Low threshold Raman effect in high power
narrowband fiber amplifier
Man Hu (胡 曼)
1,2
, Weiwei Ke (柯伟伟)
3,4
, Yifeng Yang (杨依枫)
1
, Min Lei (雷 敏)
1
,
Kai Liu (刘 恺)
1
, Xiaolong Chen (陈晓龙)
1
, Chun Zhao (赵 纯)
1
, Yunfeng Qi (漆云凤)
1
,
Bing He (何 兵)
1,
*, Xiaojun Wang (王晓军)
3,4
, and Jun Zhou (周 军)
1,
**
1
Shanghai Key Laboratory of All Solid-State Laser and Applied Techniques, Shanghai Institute of
Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Institute of Applie d Physics and Computational Mathematics, Beijing 100088, China
4
Key Laboratory of Science and Technology on High Energy Laser, CAEP, Mianyang 621900, China
*Corresponding author: bryanho@siom.ac.cn; **corresponding author: junzhousd@siom.ac.cn
Received August 18, 2015; accepted November 6, 2015; posted online December 17, 2015
A low-threshold Raman effect in a kilowatt ytterbium-doped narrowband fiber amplifier system is reported. The
Raman Stokes light at 1120 nm is achieved with the total output power of only ∼400 W, indicating that the
Raman threshold of this kilowatt codirectional pumped continuous wave fiber amplifier is much lower than
the predicted value estimated by the classic formula. To figure out the mechanism of this phenomenon, sim-
ulations based on the general stimulated Raman scattering (SRS) model are analyzed indicating that the key
factor is the coupling between four-wave mixing (FWM) and SRS. The simulation results are in good agreement
with our experiments.
OCIS codes: 190.0190, 140.3510, 190.5650, 190.4380.
doi: 10.3788/COL201614.011901.
Due to high efficiency, compactness, and excellent beam
quality, fiber lasers have become one of the most popular
laser technologies that are widely used in various scientific
and industrial applications
[1–3]
, such as material process-
ing, remote sensing, free space communication, harmonic
generation, medicine, and defense. Current state of con-
tinuous wave (CW) fiber laser systems based on master
oscillator power amplifiers (MOPAs) emit several 10’sof
kilowatts of output power with nearly diffraction-limited
beam quality
[4]
, but further improvement is limited by
nonlinear effects, thermal effects, and modal instabilities.
Specifically, for narrowband fiber lasers with typical line-
widths of several 10’s of gigahertz, the most limiting of
effects is the stimulated Raman scattering (SRS) because
it normally presents the lowest onset threshold
[5]
. Once the
threshold is reached, the SRS in fibers steadily transfers
part of the signal energy into the undesirable Raman
Stokes waves at longer wavelengths, and hence degrades
the laser efficiency.
Generally speaking, for CW double-clad fiber lasers or
amplifiers with subnanometer linewidth, the output laser
power in the 20/400/0.06 fiber is estimated to be several
kilowatts to exceeds the Raman threshold using the classic
formula P
th
¼ 16A
eff
∕g
R
L
eff
as defined in Ref. [6], where
P
th
is the output power of the fiber laser that may induce
considerable nonlinear power transferring, A
eff
is the effec-
tive area of the laser wavelength in the fiber, g
R
is the
peak value of the gain spectrum of the SRS effect, and
L
eff
is the effective non linear interaction length. By apply-
ing the parameters (1064 nm LP
01
in 20/400/0.06 fiber),
L
eff
¼ 14 m, g
R
¼ 10
−13
m∕W, the calculated Raman
threshold power is 2.8 kW. The recent new formula
P
P
0
≈ ½ð16A
eff
∕g
R
L − P
signal
0
ÞL·α
signal
∕½α
signal
ðe
−ζ·L
−1Þ∕
ζ − ðe
−α
signal
L
− 1Þ , with η ¼ 0.01 (η is the ratio between
the Raman power and the signal at the output of the fiber
at the threshold) proposed by Jauregui
[7]
, where P
signal
0
is
the input signal power to the amplifier, α
signal
is the at-
tenuation factors of the fiber at the laser signal wave-
lengths, ζ is a constant related to the doping ions, and L
is the fiber length, provides the Raman threshold of about
1.5 kW. To the best of our knowledge, SRS generated in
a 20/400 ytterbium-doped narrowband fiber amplifier
with only hundreds of watts has been scarcely reported.
The current study reports on the strong SRS effect ob-
served in a 1.3 kW narrowband ytterbium-doped fiber am-
plifier (YDFA) system constructed with common 20/400/
0.06 fiber. The Stokes band increases at a low power level
of 400 W, which is sufficiently lower than the current
theory prediction. This phenomenon is due to the coupling
between four-wave mixing (FWM) and SRS. The coupling
greatly enhances the Raman gain (RG). Such a Raman
enhancement effect may lead to a significant improvement
in obtaining low threshold, high power Raman fiber
lasers.
The experimental setup contains an all-fiber narrow-
band master oscillator (MO) seed source based on fiber
Bragg gratings (FBGs) (called FBGs MO seed source) and
a three-stage YDFA, which is shown in Fig.
1. The MO
consists of a highly reflective (HR) FBG with more than
99% reflectivity and 0.18 nm bandwidth at 1064 nm, a
lowly reflective (LR) FBG with 19% reflectivity and
0.07 nm bandwidth, and a 4 m 10/125 μm large-mode area
COL 14(1), 011901(2016) CHINESE OPTICS LETTERS January 10, 2016
1671-7694/2016/011901(5) 011901-1 © 2016 Chinese Optics Letters