Manipulation of operation states by polarization
control in an erbium-doped fiber laser with a
hybrid saturable absorber
Kuei-Huei Lin1, Jung-Jui Kang2, Hsiao-Hua Wu3, Chao-Kuei Lee2, and Gong-Ru Lin4
1
Department of Science, Taipei Municipal University of Education
1, Ai-Kuo West Rd., Taipei 100, Taiwan
2
Department of Photonics, National Sun Yat-Sen University
70, Lien-Hai Rd.,Kaohsiung 804, Taiwan
3
Department of Physics, Tunghai University
181, Sec. 3 Chung Kang Rd., Taichung 40704, Taiwan
hhwu@thu.edu.tw
4
Graduate Institute of Photonics and Optoelectronics, and Department of Electrical Engineering,
National Taiwan University,
1, Roosevelt Rd. Sec. 4, Taipei 10617, Taiwan
grlin@ntu.edu.tw
Abstract: We propose an operation switchable ring-cavity erbium-doped
fiber laser (EDFL) via intra-cavity polarization control. By using a
semiconductor saturable absorber mirror in the EDFL cavity, stable
Q-switching, Q-switched mode-locking, continuous-wave mode-locking,
pulse splitting, and harmonic mode-locking pulses can be manipulated
simply by detuning a polarization controller while keeping the pump power
at the same level. All EDFL operation states can be obtained under the
polarization angles detuning within 180°. Continuous-wave mode-locking
of EDFL with 800-fs pulsewidth repeated at 4 MHz has been obtained, for
which the output pulse energy is 0.5 nJ and the peak power is 625 W.
Interaction between solitons and the accompanied non-soliton component
will lead to either pulse splitting or 5th-order harmonic mode-locking at
repetition rate of 20 MHz.
2009 Optical Society of America
OCIS codes: (140.3510) Lasers, fiber; (140.3540) Lasers, Q-switched; (140.4050)
Mode-locked lasers; (190.5970) Semiconductor nonlinear optics including MQW
References and links
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6.
7.
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9.
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#106038 - $15.00 USD
(C) 2009 OSA
Received 6 Jan 2009; revised 18 Feb 2009; accepted 21 Feb 2009; published 11 Mar 2009
16 March 2009 / Vol. 17, No. 6 / OPTICS EXPRESS 4806
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1. Introduction
High peak power and short pulsewidth fiber lasers are of interested in laser science and
technologies because they have many practical applications in various domains. The
operation of short-pulse lasers can be divided into three major types: Q-switched [1-3],
continuous-wave mode-locking (CML) [4-10], and Q-switched mode-locking (QML) [11-13].
Various active or passive technologies have been utilized in the construction of short-pulse
fiber lasers. All active pulsed lasers contain bulk elements, which make their design rather
complicated; therefore, much attention has been paid to the development of passive fiber
lasers. An all-fiber passively Q-switched erbium laser with a Co2+:ZnSe crystal as a
saturable absorber is demonstrated experimentally [1]. Giant pulses with energy of 3.6 nJ
and peak power of 0.7 mW have been obtained. Zenteno et al. report the generation of
Q-switched mode-locked pulses from an Nd-doped fiber laser that uses a solid-state solution
of BDN-I dye as the saturable absorber [11]. For an absorbed pump power of 110 mW,
pulses of 8-ns duration at a repetition rate of 14 MHz can be generated under an 800-ns-wide
Q-switched envelope at a repetition rate of 10 kHz, yielding an average output power of 8
mW near 1.06 µm.
