Ultrahigh contrast from a frequency-doubled
chirped-pulse-amplification beamline
David Hillier,* Colin Danson, Stuart Duffield, David Egan, Stephen Elsmere,
Mark Girling, Ewan Harvey, Nicholas Hopps, Michael Norman,
Stefan Parker, Paul Treadwell, David Winter, and Thomas Bett
Atomic Weapons Establishment (AWE) plc, Aldermaston, Reading, Berkshire RG7 4PR, UK
*Corresponding author: [email protected]
Received 6 March 2013; accepted 7 May 2013;
posted 13 May 2013 (Doc. ID 186469); published 17 June 2013
This paper describes frequency-doubled operation of a high-energy chirped-pulse-amplification beamline. Efficient type-I second-harmonic generation was achieved using a 3 mm thick 320 mm aperture
KDP crystal. Shots were fired at a range of energies achieving more than 100 J in a subpicosecond,
527 nm laser pulse with a power contrast of 1014 .
OCIS codes: (140.3515) Lasers, frequency doubled; (140.3530) Lasers, neodymium; (190.2620)
Harmonic generation and mixing.
http://dx.doi.org/10.1364/AO.52.004258
1. Introduction
The Orion laser [1] is an entirely new facility for
performing high energy density plasma physics experiments. Orion consists of two synchronized lasers:
10 “long-pulse” beamlines generate a total of 5 kJ at
351 nm in a 100 ps–5 ns temporally shaped pulse [2].
Two short-pulse beamlines each produce ∼500 J in
0.5 ps, one petawatt, at 1054 nm.
The short-pulse beamlines are seeded by a modelocked oscillator producing 150 fs, 3 nJ pulses centered at 1054 nm. These pulses have a positive chirp
imposed upon them by an Offner triplet stretcher [3]
increasing their duration to 6 ns. A single output
pulse is selected by a Pockels cell and amplified using
an optical parametric amplifier (OPA) pumped
by a commercial pump laser. The OPA produces a
180 mJ (1.5% RMS), 18 nm bandwidth pulse with
a super-Gaussian temporal and spatial profile
[4]. The OPA output pulses are spatially shaped
to a 30th power super Gaussian profile and then
amplified by four-passed Nd:glass (Nd:silicate and
Nd:phosphate) rod amplifiers followed by a series
of Nd:phosphate disk amplifiers of increasing aperture. The amplified pulse is then expanded to
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600 mm diameter and compressed to 500 fs using
a pair of gold diffraction gratings before being
focused onto a target with an f∕3 off-axis parabola.
Wavefront correction is achieved with a closed-loop
adaptive-optics system.
One characteristic of chirped-pulse-amplification
(CPA) beamlines, such as those on Orion, is that
the compressed pulse sits on top of a pedestal
formed by parametric fluorescence from the OPA
preamplifier. This pedestal has a characteristic
temporal profile linked to the pump pulse duration
and any spectral effects in the beamline (gain narrowing and spectral clips in the compressor). Prepulses can also be present. These may be
generated by badly suppressed pulses from the
master oscillator, spurious reflections from the
beamline, or spectral clipping on the compressor
gratings. If the main laser pulse sits on top of a
pedestal or is preceded by low-intensity pulses,
preplasmas can be formed, which can adversely
affect interactions between the main laser pulse
and target [5].
Various methods of improving the temporal
contrast of CPA lasers have been implemented in
other facilities:
Double chirped-pulse amplification [6] (DCPA),
where a preamplified pulse is compressed, passed
through some nonlinear filter, restretched, and then
sent to the main amplifier chain, is used extensively
in small-scale, high-repetition-rate systems. Crosspolarized wave (XPW) generation [7,8] and low-gain
optical parametric amplification [9] have both been
used as nonlinear filters in systems operating in a
similar regime to Orion. DCPA has the disadvantage
that it is inherently inefficient due to the losses in the
nonlinear processes and the additional compressor
and stretcher.
OMEGA MTW [10] and Vulcan PW [11] both use
the same technique to reduce the energy within their
fluorescence pedestal; here the seed pulse is amplified in a picosecond OPA prior to the main stretcher.
This reduces the gain required from the nanosecond
OPA and hence increases the nanosecond signalto-noise ratio by the same amount. The pedestal
remains at the same level, albeit for a few picoseconds in front of the pulse.
The pulse can also be cleaned after amplification
and compression either by the use of a plasma mirror
[12] or, as is reported here, by frequency conversion.
