IEEE PHOTONICS JOURNAL, VOL. 13, NO. 4, AUGUST 2021 7200506
Design of Ring-Core Few-Mode-EDFA With the
Enhanced Saturation Input Signal Power
and Low Differential Modal Gain
Yihong Fang , Yan Zeng, Yuwen Qin ,OuXu , Jianping Li , and Songnian Fu
Abstract—We propose a design of ring-core few-mode fiber (RC-
FMF) with the erbium doping at the cladding region near the outer
ring edge. After the parameter optimization of the RC-FM-EDF,
the intensity overlapping difference between LP
01
and LP
11
modes
can be minimized, leading to a DMG of 0.22 dB for the corre-
sponding ring-core few-mode erbium-doped fiber amplifier (RC-
FM-EDFA). Meanwhile, in comparison with uniform core doping
design, the saturation input signal power of the RC-FM-EDFA can
be enhanced from −17.7 dBm and −16 dBm to −8.5 dBm for
LP
01
and LP
11
modes, respectively. Consequently, our proposed
RC-FM-EDFA is characterized by both low differential modal gain
(DMG) and the enhanced saturation input signal power. Those new
characteristics of the RC-FM-EDFA is ideally desired for future
agile networks.
Index Terms—Few-mode erbium-doped fiber amplifier (FM-
EDFA), ring-core fiber, differential modal gain (DMG), saturation
input signal power.
I. INTRODUCTION
W
ITH the rapidly increasing requirement of transmission
bandwidth, space-division-multiplexing (SDM) tech-
nique based on either few-mode fiber (FMF) or multi-core fiber
(MCF) have become a hot research topic around the world, in
order to overcome the capacity crunch of traditional single-core
single-mode fiber (SMF) [1]–[4]. To realize mode-division mul-
tiplexing (MDM) transmission over the long-haul FMF, inline
Manuscript received June 11, 2021; revised July 2, 2021; accepted July 2,
2021. Date of publication July 7, 2021; date of current version August 5,
2021. This work was supported in part by the National Key R&D Program
of China under Grant 2018YFB1800903, in part by the National Natural Sci-
ence Foundation of China under Grant 62022029, in part by the Guangdong
Introducing Innovative and Entrepreneurial Teams of “The Pearl River Talent
Recruitment Program” under Grant 2019ZT08X340, and in part by the Research
and Development Plan in Key Areas of Guangdong Province under Grant
2018B010114002. (Corresponding author: Yuwen Qin.)
Yihong Fang, Yan Zeng, Ou Xu, Jianping Li, and Songnian Fu are with
the Advanced Institute of Photonics Technology, School of Information En-
gineering, Guangdong University of Technology, Guangzhou, Guangdong
510006, China, and also with the Guangdong Provincial Key Laboratory
of Information Photonics Technology, Guangdong University of Technol-
ogy, Guangzhou, Guangdong 510006, China (e-mail: fongyathong@yeah.net;
2112003001@mail2.gdut.edu.cn; xuou@gdut.edu.cn; jianping@gdut.edu.cn;
songnian@gdut.edu.cn).
Yuwen Qin is with the Advanced Institute of Photonics Technology, School
of Information Engineering, Guangdong University of Technology, Guangzhou,
Guangdong 510006, China, with the Guangdong Provincial Key Labora-
tory of Information Photonics Technology, Guangdong University of Tech-
nology, Guangzhou, Guangdong 510006, China, and also with the Synergy
Innovation Institute of GDUT, Heyuan, Guangdong 517000, China (e-mail:
qinyw@gdut.edu.cn).
Digital Object Identifier 10.1109/JPHOT.2021.3095123
optical amplifiers are indispensable. It has been reported that
few-mode erbium-doped fiber amplifiers (FM-EDFAs) can be
applied for the long-haul FMF transmission with good perfor-
mances [5]–[21]. One of the most important FM-EDFA char-
acteristics is the differential modal gain (DMG), which needs
to be carefully mitigated, for the successful implementation of
multiple-input multiple-output (MIMO) signal processing at the
receiver side. Since it is revealed that the DMG is determined
by the intensity overlapping between the guided mode and the
pump mode, together with the erbium doping distribution [16],
the existing schemes to mitigate the DMG can be generally
divided into three categories. First, the mode profile of the pump
laser can be optimized, and the cladding pumping becomes a
popular choice because it can provide more uniform pump power
distribution over the optical fiber cross section [5]. Secondly, the
optimization of the erbium doping distribution is helpful [10]–
[12], including the convenient step-index FMF with the ring
doping [13] and the doping combination at both ring and cen-
tral areas [14]. Finally, the corresponding refractive index (RI)
profile can be designed in order to manipulate various intensity
profiles of guided modes [21]. Since various mode profiles are
concentrated for the ring-core erbium-doped fiber (RC-EDF),
DMG is expected to reduce to a low level. A RC-FM-EDFA has
been numerically investigated, by comparing the performance
between the core and the cladding pumping [17]. Moreover,
small DMG of 1.6 dB has been achieved for the two-mode RC-
EDFA [18], and a procedure of the RC-FM-EDFA design has
been reported to achieve a DMG of around 1 dB [19]. Moreover,
a RC-FM-EDFA is numerically investigated to obtain an DMG
of less than 0.6 dB by the use of extra annulus doping [20],
and the DMG suppression can be realized via additional trench
configuration of the RC-FM-EDF [21]. Meanwhile, there occurs
another headache for the gain saturation effect arising in the
EDFA, due to the small saturation input signal power. Therefore,
the total input power must set less than the saturation input
signal power, in order to secure the linear optical amplification.
Researchers are focused on optimizing the erbium doping profile
to reduce the gain compression in a s ingle-mode EDFA, and a
single-mode EDFA with 4 dB gain compression is demonstrated
under a cladding pumping power of 25W [22].
In this paper, we design a RC-FM-EDF and investigate the
characteristic of corresponding optical amplifier, under the con-
dition of the cladding pumping. The DMG between LP
01
and
LP
11
modes can be reduced to 0.22 dB, due to the parameter
This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/
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