948 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 10, MAY 15, 2013
Generation of Soliton Molecules in a
Normal-Dispersion Fiber Laser
Junsong Peng, Li Zhan, Shouyu Luo, and Qishun S. Shen
Abstract—Though normal-dispersion mode-locked fiber lasers
generally work on a single-pulse regime due to the large chirp
of the pulses, we observe bound states of one kind of dissipative
solitons in the laser. The separation between the bounded pulses
in such soliton molecules increases with the pump power, and
also varies with the settings of polarization controllers, which is
consistent with other kinds of bounded pulses such as solitons.
Our study indicates that the bound state of pulses is an intrinsic
feature of mode-locked lasers and is independent of pulse profiles,
stimulated by overdriving of mode-locking mechanisms.
Index Terms— Dissipative soliton, ultrafast, mode-locking.
I. INTRODUCTION
M
ULTIPLE pulses can be formed as a result of over-
driving of mode-locking mechanisms in ultrashort fiber
lasers [1]. When multiple pulses are closely spaced, they can
be termed as soliton molecules or bound states. There are
several kinds of DSs observed in mode-locked fiber lasers
including square solitons [2]–[4], gain-guided solitons [5] and
so on. Among these, one kind of pulse with a special spectrum
which has a dip in the top and is steep at the edges has
drawn much attention. The pulse was firstly reported by A.
Chong et al [6]. It is highly chirped in the oscillator with a
large energy, which can be compressed to sub-100 fs outside
the oscillator [7]. Benefiting from large chirp, this kind of
DS can contain much larger energy than the ones of solitons
and stretched-pulses. However, pulse peak power is clamped
by mode-locking mechanisms [1]. Thus, it is expected that
soliton molecules involving this kind of DSs should also exist.
Recently, the pulse pairs were observed in a DS laser, but
the two pulses are not bounded [8]. In addition, bound states
of gain-guided solitons [5] and parabolic pulses [9] were
observed. Temporal dynamics evolution of soliton molecules
was studied in the normal-dispersion regime [10].
Here, we report for the first time, soliton molecules involv-
ing more than two pulses in a normal-dispersion fiber laser.
Manuscript received January 17, 2013; revised March 24, 2013; accepted
April 6, 2013. Date of publication April 12, 2013; date of current version
April 25, 2013. This work was supported in part by the National Natural
Science Foundation of China under Grants 10874118 and 11274231, in part
by the Key Project of the Ministry of Education of China under Grant 109061,
and in part by the Eagle Project of the Academy of Opto-Electronics, Chinese
Academy of Science. (Corresponding author: L. Zhan.)
The authors are with the Department of Physics and Key Laboratory for
Laser Plasmas, State Key Lab of Advanced Optical Communication Systems
and Networks, Shanghai Jiao Tong University, Shanghai 200240, China
(e-mail: lizhan@sjtu.edu.cn).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2013.2257720
PDI
Output
EDF
PC2PC1
Pump 2
Pump 1
OC
WDM
WDM
Fig. 1. Experimental configuration of the laser.
In contrast to previous studies in which soliton molecules
can only involve two pulses in the normal-dispersion regime
[5], [9], [10], up to five pulses are bounded in our study.
Soliton molecules involving more than two pulses have been
reported in dispersion-managed fiber lasers [11], [12]. Our
work gives the counterpart in the normal-dispersion regime.
The separation between the bounded pulses increases with
the pump power, and varies with the settings of polarization
controllers, which is consistent with other kinds of bounded
pulses such as solitons [13], [14], stretched-pulses [15] and
similaritons [16]. Our study implies that though DSs can
contain high energy, nonlinearity management is crucial to
further increase single-pulse energy.
II. E
XPERIMENTAL SETUP AND PRINCIPLES
DSs exist in all-normal-dispersion cavities, and also can be
found in dispersion-managed cavities with large net normal
dispersion. We construct an Er-doped fiber (EDF) ring laser
with strong net normal dispersion as shown in Fig. 1. The
ring cavity is made of a 250 cm EDF with group veloc-
ity delay (GVD) of −51ps/(nm·km) at 1550 nm (80 dB/m
absorption ratio at 1530 nm), which is bidirectionally pumped
by two 976 nm laser diodes through the wavelength division
multiplexers (WDMs). Note that normal dispersion of the
EDF is crucial for DS generation. A polarization dependent
isolator (PDI) sandwiched with two PCs (PC1 and PC2)
is used as the mode-locking component in the cavity. The
10:90 optical coupler (OC) is used to output the laser. For
reducing anomalous dispersion to guarantee strong net normal
dispersion, the OC is made of dispersion-shifted fiber (DSF),
as is the fiber wrapped on the two PCs, and the two WDMs
are made of Nufern 980 fiber. The GVD parameters of the
fibers are 7(DSF), and 4.5(Nufern 980 fiber) ps/(nm·km) at
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