LASER SOURCE HAVING A PEAK POWER OF MORE THAN 100 TERAWATTS
AND HIGH CONTRAST
The field of the invention is that of lasers with pulses of very high
5 peak power, from at least 100 TeraWatt to a few tens of PetaWatt and even
more. At this level of peak power, the pulses are generally ultra-short i.e. of a
duration below 200 femtoseconds.
Such laser sources are in particular used for laser-material
10 interactions consisting for example in accelerating particles (protons,
electrons, ions) or in generating secondary radiation in the region of far-UVs,
X- or y-rays. The pulses are focused on a generally solid target with the aim,
for example, of creating a plasma at the surface of the latter.
These laser sources with very high peak power are essentially
15 based on solid-state laser source technologies and the principle of chirped
pulse amplification, whether this involves amplification by laser effect, or
"CPA, the acronym for the expression Chirped Pulse Amplification, or
amplification by non-linear optical effect such as the amplification of
parametric fluorescence or "OPCPA, the acronym of the expression Optical
20 Parametric Chirped Pulse Amplification.
From the moment that lasers with very high peak powers are used,
it is necessary to pay a great deal of attention to the problem of temporal
contrast of the pulses. Indeed, given the multiple-amplifier configurations of
these laser sources with high peak power, there always remains a residual
25 light signal generated by the amplification of the spontaneous emission (or
"ASE) in the case of laser amplification (CPA) or by the amplification of the
parametric fluorescence in the case of parametric amplification (OPCPA).
This parasitic signal shown in figure 1 has a much larger temporal width than
the main pulse; it typically exists during the pumping pulses, which generally
30 have pulse durations of about a few nanoseconds. And most importantly, it is
already present before the main pulse. The temporal contrast is the ratio
between the peak power PI of the main pulse and the peak power PO of the
ASE pulse; the target values and the measurements for PO are generally
situated in a time interval between 50 and 100 picoseconds before the main
35 pulse, in such a way as to remove other effects such as imperfect
compression. The problem is the same if the ratio is very high, in the case of
a main pulse of very high peak power, the ASE pulse will still have a
significant peak power. If one considers the case of a main pulse of 1
PetaWatt with a temporal contrast of lo9:l (PIIPO = lo9), the ASE will have a
5 peak power of 1 Megawatt, enough in the case of focusing on a target, to
create a plasma on the latter (notably if it is solid) before the arrival of the
main pulse which is disastrous for the intended applications.
It is therefore necessary to optimize the temporal contrast of the
pulses, knowing that to obtain satisfactory results with any solid material
10 forming the target, it is necessary to aim for temporal contrasts with respect to
the ASE of the order of 10l2 :I for a laser of 1 petawatt and of loq3 :I for a
laser of 10 petawatts.
Until now, the technologies used to increase the temporal contrast
15 are:
- the addition of a saturable absorber, or
- the implementation of a non-linear filter based on the technology of
generating a cross-polarized wave in a non-linear crystal, subsequently
denoted by "XPW for Cross-Polarized Wave, or
20 - the use of plasma mirrors.
A saturable absorber is very simple to implement but its
contribution is limited because it does not make it possible to improve the
contrast of the pulses by more than one or two orders of magnitude. This is
25 mainly due to the fact that the laser damage threshold of these materials are
relatively low.
The XPW filter technique was demonstrated for the first time in a
laser chain with high peak power in 2004. The architecture of a XPW filter is
30 relatively simple but its efficiency is not very high (at the output a maximum of
30% of the energy of the input pulse is obtained) and the theoretical increase
in contrast (output contrast = cube of the input contrast) is heavily limited by
the extinction ratios of the polarizers used, which means that the net gain is
only of 4 to 5 orders of magnitude, which is still clearly better than the
saturable absorber. A XPW filter includes two polarizers and one or two nonlinear
crystals between the two polarizers.
Plasma mirror technology has been used for a few years now to
5 improve the contrast of laser chains with high peak power. The principle is
based on the use of the beam at the output of the chain, therefore after the
final temporal compression. The beam is focused on a transparent medium;
the ASE pulse is therefore transmitted but from the start of the main pulse
there is enough intensity to create a plasma at the surface of the transparent
10 material. This plasma is reflective, thus forming a plasma mirror, and it will
reflect around 70% of the main pulse which will be "rid" of a large part of the
ASE pulse which will have been transmitted before the creation of the
plasma. However, it will be necessary to repeat the operation a second time
to obtain an increase in the contrast of around 4 to 5 orders of magnitude.