Hakulinen et al. demonstrate that resonant high-modulation-depth saturable absorbers
allow efficient pulse shortening in Q-switched lasers [3]. Using a 70% modulation depth
resonant saturable absorber mirror (SESAM) they achieved 8 ns pulses that are close to the
limit set by the cavity length. A resonant SESAM with a high reflectivity change also allows
reliable start-up of passive mode locking in a wide range of normal or anomalous cavity
dispersion [4-5]. With SESAMs, the mode-locked regime can be achieved for different
values of cavity dispersion for a broad spectrum ranging from 800 to 1600 nm. Grudinin et
al. studied passive harmonic mode-locking (HML) in soliton fiber lasers [14]. They
demonstrated that the laser performance could be further improved by the use of a SESAM in
combination with a nonlinear amplifying loop mirror. The SESAM acts not only as a fast
saturable absorber but also as a passive phase modulator. They demonstrated that such a
laser is capable of generating 500-fs pulses at repetition rates exceeding 2 GHz. For
diode-pumped solid-state lasers (DPSL), the laser operation state was found to be dependent
on the pump power [6]. At low pump powers, the laser operates in the continuous-wave
(CW) state. As the pump power is increased, the laser reaches QML state. At even higher
pump powers, the laser turns into CML state. However, the pump power for stable QML
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(C) 2009 OSA
Received 6 Jan 2009; revised 18 Feb 2009; accepted 21 Feb 2009; published 11 Mar 2009
16 March 2009 / Vol. 17, No. 6 / OPTICS EXPRESS 4807
operation with regular QML pulses is limited within a small range. For certain applications,
switching between different short-pulse states while keeping the same averaged output power
will be useful.
In this paper, we propose a method for manipulating the output of a ring-cavity
erbium-doped fiber laser (EDFL), by which the laser can be operated in CW and various
short-pulse states. By use of a semiconductor saturable absorber mirror and polarization
control in the laser, different operation states such as continuous-wave, stable Q-switching,
Q-switched mode-locking, continuous-wave mode-locking, pulse splitting, and harmonic
mode-locking can be obtained while the pump power remains unchanged. All EDFL
operation states can be realized within polarization tuning angles of 180°. The role of
polarization control in the formation of Q-switching, Q-switched mode-locking, and CML
together with the mechanism bring about pulse splitting and harmonic mode-locking are
discussed.
2.
Experiments
Figure 1 shows the experimental setup of the EDFL. The output port of a tunable C-band
erbium-doped fiber amplifier (EDFA, SDO Corp.) was connected to its input port through a
2×2 3-dB coupler, a polarizer and a polarization controller, by which a ring cavity EDFL was
constructed. The cavity length of EDFL is about 50 m. All the fiber components in the
laser cavity are linked via single-mode fibers with FC/PC or FC/APC connectors. One of the
coupler ports was connected to the SESAM (Batop, SAM 1550-30-10ps-FC/APC), which has
an unsaturated absorption of A0 = 30% and modulation depth of ∆R = 18% at 1550 nm, with
saturation fluence of 70 µJ/cm2. The low intensity spectral reflectance of this SESAM is
monotonically increasing with respect to wavelength between 1530 nm and 1620 nm.
Another port of the 3-dB coupler was used as the EDFL output, which was then connected to
the measurement instruments.
EDFA
SESAM
Polarization
Controller
50/50
Coupler
Polarizer
Output
Fig. 1. Schematic diagram of the erbium-doped fiber laser.
The EDFL output was measured by a power meter (ILX OMM-6810B), a high speed
InGaAs detector (Electro-Physics Technology, ET 3000) that was connected to an
oscilloscope (Tektronix TDS 2022), and an optical spectrum analyzer (Ando AQ6317B).
While manually tuning the polarization controller, the laser operating states were monitored
by using the InGaAs detector and the oscilloscope, and both of the temporal and spectral
behavior of the EDFL was recorded. A noncollinear autocorrelator (Femtochrome
FR-103XL) was used to measure the width of mode-locked pulses.
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Received 6 Jan 2009; revised 18 Feb 2009; accepted 21 Feb 2009; published 11 Mar 2009
16 March 2009 / Vol. 17, No. 6 / OPTICS EXPRESS 4808
3.
Results and discussion
The EDFL can firstly be tuned to operate in the CW state [Fig. 2(a)]. With laser diode (LD)
pump current of 70 mA (corresponding to LD pump power of 42 mW), the EDFA output
power is 10.2 dBm. When operated in CW state, the EDFL output power is 3.3 dBm and the
laser wavelength is 1559 nm, which is different from the ASE peak of this EDFA at 1532 nm.
