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2230 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 29, NO. 24, DECEMBER 15, 2017
Polarization Domains and Polarization Locked
Vector Solitons in a Fiber Laser
Mengmeng Han, Xingliang Li, and Shumin Zhang
Abstract— We have experimentally observed, for the first time,
the coexistence of polarization domains (PDs) and polarization
locked vector solitons (PLVSs) in an Er-doped fiber laser with
microfiber-based graphene as a saturable absorber (SA). Taking
advantage of the high nonlinearity and the saturable absorption
of the microfiber-based graphene SA, we observed both PDs
and their splitting into regularly or irregularly distributed
multiple PDs under relatively high pump power; at lower pump
power with careful adjustment of the intra-cavity polarization
controllers, we observed PLVSs either as disordered soliton
bunch or with harmonic mode locking. The conditions under
which these patterns formed have also been experimentally
investigated in detail.
Index Terms—Optical fiber laser, polarization domain, vector
soliton, graphene.
I. INTRODUCTION
FIBER lasers have been widely applied in optical com-
munication, biomedical research, spectroscopy, metrol-
ogy [1]–[4], etc. because of their simple implementation,
low cost and compactness. Due to the existence of bire-
fringence, fibers can support two degenerate modes that are
polarized in orthogonal directions, and in each direction,
the fiber laser always oscillates in multiple longitudinal cavity
modes. If the laser cavity does not include a polarization
dependent device, the cross coupling between these two
orthogonal polarization modes can lead to the formation of
polarization domains (PDs). Since Zakharov and Mikha˘ılov [5]
first theoretically predicted the formation of PDs in non-
linear optics, research on PDs has attracted great attention.
Wabnitz et al. theoretically investigated and experimentally
confirmed PD formation between counter-propagating beams
in nonlinear optical fibers [6], [7]. The formation of PDs in
optical fibers was further confirmed experimentally [8], [9],
and subsequently, researchers have observed PDs in Er-doped
fiber (EDF) lasers. Gao et al.[10] experimentally observed PDs
in an EDF ring laser. Recently, Lecaplain et al.presenteda
simple theoretical model to explain the PD formation and
Manuscript received September 5, 2017; revised October 24, 2017; accepted
November 6, 2017. Date of publication November 10, 2017; date of current
version November 17, 2017. This work was supported in part by the National
Natural Science Foundation of China under Grant 11374089 and Grant
61605040, in part by the Hebei Natural Science Foundation under Grant
F2017205162, Grant F2017205060, and Grant F2016205124, and in part by
the Program for High-Level Talents of Colleges and Universities in Hebei
Province under Grant BJ2017020. (Corresponding author: Shumin Zhang.)
The authors are with the Hebei Advanced Thin Films Laboratory,
College of Physics Science and Information Engineering, Hebei Normal
University, Shijiazhuang 050024, China (e-mail: hmm19880427@163.com;
lixingliangkaoyan@163.com; zhangsm_optics@126.com).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2017.2772290
PD wall complexes in EDF lasers operating with either a
normal path-averaged dispersion or an net anomalous dis-
persion [11], [12]. Tang et al.[13] experimentally observed
and theoretically analysed PD formation between two linearly
polarized beams co-propagating in a weakly birefringent,
negative dispersion EDF laser, and pointed out the PD for-
mation was a general feature of quasi-isotropic cavity fiber
lasers. In 2015, our group obtained PDs in an Yb-doped fiber
laser [14].
If a saturable absorber (SA) is inserted into a fiber laser,
the phase of the multiple longitudinal cavity modes will be
locked, and a mode locked pulse will form. In this case,
if the cavity still has no polarization-sensitive devices, another
polarization dynamic pattern, vector solitons (VSs) can be
obtained because of the birefringence in the fiber. If the VSs
maintain both their temporal and polarization state profiles
during propagation, such VSs are known as polarization locked
vector solitons (PLVSs). Mou et al. experimentally observed
single-pulse PLVS and bound state PLVSs in EDF lasers
mode locked using a carbon nanotube (CNT) SA [15], [16].
