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OPTICS LETTERS / Vol. 37, No. 10 / May 15, 2012
Water vapor concentration measurement in air using
filament-induced fluorescence spectroscopy
Tie-Jun Wang,1,* Huailiang Xu,2,4 Jean-François Daigle,1,3 Aravindan Sridharan,1
Shuai Yuan,1 and See Leang Chin1
1
Centre d’Optique, Photonique et Laser (COPL) and Département de physique, de génie physique et d’optique,
Université Laval, Québec, Québec G1V 0A6, Canada
2
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering,
Jilin University, Changchun 130012, China
3
AEREX Avionique Inc., Breakeyville, Québec, G0S 1E1, Canada
4
e-mail: [email protected]
*Corresponding author: [email protected]
Received January 4, 2012; revised March 6, 2012; accepted March 9, 2012;
posted March 12, 2012 (Doc. ID 160909); published May 14, 2012
Water vapor fluorescence in air induced by femtosecond laser filaments was systematically investigated. The fluorescence signal intensity was found to be linearly proportional to the water vapor concentration, which opens up the
possibility of absolute humidity measurements, even remotely. © 2012 Optical Society of America
OCIS codes: 190.4180, 190.5530, 300.2530.
Femtosecond laser filamentation [1], since it was observed in 1995 [2], has attracted a lot of interest. Filamentation of an intense laser pulse in air is accepted as a
dynamic equilibrium between Kerr self-focusing and
defocusing by the self-generated low-density plasma produced by multiphoton/tunnel ionization of air molecules
[3–8]. Laser intensity inside the filament is clamped to
∼5 × 1013 W∕cm2 . This intensity is high enough to dissociate/ionize other gas molecules, and excites the ions
and molecules into some highly excited states, resulting
in the emission of characteristic fingerprint fluorescence.
The filament-induced fluorescence spectra were shown
to be very “clean”; i.e., practically free of plasma continuum [9]. Since the onset of laser filament can be controlled at long distances by adjusting the initial laser
parameters, such as beam diameter, divergence and
pulse duration [3–8], remote multicomponent sensing
by filament-induced characteristic fluorescence spectroscopy has been demonstrated [9,10].
Recently, Rohwetter et al. in [11] have observed a
water vapor condensation phenomenon induced by femtosecond laser filamentation in a cloud chamber. This
laser-assisted water condensation was subsequently
demonstrated in the atmosphere [12,13]. More recently,
a laser-filamentation induced snow formation in a cloud
chamber using high-repetition rate pulses has been reported [14]. Among the interaction between laser pulses
and water vapor, humidity measurement is particularly
important because of its relevance regarding the changes
of the state of water content in the atmosphere [15]. Any
instrument for measuring humidity is known as a hygrometer. So far, most hygrometers sense relative humidity
rather than the absolute amount of water present [15].
But relative humidity is dependent on both temperature
and absolute moisture content. Small temperature variations within the air in a test chamber, or in the atmosphere, will translate into relative humidity variations.
In this work, a method based on water vapor fluorescence induced by femtosecond laser filaments is
0146-9592/12/101706-03$15.00/0
proposed to measure absolute water vapor concentration
in air, even remotely.
The experiments were performed by using a
2 mJ∕40 fs, 1 kHz Ti:sapphire laser beam. The schematic
of the experimental setup is illustrated in Fig. 1(a). The
laser pulses focused by a lens of 50 cm focal length created a 5 cm long filament in air. A conical flask, with open
area of 3 cm in diameter under the filament, was filled
with distilled water, which was used to create a water
vapor density gradient. The water temperature was controlled by a heating plate underneath. The distance
between the filament and water surface can be controlled by moving the conical flask and heating plate
using a lifting platform. The filament-induced fluorescence of water vapor was collected at right angles by
a fused silica lens (5.08 cm in diameter, 10 cm focal
length) and imaged onto the tip of a fiber coupler into
an ICCD-gated spectrometer. The ICCD gate was opened
1 ns before the laser pulse arrived and the gate width was
set to 100 ns. For each measurement, the signal has been
accumulated for 10,000 shots. The fluorescence detection system and the filament formation condition were
fixed during the experiments.