By manually adjusting the polarization controller, the EDFL can be operated in the
Q-switching state [Fig. 2(b)], and the pulse repetition rate is dependent on the LD pump power.
The repetition rate of passively Q-switched laser pulses increases with the EDFL intracavity
power. We have observed that even with fixed LD pump power, the Q-switched pulse
repetition rate can be slightly tuned by adjusting the polarization controller. For cavity
length of 50 m and LD pump power of 42 mW, the range of repetition rate for Q-switched
pulses is 18.4-20.8 kHz, and the pulsewidth is 4.1 µs. The center wavelength of Q-switched
EDFL is 1532 nm, which is also the ASE peak of the EDFA. As the polarization controller
was adjusted while maintaining the LD pump power, the EDFL operation state gradually
changed from Q-switching to Q-switched mode-locking state [Fig. 2(c)]. The repetition rate
for the Q-switching envelope is about 20 kHz, while the QML pulse repetition rate can be 4
MHz or higher-order harmonics (64 MHz has been achieved). The QML sate can either be
operated at center wavelength of 1532 nm or 1558 nm, depending on the polarization
adjustment. This adjustment is capable of manipulating central wavelength dynamics in
such EDFLs [15]. For short cavity configurations and low pump powers, the optical
feedback from fiber connections outside the EDFL cavity could switch the QML to
Q-switched state, which is improved by connecting the output port of 3-dB coupler to an
optical isolator. As shown in Fig. 2(c), a pedestal is superimposed on the QML pulses.
This can be improved by increasing the LD pump current to 100 mA and removing the
polarizer and polarization controller.
0.08
(a)
Intensity (a.u.)
Intensity (a.u.)
0.08
0.06
0.04
0.02
0.06
0.04
0.02
0.00
0.00
-150
(b)
-100
-50
0
50
100
-150
150
-100
-50
Time ( µs)
0.08
(c)
50
100
150
(d)
0.6
Intensity (a.u.)
Intensity (a.u.)
0
Time ( µs)
0.06
0.04
0.02
0.4
0.2
0.0
0.00
-20
-10
0
10
Time ( µs)
20
30
-1000
-500
0
500
1000
Time (ns)
Fig. 2. (a) CW lasing signal on oscilloscope; (b) Q-switched EDFL output pulse train; (c)
Q-switched mode-locking EDFL output pulse train; (d) CW mode-locking EDFL output
pulse train.
The result for polarizer-free Q-switched mode-locking EDFL are shown in Fig. 3, where
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Received 6 Jan 2009; revised 18 Feb 2009; accepted 21 Feb 2009; published 11 Mar 2009
16 March 2009 / Vol. 17, No. 6 / OPTICS EXPRESS 4809
the QML envelope repetition rate has been increased to about 115 kHz due to the increased
intracavity power and the QML pulse spacing is reduced to 200 ns due to the shortening of
laser cavity length. Further adjustment of the polarization controller eventually turns the
QML state into continuous-wave mode-locking state [Fig. 2(d)]. The pulse repetition rate is
again 4 MHz, corresponding to cavity round-trip time of 250 ns and cavity length of about 50
m. Figures 4(a) and 4(b) show the autocorrelation trace and spectrum of the CML pulses
respectively. The EDFL operated at CML mode exhibits central wavelength of 1531 nm
associated with two narrow-band peaks at 1524.2 and 1528.5 nm. Since we do not
incorporate dispersion compensation components in the cavity, the total intracavity dispersion
is anomalous at 1550 nm band. The measured CML-EDFL pulsewidth is 800 fs with a
spectral linewidth of 8 nm, giving rise to a time-bandwidth product deviated from
transform-limit situation. Therefore, the CML-EDFL pulses are not transform-limited at
current stage, and a shorter pulsewidth can be expected by utilizing intracavity/extracavity
dispersion compensation method. The EDFL output pulse energy is 0.5 nJ and the peak
power is calculated to be 625 W. At some particular polarization adjustments, we have
observed that the mode-locked EDFL output spectrum spans from 1530 nm to 1560 nm, but
this operation state depends strongly on the environments.
