Real-time observation of vortex mode switching
in a narrow-linewidth mode-locked fiber laser
JIAFENG LU,
1
FAN SHI,
1
LINGHAO MENG,
1
LONGKUN ZHANG,
1
LINPING TENG,
1
ZHENGQIAN LUO,
2
PEIGUANG YAN,
3
FUFEI PANG,
1,4
AND XIANGLONG ZENG
1,
*
1
Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Joint International Research Laboratory of Specialty Fiber Optics and
Advanced Communication, Shanghai University, Shanghai 200444, China
2
Department of Electronic Engineering, School of Information Science and Engineering, Xiamen University, Xiamen 361005, China
3
Shenzhen Key Laboratory of Laser Engineering, Shenzhen University, Shenzhen 518060, China
4
e-mail: ffpang@shu.edu.cn
*Corresponding author: zenglong@shu.edu.cn
Received 6 January 2020; revised 28 April 2020; accepted 25 May 2020; posted 26 May 2020 (Doc. ID 386954); published 1 July 2020
Temporal and spatial resonant modes are always possessed in physical systems with energy oscillation. In ultrafast
fiber lasers, enormous progress has been made toward controlling the interactions of many longitudinal modes,
which results in temporally mode-locked pulses. Recently, optical vortex beams have been extensively investigated
due to their quantized orbital angular momentum, spatially donut-like intensity, and spiral phase front. In this
paper, we have demonstrated the first to our knowledge observation of optical vortex mode switching and their
corresponding pulse evolution dynamics in a narrow-linewidth mode-locked fiber laser. The spatial mode switch-
ing is achieved by incorporating a dual-resonant acousto-optic mode converter in the vortex mode-locked fiber
laser. The vortex mode-switching dynamics have four stages, including quiet-down, relaxation oscillation, quasi
mode-locking, and energy recovery prior to the stable mode-locking of another vortex mode. The evolution
dynamics of the wavelength shifting during the switching process are observed via the time-stretch dispersion
Fourier transform method. The spatial mode competition through optical nonlinearity induces energy fluctuation
on the time scale of ultrashort pulses, wh ich plays an essential role in the mode-switching dynamic process.
The results have great implications in the study of spatial mode-locking mechanisms and ultrashort laser
applications.
© 2020 Chinese Laser Press
https://doi.org/10.1364/PRJ.386954
1. INTRODUCTION
Transient phenomenon dynamics are of significance for
revealing the evolution mechanism of numerous nonlinear sys-
tems [1–4]. Mode-locked lasers can exhibit profo und nonlinear
optical dynamics and have moved into the spotlight of optical
research due to their unique, intriguing properties of temporal
and spatial oscillations [5–9]. Recent years have seen increased
interests in mode-locked fiber lasers, largely in anticipation
of particle manipulation [10,11], ultrafast laser fabrication
[12], high-capacity optical communication [13], and Bose–
Einstein condensates [14 ].
The mode-locked fiber laser provides an ideal platform for
exploring ultrashort nonlinear dynamics, where mode-locking
(ML) pulses arise from the balance among optical nonlinearity,
dispersion, and intracavity gain and loss [15]. Before the ulti-
mate stable ML state, mode-locked lasers experience a series of
unstable phenomena when detuned from a steady state or
evolve into stable ML pulses from the noise [16]. These insta-
bilities are important, as they reveal the landscape of the pulse
evolution during the self-starting process and ML process. The
self-starting dynamics in the passively mode-locked fiber lasers
have been established by a rich variety of theoretical and exper-
imental results [17–20]. Recently, researcher s have pioneered
the time-stretch dispersive Fourier transform (TS-DFT) tech-
nique for the exploration of build-up dynamics in ML fiber
lasers [21–24]. They have revealed detailed information about
underlying dynamics in the self-starting process and have gen-
eralized the build-up process into three stages: relaxation oscil-
lation, quasi-mode-locking (Q-ML), and stable ML. Polarization
change in the laser cavity, energy fluctuation of pump power,
and extra environmental perturbations can influence the forma-
tion dynamics of the ML pulses. Therefore, the entire obser-
vation of the ML process has great implications in the laser
operation.
Temporal mode-locked fiber lasers have major influence on
both laser physics and practical applications through lasing
spatially at the fundamental mode. However, the so-called
“capacity crunch” is anticipated, whereby the single-mode fibers
Research Article
Vol. 8, No. 7 / July 2020 / Photonics Research 1203
2327-9125/20/071203-10 Journal © 2020 Chinese Laser Press
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