
a remote experiment in a laboratory scale, because methane
is explosive and also because harmful to human (asphyxiation). Here we can make an estimate of the distance at which
the pollutant can be detected in air. However, measurements
using the time-resolved technique will be limited when the filament is long; the technique air background subtraction from
the measured signal has to be used. The estimate is based on
the current detection system and experimental conditions. In
this experiment, the filament length L can be obtained from
z f
the relation L = f − z f f+ f [22], where f is focal length of the
focusing lens and z f can be expressed as [25]
z f = 0.367ka2
1/2 ,
2
1/2
P
− 0.852 − 0.0219
Pcrit
(1)
where k and a represent, respectively, the wave number and
the radius at the 1/ e level of the beam profile; P is the input laser power and Pcrit , the critical power for self-focusing.
The CH radical signals (rectangular points) as a function of
CH4 concentration together with a linear fit (solid line)
FIGURE 5
658
Applied Physics B – Lasers and Optics
In this experiment, a ≈ 4.2 mm and since the mixture is
mostly composed of air (≤ 5%), Pcrit is set to the critical
power in air (about 10 GW) [26]. Therefore, we obtained
a filament length of L = 4.6 cm with an excitation pulse of
5 mJ and a 1 m focusing lens. Now we assume that the filament is 20 m long and the whole filament passes through
the region containing CH4 gases. This is a reasonable assumption which has been observed when a femtosecond laser
pulse propagates freely in air with a single pulse energy
of about 50 mJ [27]. The intensity inside this long filament
would be similar to that in the present experiment due to
intensity clamping inside the filament. However, it should
be pointed out that in the assumption the filament length of
L = 20 m is an effective filament length, that is, it might be
longer than 20 m due to multiple refocusing if single filament occurs, or shorter than 20 m when multi-filamentation
occurs.
Using the LIDAR equation I ∝ RL2 (where I is the signal
intensity, L the effective filament length and R the distance
between the end of the filament and the detector) [28] and
the parameters obtained in the present work ( I = 49500 for
L = 4.6 cm and a CH4 concentration of 5% at R = 1.5 m), we
estimate that the detection limit of 3σ = 66 will be reached
at about 0.85 km for the filament length of L = 20 m. This
concentration of 5% corresponds to the practical limit at
which it will become potentially explosive (5% to 15%, potentially explosive for methane in air) [17]. Moreover, if we
consider a 200 m long filament, which has been observed
by the Teramobile team [29], the detection limit could be
further extended up to about 2.7 km. To achieve these detection limits requires control over the onset of the filament.
This could be done by optimizing experimental parameters such as pulse chirping, beam size, and divergence (see,
e.g., [6, 30]).
In summary, we have demonstrated an analysis of remote sensing CH4 based on filamentation induced by the
non-linear propagation of femtosecond laser pulses in the
atmosphere. Inside the filaments, intense femtosecond laser
pulses dissociate pollutant molecules into small fragments,
which emit characteristic fluorescence. The latter can be used
to remotely analyze the pollutant concentration from direct
measurement of the characteristic spectral lines. With reasonable laser energies, it could be possible to extend the
detection limit up to the kilometer range. This technique
would thus be practical for remote analysis of hazardous
or unreachable spots, such as deep mine wells, or chemical
leakages.
ACKNOWLEDGEMENTS This work was supported in part
by NSERC, CFI, CIPI, DRDC-Valcartier, Femtotech, Le Fonds FCAR and
Canada Research Chairs.
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