Beam Combining of SOA-based Bidirectional Tunable Fiber
Compound-ring Lasers with External Reflectors
Muhammad A. Ummy
1
, Simeon Bikorimana
2
and Roger Dorsinville
2
1
Department of Electrical & Telecommunications Engineering Technology, New York City College of Technology,
City University of New York, 300 Jay St, Brooklyn, NY 11201, U.S.A.
2
Department of Electrical Engineering, City College of New York, City University of New York,
160 Convent Avenue New York, NY 10031, U.S.A.
Keywords: Beam Combining, Power Scalability, Sagnac Interferometer, Laser Tuning, Ring Laser, Semiconductor
Optical Amplifiers.
Abstract: A simple, stable and inexpensive dual- output port widely tunable semiconductor optical amplifier-based
fiber compound-ring laser structure is demonstrated. This unique nested ring cavity enables high optical
power to split into different branches where amplification and wavelength selection are achieved by using
low-power SOAs and a tunable filter. Furthermore, two Sagnac loop mirrors which are spliced at the two
ends of the ring cavity not only serve as variable reflectors but also channel the optical energy back to the
same port without using any high power combiner. More than 98% coherent beam combining efficiency of
two parallel nested fiber ring resonators is achieved over the C-band tuning range of 30 nm. Optical signal
to noise ratio (OSNR) of + 45 dB, and optical power fluctuation of less than ± 0.02 dB are measured over
three hours at room temperature.
1 INTRODUCTION
All-single-mode fiber resonators of different types,
such as linear (He et al., 2009), Fox-Smith (
Barnsley
et all., 1988), ring, and compound fiber ring (Zhang
and Lit, 1994;) cavities have been theoretically and
experimentally explored in designing various kinds
of fiber laser sources with single-longitudinal mode
operation for low and high optical power
applications for optical communication systems,
scientific, medical, material processing and military
purposes (Shi et al., 2014). Adjustable, scalable
output optical power and wavelength tunability
properties of a fiber laser source are of a great
interest in the aforementioned applications that
require high optical power. Complex and expensive
in-line variable optical attenuators (VOA) with
adjustable insertion losses are usually used to control
the output power level of laser sources. Mechanical,
micro-electromechanical (Syms et al., 2004;),
acousto-optic (Li et al., 2002) methods as well as
optical fiber tapers (Benner et al., 1990) and hybrid
microstructure fiber-based techniques (Kerbage et
al., 2001) are widely used to adjust the insertion
losses of the in-line fiber-based VOA. However, all-
fiber based low power variable reflectors, such as
Sagnac loop mirrors (SLMs) can also be used to
control the amount of optical power from low and
high power fiber laser sources. All-single-mode
fiber-based SLMs have been widely used in high
sensitivity sensors, such as temperature (Lim et al.,
2010), and strain (Sun et al., 2007) sensors.
Moreover, the SLMs with adjustable reflectivity
have been used to form Fabry-Perot linear resonators
(Ummy et al., 2011) where the amount of the output
optical power from the resonators depends on the
reflectivity of the SLMs (Mortimore, 1988). In this
work, two SLMs were used to control the amount of
optical power delivered from two output ports of the
proposed fiber compound-ring laser. In addition,
inexpensive two low power semiconductor optical
amplifiers (SOAs) were placed in two parallel nested
ring cavities to demonstrate the possibility of
achieving a highly power scalable, adjustable and
switchable fiber laser structure based on multiple
nested compound-ring cavities formed by NxN fiber
couplers with two SLM output couplers.
Different methods have been utilized to scale up
optical power of laser sources where beam
combining has shown to be a promising alternative
technique of achieving high power by scaling up
multiple combined laser elements. High power laser
230
A. Ummy M., Bikorimana S. and Dorsinville R.
Beam Combining of SOA-based Bidirectional Tunable Fiber Compound-ring Lasers with External Reflectors.