Tang et al.[17] experimentally observed high-order PLVSs
in an EDF laser with a semiconductor saturable absorber
mirror (SESAM). Recently, our group achieved polarization
locked noise-like pulses and a rich set of PD wall pulses
in an EDF laser using a microfiber-based topological insula-
tor (TI) SA [18]. Compared with CNTs, TI, and an SESAM,
a graphene saturable absorber (GSA) has lower losses, ultrafast
recovery time and a broad bandwidth. Consequently, single-
pulse PLVSs [19], bound state PLVSs [20], and multi-pulse
PLVSs [21] have all been observed in graphene mode locked
EDF lasers. Usually, the SAs mentioned above were deposited
onto a fiber end-face or a side-polished fiber (also called
“D-shaped fiber”). Recently, a microfiber-based GSA has been
demonstrated to possess a high thermal damage threshold,
low polarization dependent losses, and a strong nonlinear
optical response [22], [23], which has been employed for pulse
shaping in EDF lasers [24]–[27].
As mentioned above, the observations of polarization locked
mode locked pulses and PDs were in two separate systems.
Therefore, the question arises as to whether or not the
PDs and the PLVSs can coexist in a same microfiber-based
GSA EDF laser. This was the initial motivation for our
work.
In this letter, we investigated different polarization dynamic
states in a mode locked EDF laser with a microfiber-based
GSA. Both the PDs, together with their splitting into regularly
and irregularly distributed multiple PDs, and the PLVSs,
including the fundamental pulse, disordered soliton bunch, and
1041-1135 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
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HAN et al.: PDs AND PLVSs IN A FIBER LASER 2231
Fig. 1. The experimental setup of the fiber laser (a). The nonlinear trans-
mission curve of the microfiber-based GSA (b).
harmonic mode locking, have all been obtained in our laser
cavity.
II. EXPERIMENTAL SETUP
The experimental setup of the proposed fiber laser is shown
in Fig. 1(a). It contains a 5-m long EDF (Fibercore M-12
(980/125)) with a dispersion of 32 ps2/km and a 15.9 m long
standard single mode fiber with a dispersion of −22 ps2/km.
The net cavity dispersion is −0.19 ps2. A 980-nm laser
diode (LD) with a maximum output power of 850 mW was
used to pump the EDF through a 980/1550 nm wavelength-
division multiplexer (WDM). A polarization-insensitive
isolator (PI-ISO) with an isolation of 45 dB was employed
to force the unidirectional operation. The polarization con-
troller (PC1)andPC
2were used to adjust the polarization state.
A 90:10 OC1was used to extract 10% signal lasing. In order
to observe the vector characteristics, another PC3and a fiber-
based polarization beam splitter (PBS) were connected to a
3-dB OC2. An optical spectrum analyzer (Yokogawa
AQ6317C) with a maximum resolution of 0.01 nm, a 33-GHz
real-time oscilloscope (Agilent Technologies, DSA-V-334A)
with two 45-GHz photodetectors (Newport), and a radio fre-
quency (RF) spectrum analyzer (Agilent N9020A) were used
to observe the optical spectrum, temporal pulse shape, and the
stability.
The GSA was made by depositing a homogeneous graphene
ethanol solution of concentration 0.5 mg/ml onto a microfiber
with waist diameter of ∼7µm stretched by a flame-
heated taper drawing device. The detailed operation of the
microfiber-based GSA was similar to that described in [28].
A microscope image of the microfiber-based GSA is shown
in the dashed box of Fig. 1(a). Fig. 1(b) shows the nonlinear
transmission curve of the microfiber-based GSA. The modula-
tion depth is ∼6.3%, the saturable intensity is ∼27.6 MW/cm2,
and the nonsaturable loss is ∼60.7%. This high nonsaturable
loss could be decreased by optimizing the fabrication qual-
ity of microfiber and improving the non-uniformity of the
graphene deposited on the microfiber.
