Passive Q-switching in an erbium-doped fiber laser using
tungsten sulphoselenide as a saturable absorber
H. Ahmad*, Z. C. Tiu, and S. I. Ooi
Photonics Research Centre, University of Malaya, Kuala Lumpur 50603, Malaysia
*Corresponding author: harith@um.edu.my
Received October 30, 2017; accepted December 22, 2017; posted online January 31, 2018
A highly stable Q-switched laser incorporating a mechanically exfoliated tungsten sulphoselenide (WSSe) thin
sheet saturable absorber (SA) is proposed and demonstrated. The SA assembly, formed by sandwiching a thin
WSSe sheet between two fiber ferrules within the erbium-doped fiber laser, is used to effectively modulate the
laser cavity losses. The WSSe-based SA has a saturation intensity of ∼0.006 MW∕cm
2
and a modulation depth
of 7.8%, giving an optimum Q-switched laser output with a maximum repetition rate of 61.81 kHz and a mini-
mum pulse width of 2.6 μs. The laser’s highest output power of 0.45 mW and highest pulse energy of 7.31 nJ are
achieved at the maximum pump power of 280.5 mW. The tunability of the cavity’s output at the maximum
pump power is analyzed with a C-band tunable bandpass filter, giving a broad tunable range of ∼40 nm, from
1530 nm to 1570 nm. The output performance of the tunable Q-switched laser correlates well with the gain
spectrum of erbium-doped fibers, with the shift in the gain profile as a result of the saturated SA.
OCIS codes: 140.3500, 140.3510, 140.3540, 140.3600.
doi: 10.3788/COL201816.020009.
In recent years, two-dimensional (2D) semiconductor
materials such as graphene
[1]
, transition metal dichalcoge-
nides (TMDs)
[2]
, and topological insulators (TIs)
[3]
have re-
ceived significant attentio n for various applications in the
field of photonics and optoelectronics. These materials
find a use as saturable absorbers (SAs) for the passive gen-
eration of pulsed laser outputs due to their low saturation
intensity, ultrafast carrier dynamics, as we ll as great pho-
toluminescence of 2D materials
[4]
. The ability of 2D mate-
rials to modulate intracavity losses in a fiber laser system
contributes to their successful incorporation in various
Q-switching and mode-locking systems
[1,2]
. Specifically,
TMDs have seen more applications in Q-switching and
mode-locking systems compared to other 2D materials
due to their better performance as well as their layer-
dependent absorption properties, which gives TMDs an
advantage over most other 2D materials.
TMD materials are generally represented by the for-
mula MX
2
, where M represents the transition metal atom
and X represents the chalcogen atom
[5]
. Within the TMD
layer, the two X atoms hold onto each M atom by a strong
covalent bond, while only a weak van der Waals force
holds the TMD layers to each other. The weak interlayer
forces of TMDs make them easy to fabricate into a single
or few layer form using simple exfoliation methods
[6]
.
Furthermore, by changing the number of layers in these
TMD materials, the energy bandgap can be tuned from
indirect to direct feature
[7]
. This tuning ability, combined
with high third-order optical nonlinearities, and ultrafast
carrier dynamics give TMDs a high potential for various
broadband absorber photonics and optoelectronics appli-
cations. In particular MoS
2
, MoSe
2
,WS
2
, and WSe
2
have
shown a tremendous potential for use as SAs in photonics
applications.
Recently, breakthroughs in the preparation and fabrica-
tion process of TMDs for SA applications have been able to
further enhance their optical properties. The improvements
in the sample preparation process have lead to the existence
of TMD alloys with the chemical formula MX
2ð1−xÞ
X
0
2x
,
where x represents the composition ratio
[8]
.Thisnew
category of TMD alloys has a bandgap tuning range over
the absorption wavelength that is dependent on the
composition ratio x
[8]
. Furthermore, these TMDs are very
stable due to their ordered alloy structures
[9]
.Owingto
their attractive output characteristics, TMD alloys have
been studied in previous works
[10]
, but to date there has been
no known exploration of the feasibility of the WS
2ð1−xÞ
Se
2x
alloy as an SA.
In this work, a van der Waals heterostructure-based
WS
2ð1−xÞ
Se
2x
composite SA with a composition ratio of
0.5 is fabricated from a bulk crystal by simple mechanical
exfoliation. The exfoliated few-layer WSSe SA is then in-
corporated into an erbiu m-doped fiber laser (EDFL) cav-
ity operating at 1.56 μm. From the nonlinear optical
characterization of the WSSe SA by the twin-detection
technique, a saturation intensity of 0.006 MW∕cm
2
and
a modulation depth of ∼7.80% were observed. The ability
of the WSSe SA to modulate intracavity losses results in
the passive generation of Q-switched pulses in the pro-
posed EDFL cavity.
A sample of the mechanically exfoliated few-layer WSSe
crystal is first characterized by X-ray diffraction (XRD)
using a PANalytical EMPY REAN X-ray diffractometer.
Figure
1 shows the detailed XRD patterns of the charac-
terized WSSe sample at an excitation wavelength of
1.5406 Å. The WSSe sample shows diffraction peaks
at 13.80°, 29.15°, 38.76°, 42.80°, 48.60°, and 57.84°
along (002), (004), (103), (006), (105), and (110),
COL 16(2), 020009(2018) CHINESE OPTICS LETTERS February 10, 2018
1671-7694/2018/020009(5) 020009-1 © 2018 Chinese Optics Letters
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