Intensity (a.u.)
0.05
0.04
0.03
0.02
0.01
0.00
-15
-10
-5
0
5
10
15
Time ( µs)
Fig. 3. Pedestal-free Q-switched mode-locked EDFL output pulse train, in which the LD
pump current is increased, and the polarizer and polarization controller are removed.
0.020
(a)
(b)
1.0
Intensity (a.u.)
Intensity (a.u.)
1.2
0.8
0.6
0.4
0.2
0.015
0.010
0.005
0.000
0.0
0
5
10
15
20
Time (ps)
25
30
35
1500
1510
1520
1530
1540
1550
Wavelength (nm)
Fig. 4. (a) Autocorrelation trace and (b) spectrum of the CML pulses.
pulsewidth is 800 fs and the spectral width is 8 nm.
The measured
The switching between EDFL operation states could be attributed to the incorporated loss
modulation given by the intensity-dependent polarization evolution under the control of
intracavity polarizer and SESAM. In our case, the effective intensity-dependent loss
modulator results from the combining effects of the polarization controller, the erbium-doped
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Received 6 Jan 2009; revised 18 Feb 2009; accepted 21 Feb 2009; published 11 Mar 2009
16 March 2009 / Vol. 17, No. 6 / OPTICS EXPRESS 4810
fiber (EDF), the intracavity single-mode fiber (SMF) links, and the polarizer, as
schematically shown in Fig. 5. In principle, the nonlinear phase shift induced polarization
evolution in the EDFL configured with SMF is mainly caused by self-phase modulation
(SPM) and cross-phase modulation (XPM) [17]. In addition, the polarized electric field of
the pumping source also introduces a birefringence in the EDFL for intensity-dependent
phase retardation. As a result, the CW and short-pulse states will experience different phase
retardation during intracavity circulation. The polarization controller works as a manually
tunable phase retarder to provide pre-defined phase retardation, while the polarizer works as
an analyzer. The intensity-dependent loss will be encountered when passing the optical
field through such an effective modulator, providing different transmission losses for the lowand high-intensity components. By properly tuning the polarization controller, one kind of
the operation states can benefit a smaller loss for lasing. While operating in CML or HML
states, the EDFL should be considered as a passive mode-locked laser with dual or hybrid
mode-locking mechanism. Recently, Xu et al. calculated the cavity transmission coefficient
for a CML-EDFL using the nonlinear polarization rotating technique [15], in which the cavity
transmission coefficient is express as a function of the net phase delay. In the numerical
calculation they have taken into account the nonlinear phase change, the phase delay induced
by the polarization controller, and the fiber birefringence.
They have observed
experimentally the evolution of center wavelength with different operation state. Using this
concept, we can explain the wavelength switching of the QML sate between 1532nm and
1558 nm, as well as the observation that the mode-locked EDFL output spectrum could span
from 1530 nm to 1560 nm at particular polarization adjustments. The mechanism
responsible for the nonlinear polarization evolution can also explain that the EDFL operation
states strongly depend on the initial bending or twisting of the intracavity fiber components.
