Mode-locking of fiber lasers using novel two-dimensional
nanomaterials: graphene and topological
insulators [Invited]
Grzegorz Sobon
Laser & Fiber Electronics Group, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw,
Poland (grzegorz.sobon@pwr.edu.pl)
Received January 6, 2015; revised February 15, 2015; accepted February 18, 2015;
posted February 18, 2015 (Doc. ID 231874); published March 26, 2015
The paper summarizes the recent achievements in the area of ultrafast fiber lasers mode-locked with so-called low-
dimensional nanomaterials: graphene, topological insulators (Bi
2
Te
3
, Bi
2
Se
3
, Sb
2
Te
3
), and transition metal sulfide
semiconductors, like molybdenum disulfide (MoS
2
). The most important experimental achievements are de-
scribed and compared. Additionally, new original results on ultrashort pulse generation at 1.94 μm wavelength
using graphene are presented. The designed Tm-doped fiber laser utilizes multilayer graphene as a saturable
absorber and generates 654 fs pulses at 1940 nm wavelength, which are currently the shortest pulses generated
from a Tm-doped fiber laser with a graphene-based saturable absorber. © 2015 Chinese Laser Press
OCIS codes: (140.4050) Mode-locked lasers; (140.3510) Lasers, fiber; (060.3510) Lasers, fiber.
http://dx.doi.org/10.1364/PRJ.3.000A56
1. INTRODUCTION
Ultrashort pulsed sources of optical radiation in the infrared
range are currently on demand for many industrial, military,
and medical applications, such as laser micromachining [
1],
surgery [
2], terahertz-wave generation [3], optical imaging
[
4], or supercontinuum generation [5]. Fiber lasers, thanks
to their compactness, cost-effectiveness, and robustness,
are excellent candidates for this type of application. In order
to force a fiber laser to generate ultrashort pulses, it is neces-
sary to synchronize the phases of the longitudinal modes in
the resonator. This might be done in two ways: by placing
a saturable absorber (SA) device inside the cavity or by taking
advantage of nonlinear effects occurring in optical fibers: non-
linear polarization rotation (NPR) [
6,7] or nonlinear amplifica-
tion loop mirror (the so-called figure-8 configuration [
8]). The
currently most common and popular SA—semiconductor
SA mirror (SESAM), widely used in commercially available
systems—is based on quantum wells in semiconductors [
9].
It has a very rich, 20-year history and is still developed, espe-
cially for thin-disk lasers [
10]. However, SESAMs suffer from
relatively narrowband operation—due to the energy bandgap
in semiconductors, each SESAM needs to be designed for a
specific wavelength. Thus, it is not possible to mode-lock
lasers operating in different wavelength bands or to synchron-
ize two oscillators using one common SA. Figure-8 and
NPR-based lasers tend to be environmentally unstable, are
vulnerable to fiber movement, and do not always provide self-
starting pulsed operation. All those drawbacks forced the
ultrafast laser community to search for new types of SAs,
based on novel materials.
In the recent five years, so called two-dimensional (2D)
nanomaterials revolutionized the field of mode-locked lasers.
A 2D material is often defined as an atomically thin layered
material, and its thickness is reduced down to single or only
a few layers [
11–13]. In such a material, strong intralayer
covalent bonding and weak interlayer van der Waals forces
are present [
14]. In the absence of interlayer perturbation,
the electron motion is limited to two dimensions [
15], which
may lead to many new and unexpected electronic and optical
properties. The most popular example of a 2D material is
graphene, which is composed of a one-atom-thick layer of
carbon forming a 2D honeycomb lattice. Graphene is a
fundamental building block of three-dimensional (3D) graph-
ite. Other 2D materials usable in laser technology include
topological insulators (TIs; Bi
2
Te
3
,Bi
2
Se
3
,Sb
2
Te
3
) and tran-
sition metal sulfide semiconductors, like molybdenum disul-
fide (MoS
2
). The growing interest in novel 2D materials
started in 2009, when the groups from Singapore and United
Kingdom independently developed the first graphene-based
fiber lasers [
16,17]. In the next five years, hundreds of new
laser setups were reported in the literature, utilizing not
only graphene but also TIs and recently transition metal
sulfides. The field of fiber lasers has experienced an unprec-
edented and very fast progress, leading to many outstanding
achievements.
In this paper, the most important recent achievements in
the field of fiber lasers incorporating 2D materials are summa-
rized. Additionally, original results on ultrashort pulse gener-
ation at 1.9 μm using multilayer graphene are presented.
2. SATURABLE ABSORBERS BASED ON 2D
NANOMATERIALS
2D nanomaterials are currently intensively investigated
among the applied physics and photonics community. The
most important materials that might be used as SAs for
mode-locked fiber lasers include carbon nanotubes (CNTs),
graphene, TIs (Bi
2
Te
3
,Bi
2
Se
3
,Sb
2
Te
3
), and transition metal
sulfide semiconductors such as MoS
2
or tungsten disulfide
A56 Photon. Res. / Vol. 3, No. 2 / April 2015 G. Sobon
2327-9125/15/020A56-08 © 2015 Chinese Laser Press