All-fiber polarization locked vector soliton
laser using carbon nanotubes
C. Mou,* S. Sergeyev, A. Rozhin, and S. Turistyn
Photonics Research Group, School of Engineering and Applied Science, Aston University, Birmingham, UK, B4 7ET
*Corresponding author: mouc1@aston.ac.uk
Received June 13, 2011; accepted August 1, 2011;
posted September 7, 2011 (Doc. ID 149152); published September 23, 2011
We report an all-fiber mode-locked erbium-doped fiber laser (EDFL) employing carbon nanotube (CNT) polymer
composite film. By using only standard telecom grade components, without any complex polarization control
elements in the laser cavity, we have demonstrated polarization locked vector solitons generation with duration
of ∼583 fs, average power of ∼3 mW (pulse energy of 118 pJ) at the repetition rate of ∼25:7 MHz. © 2011 Optical
Society of America
OCIS codes: 060.3510, 140.4050, 250.5530.
Mode-locked ultrafast erbium-doped fiber lasers
(EDFLs) have shown many advantages over conven-
tional solid-state light sources in various areas such as
optical communication, metrology, sensing, medical ap-
plication, etc. A range of approaches have been used to
achieve mode-locking in EDFLs. The two currently most
popular techniques are nonlinear polarization rotation
(NPR) [1] and semiconductor saturable absorber mirrors
(SESAMS) [2,3], though other methods of mode-locking
are actively studied. In practical terms, the search is for
an optimal balance between laser performance and cost.
Recently, carbon nanotube (CNT) based saturable ab-
sorbers have attracted a great deal of attention due to
their inherent nonlinear optical properties, such as fast
recovery time, low cost, wide band absorption, ease of
fabrication, and low cost [4,5]. So far, various mode-
locked lasers implemented by CNT have been demon-
strated, including fiber lasers [6–11], waveguide lasers
[12], and semiconductor lasers [13]. Though polarization
has been taken into consideration in some works [14–17]
when designing or applying CNT in the laser cavity, in
general, polarization properties of the CNT mode-locked
lasers have not yet been fully explored.
In this Letter, we use a CNT-polyvinyl alcohol (PVA)
thin film saturable absorber with absorption peak at
1:5 μm region to implement an ultrafast mode-locked
EDFL with stable single polarization output. The gener-
ated pulses present vector solitons [18–25] in which po-
larization state is locked and stabilized by nonlinearity.
Efficient absorption of CNTs at specific wavelength is de-
termined by the band gap of the specific chiralities of
semiconducting single wall CNTs. We use commercially
available purified HiPCO single wall CNTs (Unidym)
grown through high-pressure CO conversion. HiPCO
CNTs have tube diameter 0:8–1:3 nm, which give a strong
optical absorption at the 1000–1600 nm spectral region.
For preparation of the saturable absorber, the 2 mg of
CNTs are placed in 10 ml of deionized water containing
10 mg of sodium dodecylbenzene sulfonate (Sigma-
Aldrich) surfactant and then subjected to sonication
using commercial ultrasonic processor (Nanoruptor,
Diagenode) for one hour at 200 W and 20 kHz. In order
to remove residual bundles and impurities, the resulting
solution is subjected to ultracentrifugation, 25000 RPM
during one hour with Optima Max-XP ultracentrifuge
(Beckman Coulter). We mix the resulting solution with
PVA powder and placed in the Petri dish. The CNT-PVA
film was then obtained after drying the sample in the de-
siccator for a few days. The film has a homogeneous dis-
tribution of single wall carbon nanotubes on a submicron
scale, as confirmed by a featureless surface in optical mi-
croscopy. The absorption spectrum of the CNT-PVA sa-
turable absorber shown in Fig. 1 is measured using a
Lambda 1050 UV-NIR spectrometer (Perkin Elmer). It
has typical features of HiPCO CNTs between 1000 and
1600 nm with the optical density about 0.4 at 1560 nm.
Figure 2 illustrates the schematic configuration of the
CNT mode-locked EDFL. The EDFL constitutes ∼
2 mof
highly concentrated erbium-doped fiber (EDF Er80-8/
125 from Liekki) as the gain medium. A fiber pigtailed iso-
lator (OIS) is employed to ensure single direction oscilla-
tion of the laser. The laser is pumped viaa gratingstabilized
980 nm laser diode (LD) using a 980 nm=1550 nm wave-
length division multiplexing (WDM). A set of commercial
diode laser driver and controller is employed to stabilize
the performance of the pump.
The nonlinear effects were reduced by minimization of
the length between the EDF and the location of a stan-
dard fused fiber coupler where 90% of light is coupled out
the laser cavity. An in-line polarization controller was in-
corporated in the laser cavity for optimization purpose.
The involvement of polarization controller does not af-
fect the laser to be self-started. The CNT mode-locker
was sandwiched between two standard fiber connector
400 600 800 1000 1200 1400 1600 1800
0.2
0.4
0.6
0.8
1.0
1.2
Absorbance (a.u)
Wavelength(nm)
Fig. 1. Absorption spectrum of the CNT-PVA saturable
absorber measured by a wideband spectrometer.
