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Author(s):
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High-temporal
contrast
using
low-gain
optical
parametric
amplification
Rahul
C.
Shah,
Randall
P.
Johnson,
Tsutomu
Shimada,
Kirk
A.
Flippo,
Juan
C.
Fernandez,
and
B.
M.
Hegelich
Optics
Letters
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Form 636 (7/06)
Author
List Continued (Shah, Form 678)
Last
First
Middle
ZNo.
Group
Fernandez
Juan
C.
098757
P-24
Hegelich
Bjorn
M.
186672
P-24
High-temporal
contrast
using
low-gain
optical
parametric
amplification
Rahul
C.
Shah,
Randall
P.
Johnson*,
Tsutomu
Shimada,
Kirk
A.
Flippo,
Juan
C.
Fernandez,
and,
B.
M.
Hegelich
Los
Alamos
National Laboratory, Los Alamos,
NM
87545 USA,
* Co-rresponding author: rpjohnson@lanl.gov
Compiled
September
19, 2008
We
demonstrate
the
use
of
low-gain
optical
parametric
amplification
(OPA)
as
a
means
of
improving
temporal
contrast.
250
J.lJ,
500
fs
pulses
of
1053
nm
are
frequency
doubled
and
subsequently
restored
to
original
wavelength
by
the
OPA
with>
10% efficiency. © 2008
Optical
Society
of
America
oels codes: 190.4970, 230.4480, 170.7160, 140.3538
(a)
(b)
15
~
~
~
A
>-
10
0
c
A
Q)
·u
!E
Q)
....
5
Q)
0 1 mm
A
2mm
uad. fit
:§
0
100 150
200 250
total input (IlJ)
Fig.
1.
(a) Schematic
of
low
gain OPA temporal pulse
cleaning: DL, delay line; BS, 10/90 beamsplitter; C,
2
mm
Type
I BBO crystal;
S,
signal;
and
I, idler. (b) Idler
efficiency for
both
1
and
2 mm crystals. shahFIG1.eps.
Realization of promising applications
of
relativistic light
intensities achieved with
terawatt
chirped pulse ampli-
fication (CPA) lasers
[1]
depends on
the
ability
to
con-
trol
premature
target
ionization by preceding intensity
spikes, compression quality, or amplified spontaneous
emission (ASE). As an example,
the
threshold for de-
struction of
"'nm
target
foils in laser-based ion accelera-
tion schemes
[2]
occurs
at
1010
W/cm
2
.
At
typical peak
interaction intensities of
10
20
W / cm
2
, this translates
to
a requirement
of
better
than
10-
10
intensity
contrast
a
few
ps from
the
peak. Given
the
absolute damage thresh-
old, as laser intensities continue to grow
[3],
so
to
will
the
contrast
requirement.
This
short
timescale dictates
the
use of non-linear optical processes to remove
the
ASE.
Since
the
strength
of
the
ASE depends on
the
total
gain it sees, temporal cleaning subsequent
to
initial gain
stages suffices
to
reduce its level to
the
contrast
crite-
ria. Most
current
systems
then
require temporally clean
pulses of
lOs
of
l-iJ
though
the
exact requirement depends
on
the
pulse
duration
and
final energy. Such clean pulse
energies can be
obtained
by first amplifying a temporally
stretched pulse
and
then
compressing it for cleaning with
a non-linear technique. Before subsequent amplification,
the
pulse passes through
another
stretcher. Demonstra-
tion of this scheme, called double-CPA, showed
the
ASE
reduced
to
at
least
10-
10
at
up to
25
ps before
the
pulse-
peak
[4].
In future higher power systems,
the
ASE con-
siderations will increase
the
necessary clean pulse energy.
Temporal cleaning using third-order non-linear optics
(X
3
)
requires an intensity of order
10
11
_1012W/cm2. In
the
case of a crystal for cross-polarization-wave gener-
ation
[5],
pulses > 1 ps exceed
the
ftuence damage
at
this intensity . Access
of
X
3
effects also brings
about
self-focusing effects which may degrade beam quality in
near-field application.
OPA, a second-order non-linear process
(X
2
)
in which
a
pump
wave amplifies a lower frequency signal, offers
an
alternative
using order G W / cm
2
. Optical
parametric
noise (OPN) generates an incoherent background analo-
gous
to
ASE with
the
distinction
that
its temporal du-
ration matches
that
of
the
pump. Using a
pump
laser of
<10
ps
duration
windowed
the
amplification of
the
"'ps
input
to
maintain high
contrast
[6].
In addition to am-
plification of a signal wave, OPA generates a
third
wave,
referred
to
as
the
idler. Most recently high gain
OPA
with chirp compensation generated ultra-broad band-
width idler for which
the
combination of
pump
duration
and
chirp imply
OPN
window
<1
ps
[7].
In this letter we show
that
idler generation from low
gain OPA, provides an insert able approach for improv-
1
ing temporal
contrast
. Working in
the
near-field, 250 J-d,
500
fs
pulses of 1053 nm are frequency doubled
and
sub-
sequently restored to original wavelength by
the
OPA
with>
10% efficiency.
