Del Mar Photonics currently involved in research
cooperation to study femtosecond filamentation generated by TW femtosecond
Cr:Forsterite laser JAWS and TW
femtosecond Ti:Sapphire laser
Cortes-800
Two Petawatt class laser are currently under construction:
OPCPA and Cortes-K
We are comparing our results with those obtained in NRL: http://www.nrl.navy.mil/content.php?P=03REVIEW59
Filamentation and Propagation of Ultra-Short, Intense Laser Pulses in Air
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A.C. Ting, D.F. Gordon, R.F. Hubbard, J.R. Peņano, and P. Sprangle
Plasma Physics Division
C.K. Manka
RSI, Inc.
Ultra-short (femtosecond), high-power laser pulses can exceed the threshold for
nonlinear self-focusing in air. This results in an extended propagation from the
dynamical balance between the plasma formation and the nonlinear focusing.
Experiments were performed using the chirped-pulse-amplification (CPA) lasers in
the Plasma Physics Division to study the physics of extended propagation in air
and its effects on atmospheric breakdown, laser-induced electrical discharge,
and chemical/biological (chem/bio) agent detection. Self-guiding of the laser
beams for extended distances and formation of multiple laser and plasma
filaments were observed. Time-resolved images of laser-induced electrical
discharges showed the initiation and sustention of the discharges by the plasma
filaments. Measured optical spectra of the white light generated in the laser
propagation revealed the presence of molecular plasmas that are useful for
identifying chem/bio agents. Potential applications include directed energy
weapons, remote sensing for both chem/bio defense, and environmental air
pollutant monitoring.
INTRODUCTION
Ultra-high-power lasers that can deliver intense radiation have raditionally
resided in a few, very large national laboratories. This is because more energy
is usually required as the power of the laser increases, and thus the size of
the laser correspondingly increases. Therefore, research into the physics
associated with intense radiation from these ultra-high-power lasers could only
be carried out at these large institutions. In addition, the size and cost of
the lasers severely limited the range of potential applications. This has all
changed during the last decade when a new way of generating high-power lasers
was discovered. This simple but effective "trick" to increase laser power starts
with the recognition that power is, by definition, energy per unit time. Instead
of increasing the energy carried in a laser pulse for a fixed time duration to
obtain higher power, one can produce the same laser power if one decreases the
pulse duration while maintaining the same amount of energy in it. By utilizing
ultra-short laser pulses with durations as short as a few tens of femtoseconds
(thousand-trillionth of a second), laser pulses with power as high as tens of
terawatts (trillion watt) can now be obtained by using table-top sized laser
systems. Research on these lasers can now be performed in reasonably sized
laboratories, and many potential applications are envisioned.
Many interesting phenomena are associated with the interactions of these very
intense and short laser pulses with various media. In particular, the
propagation of a short intense laser pulse in a gas such as air is very
different from that of a long or continuous wave (CW) laser pulse. For example,
the high intensity of these pulses can produce nonlinear contributions to the
index of refraction of the medium. The intensity of the laser pulse could also
become so high that the air molecules would ionize and form a plasma. The
inter-play between the laser pulse and the plasma that it creates can be very
complicated and can profoundly affect the evolution of the laser pulse as it
propagates through the atmosphere. Experiments using ultra-short (~100 fs), high
intensity (>1013W/cm2) laser pulses have demonstrated long-distance self-guided
atmospheric propagation,1 air breakdown, filamentation, and white light
generation. Intense, directed white light pulses have been generated and
backscattered from atmospheric aerosols. The generation of pulsed THz radiation
in plasma channels formed by femtosecond pulses has also been observed and
analyzed. Although many of the observations cannot be completely explained, the
experimental, theoretical, and numerical results obtained to date indicate
potential applications for both passive and active remote sensing and induced
electric discharges, among others. In addition, the individual micropulses in a
shipboard free electron laser (FEL) system may exhibit short-pulse propagation
characteristics. To achieve these potential applications, it is necessary to
have a comprehensive and quantitative understanding of the physical mechanisms
that govern the propagation of intense, short laser pulses in air.
The following sections begin with a description of the table-top
ultra-high-power lasers in the laboratory. Next, the physics of propagating
femtosecond terawatt laser pulses in air is discussed, with experimental
demonstration of the novel phenomena of self-guided laser filaments and
numerical verification of the experimental results. These filaments and the
associated broadband radiation that they generate can be used in the remote
sensing of cal/biological agents in defense or anti-terrorism applications or
detecting hazardous air pollutants in environmental monitoring and enforcement.