Previous investigations of type-I frequency doubling
of femtosecond and picosecond laser pulses from Nd:
glass [13–17] and Ti:sapphire [18,19] lasers have
been performed at smaller apertures, lower energies,
and/or lower intensities than are demonstrated here.
Type-I doubling is chosen to simplify implementation, as it produces a beam polarized along one of the
principal axes of the target plane, and therefore
avoids the use of large-aperture waveplates post
compression. It also removes the need to compensate
for the temporal walk off between the e and o rays,
which is present in type-II doubling [13]. Frequency
doubling the beamline has allowed us to provide a
facility to study laser–matter interactions in a new
regime [20] and improve the temporal contrast ratio
of the compressed pulse by a factor of 106 leading to a
power contrast of 1014.
2. Crystal Selection
A spatially and temporally resolved code has been
developed to model the performance of our shortpulse beamlines [21]. This code includes a functionality to simulate the performance of potassium
dihydrogen phosphate (KDP) for doubling the laser
pulses post compression. The code was used to simulate the frequency conversion of the beamline operating at nominal maximum fluence for a range of
crystal thicknesses and pulse durations (set by simulated detuning of the stretcher) shown in Fig. 1.
A 3 mm crystal was chosen as the best option
for delivering maximum energy to the target for
pulse durations of 0.5–5 ps with a bias toward the
shorter pulses that will be used most often.
3. Experimental
Frequency doubling is achieved at a large aperture
using a 320 mm diameter, 3 mm thick KDP crystal
Fig. 1. Model predictions for second-harmonic output energy at
different input pulse lengths; the simulated input energy is 150 J.
[supplied by Gooch and Housego (Ohio)] cut for
type-I phase matching with its surface at 41° to
normal such that phase matching occurs with the
beam at normal incidence to the face. The crystal
is the largest high-aspect-ratio KDP crystal available
with suitable transmitted wavefront properties. The
crystal is solgel coated to achieve optimal transmission, being antireflection coated at 1054 nm on the
incident face, with the exit face similarly coated
for 527 and 1054 nm.
The doubling crystal and associated optics are inserted between the compressor and target chambers
as shown in Fig. 2(a). The entire beamline from the
compressor input onwards is enclosed in a single vacuum volume to avoid the deleterious effect of passing
the compressed pulse through a vacuum window. In
order to enhance operational efficiency, all of the
optics required for frequency-doubled operation of
the beamline except for the parabola are contained
within a single, isolatable, chamber.
After compression, the beam is passed to the chamber [Fig. 2(b)], where the 600 mm diameter beam is
apodized to 300 mm, matching the aperture of the
doubling crystal. The apodizer is centered on the region of highest intensity of the (laterally dispersed)
beam achieving a transmission of ∼30%. The apodizer is formed by four segmented quadrants of
absorbing glass. Each is aligned such that any reflections are dumped into the compressor vessel walls
while ensuring that no caustics hit the diffraction
gratings. A solid metal block is placed behind the
absorbing glass to prevent any transmitted firstharmonic light from reaching the target.
After apodization, the beam is reflected off a
1054 nm highly reflective (HR) mirror, and passes
through the doubling crystal, and then off three dichroic mirrors, which are HR at 527 nm and highly
transmissive (HT) at 1054 nm. The beam then leaves
the chamber and reflects off two further mirrors,
which are specified to be HR at both 527 and
1054 nm. Focus onto the target is realized with an
f∕6 off-axis parabola, which is also designed to be
HR at 527 nm and HT at 1054 nm. Each dichroic
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4259
Fig. 2. The optics required to frequency double the Orion shortpulse beam are inserted into the beamline in a single vacuum
chamber in the compressor hall.
mirror reduces the reflected power of the fundamental by ∼2 orders of magnitude, achieving a total
extinction ratio of ∼108.
The majority of the unconverted, 1054 nm portion
of the pulse passes through the back of the first turning mirror and is dumped into a slab of absorbing
glass, which is angled to avoid back reflections. Leakage through the back of the second turning mirror is
reduced in size with a Galilean telescope and used for
diagnostics on the 527 nm beam. On the diagnostics
table the 527 nm pulse is separated from any
residual 1054 nm light by a series of dichroic mirrors
that are HR at 527 nm and HT at 1054 nm. In order
to prevent any scattered 1054 nm light from reaching
the diagnostics, BG39 cutoff filters, which transmit
527 nm light and block 1054 nm, are placed in front
of each diagnostic. The beam is split between three
diagnostics stations: a near-field camera that is used
to monitor the spatial profile of the beam and look for
any localized deleterious nonlinear effects, a pyroelectric energy meter, and a contrast measurement
station.
