1 Femtosecond Laser Filamentation Induced Phenomena and Applications
11
water aerosols containing (NaCl, PbCl 2 , CuCl 2 and FeCl 2 ), smoke from mosquito
coil, metallic targets (lead, copper and aluminum). All of them showed distinctive
finger print fluorescence.
In practice, there are some technical challenges. One is the well-known problem
of multiple filamentation [2–7, 10]. In long distance detection, terawatt level laser
pulses would have to be used. But the mode quality of such lasers is usually not
good and not stable so that the filaments would fluctuate. Even if the mode quality
were good and stable, one might have to deal with atmospheric turbulence that would
disturb the filament pattern. As such, the return signal during a stand-off detection
would be fluctuating. Quantitative measurement would be a big challenge.
The other challenge is the adaptation of the technique of propagation to the intrinsic physics of filamentation. If we send the laser pulse vertically into the atmosphere
with the intention of using filamentation to detect pollutants or other materials at a
certain altitude, one needs to be aware of the change of pressure vertically. This is
because the clamped intensity is pressure independent [13]. Since the critical power
for self-focusing P c is given by [48]
P c =
3.77λ
2
8π n 2 n 0
(1.8)
where n 2 is the coefficient of Kerr nonlinear index of refraction, n 0 , the linear index
of refraction and λ, the wavelength. Since n 2 is proportional to the air density [49],
from (1.8), P c would have to increase at higher altitude where the pressure is lower.
This would mean that at higher altitude, the input peak power of the laser pulse would
have to increase. Since
I clamped =
P clamped
A
∝
P c
A
(1.9)
where I clamped and P clamped are the clamped intensity and the corresponding clamped
peak power of the laser pulse given a cross sectional area A of the filament. Since
I clamped does not change and P c increases at higher altitude (lower pressure), (1.9)
shows that the cross sectional area of the filament A has to increase thus increasing
the filament volume. Essentially, the peak power or the energy content of the pulse
would have to increase. Because the beam quality of a fs Ti-sapphire laser pulse
at the multi-terawatt level is normally not perfect, increasing the power or energy
content of the pulse at sea level would lead to multiple filamentation at sea level
before propagating upward. Special care or design would have to be taken by the
practitioner.
11
water aerosols containing (NaCl, PbCl 2 , CuCl 2 and FeCl 2 ), smoke from mosquito
coil, metallic targets (lead, copper and aluminum). All of them showed distinctive
finger print fluorescence.
In practice, there are some technical challenges. One is the well-known problem
of multiple filamentation [2–7, 10]. In long distance detection, terawatt level laser
pulses would have to be used. But the mode quality of such lasers is usually not
good and not stable so that the filaments would fluctuate. Even if the mode quality
were good and stable, one might have to deal with atmospheric turbulence that would
disturb the filament pattern. As such, the return signal during a stand-off detection
would be fluctuating. Quantitative measurement would be a big challenge.
The other challenge is the adaptation of the technique of propagation to the intrinsic physics of filamentation. If we send the laser pulse vertically into the atmosphere
with the intention of using filamentation to detect pollutants or other materials at a
certain altitude, one needs to be aware of the change of pressure vertically. This is
because the clamped intensity is pressure independent [13]. Since the critical power
for self-focusing P c is given by [48]
P c =
3.77λ
2
8π n 2 n 0
(1.8)
where n 2 is the coefficient of Kerr nonlinear index of refraction, n 0 , the linear index
of refraction and λ, the wavelength. Since n 2 is proportional to the air density [49],
from (1.8), P c would have to increase at higher altitude where the pressure is lower.
This would mean that at higher altitude, the input peak power of the laser pulse would
have to increase. Since
I clamped =
P clamped
A
∝
P c
A
(1.9)
where I clamped and P clamped are the clamped intensity and the corresponding clamped
peak power of the laser pulse given a cross sectional area A of the filament. Since
I clamped does not change and P c increases at higher altitude (lower pressure), (1.9)
shows that the cross sectional area of the filament A has to increase thus increasing
the filament volume. Essentially, the peak power or the energy content of the pulse
would have to increase. Because the beam quality of a fs Ti-sapphire laser pulse
at the multi-terawatt level is normally not perfect, increasing the power or energy
content of the pulse at sea level would lead to multiple filamentation at sea level
before propagating upward. Special care or design would have to be taken by the
practitioner.