Clear fluorescence was found in the spectral range of
306–309 nm with water temperature of 80 °C as shown in
Fig. 1(b), which was identified as the hydroxyl (OH)
Fig. 1. (Color online) (a) Schematic layout of experimental
setup and (b) typical spectrum in the range of 303–322 nm
for filament-induced fluorescence of water vapor in air. The
spectrum was taken at the water temperature of 80 °C and
filament-to-water surface of 5 mm.
© 2012 Optical Society of America
May 15, 2012 / Vol. 37, No. 10 / OPTICS LETTERS
radical [16]. These OH radicals are ascribed to the dissociation of water vapor molecules in air. Typical nitrogen
fluorescence from inside the filament in air [10] was also
observed [Fig. 1(b)]. The closer the filament was to the
water surface, the stronger the OH fluorescence was.
At the same time, the nitrogen fluorescence decreased.
The spectral peak intensities of 308.9 nm from OH
and 315.9 nm from N2 were chosen for the following
analysis.
The dependence of the fluorescence on the distance
between the filament and water surface was first investigated with water temperature at 80 °C. As shown in
Fig. 2(a), the OH fluorescence exhibits two distinguished
different stages: it becomes stronger as the distance between the filament and water surface increases from 0 to
∼5 mm, which is the first stage; it decreases to a constant
as the distance increases more, which is the second
stage. As for the fluorescence from air molecules, as
shown in Fig. 2(b), the fluorescence increases as the distance increases and reaches a constant when N2 is at
315.9 nm. Both OH and N2 fluorescence reach the constant values at a distance of ∼15 mm, which are equal
to the corresponding fluorescence intensities when the
water bath was removed. The relative humidity in the
lab was 46%. Similar phenomenon was observed when
water was at other temperatures. At the room temperature of 21 °C, as shown in Fig. 2(c), the first stage of OH
fluorescence was not observed. These observations
could be interpreted based on the distribution of water
vapor concentration in air. When water is as hot as
80 °C, water vapor is oversaturated near the water surface so that water droplets are formed, which can be seen
by eyes, even without filaments. Laser filament may also
contribute to water droplet formation [11]. These droplets may absorb or scatter the OH fluorescence,
resulting in the decrease of the OH fluorescence when
compared to that obtained from saturated water vapor.
As the distance between the filament and water surface
increases further water vapor concentration undergoes
Fig. 2. (Color online) Fluorescence intensities of OH 308.9 nm
[(a) and (c)] and N2 315.9 nm [(b) and (d)] lines as a function of
the distance between the filament and water surface with water
temperature at 80 °C and 21 °C. Blue dashed lines indicate the
spectral intensity when water container was removed from the
interaction zone. Red solid lines are exponential fits.
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saturation without the formation of water droplets, and
then decreases gradually to the concentration level in the
room. When the distance increases further, water vapor
concentration is constant. However, a totally different
behavior is observed in the nitrogen fluorescence. These
water droplets and vapor will displace the air molecules
at a constant pressure so that N2 fluorescence will increase as the distance increases. This is confirmed by
the behavior of N2 fluorescence at 315.9 nm. When the
distance between the filament and water surface is as
far as that when vapor concentration is equal to the room
vapor concentration, the fluorescence yields of OH and
N2 become constant. Note that water absorption in this
wavelength range is negligibly weak.
Figure 3 shows the water temperature dependence of
the filament-induced fluorescence of water vapor. The
distance between the filament and the water surface was
fixed at 2 mm. When the water temperature increases,
water vapor concentration goes higher, so that OH fluorescence gets stronger. As the temperature increases to
50–60 °C the vapor concentration is saturated. Further
increase in the temperature results in the oversaturated
concentration and the formation of water droplets, which
leads to the decrease of fluorescence at 70 °C. This
observation confirms our conjecture mentioned above.