III. EXPERIMENTAL RESULTS
A. The Polarization Domains
When the pump power was increased to 86.3 mW, self-
pulsing was observed. By further increasing the pump power
to 147.1 mW, the fundamental PD was achieved with an output
power of 4.8 mW. Fig. 2(a) shows the temporal characteristics
of the PDs, in which the extents of the domain along the
Fig. 2. The two orthogonal PDs (a) and (c) with different durations; the
corresponding optical spectra (b) of (a).
x axis and y axis are 1.3 ns and 99.5 ns, respectively. The
corresponding spectra are shown in Fig. 2(b). The central
wavelengths of the orthogonal polarization components are
1566.44 nm (x axis) and 1566.81 nm (y axis), respectively,
and the wavelength separation is 0.37 nm, which indicates
that the PDs are incoherently coupled in the cavity. The
results showed that when the oscillation along one optical
axis was ‘on’, the other was complete ‘off’. Since the cav-
ity loss can be changed through changing the linear cavity
birefringence, the width of a PD can be tuned by rotating the
intracavity PCs, and as there existed only two domains in the
cavity, increasing of the PD duration in one polarization state
equally decreased the duration in the other polarization state
asshowninFig.2(c).
Further increasing the pump power to 189.9 mW and rotat-
ing the intra-cavity PCs simultaneously, resulted in splitting of
the PD. In general, the domain widths and the space between
the adjacent domains were all different. For example, Fig. 3(a)
shows the irregularly distributed PDs, where two extra PDs are
formed in one cavity period. The corresponding optical spectra
are shown in Fig. 3(b). The central wavelengths separation
between the x axis and y axis is 0.09 nm, which indicates that
the PDs are also coupling incoherently in the cavity.
On further increasing the pump power to 207.5 mW and
slightly rotating the intra-cavity PCs, regularly distributed PDs
with the same domain width were obtained. Fig. 3(c) shows
the 8th harmonic PDs, with their corresponding optical spectra
shown in Fig. 3(d). In this case, the central wavelengths
separation between the x- and y-axis has changed to 0.72 nm,
and the regularly distributed PDs are still incoherent coupling.
B. The Polarization Locked Vector Solitons
Under relatively low pump power and by carefully adjusting
the intra-cavity PCs, we observed PLVSs. Fig. 4(a) shows the
optical spectra before and after passing through the PBS of the
fundamental mode-locked VSs obtained with a pump power
of 36.9 mW. The output power is 1.08 mW. The obvious
Kelly sidebands of the spectra confirm that the fiber laser
operated in the negative dispersion regime [29]. The central
wavelengths of the orthogonal polarization modes are both
1565.97 nm, the same as the initial central wavelength, which
indicates that the orthogonal polarization modes maintain
2232 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 29, NO. 24, DECEMBER 15, 2017
Fig. 3. The irregular PDs (a) and their spectra (b) along the x axis and
yaxis;the8
th harmonic PDs (c) and their spectra (d) along the x and y axis.
Fig. 4. The characteristics of fundamental PLVSs. The initial and x/y-axis
spectra (with 0.5-nm resolution) (a), the inset shows the autocorrelation trace
of the initial signal; the temporal traces (b) of the initial and x/y-axis outputs.
both their temporal and polarization state during propagation,
as expected for PLVSs [30], [31]. In addition, the CW on
the spectrum is high, which may be due to the shallow
modulation depth of the GSA [32]. The inset shows the auto-
correlation trace of the initial signal. Assuming a sech2pulse
profile, the full width at half maximum is about 1.47 ps. The
3-dB spectral width of the initial pulse is 2.94 nm, and the
corresponding time-bandwidth product is about 0.529, which
indicates that the pulses are chirped. The temporal pulse traces
of the initial and the orthogonal laser outputs are shown
in Fig. 4(b). The fundamental repetition period of the laser
is 101 ns, corresponding to the cavity length of 20.9 m.
Increasing the pump power to 42.7 mW, multiple VSs
appeared in the cavity. With different orientations of the PCs
and pump powers, the VSs might occupy either all or part of
the available space along the cavity. For example, Fig. 5(a)
shows the VS bunch. In this case, many pulses grouped
themselves into a tight packet. The two polarization compo-
nents had the same bunched pulse envelope as the initial one.
Fig. 5(b) shows the spectra before and after passing through
the PBS. The symmetric Kelly sidebands of the spectra can
also be observed. The central wavelengths of the initial signal
and the orthogonal polarization modes are all 1566.20 nm,
which indicates the soliton bunch is the PLVS bunch.