DOI: 10.5220/0006149002300236
In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 230-236
ISBN: 978-989-758-223-3
Copyright
c
2017 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
sources with high beam quality have been
demonstrated by using complex coherent and
spectral beam combining techniques in external
cavities (Klingebiel et al., 2007; Augst et al., 2009).
In addition, incoherent beam combining method
(Sprangle et al., 2009) has been used to scale up the
optical power by combining individual laser
elements as well. Michelson, Mach-Zehnder
resonators were mostly used in coherent beam
combining in order to achieve high combining
efficiency and nearly diffraction-limited beam
quality (Sabourdy et al., 2003). Moreover, ring
resonators have proven to be efficient and stable
(Jeux et al., 2012) for passive coherent beam
combining method.
In order to achieve high power laser sources
with high beam combining efficiency, all the
aforementioned methods require sophisticated high
power external optical components such as micro-
lenses, isolators, circulators, photonic crystal fibers
and master oscillator pre-amplifiers that are usually
complex and very expensive. In addition, rare-earth,
ytterbium doped fiber amplifiers (Jeux et al., 2012)
or erbium doped fiber amplifiers (EDFAs) (Kozlov
et al., 1999) that are usually used as gain media for
beam combining to achieve high power laser
systems also need to be pumped with other types of
laser sources, which makes them very inefficient.
However, by using nested compound-ring cavities
where circulating beams are equally split in N-
number of low power beams that can be amplified
by N-number of low power SOAs, one can achieve
high efficient and high power laser system that does
not require the extra pump lasers or master oscillator
pre-amplifiers or other expensive external high
power optical components.
Semiconductor optical amplifier (Moon et al.,
2007), stimulated Raman scattering (SRS) amplifier
(Kim et al., 2003) and stimulated Brillouin scattering
(SBS) amplifier (Smith et al., 1991) have also been
used as gain media in different fiber laser systems.
However, the SOAs are more advantageous than
their above-mentioned counterparts, because they
are compact, light, less expensive, efficient, and
available for different operational regions from a
wide range of wavelength spectrum. Moreover,
when the SOAs are used in bidirectional fiber ring
resonators, they do not require the extra optical
components such as optical isolators and optical
circulators; as a result, they are easy to integrate
with other optical components for compact fiber
laser systems.
In this work, we demonstrate a novel technique
for coherent beam combining method based on
passive phase-locking mechanism (Bruesselbach et
al., 2005) of two C-band low power SOAs-based all-
single-mode fiber compound-ring resonators by
exploiting beam combining (i.e. interference) at 3dB
fiber couplers that connect two parallel merged ring
cavities. Unlike previous work (Ummy et al., 2016)
where non-adjustable multimode fiber laser output
formed by a high power and expensive power
combiner with a multimode output fiber (i.e. , low
brightness) has been replaced by two low power
Sagnac loop mirrors to create all-single-mode fiber-
based dual-output port laser structure with
switchable and adjustable output power. In addition,
the single-mode performance is maintained in order
to improve the brightness at the proposed fiber
compound-ring laser output port. The output power
of the proposed combined fiber compound-ring
resonators with two low power SOAs was almost
twice as large as the output power obtained from a
single SOA based- fiber ring or Fabry-Perot linear
resonator (Ummy et al., 2011). More than 98%
beam combining efficiency of two parallel nested
fiber ring resonators is demonstrated over the C-
band tuning range of 30 nm. Optical signal to noise
ratio (OSNR) + 45 dB, and optical power fluctuation
of less than ± 0.02 dB are measured over three hours
at room temperature.