15 This main pulse, reflected twice, is then focused on the target.
This technology has several drawbacks. The energy loss is
therefore of the order of 50% and it is definitive since there are no further
amplifiers afterwards, unlike in the case of the XPW filters. Moreover, this
technology is relatively complex to implement. Firstly, the assembly of the
20 device is under vacuum since it involves a compressed beam and the
assembly is quite bulky given the size of the beam. secondly, it is obviously
necessary, after each shot, to move the plasma mirrors since the light spot of
the focused laser has produced a highly reflective plasma but has also locally
produced irreversible damage to the optical surface. This therefore entails
25 the installation of precise motorized parts that are compatible with the
vacuum.
To obtain temporal contrasts of the order of 10":l or even of
10'~:flo r a solid target, at the minimum a few tens of picoseconds before the
30 main pulse, none of these techniques considered stand-alone is sufficient and
it will therefore be necessary to combine them. Due to this fact, given the
relatively small contribution of saturable absorber technology to the
improvement of contrast, a combination of the saturable absorber with one of
the two other techniques cannot suffice insofar as the "natural" contrast at the
35 start of the laser chain before the use of these devices is of the order of lo5 to
lo6 :I. It is therefore necessary to combine a XPW filter and a double plasma
mirror, to obtain the required level of contrast, knowing that the latter device
has the aforementioned drawbacks.
5 As a consequence, there remains to this day a need for a system
simultaneously satisfying all the aforementioned requirements in terms of
peak power, temporal contrast, energy, and simplicity of implementation.
The basic idea of the invention is to associate at least two XPW
10 filters in order to improve the contrast by 7 to 8 orders of magnitude with an
extremely simple, low cost and entirely passive device, i.e. one without parts
to move or replace. Compensation for the dispersion introduced by the first
XPW filter, i.e. by the propagation through the non-linear crystals and the
polarizers, is achieved by means of an intermediate compressor in such a
15 way as to produce a pulse with optimized characteristics (duration and
spectral phase) at the input of the second XPW filter.
More precisely, the subject of the invention is a laser source
capable of emitting energy pulses greater than or equal to 100 TeraWatt,
consisting of a laser chain that comprises, in cascade:
20 - a solid-state laser oscillator,
- a first and a last amplification stage with frequency chirping, each
including in cascade a stretcher, an amplifying chain and a
compressor,
- a first filter with one or two non-linear crystals and third order non-linear
optical susceptibility, capable of generating a cross-polarized wave,
known as non-linear cross-polarization filter, inserted between these
two amplification stages.
It is mainly characterized in that the laser chain comprises between
the first and the last amplification stage, at least one other non-linear cross-
30 polarization filter, i.e. N filters in the laser chain with N2 2 and N-I dispersion
compensator(s), placed at the output of the first filter@) (respectively).
It optionally comprises a third amplification stage with frequency
chirping inserted between two non-linear cross-polarization filters, this third
amplification stage including in cascade a stretcher, an amplifying chain and a
35 compressor, this compressor also being a dispersion compensator.
A dispersion compensator can be a mirror compressor with
controlled dispersion.
An amplification stage is typically a CPA or an OPCPA amplifier.
5 Other characteristics and advantages of the invention will become
apparent upon reading the following detailed description, given by way of nonlimiting
example and with reference to the appended drawings wherein :
figure 1 schematically shows the peak power of a main pulse and
of the ASE as a function of time,
10 figure 2 schematically represents a first example of a laser source
according to a first embodiment of the invention,
figure 3 schematically represents a second example of a laser
source according to the invention.
15 The basic idea of the proposed solution is to combine at least Wo
XPW filters in order to improve the contrast of the pulses by 7 to 8 orders of
magnitude.
More precisely, the laser source 100 according to the invention, an
example of which is shown in figure 2, comprises in cascade in a basic
20 configuration:
- a solid-state laser oscillator 1,
- a first CPA or OPCPA amplification stage 2 with frequency
chirping, at the output of which the contrast is of the order of lo5 or
1 06,
25 - a first XPW filter 3, preferably a XPW HE filter,
- a dispersion compensator 4 including for example a pair of mirrors
with controlled dispersion,
- another XPW filter 5,
- a last CPA or OPCPA amplification stage 6 with frequency
30 chirping, at the output of which the contrast is of the order of 1012 or
lot3.
It will be recalled that an amplifier with frequency chirping includes
in cascade a stretcher, an amplifying chain with one or more amplifiers, and a
compressor. In each amplifier, there are one or more passes of the laser
beam.
