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Q-switched and mode-locked Er-doped fiber laser

using PtSe

as a saturable absorber

KANG ZHANG,

MING FENG,

*YANGYANG REN,

FANG LIU,

XINGSHUO CHEN,

JIE YANG,

XIAO-QING YAN,

1,2

FENG SONG,

AND JIANGUO TIAN

Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, School of Physics and TEDA Applied Physics School,

Nankai University, Tianjin 300071, China

e-mail: yanxq01@nankai.edu.cn

*Corresponding author: mingfeng@nankai.edu.cn

Received 14 June 2018; revised 25 July 2018; accepted 25 July 2018; posted 25 July 2018 (Doc. ID 335211); published 23 August 2018

We report a passively Q-switched and mode-locked erbium-doped fiber laser (EDFL) based on PtSe

, a new

two-dimensional material, as a saturable absorber (SA). Self-started Q-switching at 1560 nm in the EDFL was

achieved at a threshold pump power of 65 mW, and at the maximum pump power of 450 mW, the maximum

single Q-switched pulse energy is 143.2 nJ. Due to the polarization-dependent characteristics of the PtSe

-based

SA, the laser can be switched from the Q-switched state to the mode-locked state by adjusting the polarization

state. A mode-locked pulse train with a repetition rate of 23.3 MHz and a pulse width of 1.02 ps can be

generated when the pump power increases to about 80 mW, and the stable mode-locked state is maintained

until the pump power reaches its maximum 450 mW. The maximum single mode-locked pulse energy is

0.53 nJ. This is the first time to our knowledge that successful generation of stable Q-switched and mode-locked

pulses in an Er-doped fiber laser has been obtained by using PtSe

Press

OCIS codes: (140.3510) Lasers, fiber; (140.7090) Ultrafast lasers; (140.3380) Laser materials; (160.4670) Optical materials.

https://doi.org/10.1364/PRJ.6.000893

1. INTRODUCTION

Q-switching and mode-locking are two main ways to generate

pulses [1–4]. Among the two technologies, saturable absorbers

(SAs) play a vital role in passive Q-switching and mode-locking

[5–13]. Fiber lasers have the advantages of simple structure,

small size, low price, and high environmental stability [12–17].

Therefore, it is clear that the development of pulsed lasers

depends greatly on the development of saturable absorbing

materials. Traditional SAs, such as semiconductor saturable

absorber mirrors (SESAM), face many defects, such as narrow

working bandwidth and complex manufacturing packages,

which greatly limit the developm ent of pulsed lasers. With

the advancement of material science, new nanomaterials such

as carbon nanotubes (CNTs) [8,9], graphene [10–12], gra-

phene oxide [13], black phosphorus (BP) [17,18], and topo-

logical insulators (TI) [19–22] have emerged one after another,

and their performance has been qualitatively improved as their

prices have declined. Recently, transition-metal dichalcogenides

(TMDs) (e.g., MoS

[23], WS

[24,25], TiS

[26], MoTe

[27,28]) have attracted much attention from laser researchers

due to their thickness-dependent band gap and unique absorp-

tion property [29 ,30]. PtSe

also attracts our attention as a

new member of the layered TMDs family [31–35]. PtSe

’s

characteristic of having a widely tunable band gap allows it

to effectively respond to near-infrared light, and its photo-

responsivity is comparable with that of BP [36]. Monolayer

PtSe

has an indirect band gap of about 1.2 eV, while the band

gap of double-layered PtSe

is reduced to 0.21 eV. Three or

more layers of PtSe

have a zero band gap, and the macroscopic

properties are represented by semimetals [31]. Compared to

MoS

, a TMD material that has been widely studied in the

field of laser mode-locking, PtSe

has a higher carrier mobility

that is comparable to that of graphene [35–38]. Therefore, it

can produce a fast nonlinear response to incident light and can

achieve narrower pulses. In addition, the narrower energy band

gap of PtSe

allows it to have nonlinear effects in a wider wave-

length range. Moreover, the characteristics of the zero bandgap

of multi-layered PtSe

and the high carrier mobility are similar

to graphene, although there is a large difference in the band

structures. Therefore, PtSe

has the potential to substitute for

graphene as an excellent SA.

In this paper, we report an erbium-doped fiber laser (EDFL)

based on PtSe

as an SA with obvious polarization-dependent

saturable absorption, and we have obtained both passive

Q-switching and mode-locking pulses. As far as we know,

Research Article

Vol. 6, No. 9 / September 2018 / Photonics Research 893

it is the first time that PtSe

has been used as an SA for both

Q-switching and mode-locking in a fiber laser.