The plasma filaments associated with a self-guided femtosecond intense laser
pulse can also be used for triggering high-voltage electrical discharges. This
phenomenon is next discussed, with emphasis on the discharge initiation
mechanism, by studying the time evolution of the discharge. There is
considerable interest worldwide in studying this phenomenon so that it can be
applied to areas such as lightning arrest around power plants. The concluding
section summarizes research efforts at NRL in studying ultra-short intense laser
pulse propagation in air.
NRL T3 AND TFL LASERS
The NRL High Field Physics Laboratory was one of the first laboratories to have
a table-top terawatt (TW) laser system soon after the invention of the chirped
pulse amplification (CPA) method. The T3 laser was installed in 1992 as the
first commercial CPA laser ever built. It has been upgraded several times over
the years and is still a state-of-the-art CPA laser. It is a solid state laser
involving two lasing media, titanium-doped sapphire (Ti:sapphire) and
neodymium-doped glass (Nd:glass). The lasing wavelength is in the infrared at
1054 nanometers (nm). Like most lasers, it consists primarily of a laser
oscillator that generates the seed laser pulse and then a series of laser
amplifiers to boost the energy in the pulse. The difference is that the seed
laser pulse has a pulse length of only 100 femtoseconds (fs). As the laser pulse
is amplified, the power and intensity of the pulse continuously increases and
eventually will reach the breakdown threshold of the laser glass medium. To
avoid such disastrous consequences, the CPA technique stretches the laser pulse
after the oscillator with a diffraction grating to ~10,000 times and thus
reduces the laser intensity and power by the same factor. The stretched pulse
can now be safely amplified to the desired high energy per pulse. The stretching
process is then reversed by re-compressing the amplified pulse with diffraction
gratings in air or vacuum to produce a high-power, ultra-short pulse. One
interesting observation is that in the stretched pulse, the frequency content of
the pulse is arranged such that the high-frequency (pitch) components are moved
to the back of the stretched pulse, reminiscent of the chirped tune of a singing
robin, and hence the "chirped" pulse amplification technique.
The NRL TFL laser shown in Fig. 2 is a TW laser with a smaller footprint than
the T3 laser, and most importantly, it can be rep-rated at 10 times a second (10
Hertz). It is based entirely on Ti:sapphire technology, and it is lasing at the
infrared wavelength of 810 nm. The laser pulse width is 50 fs with 50 milli-Joules
(mJ) of energy in each pulse. Both lasers are located in our well-equipped
laboratory, with extensive optical and electronic diagnostic equipment that can
be used to study the effects and mechanisms of these ultra-short intense laser
pulses interacting with various media.
FIGURE 2
The NRL TFL laser is a table-top Ti:sapphire CPA laser system that produces
laser pulses at a wavelength of 810 nm with 50 mJ of laser energy in a 50-fs
pulse. It is operated at a repetition rate of 10 Hz. The amplified stretched
pulse is compressed in a portable compressor (not shown in the picture), which
allows optimized positioning of the laser for air propagation experiments.
PROPAGATION OF INTENSE SHORT LASER PULSES IN AIR
The propagation of intense, short laser pulses in the atmosphere involves a
variety of diverse linear and nonlinear optical processes. The nonlinear
processes are the consequences of the high intensity of the laser pulse.
Processes affecting the laser spot size include diffraction, nonlinear
self-focusing, ionization, and plasma defocusing. In addition, self-phase
modulation, stimulated Raman scattering, and plasma formation also contribute to
considerable spectral broadening and white light generation by the laser pulse.
On the other hand, the ultra-shortness of the laser pulse also needs to be taken
into consideration. The physics governing the atmospheric propagation of short
intense laser pulses can be very different from that of long laser pulses. For
example, the Raman instability associated with the excitation of molecular
rotational modes, which can disrupt the long-distance propagation of long, e.g.,
nanosecond (ns) pulses, may not be as disruptive for laser pulses that are
shorter than the characteristic picosecond (ps) period of the rotational mode.
The implication of this observation is that the nonlinear refractive index of
air could be a function of the laser pulse length. A 100-fs pulse could have an
effective nonlinear refractive index several times smaller than that of a
picosecond pulse. The inherently large spectral bandwidth of a short pulse also
renders it more susceptible to dispersion effects in the atmosphere. Finally,
the broad spectrum of the short laser pulse could affect the absorption
characteristic of the laser in the atmosphere. In conventional narrow bandwidth,
long-pulse lasers that are used in laser radar (LIDAR) applications, the laser
line can be positioned between absorption lines to minimize attenuation in the
atmosphere. However, the broad spectrum of a short pulse could be overlapping
several individual absorption lines, and this could affect the thermal blooming
process, which is a sensitive function of the absorption rate. These effects
could be important for proposed shipboard FEL systems.