The contrast measurement station consists of two
photodiodes: one is heavily attenuated such that it
can measure the main laser pulse. The other is used
to observe any low-intensity prepulses or structure
present before the main pulse. The contrast of this
structure can be calculated from the duration of
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the main pulse (measured in the first harmonic),
the impulse response of the photodiode, and the attenuation of the unsaturated photodiode.
The system is aligned using a 532 nm CW laser
launched through the back of the first mirror in
the frequency conversion chamber shown in Fig. 2(b).
Colinearity and cofocusing between the alignment
beam and the main beam are set initially using an
insertable subaperture spherical mirror. This is then
optimized by viewing pulses from the OPA (which are
sufficiently intense to achieve low levels of frequency
conversion) and the (attenuated) CW beam on a
camera that images target chamber center from
the far side of the focal plane.
The crystal was angle tuned for optimal frequency
conversion in situ using the doubled OPA light. The
tuning curve was found to be broad enough to allow
us to detune the crystal in its slow axis by 1 mrad
such that any back reflections were stopped by the
pinhole in Orion’s final spatial filter. While it is
known that the sensitivity of second-harmonic power
to angle increases at higher intensities due to χ 3 effects within the doubling crystal [22], we found that
the angular variation in conversion efficiency even at
high intensities was small enough that the 1 mrad
detuning had no ill effects.
Transition between operation at 527 and 1054 nm
is achieved by the removal or replacement of the
beam apodizer and two turning mirrors.
4. System Operation
Full system shots were fired at progressively increasing energies with a 0.5 ps pulse duration. The system
was then run for several months to generate highcontrast laser pulses for an experiment to study
ionization potentials in hot dense plasmas [23]. In
this period the energy was set to meet the demands
of the experiment. Energy conversion efficiencies
of up to 75% were achieved [once the transmission of
the apodizer is taken into account Fig. 3(a)] generating 527 nm laser pulses of up to 100 J (Fig. 4). The
conversion efficiencies are consistent with those seen
in our previous work using 2 mm thick crystals [17]
and those by others using 4 mm thick crystals
[13–16] at similar intensities. While our modeling
captured enough of the relevant physics to specify
Fig. 3. The first-harmonic beam (a) is apodized and frequency
doubled to produce a 300 mm diameter 527 nm beam (b).
Fig. 4. Second-harmonic energy and conversion efficiency as
measured for a range of input energies over several months of
operation.
A CCD camera imaging target chamber center is
used to observe focused frequency-doubled OPA
pulses. This shows, in Fig. 5(a), a ∼10 μm diameter
near-diffraction-limited focal spot. By comparing this
with the instrument-limited, 15 μm diameter x-ray
spot generated by a full energy system shot shown
in Fig. 5(b) (measured using an x-ray pinhole camera
with a 10 μm pinhole), we can see this is directly
indicative of the focal spot achieved on a full target
shot giving an intensity on target of ∼1020 W cm−2.
It is interesting to note that we have not encountered any of the deleterious effects on the focal spot
observed by Neely et al. [14,16], who used a 4 mm
thick crystal KDP crystal at comparable intensities.
We also do not observe any localized structure or
beam break-up effects due to nonlinear effects in
our doubling crystal or diffraction from the hard clip
on the apodizer as shown in Fig. 3(b).
5. Contrast Enhancement
Fig. 5. The low-intensity optical focal spot (a) from the doubled
OPA is consistent with the x-ray spot and (b) generated by a full
energy (85 J on target) shot.
the doubling crystal, any effects not included (such as
χ 3 effects [22], local variations in beam intensity or
wavefront and imperfections in the crystal) will have
all contributed to the lower-than-calculated conversion efficiencies.
Measurements taken using a two-photodiode prepulse monitor on the compressed first-harmonic
beam show that the beamline has a 3 ns pedestal
of 10−8 the intensity of the main pulse (Fig. 6). This
pedestal is due to parametric fluorescence in our
OPA and is comparable with those seen at other
Nd:Glass pettawatt laser facilities [9,10]. Similar
measurements taken after frequency conversion
can be seen in Fig. 6 and indicate that the pedestal
has been reduced to 10−14 . In both measurements,
biased silicon photodiodes (Newport 818-21A) were
used; this made the 527 nm measurement ∼5 times
more sensitive than the 1054 nm due to the greater
responsivity at this wavelength. To reach this dynamic range the photodiodes required extensive baffling, spectral filtering, and shielding to prevent any
stray reflections or scattered light within the diagnostics enclosure swamping the signal. Two artifacts
from scattered light within the diagnostics station,
indicated in Fig. 7, proved impossible to remove
Fig. 6. Contrast enhancement by frequency doubling the beamline.