Low
gain (2-20x) allows
thinner
crystals
and
minimizes bandwidth loss due to energy ve-
locity mismatch.
The
results agree with expected tem-
poral cubing
of
th
e
input
signal,
show stability on
par
with
that
of
the
input,
and
preserve spectral
width
and
spatial quality.
The
cubic relationship between
the
temporal profile
of
the
idler
and
signal becomes
apparent
from analytical
OPA
solution neglecting
pump
depletion with low gain.
Let
hIs,
Ip
refer
to
the
idler, signal
and
pump
intensi-
ties respectively. For
gz /2 < 1 where z refers to propa-
gation
length
and
9
oc
Ao
,one finds Ii
~
Iso(g z /2)2.
The
cubic dependence occurs if
the
pump
forms by dou-
bling a portion of
the
original signal giving
Ip
oc
Is
2.
The
non-linear restoration
of
the
original wavelength im-
proves
the
temporal
contrast
beyond
that
of
the
initial
up-conversion. Even with larger gain,
the
exponential
gain of
the
pump
will fall into this cubic approximation
in
the
temporal wings.
The
same
holds
for
saturation
ef-
fects. From Manley-Rowe,
the
idler energy matches
that
added
to
the
signal pulse. Assuming 50% doubling
ef-
ficiency due
to
Gaussian beam profile, picking off 10%
of
the
initial energy as signal corresponds
to
an unsat-
urated
gain of
",,3
for complete
pump
extraction. For
Type
I phase matching, a slightly non-collinear geome-
try
allows separation of
the
signal
and
idler. Using high
quality surfaces, free propagation
and
a well-collimated
beam minimizes
scatter
from
the
signal which would de-
grade
the
contrast.
Fig.
l(a)
presents
the
low-gain OPA pulse cleaning
scheme more clearly. In
the
final version, a
90/10
beam-
splitter divides near-transform limit 1053 nm, 500
fs
pulses of up to 250
j.LJ
.
The
larger portion doubles by
passing through 2 mm,
Type
I BBO with maximum
ef-
ficiency near 70%.
The
s-polarized 527 nm light
and
the
seed fundamental light then mix
at
""
4° in
an
identi-
cal BBO crystal
for
which an adjustable delay in
the
infra-red
path
allows temporal superposition.
The
3 mm
input
beam diameter satisfies
the
geometric
constraint
imposed by pulse
duration
and
separation
angle.
With
Ip
",,4
GW/cm
2
,
th
e
unsaturated
gain measures 11. To
demonstrate
th
e scaling in
the
un-saturated
regime,
data
using a 1
mm
crystal,
cut
for larger angle mixing,
is
also
presented. In
that
case,
the
pulses were only compressed
to
""
2 ps
and
th
e mixing occurred
at
""
10° with con-
verging beams
at
diameter""
1 mm.
The
scaling
data
are presented in Fig. 1 (b) with repre-
sentative measurement error bars. For
the
thinner
crys-
tal
the
measured efficiency fits a
quadratic
scaling for all
data
points with gain<
2.
For a cubic process
the
shot-
to-shot stability scales as 3x
that
of input. In
the
case of
the
2 mm crystal,
the
efficiency curve evidences
strong
saturation
which results in
1:1
relative stability between
input
and
output
. For
the
latter,
the
extraction
efficiency
of
the
pump
reaches
50
%
and
overall efficiency 15%.
0.01
--signal
--
--
idler
(a)
2'
0.01
'i:
::J
1E-4
.ci
....
~
~
1E-6
'iii
c:
Q)
'E
1E-S
1E-10
-15
-10 -5
o
5
delay (ps)
1.0
(b)
(j)
=e
O.S
::J
.e
0.6
~
~
0.4
'iii
--sig
c:
Q)
--
idler
'E
0.2
0.0
.......
""
____
.....
..........
1045 1050 1055 1060 1065
wavelength (nm)
Fig.
2.
(a) Temporal
contrast
measurement of
both
un-
amplified signal
and
idler with calculated cube of signal.
Inset shows larger temporal range: a, known artifact;
e,
etalon reflection;
u,
unknown peak, presumed to be ar-
tifact. (b) Measured spectra. shahFIG2.eps.
A commercial third-order auto-correlator (Del Mar
Photonics,
San Diego) measured
the
temporal
contrast
with (idler) and
without
cleaning (signal with
pump
blocked) using ",,30
j.LJ
. As a fiducial,
the
beam passed a
5 mm etalon prior
to
the
pulse cleaning setup.
The
in-
set
of Fig. 2(a) presents a 150 ps window, representative
of
longer scans
(1
ns maximum) .
The
first order etalon
reflection
at
50 ps with intensity
contrast
7 x
10-
4
seen
in the direct measure of
the
regenerative amplifier ap-
pears in
the
idler scan as a peak
of
4 x
10-
10
.
There
is
remarkable agreement with
the
anticipated cubic rela-
tion from which one expects etalon peak of 3 x
10-
10
in
the
cleaned pulse.