2 EXPERIMENATL SETUP
Fig.1 illustrates the experimental diagram of the C-
band SOA-based tunable fiber laser with two nested
ring cavities (i.e., compound-ring cavity) and two
broadband SLMs that serve as either dual output
ports or a single output port according to the
reflectivity settings of each of the SLMs. Each ring
cavity consists of two branches I-II, I-III, for the
inner and outer ring cavity, respectively. Both ring
cavities share a common branch, I, which contains
an SOA
1
, (Kamelian, OPA-20-N-C-SU), a tunable
optical filter (TF-11-11-1520/1570), and a
polarization controller, PC
1
. Branch II contains an
SOA
2
(Thorlabs, S1013S), and a polarization
controller, PC
2
. Branch III contains only a
polarization controller, PC
3
. Note that the branch has
no SOA due to the limited number of SOAs
available during the time of our experiment. All the
three branches, I, II and III, are connected by two
3dB fiber couplers, C
1
and C
2
. Each 3dB fiber
coupler, C
1
and C
2,
is also connected to a Sagnac
loop mirror, SLM
1
and SLM
2
, respectively as shown
in Fig. 1. These Sagnac loop mirrors with a
polarization controller placed in each loop act as
variable reflectors. By adjusting the polarization
Beam Combining of SOA-based Bidirectional Tunable Fiber Compound-ring Lasers with External Reflectors
231
controller (i.e., PC
4
or PC
5
), one can change the
reflectivity of the loop mirrors and thereby, can
switch from single to dual-output port configuration.
The low power tunable optical filter (TF), which is
placed in the common branch I, is used for selecting
and tuning the operating wavelength of the proposed
fiber laser.
Figure 1:
Experimental setup of the dual Sagnac loop
mirror SOA- based tunable fiber compound-ring laser.
3 PRINCIPLE OF OPERATION
When the pump level (i.e., bias current threshold
level) of either SOA is more than the total fiber
compound-ring cavity losses, amplified spontaneous
emission (ASE) emitted from SOAs either
propagates in the forward and backward directions.
For instance, when a bias current I
B
of around 75
mA) is injected into the SOA
1
(branch I), the emitted
ASE emitted by the SOA
1
(branch I) circulates in
clockwise (cw) direction by propagating through a
tunable optical filter, which selects a passband of
certain wavelengths. The selected wavelengths reach
a 3dB fiber coupler C
2
after propagating through a
polarization controller, PC
1
. Then, the selected light
beam that arrives at port 1 of the 3dB fiber coupler
C
2
, is equally split into two branches, II and III at
port 2 and port 3, respectively. The light beam that
circulates into branch II propagates through a
polarization controller, PC
2,
before it is amplified by
SOA
2
when its bias current level I
B
is around 180
mA. Then, the amplified light beam arrives at port 2
of the 3dB fiber coupler C
1
where it is equally
divided between port 1 and port 4. Similarly, light
beam from Branch III reaches port 3 after passing
through a polarization controller, PC
3
. A half of the
light beam coupled into port 1 of the 3dB fiber
coupler C
1
is further amplified by SOA
1
. Thus, a
round-trip is completed in the fiber compound-ring
structure and allows lasing to occur. Furthermore,
the remaining 50% of the light beam is coupled into
the output port 4 of the 3dB fiber coupler C
1
is
injected into the input port 4 (i.e., I
in
) of the Sagnac
loop mirror, SLM
1
. The polarization controller, PC
4
,
controls the reflectivity of the SLM
1
and it is
achieved by adjusting the state of polarization of the
light beams propagating through the loop mirror. For
a single output port configuration, the polarization
controller PC
4
of SLM
1
is adjusted for minimum
power at the output port 1 (OUT1). The counter-
clockwise and clockwise light beams interfere
destructively at the output port 1 while interfere
constructively at the port 4 of the 3dB fiber coupler,
C
3,
and thus it channels all the power back to the
compound-ring cavity.
As there is no optical isolator placed in any of
the three branches of the fiber compound-ring
resonator, the two counter-propagating light beams
circulate in the nested ring cavities as shown in Fig.
1. The counter-clockwise (ccw) propagating beam
from the SOA
1
reaches the port 1 of the 3dB coupler
C
1
, splits into two equal light beams (i.e., 50%) and
is transmitted into the ports 2 and 3. The light beam
that propagates into branch II undergoes
amplification by SOA
2.