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Fig. 7. Direct photodiode measurement showing artifacts from
the diagnostics.
entirely. These measurements were all performed
with no protection in front of the photodiodes (water
cells, etc.), which shortened their life significantly.
6. Conclusions
We have integrated a large-aperture type-I
frequency-doubling system into one of the petawatt
beamlines on the Orion laser facility. Secondharmonic, 0.5 ps laser pulses are generated at up
to 100 J, ∼0.2 PW; this is, to our knowledge, the most
powerful frequency-doubled beamline to be reported.
The beamline is well optimized with target intensities of ∼1020 W cm−2 being achieved with a power
contrast of 1014. The beamline is now routinely
run in the second harmonic providing a high-contrast
ultrahigh-intensity facility for performing plasma
physics experiments.
The authors thank all of those involved in the
conversion of the beamline, especially Peter Allen,
Colin Brown, Matt Hill, David Hoarty, Lauren
Hobbs, and Steven James, who provided the x-ray
focal spots, and the Orion operations team for all
of their hard work.
References
1. N. W. Hopps, C. N. Danson, S. J. Duffield, D. A. Egan, S. P.
Elsmere, M. T. Girling, E. J. Harvey, D. I. Hillier, M. J.
Norman, S. J. F. Parker, P. A. Treadwell, D. N. Winter, and
T. H. Bett, “Overview of laser systems for the Orion facility
at AWE,” Appl. Opt. 52, 3597–3607 (2013).
2. D. I. Hillier, D. N. Winter, and N. W. Hopps, “Pulse generation
and preamplification for long pulse beamlines of Orion laser
facility,” Appl. Opt. 49, 3006–3013 (2010).
3. G. Cheriaux, P. Rousseau, F. Salin, J. P. Chambaret, B. Walker,
and L. F. Dimauro, “Aberration-free stretcher design for ultrashort-pulse amplification,” Opt. Lett. 21, 414–416 (1996).
4. M. T. Girling, S. J. F. Parker, D. Hussey, and N. W. Hopps,
“High energy and high stability optical parametric chirped
pulse amplifier for seeding the petawatt beamlines of the
Orion laser facility,” Proc. SPIE 7721, 772122 (2010).
5. M. Natel, J. Itatani, A. Tien, J. Faure, D. Kaplan, M. Bouvier,
D. Buma, P. Van Rompay, J. Nees, P. P. Pronko, D. Umstadter,
and G. A. Mourou, “Temporal contrast in Ti:sapphire lasers:
characterisation and control,” IEEE J. Sel. Top. Quantum
Electron. 4, 449–458 (1998).
4262
APPLIED OPTICS / Vol. 52, No. 18 / 20 June 2013
6. M. P. Kalashnikov, E. Risse, H. Schönnagel, and W. Sandner,
“Double chirped-pulse amplification laser: a way to clean
pulses temporally,” Opt. Lett. 30, 923–925 (2005).
7. A. Jullien, O. Albert, F. Burgy, G. Hamoniaux, J. P. Rousseau,
J.-P. Chambaret, F. Auge-Rochereau, G. Cheriaux, J.
Etchepare, N. Minkovski, and S. M. Saltiel, “10−10 temporal
contrast for femtosecond ultraintense lasers by crosspolarized wave generation,” Opt. Lett. 30, 920–922 (2005).
8. A. Cotel, A. Jullien, N. Forget, O. Albert, C. Chériaux, and C.
Le Blanc, “Nonlinear temporal pulse cleaning of a 1 μm optical
parametric chirped-pulse amplification system,” Appl. Phys. B
83, 7–10 (2006).
9. R. C. Shah, R. P. Johnson, T. Shimada, K. A. Flippo, J. C.
Fernandez, and B. M. Hegelich, “High-temporal contrast
using low-gain optical parametric amplification,” Opt. Lett.
34, 2273–2275 (2009).
10. C. Dorrer, I. A. Begishev, A. V. Okishev, and J. D. Zuegel,
“High-contrast optical-parametric amplifier as a front end
of high-power laser systems,” Opt. Lett. 32, 2143–2145
(2007).