Polarization Controller
Intracavity
SMF Links
Polarizer
Erbium-Doped Fiber
Fig. 5. An effective intensity-dependent loss modulator consists of a polarization controller,
a section of erbium-doped fiber, the intracavity fiber links (dashed line), and a polarizer
In more detail, Okhotnikov et al. have stated that the pulse evolution in EDFL cavity can
be described by modifying the rate equation of time-dependent saturable loss of the SESAM
with Ginzburg-Landau equation [5]:
∂δ s (t )
δ (t ) − ∆R | ψ (t ) |2
(1)
=− s
−
δ s (t ) ,
∂t
E sat , A
τ rec
where δs(t) and ∆R denote the time-dependent and stable saturable losses of SESAM, τrec is
the recovery time of SESAM, ψ(t) is the optical intensity, and Esat,A is the saturation energy of
SESAM. According to the results given by Okhotnikov and co-workers, the critical
intracavity pulse energy Ec required for the stable operation of CML against
Q-switching/QML should obey the criterion of
(2)
Ec = Esat ,G E sat , A ∆R ,
where Esat,G is the gain saturation energy of the EDFL. For a stable CML operation, the
intracavity pulse energy must be larger than Ec. As estimated, the parameters for the
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Received 6 Jan 2009; revised 18 Feb 2009; accepted 21 Feb 2009; published 11 Mar 2009
16 March 2009 / Vol. 17, No. 6 / OPTICS EXPRESS 4811
SESAM used in the EDFL are Esat,A = 0.18 nJ and ∆R = 0.18. and the gain saturation energy
is Esat,G = 10 µJ (adopted from Ref. 16). The value of Esat,GEsat,A∆R is calculated to be 324
nJ2, and the critical pulse energy is Ec = 18 pJ. The maximum intracavity CML pulse energy
Ep obtained for this EDFL is 2.5 nJ. Obviously, Eq. (2) cannot be satisfied if a SESAM
alone is used as the absorber, while the EDFL will not self-start from Q-switching instability
into continuous-wave mode-locking. In this case, either an increase in the pulse energy (by
increasing the pump power or decreasing the pulse repetition rate), or an additional
mode-locking mechanism must be introduced into the EDFL cavity for stable CML operation.
In our wok, the transition from Q-switched to mode-locked regime is carried out via the
polarization control under constant pump power.
(a)
0.15
0.10
0.05
0.00
(b)
0.20
Intensity (a.u.)
Intensity (a.u.)
0.20
0.15
0.10
0.05
0.00
-1000
-500
0
500
1000
-400
-200
0
200
400
Time (ns)
Time (ns)
Fig. 6. Adjustment on the polarization leads to (a) the splitting CML-EDFL pulse, and (b)
the HML-EDFL.
Further adjustment of the polarization controller enable us to split a CML-EDFL pulse into
several pulses [Fig. 6(a)], and the EDFL operation can evolve into the harmonic mode-locking
under proper polarization control [Fig. 6(b)]. In our experiment, stable 5th-order HML has
been observed, with pulse repetition rate of 20 MHz and pulse spacing of 50 ns. By
examining the EDFL output with a half-wave plate and a polarizer, we find that the splitted
pulses are of the same polarization state. As shown in Fig. 4(b), the spectrum for
fundamental mode-locked EDFL presents nonsoliton components in the form of narrow-band
peaks located at 1524.2 nm and 1528.5 nm. This nonsoliton component plays a significant
role for the EDFL to operate in either pulse-splitting or HML regime [14]. The interaction
between solitons and the accompanying nonsoliton component may attract or repel adjacent
solitons to form either pulse splitting or HML. In our experiments, pulse splitting with
temporal spacings of about 15 ns has been observed. However, HML higher than 6th order
are not stable against environments and we believe that higher-order HML will be obtained by
increasing intracavity laser powers and using more delicate polarization tuning mechanisms.
In particular, the polarization tuning angles for various EDFL operation states depends on the
initial bending/twisting of intracavity fiber components. Nevertheless, all operation state,
from CW state to harmonic mode-locking state, can be routinely obtained by tuning the
polarization controller. Table 1 shows typical polarization tuning angles and related output
powers for various EDFL states by using an electronic lightwave polarization controller
(FiberPro PC4002), where the angles are measured at the center of each operation range with
the CW tuning angle as a reference. We observed that all of the EDFL operation states can
be obtained within polarization tuning angles of 180°. The output powers depend slightly on
the initial bending/twisting of intracavity fiber components. Even a single polarization
controller in the EDFL cavity can maintain the operation state stable as long as the intracavity
fiber jumpers are not twisted or translated.
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Table 1. Typical polarization tuning angle and output power for various EDFL states
EDFL state
Polarizer angle
Output power
CW
0°
3.3 dBm
Q-switching
21°
3.0 dBm
QML
30°
3.4 dBm
CML
115°
2.9 dBm
Pulse splitting
167°
2.9 dBm
In brief, the optimal SESAM characteristics are quite different for mode-locked and
Q-switched laser operations [18]. Although the SESAM used in our experiment is optimized
for CML operation, we can still obtained stable CW, Q-switching, or HML operation with the
help of loss modulation mechanism induced by intracavity polarization control.
