Engineering, 2010, 2, 585-593
doi:10.4236/eng.2010.28075 Published Online August 2010 (http://www.SciRP.org/journal/eng).
Tunable Erbium-Doped Fiber Lasers Using Various
Inline Fiber Filters
Shien-Kuei Liaw1,4, Kuei-Chu Hsu2, Nan-Kuang Chen3
1Department of Electronics Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, China
2Department of Optics and Photonics, National Central University, Jhungli, Taiwan, China
3Department of Electro-Optical Engineering, National United University, Miaoli, Taiwan, China
4Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan, China
Received February 2, 2010; revised March 23, 2010; accepted March 27, 2010
Abstract
Several high-performance and tunable erbium-doped fiber lasers are reviewed. They are constructed by using
fiber Bragg gratings (FBGs) or short-wavelength-pass filters (SWPFs) as wavelength tunable components
inside the laser cavity. Broadband wavelength tuning range including C- and/or S-band was achieved, and
tunable laser output with high slope efficiency, high side-mode suppression ratio was obtained. These fiber
lasers can find vast applications in lightwave transmission, optical test instrument, fiber-optic gyros, spec-
troscopy, material processing, biophotonic imaging, and fiber sensor technologies.
Keywords: Fiber Bragg Grating (FBG), Short-Wavelength-Pass Filter (SWPF), Tunable Fiber Laser, Optical
Communication
1. Introduction
In recent years, fiber lasers have found a variety of
applications in the testing of fiber components, fiber
sensing and wavelength division multipling (WDM)
systems, in which they are used to act as a backup
source with ITU-T grids [1]. Also, fiber lasers are use-
ful for spectroscopy, sensing protection, and fiber-optic
gyro [2]. Partially because of their features, such as
low wavelength sensitivity to temperature, low-inten-
sity noise, and all-fiber construction, their advantages
over non-fiber-based laser sources are potentially low-
intensity noise, high output power, and compatibility
with fiber components. Previous works have proposed
design and/or characteristics valuation of fiber lasers,
including multiple-ring cavity fiber laser [3], two sepa-
rate erbium-doped fiber lasers [4], distributed feedback
fiber lasers [5], and Brillouin erbium-doped fiber laser
pumped using fiber Bragg grating (FBG) [6]. These fiber
lasers, however, have fixed wavelengths that are not
suitable for wavelength routing, reconfigurable switching
and/or network protection. On the contrary, tunable er-
bium-doped fiber lasers could well fit such requirements.
Nowadays, a variety of tunable fiber lasers have been
demonstrated such as tunable single-frequency fiber la-
sers [7], coherent combining tunable lasers [8], tunable
fiber-ring laser using bending effect [9], and so on.
In this paper, we overview several works regarding
tunable fiber lasers done by our groups. The first kind is
the FBG-based linear-cavity tunable fiber laser using an
optical circulator (OC) [10], or a broadband fiber mirror
(BFM) [11] as rear cavity end while the front cavity end
is based on tunable FBGs (TFBGs) which could be tuned
by applying strain. FBGs have become an enabling
technology that provides convenient, cost-effective, and
reliable solutions to a multitude of design problems in
fiber module. Using a backward pump scheme at 1480
nm in [11], stable lasing output power of 21.14-mW
measured at 1544.8 nm was obtained with a threshold
pump power of 8.0 mW. A side-mode suppression ratio
as high as 57 dB, wavelength tuning range up to 30 nm
with a step resolution of 0.5 mm/turn, and power varia-
tion less than 1.0 dB were achieved. The second kind is
the short-wavelength-pass filter (SWPF)-based tunable
fiber ring laser with lasing wavelength down to S-band
using filters adopting a side-polishing [12] or fused-
tapering [13] technique to attain the wavelength-depen-
dent fundamental-mode cutoff concept. The fiber side-
polishing and fused-tapering techniques were both em-
ployed to fabricate wavelength tunable fiber filters. The
laser can be tuned close to the short-wavelength edge of
the available erbium gain bandwidth with tuning range of

S. K. LIAW ET AL. 586
26 nm, signal-to-amplified-spontaneous-emission (ASE)
ratio of around 40 dB, and the full width half maximum
(FWHM) linewidth of about 0.5 nm. The single-longi-
tude-mode (SLM) operation will be briefly discussed in
Section 5.3. These two kinds of tunable in-line fil-
ter-based tunable fiber lasers as mentioned may broaden
wavelength tuning range in either C- and/or S-band and
will be addressed in detail. Both of them have graceful
features of simple structure, compactness, ease in con-
nection with fiber components, high-efficiency, and
continuous-tuning, which make them promising for vast
applications.
