High Power Laser Science and Engineering, (2019), Vol. 7, e5, 9 pages.
© The Author(s) 2019. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/
licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
doi:10.1017/hpl.2018.67
Dual-wavelength bidirectional pumped high-power
Raman fiber laser
Zehui Wang, Qirong Xiao, Yusheng Huang, Jiading Tian, Dan Li, Ping Yan, and Mali Gong
State Key Laboratory of Precision Measurement Technology and Instruments & Key Laboratory of Photonics Control Technology of
the Ministry of Education, Tsinghua University, Beijing 100084, China
(Received 21 September 2018; revised 12 November 2018; accepted 21 November 2018)
Abstract
In this paper, we reported both the experimental demonstration and theoretical analysis of a Raman fiber laser based
on a master oscillator–power amplifier configuration. The Raman fiber laser adopted the dual-wavelength bidirectional
pumping configuration, utilizing 976 nm laser diodes and 1018 nm fiber lasers as the pump sources. A 60-m-long
25/400 µm ytterbium-doped fiber was used to convert the power from 1070 to 1124 nm, realizing a maximum power
output of 3.7 kW with a 3 dB spectral width of 6.8 nm. Moreover, we developed a multi-frequency model taking into
consideration the Raman gain spectrum and amplified spontaneous emission. The calculated spectral broadening of
both the forward and backward laser was in good agreement with the experimental results. Finally, a 1.5 kW, 1183 nm
second-order Raman fiber laser was further experimentally demonstrated by the addition of a 70-m-long germanium-
doped passive fiber.
Keywords: fiber laser; fiber optics amplifiers and oscillators; Raman laser
1. Introduction
Lasers with wavelengths ranging from 1100 to 1300 nm
are promising for applications such as remote sensing
[1]
,
spectral beam combining
[2]
, and Tm-
[3, 4]
and Ho
[4–7]
-doped
fiber laser pumping. In general, ytterbium-doped fiber lasers
(YDFLs) are considered the most promising candidates for
achieving high output powers based on the master oscil-
lator power amplifier (MOPA) configuration
[8]
. However,
traditional YDFLs can only efficiently operate within the
band of 1000–1100 nm. The small emission cross-section
of ytterbium and the amplified spontaneous emission (ASE)
may deter YDFLs from realizing high output powers when
the lasing wavelength exceeds 1100 nm
[9]
. Nevertheless, it
has become a trend to realize lasers of a wide range of
wavelengths by taking advantage of the stimulated Raman
scattering (SRS) effect with appropriate and available pump
sources. As the fiber power coupler replaces the wavelength
division multiplexer (WDM), the Raman fiber laser has made
significant progress in recent years.
In 2014, Zhang et al. proposed a 732-W integrated
Yb-Raman fiber amplifier with dual-wavelength seeds of
1080 and 1120 nm. The length of the 20/400 µm Yb-doped
Correspondence to: Q. Xiao, Department of Precision Instrument,
Tsinghua University, Beijing 100084, China.
Email: xiaoqirong08@gmail.com
fiber (YDF) in their amplifier was 45 m
[10]
. Subsequently, in
2015, a 1.5 kW Raman fiber laser was realized by using
the same experimental configuration
[11]
. In 2014, Zhang
et al. injected a seed laser at 1120 nm into a 1080 nm Yb-
doped fiber MOPA consisting of a 12 m YDF and 70 m
germanium-doped fiber (GDF) with the same parameter
of 20/400 µm. As a result, a 1.28 kW 1120 nm Raman
laser was realized
[12]
. In 2015, Ma et al. designed linearly
polarized output Yb-Raman cascaded oscillators consisting
of a 1120 nm Raman Stokes cavity and 1080 nm laser cavity.
They obtained a 1181 W, 1120 nm laser with an optical-
to-optical efficiency of 74.3% using a 21-m-long 20/400-
µm YDF
[13]
. It should be noted that all the approaches
mentioned above adopt the forward-pumped configuration,
in which the output laser is in the same direction as the
pump laser. Both theoretical
[14]
and experimental results
[15]
prove that the backward pumping Raman threshold should
be larger than that of the forward pumping. However, the
relatively small gap between the first- and second-order
Raman threshold limits the power scaling of the first-order
Raman wave. Thus, backward pumping or bidirectional
pumping is an effective and competitive method to address
the problem, which presents yet another challenge for the
backward fiber coupler.
In 2016, our group realized a bidirectional pumped Raman
fiber laser, which consisted of a single-wavelength seed and
1