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Real-time observation of vortex mode switching

in a narrow-linewidth mode-locked fiber laser

JIAFENG LU,

FAN SHI,

LINGHAO MENG,

LONGKUN ZHANG,

LINPING TENG,

ZHENGQIAN LUO,

PEIGUANG YAN,

FUFEI PANG,

1,4

AND XIANGLONG ZENG

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

Department of Electronic Engineering, School of Information Science and Engineering, Xiamen University, Xiamen 361005, China

Shenzhen Key Laboratory of Laser Engineering, Shenzhen University, Shenzhen 518060, China

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.

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

(SMFs) are unable to meet the increasing demand of data tele-

communications. At present, the optical spatial division multi-

plexing (SDM) technique has emerged to provide a promising

solution to this potential bandwidth crisis [25,26]. The few-

mode fibers (FMFs) and multimode fibers (MMFs) exploited

in SDM systems can support a few spatial eigenmodes, each

with different propagating constants and spatial distributions.

Vortex light multiplexing has obtained great attentions due to

its new multiplexing dimension of orbital angular momentum

(OAM). Recently, terabit free-space communication employing

vortex light has been demonstrated [13]. These modes can

interact with each other in the FMFs and MMFs through

optical nonlinearity, disorder (random linear mode coupling),

and laser gain [27]. The spatial modes’ interaction in ultrafast

mode-locked fiber lasers is much more interesting and finally

evolves into the spatiotemporal ML. Previous reports mainly

focus on the overall effect and joint interaction between 10

and 100 spatial modes in spatiotemporal ML, but the specific

interaction between the identified individual spatial modes

remains to be further explored [28–31].

In this paper, we have demonstrated the first to our knowl-

edge observation of vortex mode-switching dynamics. This

result helps scientists understand the rich variety of instability

phenomena in the mode-switching dynamic process and the

influence of spatial mode interactions on ML states.

Previously, static spatial mode switching has been demonstrated

in both fiber [32] and free space [33]. Nevertheless, the static

mode switching of different spatial modes requires manual ad-

justment of a polarization controller (PC) or cavity mirror.

However, these static mode-switching methods often suffer

from the following restrictions: (1) complexity of manual ad-

justments; (2) low switching speed for capturing the dynamic

mode-switching process; and (3) environmental sensitivity.

To realize the real-time observation of mode-switching dynam-

ics, here a dual-resonant acousto-optic mode converter (AOMC)

is employed to provide a tunable mode switching with fast

switching speed in a narrow-linewidth mode-locked laser cavity.

2. THEORY: ACOUSTIC DISPLACEMENT-BASED

MODEL OF DUAL-RESONANT AOMC

The AOMC components exploit the electro-acoustic conver-

sion effect in piezoceramic (PZT) materials and the acousto-op-

tic interaction effect in silica fiber media to construct a dynamic

tunable spatial mode conversion [34–37]. In our previous re-

ports [38,39], we preliminarily utilized the AOMC in a con-

tinuous-wave fiber laser, yet complete theoretical analysis was

not demonstrated to reveal the entire property of dual-resonant

AOMC. Here we introduce the mode-switching mechanism of

AOMC into a mode-locked fiber laser to discover the mode-

switching dynamics on ML states, which broadens the practical

value of dual-resonant AOMC and enriches the ultrafast physi-

cal fundamentals.

The interaction of light beams and acoustic waves enables

optical field control in silica fibers, which is mainly derived

from the photon–phonon scattering. It is a common belief that

a travelling flexural acoustic wave (FAW) produces a periodic

micro-bending, which causes an interlaced density of fiber

medium as shown in Fig. 1(a). The FAW propagating along

the unjacketed silica fiber at the lowest-order mode (F

mode)

leads to an acoustically induced displacement vector (AIDV) of

˜u on the fiber medium, which is asymmetric with respect to

the direction of applied vibration. Figures 1(b) and 1(c) show

a schematic drawing of the AOMC component and a sche-

matic diagram of the fiber end face with an acoustic wave,

respectively. Therefore, the permittivity deformation of the

Fig. 1. Diagram of an AOMC and the simulation of the switching mechanism based on optical and acoustic birefringence. (a) The schematic

diagram of the dual-resonant AOMC and the mode-switching mechanism. (b) The setup of an AOMC component. (c) The schematic diagram of

the fiber end face. (d) The simulation of beat lengths between the LP

mode and LP

11a∕b

modes with different ellipticities of the fiber core. The

straight lines and dash lines represent the beat lengths from the LP

mode to the LP

11a

and LP

11b

modes, respectively. (e) The Δλ shifts with the

decrease of ellipticity of fiber core. The inset figure shows the wavelength separation in the transmission spectrum of a dual-resonant AOMC.

1204 Vol. 8, No. 7 / July 2020 / Photonics Research

Research Article

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