2. Wavelength Tunable Mechanisms
2.1. Tunable Fiber Bragg Grating
In principle, a wavelength shift in a FBG may be due to
the changes in temperature, strain, pressure and/or other
parameters. The shift in Bragg wavelength with strain
and temperature can be expressed as [14]
B
2
1211 12
d
()
d
2{1 ()[()]}[]
2
n
nT
nPpp T
n





 




(1)
where ε is the applied strain, Λ is the period of fiber, Pi,j
are the Pockel’s (piezo) coefficients of the stress-optic
tensor, and ν is the Poisson’s ratio. Note that n is the ef-
fective refractive index of the fiber core as defined in Eq.
(1), α is the thermal expansion coefficient of the silica
fiber with a typical value of 0.015 nm/ºC, and ΔΤ is the
temperature change in degree Celsius. The term (n2/2)
[P12－ν(P11－P12)] has a numerical value of 0.22. The
strain can be measured under a constant temperature ac-
cording to the following equation:
61 B
B
10.78 10




 (2)
where λB is the Bragg wavelength, and this value gives a
“rule-of-thumb” measurement of wavelength shift for a
FBG with strain of 1 nm per 1000 με at 1.31 μm. To de-
sign a strain tunable FBG, firstly, it is embedded in a
strip of composite thermal plastic material and then is
attached to L-shaped holders at both ends. The FBG is
then mounted on a precision translational stage with a
high-resolution micrometer. By strained or compressed
tuning of the precise screw of the micrometer, we can
apply both directions in the transverse displacement for
increasing the tuning range up to ±8 nm. Two steel rods
are attached to the sides of the FBG composite strip to
confine the applied strain or stress to the longitudinal
direction only. The micrometer has a resolution of 0.5
mm/turn and a full range of 5.0 mm in translational dis-
tance, therefore up to ten turns can be applied to tune the
FBG reflection wavelength.
The lasing wavelength as a function of turns of screw
is shown in Figure 1(a), with a linear slope of 4.82 nm/
mm translational distance. Thus, ΔλB＝0.00482 nm/μm ×
d, where d is the displacement of the translational stage
in unit of micrometers. Another way is to embed the
FBG in the outer laminar. The composite with the TFBG
embedded within is attached to a 3-point tuning device
by using instant adhesive glue. By tuning the precision
screw of the 3-point bending device either by straining or
compressing, we can apply transverse displacement in
either direction to easily attain a tunable range of ±10 nm.
This eliminates any use of complicated or bulky compo-
nents to perform the tuning function.
Figure 1(b) shows the superimposed tuning spectra of
two homemade tunable FBGs. The demonstrated tuning
range of each FBG is approximately 15 nm with reflec-
tance of 99.9%. Before tuning, FBG1 has a central re-
flection wavelength of 1540.5 nm while that of FBG2 is
Turns of Screw
-1.0-0.50.0 0.5 1.0
Wavelength
1549
1550
1551
1552
1553
1554
1555
Experimental Date
Filting Curve
(a)
1530 1540 1550 1560
-90
-80
-70
-60
-50
Second grating
1552.68nm
First grating
1540.5nm
1552.68nm
-86.12dBm
1557.6nm
-84.06dBm
1560.66nm
-83.93dBm
1535.6nm
-85.23dBm
1540.5nm
-85.3dBm
1545.47nm
-85.73dBm
1547.7nm
-85.21dBm
Wavelength (nm)
Power (dBm)
(b)
Figure 1. (a) Wavelength tunable FBG versus turns of mi-
crometer screw; (b) superposed transmission spectra of two
tunable FBGs (Total tuning range may cover the C band).

S. K. LIAW ET AL. 587
1552.7 nm. Fine tune resolution as precise as 0.2 nm
FBG can be realized.
