Revelation of the birth and extinction dynamics
of solitons in SWNT-mode-locked fiber lasers
YUDONG CUI
1
AND XUEMING LIU
1,2,3,
*
1
State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University,
Hangzhou 310027, China
2
Institute for Advanced Interdisciplinary Research, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
3
School of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan 411201, China
*Corresponding author: liuxueming72@yahoo.com
Received 16 November 2018; revised 8 January 2019; accepted 26 January 2019; posted 29 January 2019 (Doc. ID 352214);
published 13 March 2019
The dispersive Fourier transform (DFT) technique opens a fascinating pathway to explore ultrafast non-repetitive
events and has been employed to study the build-up process of mode-locked lasers. However, the shutting process
for the mode-locked fiber laser seems to be beyond the scope of researchers, and the starting dynamics under near-
zero dispersion remains unclear. Here, the complete evolution dynamics (from birth to extinction) of the conven-
tional soliton (CS), stretched pulse (SP), and dissipative soliton (DS) are investigated by using the DFT technique.
CS, SP, and DS fiber lasers mode locked by single-walled carbon nanotubes (SWNTs) are implemented via en-
gineering the intracavity dispersion map. The relaxation oscillation can always be observed before the formation
of stable pulse operation due to the inherent advantage of SWNT, but it exhibits distinct evolution dynamics in
the starting and shutting processes. The shutting processes are dependent on the dispersion condition and turn-off
time, which is against common sense. Some critical phenomena are also observed, including transient complex
spectrum broadening and frequency-shift interaction of SPs and picosecond pulses. These results will further
deepen understanding of the mode-locked fiber laser from a real-time point of view and are helpful for laser
design and applications.
© 2019 Chinese Laser Press
https://doi.org/10.1364/PRJ.7.000423
1. INTRODUCTION
Mode-locked fiber lasers delivering ultrafast pulses have been
employed in numerous applications as diverse as ophthal-
mology, micromachining, medical imaging, and precision
metrology [1–3] because fiber lasers have several inherent
advantages, e.g., high electrical efficiency, lower maintenance,
higher reliability, smaller footprint, and easier transportability
[4,5]. So far, fiber lasers have generally been implemented with
passively mode-locked techniques, such as nonlinear polariza-
tion rotation (NPR), nonlinear optical loop mirrors, semicon-
ductor saturable absorber mirrors (SESAMs), sin gle-walled
carbon nanotubes (SWNTs), and two-dimensional semicon-
ductor material s [6–11]. Among them, SWNTs are the most
promising candidate for ultrashort pulse generation because
they possess ultrafast excited state carrier dynamics and high
optical nonlinearit y [8,12].
To pursue higher pulse energy, researchers successively pro-
posed the conventional soliton (CS), stretched pulse (SP; also
named dispersion-managed soliton), and dissipative soliton
(DS) by engineering intracavity group velocity dispersion
(GVD) [7,13]. The dynamics of the mode-locked fiber lase r
and its output features depend mainly on the dispersion
map of a laser oscillator [13–16]. Soliton fiber lasers are gen-
erally constructed with an anomalous-dispersion condition, and
pulses can be sustained through the balance of nonlinear and
dispersive phase shifts [11]. So it is called a CS to distinguish it
from other types of solitons in mode-locked fiber lasers [17].
When the net dispersion is near zero, a SP forms, which can
decrease intracavity nonlinearity by periodically stretching and
compressing [14,17]. Under net-normal or all-normal cavity
dispersion, the pulse-shaping dynamics is dominated by gain
and loss with the assistance of dispersion, nonlinearity, and
spectral filtering [7]. DSs are always highly chirped, so that they
can hold higher pulse energy and nonlinear phase shift [18]. In
fact, each kind of pulse possesses unique features that are useful
for specific applications. CSs are easy to obtain and compatible
with transmission in single-mode fiber (SMF) [11]. SPs have a
wide spectral width and low phase noise, and have been em-
ployed in numerous application s [1,14]. The high-energy pulse
based on DSs greatly promotes the development of high-power
ultrafast lasers [7].
Research Article
Vol. 7, No. 4 / April 2019 / Photonics Research 423
2327-9125/19/040423-08 Journal © 2019 Chinese Laser Press
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