A XPW filter includes one or two non-linear crystals with third order
non-linear optical susceptibility and two polarizers. Among the XPW filters
5 used in the context of this invention, mention may be made of the filters
described in patent 1 662 306. This can for example be the XPW filter
comprising a first polarizer PI which makes it possible to obtain a field E with
rectilinear polarization from the pulse at the input of the filter. This field is
focused on a cubic crystal C i.e. one not having any offset in group velocity
10 between the incident field and the generated field, such as a BaF2 crystal
which is furthermore transparent over a large spectral region, from the
ultraviolet to the infrared. The field E can be focused outside the crystal C but
near the latter. This crystal C converts part of the incident field into a field E'1
with rectilinear polarization, orthogonal to that of E. Another part of the
15 incident field is transmitted by the crystal C without being converted: this
unconverted field, of same polarization as the incident field, is the carrier of
the ASE.
According to a first embodiment, the filter includes a second crystal
with cubic geometry capable of receiving at the input the field E'1 and the
20 residual field E and of generating from this residual field E a field E'2 of same
polarization as E'1 and with the same temporal properties, and therefore
capable of producing constructive interferences with the field E'1 .
According to another embodiment, the crystal C suffices to
generate this field E'2 in a multi-pass configuration.
25 A second polarizer P2 with rectilinear polarization orthogonal to
that of PI makes it possible to attenuate or even to remove the field carrying
the ASE, to only let through the field E'1+ E'2 i.e. that of the main pulse.
As indicated previously, the first of these filters 3 is preferably a
30 XPW HE (High Energy) filter, thus named because it is capable of receiving
pulses with an input energy lying between 0.5 mJ and 15 mJ, thanks to a
configuration in which the size of the beam on the crystal(s) is compatible with
this level of energy. An example of such a configuration is described in
"Efficient cross polarized wave generation for compact, energy-scalable,
35 ultrashort laser sources", Ramirez & al., Optics Express, Vol. 19, Issue 1, pp.
93-98 (201 1). The second filter 5 is a XPW HE filter or a conventional XPW
filter as described above, the input energy of which lies between 0.1 and 0.5
mJ.
Insofar as the first filter 3 introduces some dispersion induced by
5 the propagation of the beam through the polarizers and the non-linear
crystal(s) where the XPW effect takes place, it is imperative to compensate
for this dispersion in such a way as to present a beam with optimal
characteristics on the second (other) XPW filter 5. Given the relatively low
value of this dispersion, the compensation is produced by means of a
10 compressor 4 with mirrors with controlled dispersion (generally a pair of
mirrors), which are components currently available commercially and whose
dispersion characteristics can be adjusted depending on the compensation to
be produced.
There can be a dispersion compensator after the second XPW
15 filter 5, but not necessarily insofar as the beam is stretched immediately
afterwards and not insofar as the compensation can be produced by means
of adjustments of the compressor of the last amplification stage 6 with
frequency chirping.
XPW filters exist which in terms of admissible input energy (up to
15 mJ) make such a combination possible: indeed, if for example the first
XPW filter receives an energy of 3 mJ and has an efficiency of 15%, it
procures a beam of 450 pJ which given the losses, notably those of the
compressor whose mirrors have controlled dispersion, makes it possible to
25 have access to 300 pJ at the input of the second XPW filter. When this
second filter has an efficiency of 20%, it makes it possible to deliver an
energy of 60 pJ for the injection of the second amplification stage with
frequency chirping, i.e. a perfectly adequate value and one that is easily at
the level of the prior art.
30 The problem of complexity is thus solved, because the XPW
stages are very simple, consisting of a few crystals and a few optical
components, the dimensions of which remain small because the energies
involved are weak at this place in the chain.
The first filter 3 is optionally placed in a vacuum, taking the incident
35 energy into account, but it is a vacuum chamber including only the non-linear
crystal(s) and a spatial filtering device, i.e. typically a cylinder of 5cm in
diameter and of 50cm in length, and therefore a much less voluminous
chamber than in the case of the plasma mirrors of the order of one m3 ;
moreover the required level of vacuum is much lower than in the case of the
5 plasma mirrors (of the order of 2 orders of magnitude lower).
The second problem incurred by the double plasma mirror, that of
energy loss, is here again solved because, since XPW filters are placed at the
start of the laser chain, to compensate for the significant loss that they
introduce (the total transmission is only of a few %), it suffices to add one or
10 two passes from the first multi-pass amplifier of the second amplification
stage with frequency chirping.