The amplified light beam
that takes the path of branch II passes through a
polarization controller PC
2
before it reaches port 2
of the 3dB fiber coupler C
2
, while the light beam
that propagates through the branch III passes
through the polarization controller PC
3
before it
reaches port 3 of the 3 dB fiber coupler C
2
.
Half of
the light beam at the 3 dB fiber coupler C
2
is coupled
into port 1 where it propagates back into branch I to
complete one round trip, while the other half of the
beam is channelled into SLM
2
. Similarly, the light
beam that is fed into the SLM
2
exits at the output
port 1 of the 3dB fiber coupler C
4
(OUT2). The
polarization controller, PC
5
, can control the output
power. An optical spectrum analyzer (OSA),
variable optical attenuator (VOA) and optical power
meter (PM) were used to characterize the proposed
fiber compound-ring laser. Note that the path lengths
of both loops are the same since all branches have
identical length and all fiber connections are done by
using FC/APC connectors.
4 CHARACTERIZATION OF THE
FIBER RING LASER
4.1 Gain Media
The amplified spontaneous emission (ASE) of
SOA
1
, and SOA
2
were characterized by using an
optical spectrum analyzer (OSA) where both SOAs
were set at the same bias current (I
B
) of 200 mA.
Even though all two SOAs are biased at the same
current level of 200 mA, they exhibited different
ASE spectra where SOA
1
has higher gain than SOA
2
PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology
232
for the same bias current level. Thus, different bias
current levels are required in order to get the same
output power when the SOAs are individually used
in the proposed fiber compound- ring resonator.
4.2 Output Power and FWHM
Maximum and minimum insertion losses (IL) of 5.5
dB 2.2 dB were measured at 1520 nm and 1570 nm,
respectively. Similar downward trend was also
noticed in the FWHM linewidths, which varies from
0.4 to 0.32 nm at 1520 and 1570 nm, respectively.
Due to the downward trend of the insertion losses
from the tunable filter, an upward trend is also
expected in the output power of the proposed fiber
compound-ring laser for a constant gain setting of
the SOAs. Consequently, a constant output power
can be obtained over the entire wavelength tuning
range by adjusting the bias current (I
B
) of the SOAs
but at the expense of signal broadening of the fiber
laser source.
The reflectivity of both SLMs was set at less than
0.1% so that both output ports of the fiber
compound-ring lasers (i.e., OUT1 and OUT2) have
the same output power. Then, by collecting both
clockwise and counter-clockwise propagating light
beams through the SLM
1
and SLM
2
, respectively,
we measured the 3dB bandwidth at different bias
current levels at 1550 nm wavelength by using an
OSA. The 3dB-bandwidth increased from 0.1985 to
0.2182 nm as the bias current was increased to the
standard bias current of each of the SOAs (Table 1).
Table 1: 3dB-bandwidth (FWHM) at different bias current
I
B
(mA) at 1550 nm wavelength with both Sagnac loop
mirror reflectivity set at <0.1%.
SOA
1
I
B1
(mA)
SOA
2
I
B2
(mA)
P
OUT1
(dBm)
P
OUT2
(dBm)
FWHM
(nm)
75 250 3.40 3.40 0.1985
100 300 5.80 5.70 0.2075
125 350 6.73 6.75 0.2122
150 400 7.65 7.68 0.2131
175 450 8.35 8.35 0.2157
200 500 8.94 8.95 0.2182
4.3 Coherent Beam Combining
Efficiency
The principle of the proposed passive coherent beam
combining technique of two compound-ring based
fiber lasers with two adjustable output couplers (i.e.,
Sagnac loop mirrors) is based on passive phase-
locking mechanism due to spontaneous self-
organization operation (Bruesselbach et al., 2005).
