1.3 kW monolithic linearly polarized single-mode
master oscillator power amplifier and strategies
for mitigating mode instabilities
Rumao Tao,
1
Pengfei Ma,
1
Xiaolin Wang,
1,
* Pu Zhou,
1,2
and Zejin Liu
1
1
College of Optoelectric Science and Engineering, National University of Defense Technology,
Changsha, Hunan 410073, China
2
e-mail: zhoupu203@163.com
*Corresponding author: chinawxllin@163.com
Received January 12, 2015; revised March 9, 2015; accepted March 9, 2015;
posted March 13, 2015 (Doc. ID 232432); published April 17, 2015
We report on the high-power amplification of a 1064 nm linearly polarized laser in an all-fiber polarization-
maintained master oscillator power amplifier, which can operate at an output power level of 1.3 kW. The beam
quality (M
2
) was measured to be <1.2 at full power operation. The polarization extinction rate of the fiber amplifier
was measured to be above 94% before mode instabilities (MIs) set in, which reduced to about 90% after the onset of
MI. The power scaling capability of strategies for suppressing MI is analyzed based on a semianalytical model, the
theoretical results of which agree with the experimental results. It shows that mitigating MI by coiling the gain fiber
is an effective and practical method in standard double-cladding large mode area fiber, and, by tight coiling of the
gain fiber to the radius of 5.5 cm, the MI threshold can be increased to three times higher than that without coiling
or loose coiling. Experimental studies have been carried out to verify the idea, which has proved that MI was
suppressed successfully in the amplifier by proper coiling. © 2015 Chinese Laser Press
OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.2420) Fibers, polarization-maintaining;
(060.2430) Fibers, single-mode.
http://dx.doi.org/10.1364/PRJ.3.000086
1. INTRODUCTION
Many applications, such as coherent lidar systems, nonlinear
frequency conversion, and coherent beam combining archi-
tectures, require high-power linearly polarized laser sources
with near-diffraction-limited beam quality [
1–6]. Recently, a
linearly polarized fiber laser with 1 kW output power has been
achieved in a monolithic fiber Bragg grating (FBG)-based
Fabry–Perot cavity [
7], which employed a pair of high-power
FBGs. It is a technological challenge to design an FBG that
can withstand multikilowatt power, and further power scaling
may encounter some technological difficulties. Fiber laser
systems based on master oscillator power amplifiers (MOPAs)
are typically capable of reaching high output powers, while
also offering more flexibility in terms of linewidth and polari-
zation control than a simple grating-based laser [
8]. Most of
the high-power fiber laser systems with random polarized out-
put are based on MOPAs at the moment, which has achieved
output power as high as tens of kilowatts. However, power
scaling of linearly polarized MOPAs to multikilowatt level is
currently limited by the onset of mode instabilities (MIs)
[
8–10]. Although lots of work has been carried out to deal with
MI experimentally and theoretically [
11–27], few methods to
mitigate MI effectively in the all-fiber MOPA configuration
with standard step-index large mode area (LMA) fiber have
been proposed, and MI-free power scaling in an all-fiber
MOPA, which is based on standard step-index polarization-
maintaining (PM) LMA fibers, is even more challenging.
In this paper, we present a 1.3-kW-level all-fiber Yb-doped
PM fiber amplifier with linearly polarized operation and
near-diffraction-limited beam quality. We also discuss experi-
ments, coupled with numerical modeling, to estimate the fur-
ther power scaling capability of various strategies to mitigate
MI. Numerical modeling results suggest that MI-free single-
mode output powers in excess of 3 kW could be realized in
standard step-index LMA fiber.
2. EXPERIMENTAL SETUP AND RESULTS
A monolithic, all-fiber, Yb-doped PM fiber amplifier is shown
in Fig.
1. The seed laser in the experiment is a 50 mW linearly
polarized continuous-wave laser with central wavelength at
∼1064 nm, which was then amplified in two preamplifier
stages to ∼25 W. The main amplifier consists of a 20 m PM
double-clad LMA Yb-doped fiber (YDF) with 21 μm diameter/
0.064 NA core and 400 μm diameter/0.44 NA cladding, which is
coiled in a radius of 10 cm. Six multimode fiber-pigtailed
915 nm laser diodes were used to pump the gain fiber through
a 6 1 × 1 signal/pump combiner, which can provide a
maximum pump power of about 2 kW. An approximately
0.75 m long passive fiber is spliced to the gain fiber for power
delivery, the output end of which is angle cleaved in order to
prevent parasitic feedback from Fresnel reflection.
The achieved output power at different pump power levels
is shown in Fig.
2(a), which is measured after the output beam
passed through a dichroic mirror. The slope efficiency of the
amplifier is 65.3% with respect to launched pump power, and
the pump-limited maximum output power is 1261 W. The inset
in Fig.
2(a) shows the measured far-field beam profile at the
maximal operation power, and the beam quality factor M
2
is
86 Photon. Res. / Vol. 3, No. 3 / June 2015 Tao et al.
2327-9125/15/030086-08 © 2015 Chinese Laser Press