October 1, 2011 / Vol. 36, No. 19 / OPTICS LETTERS 3831
0146-9592/11/193831-03$15.00/0 © 2011 Optical Society of America
ferrules; index matching gel is applied to minimize the
transmission loss. This prepackaged saturable absorber
is then spliced into the laser cavity. The total length of the
laser cavity is ∼7:83 m. The whole cavity has an average
anomalous dispersion of about ∼16 :48 ps=nm=km that
will result in soliton output.
Figure 3 shows a typical output optical spectrum of the
EDFL centered at ∼1560 nm with a spectral bandwidth at
full width half-maximum (FWHM) of ∼3:72 nm. The pro-
nounced Kelly sidebands indicate fundamental soliton
shape of the output pulses. A typical pulse train is shown
in the inset of Fig. 3 with ∼38:9 ns interval between the two
adjacent pulses, thus giving a repetition rate of ∼25:7 MHz.
The output pulses have then been directly fed through a
commercial autocorrelator without any preamplification.
The measured autocorrelation trace corresponding to the
pulse duration of ∼583 fs is shown in Fig. 4.
The calculated time bandwidth product is ∼0:26, which
is lower than the theoretical value of 0.315. This is pos-
sibly due to the compression of pulse via the standard
fiber from the laser output to the autocorrelator. The
EDFL is pumped at ∼178 mW which allows ∼3 mW out-
put power corresponding to energy of ∼118 pJ.
The high pump power is provided by the high absorp-
tion of the gain fiber and high output coupling ratio. The
operation stability has been studied by measuring the
radio frequency (RF) spectrum of the laser with an elec-
trical spectrum analyzer. Figure. 5 plots the RF spectrum
at the fundamental frequency of laser oscillation showing
60 dB signal-to-noise ratio with 10 Hz resolution. This in-
dicates a low relative timing jitter Δt=T, where T is the
round trip time. Applying the approach described in [26]
we can estimate Δ
t=T ≈ 6 × 10
−5
, corresponding to a
timing jitter Δt ≈ 2:3 ps.
The polarization properties studied using a commer-
cial polarimeter with 1 μs resolution and measurement
interval of 1 ms which corresponds to 40–40000 round
trips are presented in Fig. 6 that shows both the exact
polarization state of the output pulse in the Poincare
sphere and the evolution of the measured state of elliptic
polarization (with the phase-difference of −π=2) locked
by vector soliton and the degree of polarization (DOP)
close to 92%.
Figure 6(a) illustrates a stable polarization operation of
the EDFL in the Poincare sphere with the axis defined by
the three Stokes parameters. Two orthogonally polarized
vector soliton states are the eigenstates for this laser and
the soliton coupling determine stable sl ow polarization
dynamics. Figure 6(b) presents the measured polariza-
tion evolution in terms of optical power and also a stable
DOP within the measured time scale. At the laboratory
conditions, the output pulses show stable elliptical polar-
ization state for 24 hours without any significant signal
degradation. Note that the output polarization state
can be controlled by tuning the PC in the laser cavity
or after the output coupler.
Fig. 2. (Color online) Schematic illustration of the proposed
CNT mode-locked EDFL.
1545 1550 1555 1560 1565 1570 1575
-65
-60
-55
-50
-45
-40
-35
-30
-25
Intensity(dBm)
Wavelength(nm)
0 100 200 300 400 500
-0.2
0.0
0.2
0.4
0.6
0.8
Voltage (a.u)
Time(ns)
Fig. 3. Output optical spectrum with pronounced Kelly
sidebands indicating soliton pulse shape. Inset shows typical
pulse train with a repetition rate of 25:7 MHz.
012345
0
100
200
300
400
500
600
700
800
Intensity (a.u)
Time (ps)
Autocorrelation Trace
Sech
2
Fit
FWHM=898fs
pulse width=583fs
Fig. 4. (Color online) Measured autocorrelation trace of the
output pulse showing pulse duration of ∼583 fs.
24 25 26 27 28
-110
-100
-90
-80
-70
-60
-50
-40
-30
Intensity (dBm)
Frequency (MHz)
~60dB
Fig. 5. Measured RF spectrum of the mode-locked EDFL at
the fundamental oscillation frequency.
3832 OPTICS LETTERS / Vol. 36, No. 19 / October 1, 2011
In conclusion, we have proposed and demonstrated
a CNT-PVA thin film mode-locked EDFL. The laser with
relatively simple design generates polarization locked
vector soliton pulses with ∼583 fs temporal width and
an output power of ∼3 mW (pulse energy of ∼118 pJ) with
a repetition rate of ∼25:7 MHz at 1560 nm.
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0 200 400 600 800 1000
1.0
1.5
2.0
2.5
3.0
3.5
86
88
90
92
Power (mW)
Time (
µ
s)
I
x
I
y
I
Total
(b)
(a)
Degree of Polarization (%)
Fig. 6. (Color online) Time evolution of polarization locked
vector solitons for the time frame of 40–40000 round trips
(1 μs–1 ms) in terms of (a) Stokes parameters at Poincare
sphere and (b) intensities of orthogonally polarized modes; la-
ser power of I
x
(black dashed line) and I
y
(red dotted line);
total intensity: I
tot
¼ I
x
þ I
y
(blue solid line) and degree of
polarization (DOP).
October 1, 2011 / Vol. 36, No. 19 / OPTICS LETTERS 3833