2. EXPERIMENTAL SETUP

In our experiment, the PtSe

thin films are grown by the

chemical-vapor-deposition (CVD) technology on a sapphire sub-

strate, which are commercial products (6 Carbon Technology,

Shenzhen, China). The optical microscope image shows that

the grown PtSe

film exhibits a good continuous uniformity over

a large area (not shown here). The surface morphology and height

profile of the PtSe

thin film are obtained from atomic force

microscopy (AFM) imaging, as shown in Fig. 1(a).Fromthe

AFM image, the thickness of the used PtSe

film is determined

to be 4.7  0.4nmby the height profile, indicating that the

number of layers of the PtSe

sample is around five. The Raman

spectrum of the PtSe

filmisshowninFig.1(b). Three character-

istic peaks could be observed: (1) the peak at 179.6cm

−1

,cor-

responding to the in-plane vibration of Se atoms in opposite

directions within a single layer, (2) the peak at 206.5cm

−1

,which

corresponds to out-of-plane vibration of Se atoms, (3) and the

longitudinal optical (LO) peak at 235.4cm

−1

, which is attributed

to the overlap of the A2u mode and Eu mode. The Raman spec-

trum observed here is in good agreement with that reported in

Ref. [39]. The linear absorption of the PtSe

film was measured

in the range from 200 to 1600 nm using a Hitachi spectropho-

tometer , and broadband absorption can be observed in Fig. 1(c).

The absorption peak characterized by a flat profile is located be-

tween 409 and 652 nm, and 1600 nm is at the absorption edge,

showing that the used PtSe

films have the potential of working

as SAs in the near-infrared region. M easureme nts on different

positions yield nearly identical results, confirming the uniformity

of the PtSe

films.

The PtSe

-based SA is fabricated with a PtSe

-covered-

microfiber structure, as shown in Fig. 2(a).First,wetransfer

the PtSe

film from the sapphire substrate onto a polydimethyl-

siloxane (PDMS) sheet. To avoid the impact of contamination

on light guiding, a new dry transfer method using a two-layer-

composite-structure of polyethylene terephthalate and silica gel

was applied to transfer the PtSe

film with a size of 3mm×

3mmfrom the sapphire substrate to the PDMS sheet [40].

With this method, the transferred 2D material possesses a cleaner

and more continuous surface, a lower doping level, and a higher

optical transmittance and conductivity than that transferred by

thermal release tape and polymethylmethacrylate (PMMA).

Therefore, there is little effect of contaminants on the experi-

ment, and the reproducibility of the experiment is satisfactory.

Then, the microfiber is sandwiched between a low-refractive-

index substrate MgF

(with a refractive index of 1.37 at a wave-

length of 1.55 μm) and the PtSe

film supported by PDMS

(with a refractive index of 1.413 at a wavelength of 1.55 μm).

The microfiber is drawn from single-mode fiber (SMF) using the

flame-brushing technique, with a waist diameter of ∼7.5 μm,

a stretching length of 20 mm, and an insertion loss of 0.1 dB.

The total length of the PtSe

film on the microfiber is about

3 mm, as shown in Fig. 2(b).

Since the microfiber-based SA is used to interact with the

evanescent field of light waves through the side of the micro-

nanofibers, the PtSe

-based SA has different absorption effects

for light with different polarization states in the fiber. In the

experiment, we measured the saturable absorption characteris-

tics of the transverse-electric (TE) and transverse-magnetic

(TM) modes of the SA by adjusting the polarization controller

in the optical path. The polarization-dependent saturable ab-

sorption, as shown in Figs. 2(c) and 2(d) , is measured by a

homemade mode-locked laser with 380 fs wide pulses at

1.55 μm. We found that the absorption of the TE mode is

significantly less than that of the TM mode at the same pump

power. This is due to the fact that the direction of the electric

field of the TE mode is parallel to the plane of the PtSe

, so that

the TE mode electromagnetic wave has less interactions with

the PtSe

plane when passing through the device, which results

in less propagation loss. The transmittance of the TE mode

increased from 24.90% to 29.80%, with a modulation depth

of 4.90%, and the saturable intensity is 0.34 GW∕cm

,as

shown in Fig. 2(c). The modulation depth of the TM mode

is 1.11%, and the saturable intensity is 1.23 GW∕cm

, which

are shown in Fig. 2(d). In Table 1, we quantitatively compare

our PtSe

-based SA with several nanomaterial-based SAs work-

ing around 1.5 μm. It can be seen that the modulation depth

of the PtSe

-based SA is comparable to reports of other nano-

materials, while the saturable intensity is larger than those of

others. The difference in saturable intensity is mainly due to

the structure that we used to fabricate the PtSe

-based SA.

With the PtSe

-covered-microfiber structure, the PtSe

inter-

acted with the evanescent field of the light wave, which is

much weaker than the field in the fiber center. Therefore,

the saturable intensity of our SA is larger than that of other

Fig. 1. (a) AFM image of the CVD-grown PtSe

film on the sapphire substrate. The red curve shows the thickness along the white dashed line.

(b) Representative Raman spectrum of PtSe

film taken at a 514 nm excitation wavelength. (c) Absorption spectrum of the PtSe

film.

894 Vol. 6, No. 9 / September 2018 / Photonics Research

Research Article

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