As
well,
the
ASE pedestal of
contrast
10-
8
and
pre-pulse
at
-50 ps in
the
un-amplified signal
fall below
the
near
10-
10
detection limit of
the
device. In
separate
experiments
we
have measured
scattering
from
signal beam to be of order
10-
4
into f /4 collection
at
ap-
proximately
the
same
separation
angle
[8].
Considering
th
e diffraction limited divergence,
the
collected scatte-
ring would reduce by an additional 10
6
.
The
net
impact
from
scattering
of a
10-
3
reflection would then occur
at
2
<
10-
13
and
that
of
the
ASE
at
10-
18
. Limitations
of
double-CPA would exceed these values. Several spikes
occur identically in
both
cleaned
and
original pulses.
Those
labeled a correspond
to
first
and
second order re-
flections
of
the
device filters.
The
origin
of
the
post
pulse
of
strength
6 x
10-
8
at
71
ps has not been identified.
The
central image
of
Fig. 2(a) shows a
temporal
zoom
of
the
inset
and
also includes
the
cubed
contrast
of
the
signal. Corrections for
saturation
do
not
improve
the
fit
of
the
cube: its
impact
is
small with respect
to
the
steep-
ness of
the
fall-off.
Though
the
temporal cubing implies
a
J3 reduction in
duration,
at
the
pulse peak,
the
du-
rations
appear
equal.
Saturation
and/or
group
velocity
mismatch (GVM) in
the
cleaning may cause this as cal-
culated
total
slip
of
the
fundamental
and
harmonic
is
270
fs
[9J
.
The
largest deviation from
the
fit occurs between
0.2
and
2.5 ps.
As
each measurement took on
the
order
of
30 minutes, known
drift
within
the
preceding
stretcher
and
compressor can result in differing pulse shapes
and
durations. Several
separate
measurements
supports
this
explanation.
The
spectra
of
both
signal
and
idler
are
shown in
Fig. 2(b). Sharpening in
the
temporal
domain
(for ex-
ample, cubing) corresponds
to
smoothing operations in
frequency. Indeed,
the
discontinuities present in
the
sig-
nal
spectrum
are
absent
in
the
spectrum
of
the
idler.
The
slight decrease (12%) in
the
spectral
width
brings
the
pulse closer to
the
transform
limit.
This
could also
mark
a limit imposed by
the
afore mentioned GVM. Us-
ing
partially
compressed pulses (in
the
1
mm
crystal
experiments)
we
have observed loss
of
bandwidth
well
matched by
the
theory presented for a X
3
process
[10J.
We find
optimal
stability,
width
and
spectral
shape
with
slight manipulation
of
pulse chirp from
that
set
by initial
pulse
duration
measurements, presumably this optimizes
compression in
the
mixing.
The
final characterization essential for laser-chain im-
plementation,
that
of near-field profile,
is
presented in
Fig.
3.
The
beams directly fall upon
the
camera
sensor
at
a distance ",1 m from
the
mixing. Image (a)
of
input
signal
and
(b)
of
idler clearly show
that
the
OPA
clean-
ing reduces the
beam
diameter,
and
the
plotted
line-
outs
(c) quantify
the
effect.
Cubing
of
the
input
inten-
sity (or steeper narrowing for larger gain) would occur
both
in time
and
space. Reduction by 1.5x indicates
sat-
uration
only slightly
fills
the
profile.
This
result
then
suggests
that
the
temporal pulse-width
stems
primarily
from
GVM.
Returning
to Fig. 3, a second key point is
the
well-maintained quality of
the
near-field profile. At
such low gains, some benefit may occur from
the
slight
separation
angle in the horizontal dimension as well as
the
Poynting
walk-off in
the
vertical dimension which
result in profile shearing on
the
order
160
and
100
J.Lm
respectively.
In
summary
we
presented
the
use
of
low gain
OPA
as
an
insertable approach for
temporal
contrast
improve-
ment.
The
measured
contrast
improvement agrees well
with expected
temporal
cubing,
and
we
find
better
than
(e)1.0
z.
.~
0.5
£
Q)
0.0 ......
,..:.,......,,....-r-P"'T--,-.r...,
-3 -2
-1
0 1 2 3
transverse coordinate (mm)
Fig. 3. Near-field
spatial
profile
of
(a) unamplified signal
and
(b) idler. shahFIG3.eps.
10% efficiency with 250
I.d input. Spectral width
and
quality have been preserved for
the
500
fs
pulses. Pulse
tilting [11,12],
or
alternatively chirp compensation, could
address
both
G VM limitations
and
geometric require-
ments
of
larger beam
aperture.
We hope
that
along
with
its
current
application within
an
intense-laser chain
[13],
our
presented findings will also benefit future systems
requiring high
temporal
contrast.
Authors acknowledge
support
of
U.S.
DOE
and
LANL
Laboratory
Directed Research
and
Development. R.S ac-
knowledges assistance of
Trident
and
P-24 staff.
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3