11. I. Musgrave, W. Shaikh, M. Galimberti, A. Boyle, C.
Hernandez-Gomez, K. Lancaster, and R. Heathcote, “Picosecond optical parametric chirped pulse amplifier as a preamplifier to generate high-energy seed pulses for contrast
enhancement,” Appl. Opt. 49, 6558–6562 (2010).
12. G. Doumy, F. Quéré, O. Gobert, M. Perdrix, P. Martin, P.
Audebert, J. C. Gauthier, J.-P. Geindre, and T. Wittmann,
“Complete characterization of a plasma mirror for the production of high-contrast ultraintense laser pulses,” Phys. Rev. E
69, 026402 (2004).
13. C. Y. Chien, G. Korn, J. S. Coe, J. Squier, and G. Mourou,
“Highly efficient second-harmonic generation of ultraintense
Nd:glass laser pulses,” Opt. Lett. 20, 353–355 (1995).
14. D. Neely, C. N. Danson, R. Allott, F. Amiranoff, J. L. Collier,
A. E. Dangor, C. B. Edwards, P. Flintoff, P. Hatton, M.
Harman, M. H. R. Hutchinson, Z. Najmudin, D. A. Pepler,
I. N. Ross, M. Salvati, and T. Winstone, “Frequency doubling
of multi-terawatt picosecond pulses,” Laser Part. Beams 17,
281–286 (1999).
15. J. Queneuille, F. Druon, A. Maksimchuk, G. Cheriaux, and G.
Mourou, “Second-harmonic generation and wave-front
correction of a terawatt laser system,” Opt. Lett. 25, 508–510
(2000).
16. D. Neely, R. M. Allott, R. L. Clarke, J. L. Collier, C. N. Danson,
C. B. Edwards, C. Hernandez-Gomez, M. H. R. Hutchinson,
M. Notley, D. A. Pepler, M. Randerson, I. N. Ross, J. Springall,
M. Stubbs, T. Winstone, and A. E. Dangor, “Frequency doubling multi-terawatt sub-picosecond pulses for plasma interactions,” Laser Part. Beams 18, 405–409 (2000).
17. D. I. Hillier, M. T. Girling, M. F. Kopec, N. W. Hopps, J. R.
Nolan, and D. N. Winter, “Full aperture, frequency doubled
operation of the HELEN CPA beamline,” AWE Plasma Physics Department Annual Report (2006). Copies of the report
can be obtained from the Corporate Communications Office
at the Atomic Weapons Establishment, Aldermaston, Reading, Berkshire RG7 4PR, UK.
18. V. Krylov, A. Rebane, A. G. Kalintsev, H. Schwoerer, and U. P.
Wild, “Second-harmonic generation of amplified femtosecond
Ti:sapphire laser pulses,” Opt. Lett. 20, 198–200 (1995).
19. M. Aoyama, T. Harimoto, J. Ma, Y. Akahane, and
K. Yamakawa, “Second-harmonic generation of ultra-high
intensity femtosecond pulses with a KDP crystal,” Opt.
Express 9, 579–585 (2001).
20. C. R. D. Brown, D. J. Hoarty, S. F. James, D. Swatton, S. J.
Hughes, J. W. Morton, T. M. Guymer, M. P. Hill, D. A.
Chapman, J. E. Andrew, A. J. Comley, R. Shepherd, J. Dunn,
H. Chen, M. Schneider, G. Brown, P. Beiersdorfer, and J. Emig,
“Measurements of electron transport in foils irradiated with a
picosecond time scale laser pulse,” Phys. Rev. Lett. 106,
185003 (2011).
21. Various papers contained within “AWE Plasma Physics
Department Annual Report 2006,” Copies of the report can
be obtained from the Corporate Communications Office at
the Atomic Weapons Establishment, Aldermaston, Reading,
Berkshire RG7 4PR, UK.
22. T. Ditmire, A. M. Rubenchik, D. Eimerl, and M. D. Perry,
“Effects of cubic nonlinearity on frequency doubling of highpower laser pulses,” J. Opt. Soc. Am. B. 13, 649–655 (1996).
23. D. J. Hoarty, P. Allan, S. F. James, C. R. D. Brown, L. Hobbs,
M. P. Hill, J. W. O. Harris, J. Morton, M. G. Brookes,
R. Shepherd, J. Dunn, H. Chen, E. Von Marley, P. Beiersdorfer,
G. Brown, and J. Emig, AWE Plc, Aldermaston, Reading,
Berkshire RG7 4PR, UK, are preparing a manuscript to be
called “Observations of the effect of ionization potential
depression in hot, dense plasma.”
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