516 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 4, APRIL 2012
Enhancement of Amplified Spontaneous Emission
Contrast With a Novel Front-End Based on
NOPAandSHGProcesses
Yi Xu, Yansui Huang, Yanyan Li, Jianzhou Wang, Xiaoming Lu, Yuxin Leng, Ruxin Li, and Zhizhan Xu
Abstract—Employing a novel front-end based on femtosec-
ond noncollinear optical-parametric amplification and second-
harmonic generation processes in a Ti:sapphire chirped pulse
amplification laser system, we first demonstrate the efficient
enhancement of amplified spontaneous emission (ASE) contrast.
Cleaned seed pulses of 100-µJ energy generated by the front-end
are amplified to 0.75 J before compressor. After compression
and under pulse peak power of 7 TW, the ASE contrast
is promoted from original ∼10
8
to ∼10
11
. The experimental
results for nanometer film target interactions also demonstrate
the improvement of the contrast. Finally, the evolution of the
ASE contrast under different seed pulse energy is demonstrated
experimentally, which can be used to design the higher power
laser with higher ASE contrast.
Index Terms— Amplified spontaneous emission contrast,
chirped pulse amplification, pulse-cleaning, seed pulse energy.
I. INTRODUCTION
T
HE development of femtosecond Ti:sapphire chirped
pulse amplification (CPA) laser system has made it pos-
sible for laser focused intensity exceeds 10
20
W/cm
2
[1-5].
Such ultra-high intensities create many exciting opportunities
for laser-matter interactions in relativistic dominated regime
[6-8]. Laser pulse temporal contrast, defined as the ratio of the
peak intensity of the main pulse to the prepulse intensity, must
be at least 10
10
to restrict destructive pre-plasma dynamics [9].
For laser plasma interaction on some special targets, such as
nanometer film targets, higher contrast is required. However,
a high-intensity Ti:sapphire CPA laser system generates not
only a femtosecond pulse but also an amplified spontaneous
emission (ASE) nanosecond background as well as prepulses.
Prepulses introduced by the incomplete compensation of
higher-order dispersion are generally not detrimental, because
Manuscript received November 15, 2011; revised January 19, 2012;
accepted January 29, 2012. Date of publication February 6, 2012; date of
current version February 17, 2012. This work was supported in part by
the Chinese Academy of Science, the National Natural Science Foundation
of China under Grant 10734080, Grant 60921004, Grant 60908008, Grant
61078037, and Grant 11134010, the National Basic Research Program of
China under Grant 2011CB808101, the International S&T Cooperation Pro-
gram of China under Grant 2011DFA11300, and the Shanghai Commission
of Science and Technology under Grant 09QA1406500.
The authors are with the State Key Laboratory of High Field
Laser Physics, Shanghai Institute of Optics and Fine Mechanics,
Chinese Academy of Sciences, Shanghai 201800, China (e-mail:
xuyi@siom.ac.cn; hyswater@163.com; yyli@siom.ac.cn;
wjzno127@163.com; xiaominglu@siom.ac.cn; lengyuxin@siom.ac.cn;
ruxinli@mail.shcnc.ac.cn; zzxu@mail.shcnc.ac.cn).
Digital Object Identifier 10.1109/JQE.2012.2187045
the arrival time scale is tens of picoseconds prior to the peak of
the main pulse, resulting in insufficient time for the preplasma
expansion to affect the laser-matter interaction dynamics.
Prepulses introduced by the Fresnel reflections or the limited
extinction ratio of polarized elements in the amplifiers can
generally be eliminated by better anti-reflection coating and
additional Pockels cells [10]. Then the main cause of contrast
problem is the amplified spontaneous emission (ASE) in
nanosecond window. Correspondingly, ASE contrast (referred
as the ratio of the peak intensity of the main pulse to the ASE
background intensity) has become one of the crucial problem
in the development of ultra-intense and ultra-fast laser systems.
Due to the limited energy and contrast, seed pulses gener-
ated by Ti:sapphire oscillator can not support the suppression
of ASE and the development of high contrast ultra-intense,
ultra-fast laser systems. Recently, several pulse-cleaning tech-
niques, based on generating high-energy cleaned seed pulses,
have been reported and applied to develop high contrast
Ti:sapphire CPA laser. In 2005, based on cross-polarized wave
(XPW) generation method, cleaned pulses with energy of
120 μJ and temporal contrast of 10
10
was reported [11].
In 2006, by applying a modified XPW method in a CPA laser,
temporal contrast of 10
11
in a 50 TW laser was reported [12].
In 2009, employing saturable absorber as pulse cleaner and an
optical parametric chirped-pulse amplifier (OPCPA) as pre-
amplifier, temporal contrast greater than 10
10
in a 60 TW
laser system was reported [13]. Later in 2010, the similar
technique was applied in a 500 TW laser system successfully
[14]. Beside of saturable absorber and XPW techniques,
optical-parametric amplification (OPA) has also been regarded
as an efficient technique for temporal contrast enhancement
[15-17]. In 2011, A novel pulse-cleaning scheme, combining
white light continuum (WLC) generation, noncollinear optical-
parametric amplification (NOPA) and second-harmonic gen-
eration(SHG), was applied to generate high energy (0.5 mJ)
and high temporal contrast (better than 10
10
) femtosecond
pulse near 800 nm [18]. Thanks to these nonlinear processes -
especially the SHG process, the temporal contrast of the
cleaned pulse can be expected to reach a higher level than
many other techniques [18]. Meanwhile, the high energy
of the cleaned pulse is sufficient for the ASE suppression.
Moreover, because of the high stability of the commercial
1 kHz CPA laser, the energy and duration fluctuation of the
cleaned pulses are very small. Owning these advantages, the
pulse cleaner based on this technique seems to be a good front-
0018–9197/$31.00 © 2012 IEEE
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