Soliton and bound-state soliton mode-locked fiber
laser based on a MoS
2
/fluorine mica
Langmuir–Blodgett film saturable absorber
RUIDONG LÜ,
1
YONGGANG WANG,
1,
*JIANG WANG,
1
WEI REN,
2
LU LI,
2
SICONG LIU,
1
ZHENDONG CHEN,
1
YONGFANG LI,
1
HONGYING WANG,
3
AND FUXING FU
3
1
School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710119, China
2
School of Science, Xi’an Institute of Posts and Telecommunications, Xi’an 710121, China
3
Key Laboratory for Surface Engineering and Remanufacturing of Shaanxi Province, Xi’an University, Xi’an 710065, China
*Corresponding author: chinawygxjw@snnu.edu.cn
Received 4 September 2018; revised 7 December 2018; accepted 27 January 2019; posted 29 January 2019 (Doc. ID 345011);
published 14 March 2019
In this article, we report on an experimentally generated soliton and bound-state soliton passively mode-locked
erbium-doped fiber laser by incorporating a saturable absorber (SA) made of MoS
2
∕fluorine mica (FM) that was
fabricated with the Langmuir–Blodgett (LB) method. The FM substrate is 20 μm thick and easy to bend or cut,
like a polymer. However, it has a higher damage threshold and a better thermal dissipation than polymers.
In addition, the LB method can be used to fabricate a thin film with good uniformity. In this study, the modu-
lation depth, saturable intensity, and unsaturated loss of the SA are measured as 5.9%, 57.69 MW∕cm
2
, and
13.4%, respectively. Based on the SA, a soliton mode-lock ed laser is achieved. The pulse duration, repetition
rate, and signal-to-noise ratio are 581 fs, 15.67 MHz, and 65 dB, respectively. By adjusting the polarization
controller and pump power, we obtain a bound-state soliton mode-locked pulse. The temporal interval between
the two solitons forming the bound-state pulse is 2.7 ps. The repetition rate of the bound-state pulses is propor-
tional to the pump power . The maximum repetition rate is 517 MHz, corresponding to the 33rd harmonic of the
fundamental repetition rate. The results indicate that the MoS
2
∕FM LB film absorber is a promising photonic
device in ultrafast fiber lasers.
© 2019 Chinese Laser Press
https://doi.org/10.1364/PRJ.7.000431
1. INTRODUCTION
Passively mode-locked fiber lasers have many widespread appli-
cations in the field of nonlinear optics, optical fiber communi-
cation, material processing, and optical sensing because of their
good stability, high beam quality, and compact design [1–3].
In recent decades, the generation of passively mode-locked
pulse lasers mainly depended on nonlinear optical loop mirrors
(NOLMs) [4], nonlinear polarization r otation (NPR) [5], and
saturable absorbers (SAs) [6,7]. However, the NOLM and NPR
techniques are susceptible to the surrounding environment and
usually need a relatively high pump power for the mode-locked
operation, thus limiting their practical applications [8,9]. As a
type of SA, semicon ductor SA mirrors (SESAMs) are considered
as practical absorbers and have been widely used in commercial
laser systems [10,11]. However, SESAMs have some disadvan-
tages such as high cost, a complex fabrication process, narrow
waveband, and long response time [12]. Two-dimensional (2D)
materials such as graphene [13,14], topologic insulators (TIs)
[15,16], transition-metal dichalcogenides ( TMDs) [17– 23],
black phosphorus [24,25], perovskite [26], MXene [27 ], and
antimonene [28] have great application potential in nonlinear
optics and ultrafast photonics. Compared with SESAMs, 2D
nonlinear materials have some advantages, such as broadband
saturable absorption, ultrafast recovery time, and a high nonlin-
ear optical absorption coefficient.
Recent reports have shown that the band gap width of
TMDs can be changed by manipulating their thickness or
introducing atomic defects, thus obtaining a series of excellent
properties [29–31]. Molybdenum disulfide (MoS
2
) is an atypi-
cal TMD material. Bulk MoS
2
is an indirect semiconductor
with a band gap of 1.29 eV, whereas monolayer MoS
2
is a
direct semiconductor with a band gap of 1.8 eV [32,33].
Few-layer MoS
2
with semiconducting properties can be excited
by absorbing one photon with a higher energy than the band
gap. At high excitation intensity, electrons in the valence band
are transferred to fill the conduction band [34]. Similar to the
zero-band gap of graphene and the surface state of TIs,
few-layer MoS
2
of the semiconductor phase can also exhibit
broadband saturable absorption with the help of the Pauli
Research Article
Vol. 7, No. 4 / April 2019 / Photonics Research 431
2327-9125/19/040431-06 Journal © 2019 Chinese Laser Press