2.2. SWPF-Based Tunable Fiber Laser
The S-band tunable erbium-doped fiber lasers were ach-
ieved by connecting the active fiber to the thermo-
optic tunable SWPFs. The mechanism of the proposed
SWPFs [12,13] is to interact with the guiding optical
fields to cause fundamental mode loss at long-wave
length, and the cutoff wavelength can be tuned when the
heating temperature applying on the filter changes. The
dispersion engineering methods had been employed by
controlling the propagation losses of lights at different
wavelengths. Both the side-polishing and the fused-
tapering techniques were adopted in our previous works
[12,13]. When the SWPFs are temperature-tuned to at-
tenuate the wavelengths longer than 1530 nm, the C+L
band ASE is suppressed and the S-band gain is obtained.
The commonly used S-band erbium-doped optical fiber
amplifiers (EDFAs) employ erbium-doped fiber (EDF)
with depressed inner cladding to achieve fundamen-
tal-mode cutoff at the longer wavelengths [15,16]. The
cutoff wavelength and the mode field diameter can be
adjusted through bending and local heating, but the fab-
rication, insertion loss, crosstalk and cost of the filters
using dispersive fibers show great difficulties for practi-
cal use. Thus, an alternative way to obtain SWPFs is to
interact with the light through the evanescent field that is
spread out of the waveguide with wavelength-dependent
properties. When the optical fiber is side-polished or
tapered, the mode field is expanded out of the fiber clad-
ding. Using dispersive liquids surrounding the side-poli-
shed fiber/taper fiber, the device can be a SWPF if the
dispersion relations are properly designed. The tunable
SWPF was achieved by tuning the temperature of the
dispersive liquids to change the dispersive curves for
obtaining different cutoff wavelengths [17,18].
In Figure 2(a), the blue and red curves are the disper-
sion curves of the Ge-doped core and the fused silica of
SMF-28, and the black curve is the dispersion for Car-
gille index-matching liquids. The refractive index dis-
persion of the core and cladding intersect at a fundamen-
tal mode cutoff wavelength which divides the wave-
lengths into bound and refracting leaky modes, and the
lights with wavelength longer than the cutoff wavelength
can be highly attenuated due to the frustrated total inter-
nal reflection. In our measurement, a broadband white
light source was launched into the fabricated SWPF,
where the optical liquid surrounding the SWPF was
heated by a thermoelectric cooler (TEC) to stabilize the
temperature. Figure 2(b) depicts the experimental and
simulated transmission spectra of a SWPF over the tun-
ing temperature of 25ºC–27ºC, where the simulation re-
sults were performed by beam propagation method. The
(a)
(b)
Figure 2. (a) Refractive index dispersion curves for index
matching liquid and materials for original fiber core (GeO2)
and cladding (pure silica); (b) experimental and simulated
spectral responses of tunable short-wavelength-pass fiber
filter at different temperatures [18].
experimental results agree well with the simulation ones.
The results include tuning efficiency of 50 nm/C,
cut-off efficiency of −1.2 dB/nm, and rejection efficiency
of 55 dB. Based on these experimental and simulated
results, the mechanism of the SWPF is proved to be
qualified for developing wavelength tunable S-band er-
bium-doped fiber lasers.
3. FBG-Based Tunable Fiber Lasers:
Configurations and Experimental/
Simulation Results
3.1. Optical Circulator as Laser’s Rear Cavity
End
The proposed configuration of linear cavity for the tun-
able laser is shown in Figure 3. The linear cavity con-
sists of a 3-port OC, two TFBGs, a segment of high-

S. K. LIAW ET AL. 588
Figure 3. Configuration of proposed OC-based linear-cavity
tunable fiber laser [10] (ISO: optical isolator; LD: laser
diode; OSA: optical spectrum analyzer).
concentration EDF, a 1480/1550 nm WDM coupler, and
one 1480 nm pumping source. The 3-port OC here acts
as a wavelength router by connecting port 3 with port 1.
In this way, the residual pumping power travels back to
the EDF for twice amplification to increase its pumping
efficiency up to 2 dB difference in laser output power. A
piece of EDF is inserted into the cavity to act as pump
absorber. At the right hand side of this cavity, there is
one 1 × 2 optical switch (OSW) and two TFBGs (i.e.,
TFBG1, TFBG2) connected to the two switched ports of
OSW. The tuning range could cover the whole C-band
by switching between the two OSW ports connected to
individual TFBG. The original reflected wavelengths of
TFBGs are 1540.5 and 1552.68 nm, respectively. Two
variable optical attenuators (VOAs) are used in each port
for the power equalization.