Adjusting the intra-cavity PCs, the soliton bunches became
unstable and occupied all the available space along the cavity
in a disorderly state. Carefully adjusting the PCs a little further
and when the CW lasing was settled in an appropriate position
in the spectrum, the harmonic mode locking formed. Different
harmonic mode locked states of the VSs have been obtained by
simply varying the pumping strength. As an example, Fig. 5(c)
shows the 61st PLVS harmonic mode locked pulses before and
Fig. 5. The initial and the polarization resolved temporal soliton bunch
(a) and the optical spectra (b) (with 0.5-nm resolution); the 61st PLVSs
harmonic mode locking (c) of the initial and x/y-axis laser outputs and their
spectra (d) (with 0.5-nm resolution), the insert shows the RF spectrum.
after passing through the PBS at a pump power of 188 mW
with a repetition rate of 603.9 MHz. We also found that the
pulse amplitudes were modulated in a basic cavity repetition
period because of the super-mode noise, which gives rise to
unequal distribution of energies among the generated optical
pulses and results in amplitude fluctuations of the output
pulses [33]. The corresponding spectra are shown in Fig. 5(d).
The central wavelengths of the initial pulse and the two
orthogonal polarization modes are all 1566.35 nm, illustrating
that the harmonic mode locked pulses are PLVSs. The inset
of Fig. 5(d) shows the corresponding RF spectrum with a
resolution bandwidth of 5 kHz between 500 and 700 MHz,
and the signal to noise ratio is 33 dB, which indicates that the
laser operated in a relatively stable regime. Since there were
no mode selecting management in the cavity [34], when the
pump power was strong enough, several longitudinal modes
would overcome the cavity loss then oscillate, which resulted
in the formation of small peaks at the center of the spectrum
for Figs. 5(b) and 5(d).
In this experiment, we have observed the coexistence of PDs
and PLVSs in an EDF laser. The formation conditions can be
understood as follows: because of the cavity birefringence,
the laser always simultaneously oscillates in two orthogo-
nal polarization modes, and along each polarization mode,
the laser is oscillating in multiple longitudinal cavity modes.
In general, these multiple longitudinal cavity modes have no
fixed phase difference, and due to the beating among the lasing
longitudinal cavity modes along the polarization, on the oscil-
loscope traces the CW state of the laser emissions looks noisy.
When the pump intensity was weak, both modes oscillated
simultaneously in the cavity. In this case the modes oscillated
independently and no coupling between them. As the pump
power was increased to a certain level, the cross coupling
between these two orthogonal polarization modes could lead
to intensity alternation between them, and the PDs formed.
Since the microfiber only has a waist diameter of ∼7µm,
corresponding to the effective mode field area of 38.48 µm2,
it can provide high nonlinearity. In addition, the nonlinear
susceptibility of graphene is as large as 10−7esu [35], these
high nonlinear effect would make the two orthogonal polariza-
tion modes have a greater cross coupling, which is beneficial
HAN et al.: PDs AND PLVSs IN A FIBER LASER 2233
to the formation of stable PD [36]. Given the periodic fast
polarization switching of the PDs, the width of a PD could be
tuned through changing the linear cavity birefringence, which
was realized by rotating the intracavity PCs. On the other
hand, when the SA begins to work by adjusting the pump
power and the PCs, the phase of the multiple longitudinal
cavity modes can be locked, and PLVSs can be formed.
In addition, because of the high nonlinear effect provided
by the microfiber and the graphene, both the PDs and the
PLVSs would split under higher pump power, and then either
regularly or irregularly distributed multiple PDs, disordered
bunched PLVSs and harmonic mode locked PLVSs would be
formed. We also found that the pump threshold for the PD
was higher than that of the PLVS while the pump conversion
efficiency was lower, which indicated that the cavity loss was
higher when PDs were present in the fiber laser.
IV. CONCLUSION
In conclusion, we have experimentally investigated the
vector characteristics of an EDF laser with a microfiber-based
graphene SA. A rich variety of dynamic states, including the
fundamental PD, irregularly and regularly distributed PDs,
the fundamental PLVS, bunched PLVSs, and harmonic mode
locked PLVSs have all been observed by adjusting the pump
power and the PCs. The formation conditions of these operat-
ing modes have been experimentally investigated in detail.
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