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Using a black phosphorus saturable absorber to generate dual wavelengths in a Q-switched
ytterbium-doped fiber laser
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2016 Laser Phys. Lett. 13 085102
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Laser Physics Letters
Astro Ltd
Laser Phys. Lett. 13 (2016) 085102 (6pp)
doi:10.1088/1612-2011/13/8/085102
Using a black phosphorus saturable
absorber to generate dual wavelengths in
a Q-switched ytterbium-doped fiber laser
F A A Rashid1, Saaidal R Azzuhri1, M A M Salim1,2, R A Shaharuddin1,
M A Ismail1, M F Ismail1, M Z A Razak1 and H Ahmad1
1
Photonics Research Centre, University of Malaya, Kuala Lumpur, Malaysia
Laser Center, Ibnu Sina Institute for Scientific and Industrial Research, Universiti Teknologi Malaysia,
Johor Bahru, Malaysia
2
E-mail: saaidal@um.edu.my
Received 6 June 2016, revised 15 June 2016
Accepted for publication 15 June 2016
Published 19 July 2016
Abstract
Using a few-layer black phosphorus (BP) thin film that acts as a saturable absorber (SA)
in an ytterbium-doped fiber laser setup, we experimentally demonstrated a passively dualwavelength Q-switching laser operation. The setup also incorporated a D-shaped polished
fiber as a wavelength selective filter. As the SA was used in the ring cavity, a dual-wavelength
Q-switch produced consistent outputs at 1038.68 and 1042.05 nm. A maximum pulse energy
of 2.09 nJ with a shortest pulse width of 1.16 µs was measured for the achieved pulses.
In addition, the repetition rate increased from 52.52 to 58.73 kHz with the increment of the
pump level. Throughout the measurement process, the results were obtained consistently and
this demonstrates that the BP film is a very good candidate to produce Q-switching pulses for
the 1 micron region.
Keywords: dual wavelength, ytterbium, D-shaped polished fiber, fiber laser, BP
(Some figures may appear in colour only in the online journal)
1. Introduction
been used to produce Q-switched pulses. It is well known that
SESAMs and carbon-based materials (CNTs and graphene)
are widely used as broadband functional SAs. Nevertheless,
both materials have drawbacks. SESAMs have a limited range
of optical responses, which restricts the Q-switching pulse
generation [13]. Meanwhile, graphene exhibits zero band gap
structure but has relatively weak optical absorption [14].
Quite recently, black phosphorus (BP) was rediscovered
as a new and interesting 2D material for various electronics
and optoelectronics applications [15, 16]. BP is a single layer
material consisting of only phosphorus atom with its own
unique electronic and optical properties. A recent report confirmed the potential use of BP as a saturable absorber, working through direct interaction with the passive operation of a
mode-locked fiber laser to generate an ultrafast mode-locking
pulsed laser [17]. BP is a high-mobility layered semiconductor, with a direct energy band-gap structure ranging from 0.3
to 2.0 eV, and it is sensitively dependent on the number of
Passively Q-switching ytterbium-doped fiber lasers (YDFL)
have recently attracted considerable attention because of their
many advantages. These include properties such as compactness, low-cost, and flexibility, and their wide area of applications, such as for environmental sensing, remote sensing,
telecommunications and medicine [1]. A number of active and
passive Q-switching methods have been performed based on
the active modulation of the Q-factor in a cavity [2]. Among
the existing passive Q-switching techniques in pulsed laser
generation, the use of a saturable absorber (SA) derived from
low-dimensional (LD) materials is a highly promising technique to convert a continuous wave (CW) laser into pulses and
modulate intracavity losses [3].
Over the past decade, many types of SAs—for instance,
semiconductor saturable absorber mirrors (SESAMs) [4],
carbon nanotubes (CNTs) [5–8], and graphene [9–12]—have
1612-2011/16/085102+6$33.00
1
© 2016 Astro Ltd Printed in the UK
F A A Rashid et al
Laser Phys. Lett. 13 (2016) 085102
Figure 1. Raman spectrum of the BP SA.
using CNT thin film as the SA, and produced a repetition rate
of 27.6 MHz in a femtosecond mode-locked fiber laser, with
a pulse duration of 0.51 ps. A 3 dB optical coupler, a variable optical attenuator (VOA), and two optical power meters
(OPMs) were also used in this setup. The mode-locked pulse
seed source was generated using an erbium-doped fiber laser
(EDFL) incorporating a CNT thin film consisting of a 3 m
erbium-doped fiber. In addition, a 974 nm laser diode (LD)—
Oclaro type, model LC96A74P-20R—was used to pump the
laser source in the cavity via the 980/1550 nm wavelength
division multiplexer (WDM) coupler. To ensure unidirectional
light propagation, an isolator was also included in this cavity. The output of the isolator was linked with a CNT film
between two fiber ferrules. A CNT film was sandwiched
between two fiber ferrules at the output of the isolator. The
generated mode-locked pulse was then amplified using a
low-dispersion in-house-built EDFA module to generate high
peak power (which can also cover the S, C, and L bands in
the 1550 nm region [21–23]) and saturate the BP sample to
a sufficient degree. Up to ~10 W of the average output power
can be generated by our in-house-built amplifier. A maximum
of 10 dB of the peak power output was then connected to the
VOA, which was used to attenuate the signal from 0 to 60 dB.