Figure 4(a) shows signal power versus 1480 nm pump
power for various lengths of EDF. With the selected
lasing signal at 1550 nm, the signal power is linearly
proportional to the pumping power. The pumping effi-
ciency increases from 3.7% to 40% as EDF length in-
creases from 0.8 m to 5 m. Although the longer EDF
length seems to have better conversion efficiency and to
generate higher laser power, it also generates more ASE
noise which results in lower signal-to-noise-ratio (SNR).
As the EDF length increases, the extra gain provided by
the pumping power is smaller than the loss attributed by
the EDF. For this linear-cavity fiber laser, the parameters
for achieving optimum performance are 1.9 m in length
for EDF and 50% reflectance for the TFBGs.
Figure 4(b) shows the superimposed output spectra of
the fiber laser. It is randomly tuned across the C-band
with power equalization function. The power equaliza-
tion can be realized by using VOAs independently. The
power uniformity within ±0.1 dB over the tuning range
has been achieved. No polarization mode competition
effect is observed partially due to the narrow linewidth of
TFBGs. The switching time for OSW is 10 ms in this
case, which depends mainly on the specification of the
mechanical OSW.
Pump power (mW)
020406080100 120 140 160
Signal power (mW)
0
10
20
30
40
50
60
EDF=0.8m
EDF=1.9m
EDF=3m
EDF=5m
(a)
(b)
Figure 4. (a) Laser output power versus pump power using
different EDF lengths; (b) superimposed output spectra of
tunable fiber laser as wavelength of TFBGs is tuned across
C-band after power equalization.
3.2. Broadband Fiber Mirror as Rear Cavity
End
The proposed BFM-based liner-cavity tunable fiber laser
in a backward pump scheme is shown schematically in
Figure 5. The laser cavity consists of a BFM, a tunable
FBG, and a piece of EDF. The BFM here acts as a
broadband wavelength reflector integrated with a TFBG
to form a laser cavity. It will lase as long as the reflected
wavelength of TFBG is within the reflective range of the
BFM. Also, a piece of EDF is inserted into the cavity to
act as pump absorber.
Figure 6(a) shows the measured experimental results
for the EDF length versus the output power at 1544.8 nm
when Figure 5 has a constant pump power of 150 mW.
We find that as the EDF length is increased from 0 to 4
m, the output lasing power is increased accordingly. If
the EDF is longer than required, there is a region of the
EDF where the pump power is relatively small, the sig-
nal has reached saturation intensity, and the gain is de-
creased according. The curves for various lengths of
EDF with threshold power of 8 mW are shown in Figure

S. K. LIAW ET AL. 589
Figure 5. Configuration of proposed BFM-based liner-cavi-
ty tunable fiber laser in backward pump scheme with use of
residual pump power [11] (PM: power meter).
EDF length (m)
012345678910
Output Power (mW)
0
5
10
15
20
25
30
35
40
Experimental Curve
@ 1544.3 nm
(a)
Pump Power (mW)
020406080100 120 140
Output Power (mW)
0
5
10
15
20
25
30
35
40
EDF length = 1m
EDF length = 3m
EDF length = 4m
(b)
Figure 6. (a) Experimental results show EDF length against
output power as pump power is set at 150 mW; (b) experi-
mental results of pump power versus lasing output power
for various lengths of EDF in BFM-based fiber laser
scheme.
6(b). The transfer efficiency versus pump power for dif-
ferent lengths of EDF is shown in Figure 7(a). We find
that the transfer efficiency is increased as the EDF length
increases in the beginning. Then it reaches a constant
value of 21.5% as the pump power is larger than 70 mW.
Pump Power (mW)
0255075100 125 150
Transfer Efficiency (%)
0
5
10
15
20
25
30
EDF length = 1m
EDF length = 3m
EDF length = 4m
(a)
(b)
Figure 7. (a) Experimental results of transfer efficiency
versus pump power for different lengths of EDF; (b) su-
perposed output spectra of BFM-based tunable fiber laser.