A preferred means of embodiment consists in the following
architecture using the base configuration, with in cascade:
15 - a titanium-sapphire oscillator 1 with mode locking,
- a first titanium-sapphire CPA 2 typically delivering 3 mJ, at the
output of which the contrast is of the order of lo5 or lo6,
- a XPW HE filter 3 with one crystal of BaF2 with a 15% efficiency
for example,
20 - a dispersion compensator 4 equipped with a pair of mirrors with
controlled dispersion, with overall transmission of the order of 70%,
- a XPW filter 5 with two crystals of BaF2 with a 20% efficiency, with
a typical input energy of 300 VJ and a typical output energy of 60 pJ,
- a second titanium-sapphire CPA 6 delivering a peak power
25 typically comprised between 100 TW and 30 PW according to the
number of amplifiers of this CPA.
Many variants are possible:
- the BaF2 can be replaced by CaF2 or SrF2 or CaBaF2, or CaSrF2
or diamond or LiF or YAG or any other material with third order nonlinear
susceptibility capable of generating a cross-polarized wave,
- the CPAs can be replaced by OPCPAs or it is possible to have a
hybrid configuration (OPCPA then CPA),
- for a CPA, the amplifying medium can be Titanium-doped
sapphire, glass (or a mixture) of Neodymium- or Ytterbium-doped
glass(es), or ytterbium-doped glass or crystals,
- the filter 5 can be a XPW HE filter,
5 - it is possible to envision a configuration with three XPW filters (two
XPW HE filters then one XPW filter for example) inserted between the
two amplification stages with frequency chirping, with a dispersion
compensator after each of the two first XPW filters; in this case, it is
possible to envision an increase in the contrast of 10 to 12 orders of
magnitude compatible with an exawatt peak power,
- it is possible to envision a configuration with three amplification
stages with frequency chirping (CPA or OPCPA) as shown in figure 3,
with one or two XPW filters (or XPW HE filters) between two
amplification stages (between the first one 2 and the middle one 8, or
between the middle one 8 and the last one 6; in the example in the
figure, there are two filters 3, 7 between the amplification stages 2 and
8), with a dispersion compensator after each of the first XPW filters,
knowing that this third amplification stage 8 can play, via its
compressor, the part of a dispersion compensator and thus replace
one compensator. When the laser chain includes N filters (N2 2 and
including the first filter), there is a dispersion compensator at the output
of the N-I first filters, i.e. N-1 compensators.
The invention can have another advantage, which is that of an
25 increase in the spectral widening inherent in XPW technology. Indeed, a XPW
stage produces a widening of the spectrum by a factor of 43 (and therefore a
reduction of the pulse duration by as much) because the non-linear
phenomenon is dependent on the cube of the intensity. The implementation
of two XPW stages is therefore of a kind to produce a widening of the
30 spectrum by a factor of three. This reduction in the duration of each pulse
makes it possible to reduce the pumping energy. Amplification stages with
frequency chirping of CPA type are then preferably used.
10
CLAIMS
1. A laser source (1 00) capable of emitting energy pulses greater than or
5 equal to 100 TeraWatt, consisting of a laser chain that comprises, in
cascade:
- a solid-state laser oscillator (I),
- a first amplification stage (2) with frequency chirping, and
- a last amplification stage (6) with frequency chirping, these first and
10 last amplification stages each including in cascade a stretcher, an
amplifying chain and a compressor,
- a first filter known as non-linear cross-polarization filter (3), with one or
two non-linear crystals and third order non-linear optical susceptibility,
capable of generating a cross-polarized wave, and wherein said crystal
or said crystals is (are) placed between two polarizers,
characterized in that the laser chain comprises:
- between the first (2) and the last (6) amplification stage, at least one
other non-linear cross-polarization filter (5, 7), i.e. N filters in the chain
with N2 2 and
20 - N-I dispersion compensator(s) (4), placed at the output of the first
filter(s) (3) (respectively).
2. The laser source as claimed in the preceding claim, characterized in
that the laser chain comprises a third amplification stage (8) with
frequency chirping inserted between two non-linear cross-polarization
filters, this third amplification stage including in cascade a stretcher, an
amplifying chain and a compressor, this compressor also being a
dispersion compensator.
30 3. The laser source as claimed in one of the preceding claims,
characterized in that at least one dispersion compensator is a mirror
compressor with controlled dispersion.
4. The laser source as claimed in one of the preceding claims,
characterized in that one (or several) amplification stage(s) is a CPA or
an OPCPA amplifier.
5 5. The laser source as claimed in the preceding claim, characterized in
that a CPA amplifier has an amplifying medium that is Titanium-doped
sapphire or glass or a mixture of Neodymium- or Ytterbium-doped
glasses or Ytterbium-doped crystals.
10 6. The laser source as claimed in one of the preceding claims,
characterized in that the non-linear crystal is a BaF2 crystal.