The intensity of OH fluorescence induced by the high
laser intensity inside a filament can be expressed as an
empirical relation with respect to the laser intensity without going into the detail of the interaction physics. That is
to say, the OH fluorescence is effectively a highly nonlinear process (multiphoton/tunneling) with respect to
the intensity:
i:e:; I FL OH ∝ N 0 I kLaser ;
I FL OH N 0 AI kLaser ; (1)
where A is the proportionality constant and N 0 is the density of water vapor molecules. I laser is the pump laser intensity. In a filament, because of intensity clamping, this
intensity is constant; k is an effective nonlinear order.
The ionization potential of water molecules (12.6 eV)
is similar to the ionization potential of air molecules
(N2 15.6 eV, O2 12.1 eV). Since the saturated partial pressure of water vapor in air is only ∼2% of the total pressure of air at room temperature, the influence of water
vapor concentration on the clamped intensity inside a filament is negligible. Thus, from Eq. (1), the intensity of
Fig. 3. Water temperature dependence on fluorescence intensity of 308.9 nm OH radical. The distance between the filament
and water surface was fixed at 2 mm.
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OPTICS LETTERS / Vol. 37, No. 10 / May 15, 2012
In summary, filament-induced fluorescence of water
vapor in air was experimentally investigated. This
water vapor fluorescence is linearly proportional to the
water vapor partial pressure. The results reported in this
Letter not only give a better understanding on the physics
of filament-induced water vapor fluorescence in air, but
also open up the possibility of the remote measurement
of water vapor concentration.
Fig. 4. (Color online) Partial pressure of water vapor in air as a
function of OH fluorescence signal at 308.9 nm at water temperature of 21 °C.
OH fluorescence is proportional to the density of water
vapor molecules. The latter is proportional to the partial
pressure of water vapor, P partial
I FL OH ∝ N 0 ∝ P partial :
(2)
The water vapor partial pressure in air with respect to the
distance to the water surface is described by [17]:
P partial P saturated e−αL ;
(3)
where P saturated is the saturated partial pressure of water
vapor in air just above the water surface, which is constant at a fixed temperature; L is the distance to the water
surface; α is the diffusion coefficient of partial pressure.
Then partial pressure of N2 in air can be calculated
through P N 2 0.78P air − P partial considering 78% N2 in
air. P air is the total pressure of air and equals to atmospheric pressure. It is worth noting that once water vapor
partial pressure is known, one can calculate the N2 partial pressure in air. In Fig. 2, the red solid lines indicate
the exponential fits based upon Eq. (3). When the vapor
partial pressure decreases to the partial pressure in the
room, the distance is defined as L0 . This distance is indicated in Fig. 2 as being the distance between the maximum OH fluorescence and the crossing point where the
exponential fit crosses the blue dashed line. By using the
averaged L0 , together with the relative humidity (RHroom )
measured in the room, which is equal to P partial ∕P saturated ,
the diffusion coefficient α can be calculated through
Eq. (3). Thus, the distance dependence of the vapor partial pressure can be calculated through Eq. (3). Combined with the experimental results similar to those
shown Figs. 2(a) and 2(c), we obtain the vapor partial
pressure as a function of water vapor fluorescence
(Fig. 4). The vapor partial pressure is linearly proportional to the fluorescence signal of water vapor. This
result provides a significant way to measure absolute humidity by monitoring water vapor fluorescence inside a
filament. Since, filament can be projected at long distances, remote sensing of water vapor concentration
can, in principle, be feasible by this fluorescence technique by measuring the fluorescence signal using the Light
Detection and Ranging (LIDAR) technique.
This work was supported in part by NSERC, DRDC—
Valcartier, Canada Research Chair, the Canada Foundation for Innovation, the Canadian Institute for Photonics
Innovation, and le Fonds Québécois pour la Recherche
sur la Nature et les Technologies. H.L. Xu would like
to acknowledge the financial supports from NCET–09–0429) and the Basic Scientific Research Foundation
of Jilin University. Technical support from Mr. M. Martin
is also acknowledged.
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