Alternatively, the EDFL operation states can be expected to be changed by tuning the
intracavity laser power, as that for diode-pumped solid-state lasers [6], or by changing the
total cavity length. While increasing the pump power, so that the intracavity pulse energy
being larger than Ec, the transition from Q-switched to mode-locked regime is realized.
Without the insertion of polarization and polarization controller, the EDFL can be tuned to
operate in Q-switched or QML states for shorter cavity lengths and lower laser powers;
however, the laser cannot be tuned to CML or HML states. Under these circumstances, back
reflection from outside cavity connections becomes important and an optical isolator should
be used to stabilize the operation states. As the LD pump power is increased for an
elongated cavity, the EDFL will be gradually switched from CW through Q-switching, QML,
and finally to CML, which is similar to the solid-state lasers. However, the polarization
controlling method enable us to change the EDFL from CW state to various pulsed states
using the same LD pumping power and cavity configuration, and the EDFL output powers are
almost unchanged. The constant pumping scheme is an advantage in many applications
where a laser with different pulsewidth (or peak power) but almost the same average power is
required. Moreover, the EDFL with hybrid saturable absorber we presented in this work can
realize stable CML at lower pump power as well as shorter pulsewidths in comparison with
the EDFL using only the SESAM for mode-locking. Therefore, it can be used in low power
lasers or when the pump power is limited. Different from conventional ring-cavity fiber
lasers using two polarization controllers in the nonlinear polarization rotating technique
[16-17], only one polarization controller is used in our EDFL laser and the operation of each
laser state is quite stable. This versatile tunable EDFL could be used as a laser oscillator
stage for subsequent power amplification. It would also be useful for studying the nonlinear
interaction of optical pulses in nonlinear optical materials or devices, such as photonic crystal
fibers and supercontinuum generation [19].
4.
Conclusion
We have demonstrated a versatile tunable erbium-doped fiber laser by using semiconductor
saturable absorber mirror and intra-cavity polarization control, in which the laser can be
switched from CW state to various short-pulse states while keeping the pump power at the
same level.
By tuning the intra-cavity polarization, Q-switching, QML, CML,
pulse-splitting, and HML have been obtained for almost the same laser output powers. All
EDFL operation states can be obtained within polarization tuning angles of 180°, and the
switching between EDFL operation states could be attributed to the overall loss modulation
induced by polarizer and polarization controller, in combination with the saturable loss by
SESAM. The passively mode-locked EDFL has 800-fs pulsewidth and 4-MHz repetition
rate, with output pulse energy of 0.5 nJ and peak power of 625 W. The spectrum for
fundamentally mode-locked EDFL presents non-soliton components in the form of narrow
band peaks. Further adjustment of the polarization controller begins to split the CML pulses
and eventually lead to pulse-splitting and harmonic mode-locking of the EDFL, which is
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Received 6 Jan 2009; revised 18 Feb 2009; accepted 21 Feb 2009; published 11 Mar 2009
16 March 2009 / Vol. 17, No. 6 / OPTICS EXPRESS 4813
attributed to the interaction between solitons and the accompanying non-soliton component.
Stable 5th-order HML has been observed, with pulse repetition rate of 20 MHz and pulse
spacing of 50 ns. In contrast to conventional ring-cavity fiber lasers using two polarization
controllers in the nonlinear polarization rotating technique, only one polarization controller is
used in our EDFL laser and the operation of each laser state is quite stable.
Acknowledgments
This work is supported by the Natural Science Council of Taiwan, Republic of China, under
grants NSC 97-2221-E-002-055, 97-2221-E-133-001, and 97-2112-M-029-001-MY3.
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(C) 2009 OSA
Received 6 Jan 2009; revised 18 Feb 2009; accepted 21 Feb 2009; published 11 Mar 2009
16 March 2009 / Vol. 17, No. 6 / OPTICS EXPRESS 4814