(Wavelength is tuned across C-band using two TFBGs).
The transfer efficiency here is defined as
Las
in th
PP
()
P
PP
  (3)
where η is the laser transfer efficiency, Pp
in is the input
pump power, and Pp
th is the threshold power. Figure 7(b)
shows the superimposed output spectra of the tunable
fiber laser using two TFBGs with original wavelength of
1539.13 and 1553.0 nm, respectively. High side mode
suppression ratio (SMSR) of around 57 dB was obtained
for the entire C-band.
4. Tunable SWPF-Based Tunable Fiber
Lasers: Configurations and Experimental/
Simulation Results
4.1. SWPF-Based Tunable Fiber Laser
In this section, a continuously tunable erbium-doped

S. K. LIAW ET AL. 590
fiber laser is demonstrated by incorporating a tunable
SWPF into ring resonator. The wideband tunable SWPF
is based on dispersive evanescent tunneling from a
side-polished single-mode fiber and a dispersive optical
polymer overlay structure. In fabrication, a portion of the
fiber jacket was stripped off and the section was then
embedded and glued into the curved V-groove on a sili-
con substrate, as shown in Figure 8. The central cladding
thickness after polishing was around 2.7 μm. Finally, the
characteristics of the well-polished fibers were calibrated
by liquid-drop experiments. The effective interaction
length was estimated to be 11 mm at 1550 nm wave-
length for side-polished SMF-28. In Figure 8, the tun-
able SWPF is incorporated into the resonant cavity to
provide a wideband tunable transmission loss window.
The dispersive optical polymer overlaying the side-
polished fiber is OCK-433 (Nye Lubricants) with the
thermo-optic coefficient dnD/dT of –3.6 × 10–4/°C and is
heated by a dual TEC. The refractive index of the
OCK-433 decreases with increasing temperature. The
tuning efficiency is 7.65 nm/°C and the signal-to-ASE
ratio is around 40 dB. The EDF used here has absorption
coefficient of 12 and 30 dB/m for 1480 and 1530 nm
wavelength, respectively, and is pumped by a 1480 nm
pump laser diode (LD) in 250 mW launched power.
To investigate the influences of the sharpness of the
spectral cutoff curve, the Cargille liquids were applied on
SWPF. The spectral responses of the fiber laser are
shown in Figure 9(a). When the refractive indices of
1.456 (nD) and 1.458 (nD) were used, the lasing wave-
length moved to shorter wavelengths and the peak power
decreased following the gain profile. Subsequently, the
Cargille liquids were replaced by OCK-433 and the
spectral responses are shown in Figure 9(b). When the
temperature cooled down, the lasing wavelengths were
moved toward shorter wavelengths again. As the tem-
perature was tuned to 39.6C, the lasing wavelength was
at 1569.8 nm. Thus, the tuning range of the fiber laser is
26 nm with temperature variation of 3.4C, and typical
signal-ASE-ratio is above 40 dB and the average FWHM
is around 0.5 nm.
4.2. Thermo-Optic Tunable Erbium-Doped
FiberRing Laser
In this subsection, wideband tunable high cutoff-effici-
ency SWPFs were discretely located in standard silica-
based C-band EDF to filter out the C + L band ASE so
that the optical gain for S-band could be acquired to re-
alize fiber laser. To investigate the amplification charac-
teristics in the S-band, a 980-nm pump laser with 135-
mW output power was launched into EDF in a forward
pumping scheme. The high-cutoff-efficiency short-pass
filters in the 17.5-m-long EDF could discretely suppress
the unwanted C + L band ASE and pass the S-band signal
Figure 8. Experimental setup of erbium-doped fiber ring
laser using side-polished fiber based tunable SWPF [12].
(a)
(b)
Figure 9. (a) Spectral responses of EDF fiber ring laser in
air and using two kinds of Cargille index liquids on SWPF;
(b) spectral responses of wavelength tuning of fiber laser
when OCK-433 polymer was cool down [12].

S. K. LIAW ET AL. 591
and 980-nm pump light. Subsequently, an input power of
−25 dBm was launched into the EDF from distributed
feedback laser signals in the S-band. The input signal
spectra and amplified output signal spectra in the S-band
at 28.6°C are shown in Figure 10(a). In the S-band the
net signal gain at 1486.9 nm was measured to be 18.92
dB.