Perhaps the most prominent phenomenon observed when a high-power laser beam
propagates in air is the formation of self-guided laser filaments. When no
external focusing is provided, the wave nature of the light emitted from a laser
will naturally diffract, and the laser beam size will continuously diverge and
increase in size. However, the refractive index of air varies with the intensity
of the laser in such a way that the higher intensity portion of a laser pulse
encounters a higher value of the refractive index. Since the refractive index is
a measure of the ratio of the speed of light in vacuum to that in the medium
under consideration, a higher index of refraction signifies a slower speed of
propagation for that portion of the laser pulse, and the laser pulse will
converge (focus) onto this lower velocity portion. This has a very close analog
to the propagation of light inside an optical fiber where the core of the fiber
has a higher index of refraction. The higher intensity core portion of a laser
pulse in air now also encounters a higher index of refraction. Therefore, it
will be guided just like the light traveling down an optical fiber.
The condition for which such self-focusing can occur is governed by the initial
laser power in the pulse. When the laser power reaches a threshold value, the
nonlinear self-focusing effect can overcome the diffractive divergence of the
laser beam, and an ideal laser beam will remain at a constant size forever. For
air, the conventionally known value for this critical power is about 3 gigawatts
(GW). If the laser power is above this critical value, the laser beam will
converge instead, and theoretically it will continue to decrease in size until a
catastrophic collapse is reached.
Figure 1 shows the T3 laser. The final amplifier is in the foreground, and the
oscillator and preamplifiers are in the back and to the left. It can generate a
laser pulse 400 fs long with 5 Joules (J) of laser energy in it. The peak power
of the pulse is therefore >10 TW. It has a repetition rate of one shot every 20
minutes. Most experiments on intense laser interactions require both the high
power and high energy in the laser pulse. However, a number of interesting
behaviors of an ultra-short laser pulse are primarily the consequence of the
high power and intensity of the pulse. Since a shorter pulse with less energy
could have the same high power, a smaller laser with less energy per pulse could
be adequate for some of the high-intensity laser experiments.
FIGURE 1
The NRL T3 laser is a table-top Ti:sapphire/Nd:glass CPA laser system that
produces laser pulses at a wavelength of 1054 nm with 5 J of laser energy in a
400-fs pulse. It is operated at a repetition rate of one shot every 20 minutes.
The amplified stretched pulse is compressed with diffraction gratings inside an
evacuated chamber (not shown in the picture) to avoid nonlinear propagation
effects that would degrade the laser beam quality.
Fortunately, at high enough intensities, the air will break down and a plasma is
formed. One of the optical properties of plasma is that it has a negative
contribution to the index of refraction. Since more plasma is formed where the
laser intensity is high, the refractive index is less near the core of the laser
beam. This is exactly the opposite of the nonlinear contribution to the
refractive index before the ionization occurs. The two opposing effects can, in
certain circumstances, balance each other and result in a long-lived,
noncollapsing filament. More filaments can be formed if the laser power is many
times higher than the critical power for self-focusing. These filaments can
propagate extended distances, much longer than would be allowed if diffraction
effect alone is considered. An example is shown in Fig. 3, where a 3.56 TW laser
pulse from the 1.054-?m wavelength T3 laser was propagated for 10 meters in the
laboratory. Many tens of filaments are clearly visible. The initial laser beam
size is 4 cm in diameter, and the individual filaments have diameters of about
200 ?m. At this small size, the propagation distance for which the filament
diameter will expand by 41.4% due to diffraction (known as the Rayleigh range)
is only ~3 cm. The combined effects of nonlinear focusing and plasma formation
have kept the filament from diverging for very much longer than was expected.
Also, at the small-diameter size of these filaments, the laser intensity are in
the range of 1013 to 1014 W/cm2. At such intensities, almost all solid or liquid
media will break down and be damaged. The intense field can also generate
secondary radiation that can disrupt the operation of many electronic devices.
Therefore, these filaments are suitable for applications that involve sensor
damage or electronic countermeasure processes.
An interesting observation was that the initial power of the laser pulse was
about a thousand times more than the critical self-focusing power. Theory
predicts that 1,000 filaments could be formed. The comparatively low number of
filaments brought out the question of whether the critical self-focusing power
was correctly evaluated in the past. Since the critical power was calculated
from the nonlinear refractive index of air, one begins to wonder about the
correct value of the index for short, intense laser pulses. A literature search
reveals that, indeed, the nonlinear refractive index of air had been measured
primarily with optical methods that involved pulses much longer than a
picosecond. However, there is strong experimental and theoretical evidence that
the standard long pulse value for the nonlinear refractive index for air is not
applicable to the self-focusing of femtosecond laser pulses.