The output of the attenuator was split into two equal parts
using a 3 dB coupler. One output of the coupler was connected
directly to an OPM as a reference. Meanwhile, the pulse from
the other output port of the coupler that passed through the
two fiber ferrules consisting of the BP thin film was measured using another OPM. The differences of the OPMs were
evaluated and the modulation depth and saturable intensity of
the BP were measured as 73% and ~0.02 MW cm−2, respectively. Figure 2(b) shows saturable absorption behavior that
conforms to the following formula:
layers [18]. This unique structure helps to prevent defects in
the crystal structure from impeding the semiconductor properties [17]. In recent progress, Luo et al experimentally demonstrated saturable absorption in BP by Q-switched and mode
locked lasers with wavelengths from 0.6 to 2.0 μm [19].
In this paper, the use of BP as the SA to passively generate
dual-wavelength Q-switched YDFL is experimentally demonstrated. A side-polished fiber (also known as a D-shaped
polished fiber) acts as a wavelength selective filter in this dualwavelength pulse generation. Our results show stable output
at wavelengths of 1038.68 and 1042.05 nm, with output powers of −27.97 and −28.52 dBm, respectively. In addition, a
maximum pulse energy of 2.09 nJ was observed. In this work,
the results demonstrate that the BP determines the saturable
absorption of the laser in the generation of passively dualwavelength pulses.
2. Fabrication and characterization of BP
This experiment uses the mechanical exfoliation method to
prepare the SA thin film based on BP [20]. This method was
chosen because it has a simple fabrication process. The process does not require complex chemical procedures or the use
of expensive instruments. Using simple clear scotch tape, thin
flakes of BP were peeled off from a commercially available
99.995% purity BP crystal. The tape containing the BP was
then investigated using Raman spectroscopy. Figure 1 shows
the Raman spectrum result. An argon-ion laser (514 nm) with
a 10 mW power exposure was radiated onto the tape for 10 ms
(recorded by the spectrometer). Three distinct Raman peaks
were exhibited by the sample at 360, 438, and 465 cm−1,
corresp­onding to A1g (out-of-plane modes), together with B2g
and A2g (in-plane modes).
Figure 2(a) shows the setup that was used to measure the
characteristics of the saturable absorption of the BP SA. The
system consisted of a pulse seed source, which was generated
∆α
α =
+ α linear ,
(1)
I
1+ I
(
2
sat
)
F A A Rashid et al
Laser Phys. Lett. 13 (2016) 085102
Figure 3. Schematic representation of the dual-wavelength
Q-switched YDFL setup.
Figure 2. (a) The laser schematic for measuring the nonlinear
absorption of BP. (b) The characteristics of the saturable absorption
of the BP SA.
where αlinear, Δα, and Isat represent the non-saturation loss,
the modulation depth, and the saturable optical intensity,
respectively.
3. Laser configuration
Figure 4. Pump powers (Pp ) and repetition rates (Rr ) in the
Q-switched pulse train of (a) Pp = 129.4 mW and Rr = 55.31 kHz
and (b) Pp = 188.0 mW and Rr = 58.73 kHz.
The configuration of the proposed dual-wavelength YDFL
setup using BP is shown in figure 3. In order to induce simultaneous dual-wavelength output, the setup also consists of
a D-shape fiber that acts as a wavelength filter, and this can
also be seen in the figure. A 70 cm length of ytterbium-doped
fiber (YDF)—Fibercore, model DF1100—is used as a gain
medium. A 974 nm laser diode with a maximum output power
of 600 mW (Oclaro LC96A74P-20R) is used to pump the
laser into the cavity through a 980/1060 nm WDM coupler.
An optical isolator is placed after the YDF within the laser
cavity to direct the light propagation in a unidirectional path.