The experimental set-up of the tunable EDF ring laser
is shown in Figure 11, where the tunable fused-tapered
SWPF with the use of Cargille index liquid (nD = 1.456)
can provide a sharp filter skirt and a deep stop band re-
jection efficiency (>50 dB). However, a single local
SWPF is inefficient for the standard EDF to be operated
at the shorter wavelengths (S-band) of the gain band-
width. Consequently, we employ four-stage in-line tun-
able fused-tapered fiber SWPFs discretely located in the
standard silica-based EDF to achieve the tunable S-band
fiber laser. When the SWPF is turned on, the C + L-band
ASE is suppressed to obtain the gain for S-band lasing.
The four filters are discretely located in a 16-m-long
(a)
(b)
Figure 10. (a) Amplification spectra of the signals in S-band
at 28.6°C; (b) evolution of output laser spectra by cooling
down optical liquid and bending splicing point using the
first set of tapered fibers [13,17].
Figure 11. Experimental setup of tunable EDF ring laser
towards short-wavelength limit at 1450 nm (Each 4-m-long
EDF and short-pass filter forms a gain stage and there are
four gain stages totally in the ring cavity. The FP filter is
used for narrowing the laser linewidth down to below 0.2
nm [13]).
standard silica-based C-band EDF to substantially sup-
press the ASE at the wavelengths longer than the lasing
wavelength which can be tuned by varying the applied
temperature on SWPFs. When a 980-nm laser with pump
power of 208 mW launches into the EDF, the laser spec-
tra at different temperatures are shown in Figure 10(b).
When the applied temperature slightly decreases, the
lasing wavelength moves to shorter wavelength. The
average η for the tunable laser is measured to be as high
as 57.3 nm/°C from 1545.2 to 1451.9 nm ascribing to the
wideband tunable high-cutoff-efficiency SWPFs. The
average FWHM is 0.53 nm and the signal-to-ASE ratio
is above 40 dB.
5. Discussion
5.1. Advantages of FBG-Based Tunable Fiber
Lasers
A versatile and cost-effective laser source should have
the ability to allow the user to choose which wavelength
is needed or the desired scanning range. The wavelength
tunable FBG-based lasers we presented here can satisfy
such requirement. It is well known that the cavity of a
fiber laser may be designed based on a pair of FBGs that
work as its end mirrors and determine the resonant
wavelength. When one of the resonant wavelengths of
the FBGs is changed slightly by tension or heating, the
reflection power by the FBG pair at a new laser wave-
length will decrease due to wavelength misalignment
between them, thus it is difficult to fine-tune the FBG

S. K. LIAW ET AL. 592
pair back to the same wavelength. Nevertheless, either
the OC-based or BFM-based laser configuration could
overcome such a problem because one FBG only is used
to tune the lasing wavelength. Other advantages of
FBG-based tunable fiber lasers are: (1) Narrow laser
linewidth and near polarization-independent; (2) both the
OC-based and BFM-based tunable fiber lasers improve
the pumping efficiency by recycling the residual pump
power back to the gain medium using backward pumping;
(3) the TFBG could be used to tune the desired wave-
length precisely and quickly; (4) the proposed FBG-
based tunable fiber lasers may use one OSW pair and a
plurality of tunable FBGs to expand the output wave-
length range; (5) they are simpler and potentially less
expensive than other commercial products; and (6) the
sizes are compact and the weights are light.
5.2. Merits of SWPF-Based Tunable Fiber
Lasers
It is advantageous to explore a widely tuning fiber laser
with lasing wavelength down to S-band at a high tuning
speed. Conventionally, the silica-based EDF at room
temperature can only emit fluorescence at wavelengths
longer than 1490 nm. Thus, achieving high-performance
S-band lasers critically depends on the SWPFs. The side-
polished SWPFs were adopted because they are mecha-
nically strong, and the polishing depth and interaction
length can be precisely determined. From a different
point of view, SWPFs using the fused-tapering technique
are easy, fast, and cost-effective fabrication processes.
An optimized side-polishing/tapered fiber filter structure
can attain high-cutoff efficiency and wide tuning range.