FIGURE 3
False-color image of self-guided filaments in a 3.56 TW laser pulse produced by
the T3 laser after propagating 10 meters in the laboratory. The beam diameter is
4 cm. The pattern of the filament distribution is correlated with the initially
nonuniform transverse beam profile.
An indirect way of obtaining the value of the nonlinear refractive index of air
is to compare experimental results of filamentation with numerical results from
theoretical models that include all the relevant physics. The NRL air
propagation simulation code models atmospheric laser pulse propagation effects
with a system of three-dimensional, nonlinear equations. These include
diffraction, group velocity, and higher order dispersion, stimulated molecular
Raman scattering, photoionization, nonlinear bound electron effects, ionization
energy depletion, and propagation in a spatially varying atmosphere.2 A coupled
set of equations that was derived for the laser amplitude and electron density
is used to analyze a number of physical processes, such as optical/plasma
filamenta-tion, pulse compression, nonlinear focusing, and white light
generation. An experiment was performed using the T3 laser to generate filaments
with a known initial condition that could be simulated with the NRL air
propagation code. A circular aperture was imposed on the initial laser beam to
create a well-defined "top-hat" transverse profile suitable as an input to the
simulation code. As the shaped laser beam propagates through the atmosphere,
normal diffraction effects reshape the profile to a donut-looking form.
Nonlinear effects enhance the fluctuations in the intensity around this donut
shape and filaments are formed. At a distance of 10 m, four distinct filaments
are formed, as shown on the left-hand side of Fig. 4. The experimental laser
parameters of 400-fs pulse length and 108 GW peak power are imported into the
simulation code, and the results are compared to the experiment. The nonlinear
refractive index of air is varied in the simulation runs. It was found that in
order to match the experimental result for the same number of filaments at the
same distance, the simulation had to use a nonlinear refractive index 50% less
than the conventional value. The simulation result is shown in on the right-hand
side of Fig. 4. This is the first quantitative experimental/numerical
verification that the nonlinear refractive index of air has different values
when ultra-short intense lasers are involved.
FIGURE 4
The formation of filaments from an apertured 400-fs laser beam with peak power
of 108 GW at a distance of 10 meters. The experimental result is shown on the
left, with the simulation result on the right. The value of the nonlinear
refractive index used in the simulation to obtain the best match with the
experiment is found to be 50% of standard value for long pulses.
Another interesting phenomenon arising from the propagation of short intense
laser pulses in air is the generation of broadband radiation, often referred to
as "white light" or "supercontinuum." This radiation is the result of the
nonlinear self-phase modulation and ionization effects that are caused by the
rapid variation in the index of refraction from the front to the back of the
laser pulse. Nonlinear generation of optical frequencies outside the original
laser linewidth can be as broad as 100%. Because the laser wavelength is in the
infrared, the broadened spectrum can extend into the ultraviolet (UV) and far
infrared. Figure 5 shows a portion of the spectrum of the radiation collected
after the laser pulse from the 0.81-?m wave-length TFL laser has propagated for
about 7 meters. It shows that radiation was produced in the UV, and many of the
spectral features have been identified as those of the neutral or ionic species
of the oxygen and nitrogen molecules. These features indicate that the molecules
in the air where the laser has traversed can be excited, and it offers the
potential application of these ultra-short pulse lasers for identification and
detection of chemical and biological molecules from various airborne pollutants
or compounds. Substantial spectral broadening is also routinely observed in
simulations with the NRL air propagation code.
LASER-INDUCED ELECTRICAL BREAKDOWN
The presence of a plasma column in the filamentation of a femtosecond TW laser
pulse in air offers another interesting application for ultra-short intense
laser pulses. The plasma column is electrically conducting and can, therefore,
support a current between two electrodes that are charged to sufficiently high
voltages. Induced high-voltage breakdown has been studied using electron beams
or high-power lasers as the trigger mechanism, but the power required is usually
quite formidable and the discharge is often erratic. The breakdown is usually
caused by an ionization front (streamer) initiated by the laser that
progressively links the two electrodes until the circuit is completed for the
final discharge. The path of the discharge and the time of discharge after the
laser trigger are both quite random. The plasma columns associated with the
filaments of a propagating ultra-short pulse could generate a conducting path
that will lead to a deterministic discharge time and path for the breakdown.
