COL 10(11), 111402(2012) CHINESE OPTICS LETTERS November 10, 2012
Dependence of stimulated Brillouin scattering in pulsed
fiber amplifier on signal linewidth, pulse duration, and
repetition rate
Rongtao Su (
JJJ
777
), Pu Zhou (
±±±
ÈÈÈ
)
∗
, Xiaolin Wang (
),
Hu Xiao (
mmm
), and Xiaojun Xu (
NNN
¡¡¡
)
∗∗
College of Optic-Electric Science and Engineering, National University of Defense Technology, Changsha 410073, China
∗
Corresponding author: zhoupu203@163.com;
∗∗
corresponding author: xuxj@21cn.com
Received March 16, 2012; accepted May 17, 2012; posted online September 14, 2012
The dependencies of stimulated Brillouin scattering (SBS) threshold in pulsed fiber amplifiers on spectral
linewidth, pulse duration, and repetition rate are measured and discussed. Experimental results show
that the SBS threshold is highly related to spectral linewidth and pulse duration. Therefore, the power
handling limitation in a pulsed fiber amplifier may be the average power in some cases and the peak power
in others.
OCIS codes: 140.3538, 060.4370, 040.3280.
doi: 10.3788/COL201210.111402.
High-power pulsed fiber lasers with narrow-linewidth
can be employed widely in several applica tions such as
LIDAR, remote sensing, nonlinear frequency gener ation,
coherent beam combining, and others
[1−3]
. However,
the output power of the fiber laser is mainly limited by
its nonlinearities such as stimulated Brillouin scattering
(SBS)
[4,5]
. SBS leads to backward-propagated stokes in
an optical be am. This is potentially destr uctive as it
can reduce output optical efficiency. SBS has the lowest
threshold among nonlinear effects in the fiber amplifier
of narrow linewidth systems
[5,6]
. Thus, this topic has
gained research interest due to its importance in fiber
amplifiers. The SBS threshold in continuous wave (CW)
fiber amplifiers is predictable, because it has been widely
researched. The SBS threshold is mainly dependent on
the lase r signa l linewidth, the effective ar e a of the mode,
and the length of the fiber, among others
[5,7−9]
. How-
ever, this dependency is more complicated in pulsed fiber
amplifiers
[1,6,10−13]
. It has been proposed that SBS can
be prevented by simply r educing pulse duration to be-
low 16 ns
[5,6]
. This assumption, however, has not been
proven exper imentally. The dependence o f SBS on sig-
nal linewidth for a CW-ope rated fiber laser
[14]
, to our
knowledge, has not yet be e n done for pulsed laser.
Notably, Sj¨oberg et al.
[10]
studied SBS dependence on
pulse using a Q-switched laser with dur ations ranging
from 20 to 60 ns. However, they used passive fiber in
that study. Thus, the prop e rty of SBS in a gain medium
could not be treated as equal. As mentioned above, it
is impor tant to study in detail the dependencies of SBS
in pulsed fiber amplifier on signal linewidth, pulse dura-
tion, and repetition rate.
In this letter, we conducted a detailed experimental
study of the SBS threshold in pulsed fiber amplifiers.
We obtained pulsed lasers with different pulse durations
and repetition rates as well as CW lasers with different
linewidths using electro-optic modulato r (EOM) and a
phase modulator (P-M), respectively. These laser s were
amplified in a single-mode fiber (SMF) amplifier, and
the SBS thresho lds were measured. The dependence of
the SBS threshold in pulsed fiber a mplifiers on spectral
linewidth, pulse duration, and repetition rate were dis-
cussed based on the measurements.
The experimental s e tup for SBS threshold measure-
ments is shown in Fig. 1. In our system, we used a
commercial CW seed at 1 064 nm with a linewidth of
∼20 kHz and an output power of ∼5 0 mW
[15]
. The
LiNbO
3
P-M was connected to the CW s e e d in order to
broaden the linewidth of the CW laser
[16]
. This process
enabled more than 20-mW power to be delivered from
the P-M because P-M loss was about 3 dB. The narrow
linewidth pulsed laser was achieved when the EOM was
connected to the CW s e ed
[17]
. The EOM had an optical
handling power of 100 mW and a high bandwith of >10
GHz. The average power of the pulsed laser after the
EOM was 0.3–2 mW due to EOM loss. We used a fiber
pre-amplifier (FPA1) to amplify the average power to
about 3–1 5 mW. Both the EOM and P-M were driven
by a function generator (FG).
FPA2 was used to amplify the pulsed/CW laser to
about 50 mW. A 99/1 coupler (coupler 1) was connected
to FPA2. Both pre-amplifiers were based on single-mode
Yb-doped fiber (SM YDF: NA=0.11, core diameter is 6
µm) with a length of 1 m. The YDFs were core-pumped
by SMF pigta iled at 976-nm laser diodes (LDs) via wave-
length division multiplexing (WDM). The pulse shape
and linewidth were detected using a photoelectric de-
tector (PD) and Fabry-Perot (F-P) interferometer from
Fig. 1. Experimental setup.
1671-7694/2012/111402(4) 111402-1
c
2012 Chinese Optics Letters