Based on the proposed SWPFs, widely tunable, single-
frequency rare-earth-doped fiber lasers can be achieved.
Besides, the SWPF-based tunable fiber lasers have other
advantages such as: (1) wide tuning range covering the
S- and C-bands, (2) high power and low noise, (3) sim-
plicity and cost-effectiveness, and (4) high index sensi-
tivity up to 1  10–5 with high Q resonator.
5.3. Single-Frequency Design
To design a single-frequency tunable fiber laser, various
kinds of methods such as multiple ring cavities, FBGs,
microrings, spatial hole burning in unpumped EDF, and
nonlinear loop mirror were proposed. Also, a short cavity
length is usually required to enlarge the mode spacing.
For the linear-cavity fiber lasers as mentioned, a simpler
way to achieve single-longitudinal-mode (SLM) opera-
tion is to put a piece of EDF as pump absorber between
the WDM coupler and 1 × 2 OSW for the OC-based lin-
ear-cavity tunable fiber laser as shown in Figure 3; and
between the 1480/1550 nm WDM and TFBG for the
BFM-based linear-cavity tunable fiber laser as shown in
Figure 5, individually. On the other hand, the SWPF in
tunable fiber lasers is naturally a broadband filter that is
obviously difficult for single-longitudinal mode laser
operation. However, a SWPF made of a highly disper-
sive waveguide structure can introduce high chromatic
dispersion inside the laser cavity to significantly reduce
the cavity modes into one. One suggestion is to concate-
nate the SWPF and an additional ultra-narrowband filter
inside the cavity to attain SLM operation.
6. Conclusions
Two kinds of tunable fiber-filter-based EDF fiber lasers
have been reviewed. Both of them have broadband wave-
length tuning range including C- and/or S-band. Using
FBG in strain mechanism, we have proposed and dem-
onstrated a tunable FBG-based fiber laser that employs
one OC, two homemade TFBGs. The configuration con-
sists of a linear cavity to achieve a wavelength tuning
range of 31.5 nm with 0.05 nm linewidth and over 60 dB
SNR. The power variation over the entire tuning range is
less than 0.1 dB with power equalization by using
low-cost VOAs. Another way is to employ a BFM and
tunable FBG at either cavity end of fiber cavity. The
BFM acts as a broadband rear-end reflector both for
lasing signal and pump source. For wavelength tunable
demonstration, power variation over the whole C-band is
less than ±1.0 dB without the usage of power equaliza-
tion. The time to reach stable laser operation is less than
11 ms after switching between the two FBGs, and the
continuous tuning resolution is less than 0.2 nm in the
whole range. For the SWPF-based tunable fiber laser
using temperature tuning mechanism, two tunable
SPWFs based erbium-doped fiber lasers were reviewed.
The side-polishing and fused-tapering techniques were
used to achieve thermo-optic tunable short-wavelength-
pass function based on material dispersion discrepancy
and variations of waveguide structures. The tuning effi-
ciency is 50 nm/C, cut-off efficiency is –1.2 dB/nm, and
rejection efficiency is 55 dB, individually. The widely
tunable SWPFs were applied to achieve broadband and
high-tuning-efficiency S- and/or C-band EDF ring lasers,
which can be tuned close to the short-wavelength edge of
gain bandwidth, and the tuning range is 26 nm with the
signal-to-ASE-ratio of around 40 dB, and the FWHM
linewidth is about 0.5 nm. All of them have graceful
features of simple structure, compactness, ease of con-
nection to fiber components, high-efficiency, and con-
tinuous tunability. They are promising for vast applica-
tions in lightwave transmission, optical test instrument,
fiber-optic gyros, spectroscopy, material processing, fi-
ber sensing, WDM backup light sources, as well as in bio
photonics.

S. K. LIAW ET AL. 593
7. Acknowledgements
The authors were partially supported by the National
Science Council (NSC) (Project Nos. NSC 98–2221–E-
011-017, NSC 97-2923–E-011-001-MY3, NSC 98–2218
–E–008-004, NSC 98-2221-E-239-001-MY2). We thank
Jang W. Y., Wang C. J., Hung K. L., Jhong G. S., Chi S.,
Tseng S. M., Huang C. M., Lai Y. for discussion, T.
Wang and Z. G. Shieh for kind help.
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