A STABLE TUNABLE AND SWITCHABLE
DUAL-WAVELENGTH SINGLE-LONGITUDINAL-MODE
ERBIUM-DOPED FIBER LINEAR-CAVITY LASER
Xiaoying He, D. N. Wang and Changrui Liao
Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
Keywords: Fiber laser, Fiber bragg grating sagnac loop, Linear-cavity.
Abstract: A simple linear cavity erbium-doped linear-cavity laser is proposed and experimentally demonstrated, with
tunable and switchable multi-wavelength SLM operation. The main mode-selection components of our
system include an FBG Sagnac loop, an umpumped EDF together with a tunable FBG. The unpumped EDF
together with the tunable FBG form a super narrow-band self-tracking FBG around the reflection peak of
such tunable FBG, which ensures the SLM laser operation. By simple adjustment of two polarization
controller, the laser can be designed to operate in a stable single-wavelength or dual-wavelength with a
wavelength spacing of 0.05 nm at room temperature.
1 INTRODUCTION
There has been increased research on multi-
wavelength fiber lasers because of their potential
applications in optical communications, optical
instrument testing, and optical fiber sensors
(Libatique and Jain, 1999); (Yao et al., 2006). A
dual-wavelength single-longitudinal-mode (SLM)
fiber laser, for instance, can be used for microwave
signal generated source by beating dual-wavelength
in the photodiode (Chen et al., 2008); (Yao et al.,
2006). However, the erbium-doped fiber (EDF) is a
homogeneous broadening gain medium at room
temperature, and moreover it inevitably brings a
long cavity length for using in the fiber laser. Thus,
some issues must be addressed to achieve SLM
multi-wavelength operation. A fiber loop mirror
with a saturable absorber has been utilized as a
passive self-tracking narrow multi-band optical
filter, to counter against homogeneous broadening
gain and implement a stable SLM multi-wavelength
operation (Liu et al., 2004). Unpumped EDF as a
saturable absorber (SA) has also been used in a fiber
loop mirror (Zhang and Kang, 2008) or a standing-
wave arm (Chen et al., 2008); (He et al., 2009) for
obtaining a passive self-tracking narrow multi-band
optical filter of erbium-doped fiber lasers.
Furthermore, an ultra-narrow mode selecting
mechanism could also be utilized in erbium-doped
fiber lasers to select emitting wavelengths from the
long cavity. The ultra-narrow mode selection filter
can be obtained by using a phase shifted fiber Bragg
grating (FBG) (Yao et al., 2006), the FBG Sagnac
loop (Feng et al., 2009), a FBG-based Fabry-Perot
(F-P) filter (Zhou et al., 2008); Sun et al., 2006), etc.
In this paper, we propose a novel linear-cavity
EDF laser based on an FBG Sagnac loop mirror,
which generates a wavelength tunable and
switchable SLM multi-wavelength lasing. The
operating wavelengths and their spacing can be
selected by use of a FBG Sagnac loop mirror with a
narrow-band FBG in the laser cavity. An umpumed
EDF as an SA, together with the narrow-band FBG,
in a standing-wave arm helps in achieving stable
single- or dual- wavelength SLM operation.
2 EXPERIMENTAL SETUP AND
OPERATION PRINCIPLE
2.1 Experimental Setup
Fig.1 shows the configuration of the proposed
tunable and switchable SLM EDF laser. The EDF
(Highwave-tech EDF-741) with a length of 12
meters is used as the gain medium, pumped by a
980nm laser diode (LD) via a 980nm/1550nm
wavelength division multiplexing (WDM) coupler
123
He X., Wang D. and Liao C..
A STABLE TUNABLE AND SWITCHABLE DUAL-WAVELENGTH SINGLE-LONGITUDINAL-MODE ERBIUM-DOPED FIBER LINEAR-CAVITY LASER.
DOI: 10.5220/0003433501230127
In Proceedings of the International Conference on Data Communication Networking and Optical Communication System (OPTICS-2011), pages
123-127
ISBN: 978-989-8425-69-0
Copyright
c
 2011 SCITEPRESS (Science and Technology Publications, Lda.)

(50:50). The absorption coefficient of the EDF is ~6-
8 dB/m at 1530nm. An FBG Saganc loop (Shu et al.,
2000), used as a narrow-band transmission comb
filter to select the resonance wavelengths, is
connected at the left arm of the linear system. The
design parameters of such FBG Sagnac loop are
presented in section 2.2. The unpumped EDF
(Nufern, EDFC-980-HP) of three meter long in the
right arm of linear system is utilized as the SA
which, together with a tunable FBG, can enhance the
SLM performance and balance the optical powers of
the lasing wavelengths. The absorption coefficient of
the unpumped EDF at 1530 nm is ~ 5-7 dB/m. The
whole cavity length was measured to be
approximately 20.5m. The laser output is monitored
by an optical spectrum analyzer (OSA) (ANDO
AQ6319) with 0.01nm resolution. By adjusting the
FBG and two polarization controller, the lasing
wavelengths can be tuned and switched.