This realization is important to applications such as the arrest of lightning
discharges where precise control of the lightning path is required. There are
also other applications in which the discharge must be synchronized with other
optical and electrical signals so knowledge of the precise discharge time after
the laser trigger is crucial.
FIGURE 5
Broadband radiation spectrum in the UV region from a 100-fs, 300 GW laser pulse
propa-gating for 7 meters in air. Spectral line structures are identified as
electronic transitions of neutral and ionic molecular species in air.
An experiment has been carried out using the 0.81-?m wavelength TFL laser at
peak powers of ~100 to 400 GW to initiate electrical breakdown between two
electrodes maintaining an average electric field of ~1.5 to 2 MV/m. The
discharge was monitored with streak cameras that could record its time evolution
beginning with the passage of the laser pulse between the two electrodes. Two
classes of discharge were observed that would not be distinguishable if it were
not for the streak camera catching the discharges in their actions. They are
shown in the two pictures in Fig. 6. The pictures are essentially multiple
exposures of a discharge, with each image slightly displaced vertically. Images
near the bottom of the pictures happen earlier in time, and they move upward as
time progresses. The ground electrode is on the left, and the negatively charged
high-voltage electrode (cathode) is on the right side of the picture. The
picture on the left in Fig. 6 shows a streamer starting from the bottom left and
moving from the ground electrode toward the cathode. Because the images are
shifted upward as the streamer moves, it appears to be tracing out a parabolic
trajectory but, in reality, it moves directly across the space between the
electrodes. From the time scale indicated on the vertical axis of the picture,
the speed of the streamer can be estimated to be close to 1% of the speed of
light. The effort of this streamer apparently is not enough to cause a breakdown
between the electrodes, and one can see that more streamers are involved at
later times. Eventually the air between the electrodes breaks down, but at a
time much later than could be displayed in this picture.
The picture on the right in Fig. 6 shows a totally different behavior in time.
There appears to be no indication of any presence of streamers. Instead, there
are illuminated horizontal paths that repeat themselves at time intervals of ~5
ns as can be measured from the vertical scale. This indicates that a complete
conducting path has been formed and a current is flowing between the two
electrodes. Since the flow of current occurs at the speed of light, the images
are essentially horizontal lines in this picture. The multiple lines represent
an oscillation in the flow of current between the two electrodes, with a
frequency governed by the circuit inductance and capacitance of the experimental
setup. The lowest bright line has a gap in the middle, and it is an indication
that the conducting path is not complete at the early stage of the discharge.
The width of the gap shortens as time progresses. The speed of approach of the
two ends of the gap is found to be around 1% of the speed of light. This is
consistent with the motion of an ionization front under these experimental
conditions, as seen in the left picture in Fig. 6.
FIGURE 6
Streak camera images of laser-induced electrical discharges. The left-hand
picture shows streamer initiated discharges with streamer velocity estimated to
be ~1% of light speed. The right-hand picture shows a fully guided discharge
occurring at ~255 ns after the laser trigger. The multiple discharges are the
consequence of resonant electrical circuit oscillations.
The time delay between the laser and the triggering of the discharge for these
fully guided discharges is consistently around 200 ns. This delay is much longer
than the expected time for the plasma density to decay due to recombination.
Simulations with an NRL air chemistry code that follows a large number of
molecular, atomic, and ionic species support the following scenario to explain
the long delay time. The applied electric field drives current through the
laser-produced plasma filament and produces a sufficiently high electron
temperature to maintain the plasma electron density. Eventually, the electron
density rises rapidly due to collisional (avalanche) ionization. This leads to a
rapid, uniform, fully guided breakdown across the gap. The time for breakdown to
occur in the simulations varies with filament size and initial electron density
and is consistent with the ~200-ns delay observed in the experiment. This
observation and its verification with simulation confirm the utility of an
ultra-short intense laser to precisely triggered high-voltage electrical
discharges.
CONCLUSIONS
Experimental, theoretical, and numerical studies have been performed on the
propagation and interactions of ultra-short intense laser pulses in air.
Filamentation of the laser pulse and the formation of plasma columns and the
generation of broadband radiation in the UV region were observed. Through the
benchmark process between experimental and numerical model calculations, we have
gained valuable knowledge of the underlying principles such as the measurement
of the nonlinear refractive index of air for fs TW laser pulses and the origin
of fully guided and well-defined electric discharges triggered by these laser
pulses. There are many applications including the detection of airborne
pollutants or chemical/biological compounds and laser-triggered lightning
arrests. Further understanding can be achieved with experimental and
theoretical/numerical studies of fundamental physics such as the onset of
various nonlinear processes as a function of the laser characteristics of the
intense laser pulses.
Additional references: