COL 10(1), 010602(2012) CHINESE OPTICS LETTERS January 10, 2012
Automatic polarization compensation method for
low-repetition frequency short optical pulse
Peng Zhang (张张张 鹏鹏鹏)
∗
, Xuechun Li (李李李学学学春春春), Youen Jiang (姜姜姜有有有恩恩恩), and Guoyang Li (李李李国国国扬扬扬)
National Laboratory of High Power Laser Physics, Shanghai Institute of Optics and Fine Mechanics,
Chinese Academy of Sciences, Shanghai 201800, China
∗
Corresp onding author: darrell242@gmail.com
Received March 14, 2011; accepted June 10, 2011; posted online August 24, 2011
An automatic polarization compensation method for low-repetition frequency short optical pulse is pro-
p osed and successfully applied to the master oscillator room (MOR) in inertial confinement fusion (ICF)
systems to maintain the MOR maximum output energy. After an average of 37 shots, the MOR output
energy reaches maximum value with the sudden occurrence of polarization variation in the fibers. The
p eak-to-peak amplitude jitter of the MOR output is 9.52% at 4 h, which meets the requirement of the ICF
system.
OCIS codes: 060.2420, 060.2430, 250.3140.
doi: 10.3788/COL201210.010602.
Controlling the state of polarization (SOP) is impor-
tant for fiber systems, such as communication systems
[1]
,
fiber sensors
[2,3]
, fiber lasers
[4]
, and pulse shaping sys-
tem in inertial confinement fusion (ICF) systems
[5]
. A
number of electrooptic polarization control methods and
experiments have been reported, most of which have ei-
ther been used for or focused on controlling the SOP of
CW or high-repetition frequency pulse train
[1,3,6−10]
. For
CW and high-repetition frequency pulse train, energy can
be directly acquired by an A/D converter after energy
is received by a photodetector (PD). However, in other
fiber systems, such as Brillouin optical time domain re-
flectometry (BOTDR) sensors
[2]
, Q-switched fiber lasers,
and the ICF pulse shaping systems, propagation light is
a low-repetition frequency short pulse train. Thus, en-
ergy acquisition of the pulse is difficult, which hinders
the implementation of p olarization control.
The master oscillator room (MOR) is one of the most
important parts of an ICF system
[5,11−14]
. The MOR
creates the temporal pulse shape specified by physical
experiments. The pulse from the MOR is injected into
a regenerative amplifier (RA). The MOR consists of lots
of regular single-mode fibers and polarization sensitive
devices, such as integrated waveguide electro-optic mod-
ulators (EOM). The control of SOP is particularly impor-
tant because slow variations in the environment (temper-
ature and pressure) can change the SOP of light in fibers
and eventually reduce output energy of the MOR. After
a long period of operation without polarization control,
the output energy of the MOR will be reduced by more
than 50%.
In this letter, an automatic polarization compensation
method for low-rep etition frequency short pulse is de-
scribed. The repetition frequency of the pulse ranges
from 1 Hz to 10 kHz, and the pulsewidth ranges from
100 ps to 1 µs. This method is successfully applied to
the MOR. Through an integral circuit, the pulsewidth
of the short electrical pulse converted from the optical
pulse can be broadened to tens of microseconds. This
allows acquisition of the energy of the optical pulse by a
high speed acquisition card. Finally, by comparing the
current pulse energy with the previous pulse energy, the
pulse polarization is compensated and the stability of the
MOR output is maintained.
Figure 1 shows the configuration of the MOR. The
pulse begins in a CW Yb-doped fiber laser tuned to 1.064
µm. The CW signal from the output of the oscillator is
chopp ed by an acousto-optic (AO) modulator to a 200
ns pulse at 1 Hz. The pulse is amplified by an Yb-doped
fiber amplifier, and subsequently goes into a two-stage
amplitude modulator, which shortens the pulsewidth to
8 ns. Finally, the pulse is amplified by another Yb-doped
fiber amplifier. After passing through a 90/10 coupler,
the pulse is injected into a RA through the 90% port.
The MOR output energy is 2 nJ.
The oscillator, amplifiers, transmission fibers, and
fiber optic jumpers between devices are made up of
single-mode fibers. The SOP of the pulse in the MOR
gradually varies with the environment. The amplitude
modulator in the MOR contains a polarizer. Therefore,
the variation of SOP will lead to a fluctuation in the
MOR output and will eventually reduce the output en-
ergy of the RA.
Figure 2 shows the normalized amplitude of the MOR
output without polarization control as a function of time
at 3 h. The amplitude has dropped by more than 70%
at 1.5 h. The periodic fluctuation of the output curve is
caused by the inherent periodic fluctuation of the fiber
laser output. The ICF system requires that the long-
time peak-to-peak variation of the MOR output energy
should be less than 20%, which can lead to an amplitude
variation of less than 3% in the RA output.
Fig. 1. MOR configuration in an ICF system.
1671-7694/2012/010602(4) 010602-1
c
° 2012 Chinese Optics Letters
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