Generation and measurement of complex laser pulse
shapes in the SG-III laser facility
Dongxia Hu (胡东霞), Jun Dong (董 军), Dangpeng Xu (许党朋), Xiaoxia Huang (黄小霞),
Wei Zhou (周 伟), Xiaocheng Tian (田小程), Dandan Zhou ( 周丹丹),
Huaiwen Guo (郭怀文), Wei Zhong (钟 伟), Xuewei Deng (邓学伟)*,
Qihua Zhu (朱启华), and Wanguo Zheng (郑万国)
Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
*Corresponding author: xwdeng@caep.cn
Received December 5, 2014; accepted February 12, 2015; posted online March 13, 2015
The generation and measurement of complex ultraviolet laser pulse shapes is demonstrated in the SG-III laser
facility. Relatively high contrast ratio of 300∶1 required by the physics experiment is achieved and successfully
measured. Two continuous main shots validate the reproduction and the stability of the pulse shape, which
provide solid foundation for precise physics experiment and laser power balance.
OCIS codes: 140.3300, 350.2660, 350.4600.
doi: 10.3788/COL201513.041406.
Laser driven inertial confinement fusion (ICF) research
requires different types of laser temporal shapes
[1–4]
and
spatial distributions
[5]
. Some of the temporal shapes are
very complicated in order to produce sequential shock
waves that compress the target pellet and confine the filled
fuel
[6]
. The precision of these temporal shapes is a key com-
ponent in meeting the experim ental goals so that the tech-
nique of generating a precisely shaped laser pulse is always
a key issue in the field of high power laser systems for the
ICF
[7,8]
. There have been several huge laser drivers built or
under construction around the world, such as the NIF
[8]
in the U.S., the LMJ
[9]
in France, and the SG-III laser
facility
[10]
in China. These laser drivers adopted similar
techniques in the laser master oscillator room (MOR), also
called the front-end syste m, to generate shaped laser
pulses by using an arbitrary waveform generator (AWG).
It's reported that, the NIF was capable of producing a
shaped ultraviolet laser pulse with a contrast ratio of
150∶1 in 2007
[8]
, and this capability was upgraded leading
to an increased contrast ratio of 275∶1 in 2011
[11]
. Such a
tuning range of laser pulses provides flexibility for differ-
ent requirements of physics experiments.
The SG-III laser facility is currently the largest laser
driver for ICF research in China. It has 48 laser beams
and is designed to deliver 180 kJ of ultraviolet laser energy
onto the target in 3 ns. The SG-III laser facility is still
under construction at present, and is expected to achieve
its full output capability at the end of 2015. During its
early stage of construction and adjustment, the most used
temporal pulse shape is rectangular with a 3 ns pulse
duration. With the progress of the construction and the
physics experiment, it is more and more urgent to test
SG-III’s capability of generating complex laser pulse
shapes as well as to demonstrate the correctness of the
technical pathways of the front-end system. There has
been early work in generating shaped laser pulses in pre-
vious laser facilities, such as the SG-II laser facility
[12,13]
and
the SG-III prototype
[14]
. In this Letter, we report the latest
pulse shaping experiment results in the SG-III laser fa-
cility. The capability of generating and measuring
complex pulse shapes demanded by physics experiments
is demonstrated. At a limited ultraviolet laser energy level
of about 1.9 kJ, the required 300∶1 contrast ratio is
achieved and successfully measured. By two continuous
main shots, the reproduction of the pulse shape is
validated, which is very important for providing a stable
experiment condition and achieving power balance in the
SG-III facility.
The front-end system in the SG-III facility is an all-fiber
laser system
[15]
. A cw laser generated by a distributed feed-
back (DFB) fiber laser is then modulated to a pulse chain
by an electro-optics modulator that is driven by an AWG.
One AWG shapes 16 pulse waveforms and, thus in total,
three AWGs provide all 48 of the waveforms for the SG-III
laser facility. Each of the SG-III’s 48 beam lines is capable
of generating a unique temporal pulse shape, so it is very
convenient to compensate for the laser amplification dif-
ference
[16]
by adjusting the 48 waveforms separately. When
generating a shaped laser pulse, for example a 3 ns square
pulse, the AWG sums the output of 30 electric impulse
generators, each having 150 ps pulse width and 100 ps
temporal separation. By adjusting the amplitudes of these
30 electric Gaussian impulse generators to the same level,
we can easily get a summation result of a square pulse.
Other pulse shapes are generated in the same way by
adjusting the impulse generators’ amplitudes. Figure
1
shows the schematics of how a shaped laser pulse is gen-
erated in the front-end system of the SG-III facility.
The laser pulse shape required by the physics experiment
refers to the ultraviolet pulse shape after frequency conver-
sion in the final optics assembly (FOA). Figure
2 explains
the whole process for achieving the required laser pulse
shape. Once the required ultraviolet pulse shape is set,
it is first transferred to a corresponding fundamental pulse
COL 13(4), 041406(2015) CHINESE OPTICS LETTERS April 10, 2015
1671-7694/2015/041406(4) 041406-1 © 2015 Chinese Optics Letters