The D-shaped fiber—Phoenix Photonics side polished optical
fiber, model number SPF-S-SM—is inserted after the isolator
in order to generate a dual-wavelength laser. The D-shaped
polished fiber has a polished region of 17 mm and an insertion
3
F A A Rashid et al
Laser Phys. Lett. 13 (2016) 085102
Figure 6. (a) The repetition rate and pulse width versus pump
power and (b) the average output power and pulse energy versus
pump power.
loss of ~0.5 dB recorded at a wavelength of 1550 nm. The BP
film, which is sandwiched between the fiber ferrules, is then
inserted between the D-shaped fiber and a 90:10 fiber coupler
1 (OC1). The 90% output of the light is coupled back into
the cavity, while the 10% output is then connected to a 50:50
fiber coupler 2 (OC2) that splits the signal equally and allows
simultaneous monitoring of the output of the dual-wavelength
Q-switched pulses. The output power of the pulsed laser is
measured using an OPM, an optical spectrum analyzer (OSA),
a digital oscilloscope connected to a photo detector, and a
radio frequency (RF) analyzer to analyze the RF spectrum up
to 7.8 GHz.
4. Experimental results
The lasing threshold for the generation of dual-wavelength
Q-switching pulses is 115.2 mW, which is measured at the
output of the ring cavity. In this case, a repetition rate of
52.52 kHz was observed. When the output pump power level
was increased to 129.4 mW and 188.0 mW, respectively,
Figure 5. (a) The optical spectra of the dual wavelengths at 1038.68
and 1042.05 nm, (b) the trace of the oscilloscope in the Q-switched
pulse train with a repetition rate of 57.08 kHz and a pump power
of 158.8 mW, (c) the pulse width measurement, and (d) the output
spectrum of the RF.
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F A A Rashid et al
Laser Phys. Lett. 13 (2016) 085102
Acknowledgment
stable pulse trains with different repetition rates of 55.31 and
58.73 kHz were measured at the oscilloscope, as depicted in
figures 4(a) and (b). The results verified the Q-switching operation behavior.
The wavelength output obtained from the OSA is shown
in figure 5(a). The spectrum shows two wavelength peaks at
1038.68 and 1042.05 nm with output powers of −27.97 dBm
and −28.52 dBm, respectively. Both peaks emerged at 158.8 mW
of pump power. The wavelengths of these two peaks can be
differentiated by utilizing a wavelength selective filter, for
example, a tunable band pass filter and a fiber Bragg grating (FBG). The trace of the Q-switching pulses has a repetition rate of 57.08 kHz, while the corresponding pulse width is
shown in figures 5(b) and (c). The full half-width maximum
(FHWM), which is used to measure the pulse width, was 1.47 μs.
The trace results in the frequency domain were obtained by
using a radio frequency spectrum analyzer (RFSA), as shown
in figure 5(d). The fundamental frequency of 57.08 kHz (inset
picture) was confirmed by the spectrum. The peak-to-pedestal
ratio (PPR) of the pulse was 50 dB, which is clearly defined
in the figure.
Figure 6(a) shows the repetition rate and pulse duration results for the variation of pump power. As the power
increased from 115.2 to 188.0 mW, the repetition rate also
increased from 52.52 to 58.73 kHz. By contrast, the pulse
width reduced from 2.05 to 1.16 µs with the same increment
of the power level. In addition, the average output power and
the energy of a single pulse were also measured, as shown
in figure 6(b). The results show that the pulse energy and
the average output power increased linearly as a function
of the pump power. The maximum pulse energy of 2.09 nJ
and the maximum average output power of 0.12 mW were
recorded at the maximum pump power of 188.0 mW. The
results obtained from this work demonstrated that the
output performance was comparable to that of typical
Q-switched operations that use other nanomaterials, such
as graphene [13] and CNTs [24], in an ytterbium-doped
fiber laser.
We thank the University of Malaya for supporting this
work, which was funded by research grant nos GA010-2014
(ULUNG), RU007/2015, UM.C/625/1/HIR/MOHE/SCI/29,
LRGS(2015)/NGOD/UM/KPT, and BR002-2016.
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5. Conclusion
In this paper, we demonstrated a dual-wavelength YDFL that
generates Q-switching pulses. Two different aspects influenced these simultaneous effects; the use of the D-shaped
polished fiber generated the dual-wavelength effect, while
the Q-switching effect appeared as a result of using BP thin
film as the saturable absorber. The spectrum shows two wavelength peaks at 1038.68 and 1042.05 nm with output powers of −27.97 and −28.52 dBm, respectively. Both peaks
appeared at an LD pump level of 158.8 mW. Meanwhile, the
maximum energy of a single Q-switching pulse was 2.09 nJ,
the shortest pulse width was 1.16 µs, and the repetition rate of
the pulse varied from 52.52 to 58.73 kHz. The results obtained
from this experiment demonstrated that BP thin film has a
potential use in laser pulse generation that could prove useful
in a wide area of applications, especially in the manufacturing, medicine, military, and sensing fields.
5
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Laser Phys. Lett. 13 (2016) 085102
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6