Figure 1: Schematic diagram of the proposed tunable and
switchable Erbium-doped fiber laser with a simple linear
cavity.
2.2 Operation Principle
As shown in Fig.1, the incident light derived from
the pumped EDF has been split into two identical
waves by the 50:50 coupler in the Sagnac loop,
which forms a comb filter together with the FBG. In
the Sagnac loop, a 2.8mm-long FBG is written in
H
2
-free SMF-28 by use of femto-second laser pulse
irradiation and the phase mask. The grating has a
peak reflectivity of ~70% and a 3-dB bandwidth of
~1.2 nm. When the grating is asymmetrically located
in the Sagnac loop, a sinusoidal response occurs
within the envelope of the reflection spectrum of the
grating (Shu et al., 2000), which forms a comb filter.
The channel spacing of such Sagnac loop is
approximately expressed as (Shu et al., 2000):
2
2
eff
nL




(1)
where n
eff
is the effective refractive index of SMF-28
fiber, λ is the center wavelength of the FBG. By
controlling the fiber length difference, ΔL, we can
construct the FBG Sagnac loop with different
wavelength spacings. Here, the fiber length
difference used is ΔL=0.32cm.
The FBG Sagnac loop has been carefully
packaged in a box, which helps in keeping a
constant temperature and stable operation situation
in our system. When the pump power of the EDF is
~105 mW, the transmission spectrum of the FBG
Sagnac loop measured at point A is given in Fig. 2.
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-45
-40
-35
Power (dBm)
Wavelen
g
th
(
nm
)
Figure 2: Transmission spectrum of the FBG Sagnac loop.
The tunable FBG at the right arm of the fiber
laser system has a reflection peak at 1569.81 nm,
with peak reflectivity of over 90% and 3-dB
bandwidth of 0.71 nm. Such an FBG is also typed-II
grating written in H
2
-free SMF-28 fiber by use of
800nm/120fs femto-second laser pulses and a phase
mask (Ibsen Photonics). The laser pulse energy is
400-480 μJ, with 1/e Gaussian beam radius of 3 mm,
and exposure time of ~45 min. This type of FBG
exhibits high temperature stability and good spectral
quality.
When several wavelengths of the incident light
pass though the FBG Sagnac loop and return, in the
unpumped EDF, if the frequency and intensity of the
incident light (forward wave) are identical to that of
the light (backward wave) reflected by the tunable
FBG at the right arm, a standing wave can be
formed. That is to say, the unpumped EDF together
with the tunable FBG forms a super narrow-band
self-tracking FBG around the reflection peak of the
tunable FBG. The total ring cavity was measured to
be approximately 19.5m, which corresponds to a
longitudinal mode spacing of 10 MHz. The
unpumped EDF length is L
e
≈ 3m, and its effective
refraction index n
eff
≈ 1.45, the free spectral range
(FRS) of the narrow-band self-tracking FBG should
be less than 5.6 MHz. Moreover, when the dual-
wavelength optical signal propagates in the
unpumped EDF, the interaction between the two
wavelengths becomes negligible as the wavelength
spacing is much greater than the cutoff frequency (<
OPTICS 2011 - International Conference on Optical Communication Systems
124

1GHz) and their output powers can be balanced.
Therefore, simultaneous SLM lasing at dual-
wavelength is ensured.
3 EXPERIMENTAL RESULTS
AND DISCUSSIONS
The reflection peak of the FBG in Fig.1 can be tuned
by strain using a translation stage. The dual-
wavelength emitting spectra of the proposed fiber
laser are shown in Fig.3, where eleven pairs of dual-
wavelength lasing are tuned from 1568.334nm to
1569.654nm. The wavelength spacing is about
0.05nm. The side mode suppression ratios (SMSR)
of the fiber laser keeps ~50dB in the tuning range.
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0
1
2
3
4
5
6
7
8
9
10
11
Wavelength (nm)
Output power (dBm)
Figure 3: Experimental emitting spectra of the Erbium-
doped fiber laser by tuning the reflection peak of the FBG.
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0
5
10
15
20
T
i
m
e
(
m
i
n
.
)
Wavelength (nm)
Output power (dBm)
0 2 4 6 8 10121416182022
1568.80
1568.82
1568.84
1568.86
1568.88
1568.90
1568.92
1568.94
1568.96
1568.98
1569.00
Time
(
min.
)
Wavelength (nm)
-32
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-30
-29
-28
-27
<0.01nm
Output power (dBm)
<0.01nm
Figure 4: Measured output spectrum at fixed wavelengths
of 1568.889 and 1568.944nm at 21-time repeated scanning
spectra with a time interval of 1 minute, pump power of
134mW.
In order to investigate the laser output stability
and amplitude-equilibrium, the output power of the
dual-wavelength at 1568.89 and 1568.94 nm have
been measured for 21 minutes under the pump
power of 134 mW. In Fig. 4(a), the output power of
dual-wavelength is ~-29.5 dBm and the SMSR is
~50dB. The linewidth of the laser at wavelengths of
1568.89 and 1568.94 nm are less than 0.01nm. The
wavelength fluctuations of dual-wavelength are both
less than 0.01nm as shown in Fig. 4(b), which is due
to thermal drift, and their outputs are stable with
power fluctuation of ~3dBm and ~1dBm. With the
increase of sweeping time, the fluctuation of output
power is decreased, as the lasing would tend to a
stable operation status with the increase of sweeping
time. If the stability of the pump LD and operation
surroundings are improved, a more stable dual-
wavelength lasing can be achieved.
By carefully adjusting the state of the PC1,
single-wavelength, dual-wavelength or three-
wavelength operations can be obtained, under the
pump power of 134mW, as shown in Fig.5. Because
of birefringence chromatic dispersion of the fiber,
different wavelengths would emerge with different
polarization states in the fiber laser cavity.
Moreover, the cavity loss mainly depends on the
polarization state of the incident wave and that of the
polarizer. Hence, only the wavelengths in which loss
is low enough to match their gain produced by
pumped EDF would be lasing. In our fiber laser,
PC1 is used to change the cavity loss of each
resonance wavelength by adjusting the polarization
states of the incident wave, and then to realize
wavelength-switching. The function of the PC2 is
mainly used to adjust the polarization states of
resonance lights in the FBG Sagnac loop. Therefore,
the FBG Sagnac loop with PC2 can be utilized to
select the resonance wavelengths as well as their
polarization states and hence each transmission
wavelength has its own polarization state, which
leads to the enhancement of the polarization hole
buring (PHB). Such PHB greatly decreases the
homogeneous gain broadening of EDF, and thus
reducing the wavelength competition. It is then
possible to obtain stable and uniform amplitude
dual-wavelength lasing at room temperature. Fig. 6
shows that a uniform amplitude dual-wavelength
laser operation is achieved by carefully adjusting
PC2, with the same pump power of 134mW.
By simple adjustment of the PC1, a three-
wavelength lasing has been presented however, it is
unstable. As shown in Fig. 7, through 3-minute
repeated scanning, the laser system has
automatically switched from three-wavelength
lasing to dual-wavelength lasing. This is due to the
fact that, when the fiber laser operates in three-
A STABLE TUNABLE AND SWITCHABLE DUAL-WAVELENGTH SINGLE-LONGITUDINAL-MODE
ERBIUM-DOPED FIBER LINEAR-CAVITY LASER
125

wavelength lasing, the PHB in the laser is not high
enough to reduce the mode competition caused by
the homogeneous gain broadening of EDF, and
accordingly the whole system quickly switches to
the stable dual-wavelength lasing.
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Output power (dBm)
Wavelength (nm)
(a)
(d)
(c)
(b)
Output power (dBm)
Wavelength (nm)
Output power (dBm)
Wavelength (nm)
Output power (dBm)
Wavelength (nm)
Figure 5: Wavelength switching of the fiber laser by
adjusting the PC1, pump power of 134mW.
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-90
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-60
-50
-40
-30
-20
Output power (dBm)
Wavelength (nm)
Output power (dBm)
Wavelength (nm)
Output power (dBm)
Wavelen
g
th
(
nm
)
Output power (dBm)
Wavelen
g
th
(
nm
)
Figure 6: Dual-wavelength emission of the fiber laser by
adjusting the PC2, pump power of 134 mW.
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0
1
2
3
T
i
m
e
(
m
i
n
.
)
Wavelen
g
th
(
nm
)
Output power (dBm)
Figure 7: Measured output spectrum at three-wavelength
lasing of 1569.408, 1568.460nm and 1569.512nm every
1minute for 3 minutes, pump power of 134mW.
4 CONCLUSIONS
A novel EDF linear-cavity laser is proposed and
experimentally demonstrated, with tunable and
switchable multi-wavelength SLM operation. The
main mode-selection components of our system
include an FBG Sagnac loop, an umpumped EDF
together with a tunable FBG. The unpumped EDF,
together with the tunable FBG, form a super narrow-
band self-tracking FBG, which ensures the proposed
SLM fiber laser system. By simple adjustment of
two polarization controller, the laser can be operated
in a stable single-wavelength or dual-wavelength
scheme at room temperature. The SMSR is over 50
dB, and the wavelength fluctuation is as low as
0.01nm, and the output power variation are less than
3dB within 21 minutes. Eleven pairs of dual-
wavelength lasing could be obtained in a tunable
wavelength range from 1568.334nm to 1569.654nm.
ACKNOWLEDGEMENTS
This work is supported by Hong Kong SAR
government through a GRF grant PolyU 5298/10E,
the Hong Kong Polytechnic University research
grants G-YX2N and A-PJ22.
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A STABLE TUNABLE AND SWITCHABLE DUAL-WAVELENGTH SINGLE-LONGITUDINAL-MODE
ERBIUM-DOPED FIBER LINEAR-CAVITY LASER
127