Due to wide bandwidth of the SOAs, the passive
phase-locking mechanism allows the fields’ self-
adjustment to select common oscillating modes or
resonant frequencies of the counter-propagating (i.e.,
clockwise and counter-clockwise) light beams in the
two merged ring cavities and optimize their in-phase
locking state conditions without any active phase
modulating system.
In order to determine the beam combining
efficiency of the proposed fiber laser structure, we
first used each individual SOA as gain medium in
the common branch I of the compound-ring cavity
and measured the output power produced by the
fiber laser system at its both output couplers, OUT1
and OUT2. Then, we placed at the same time both
SOAs, SOA
1
(Kamelian model) and SOA
2
(Thorlabs,
S1013S), in the compound-ring cavities (branch I
and II, respectively). Similarly, we measured the
output power delivered at both output couplers of
the proposed fiber laser. Note that the reflectivity of
the Sagnac loop mirrors, SLM
1
and SLM
2
,
was
adjusted to maximum (i.e. > 99.9 %) and minimum
(i.e., < 0.1%), respectively. The tunable filter was
manually adjusted from 1535 to 1565 nm and each
semiconductor optical amplifier, SOA
1
and SOA
2
,
was driven and kept constant at its standard bias
current, 200 and 500 mA, respectively. Fig. 2
illustrates the passive coherent beam combining
efficiency spectrum (right vertical axis) and the
output power spectrum (left vertical axis) from the
proposed fiber compound fiber –ring laser operating
with individual SOAs as well as both SOAs over
the C-band tuning range of 30 nm.
The beam combining efficiency (filled circles)
was obtained by dividing the optical power
measured at the output port (OUT2) when the fiber
laser was operating with both SOAs by the power
summation (unfilled triangles) of the same output
port of the fiber laser while operating with
individual SOA, SOA
1
(filled squares) and SOA
2
(unfilled circles). The leakage optical power
spectrum (unbroken line crosses) at the other output
port (OUT
1
) remained below -28.5 dBm. The
maximum output power delivered by the fiber laser
operating with a single SOA, SOA
1
(Kamelian
model) and SOA
2
(Thorlabs model), was +8.91 and
+8.90 dBm at 1565 nm, respectively. On the other
hand, when both SOAs were placed in the
compound-ring cavities, the maximum measured
output power (broken line crosses) obtained at the
output port, OUT2, was + 11.9 dBm at 1565 nm,
which is almost double of the output power obtained
with either individual SOA placed in the fiber
Beam Combining of SOA-based Bidirectional Tunable Fiber Compound-ring Lasers with External Reflectors
233
compound-ring laser cavity. Moreover, the
maximum output power obtained by just adding the
optical power (triangles) from single SOA fiber laser
operation at the output port, OUT2, was +11.91 dBm
vs. +11.90 dBm measured output power from the
fiber laser operating with both SOAs at 1565 nm
wavelength. This is where the insertion losses of the
tunable filter were the lowest.
The maximum and minimum obtained
combining efficiency (filled circles) was 99.76% and
98.06 % at 1565 nm and 1555nm, respectively, as
shown in Fig.5 (right vertical axis).
Figure 2: Shows individual SOA output power spectrum:
SOA
1
(filled squares), SOA
2
(unfilled circles), output
power summation spectrum of both SOAs (unfilled
triangles), and actual measured output power (crosses) at
the output port , OUT2 with SOA
1
and SOA
2
driven at 200
mA and 500 mA constant bias current. The PC
1
and PC
2
were maximized for each wavelength.
4.4 Fiber Laser Power Tunability and
Its Switchable Dual-Output Power
Operation
The proposed fiber compound-ring laser has a
feature of operating with two adjustable and
switchable output ports (i.e., OUT1 and OUT2). The
output power from either output port can be tuned by
adjusting the gain of the semiconductor amplifiers,
SOA
1
and SOA
2
, by controlling their bias current
levels (see Table 1) or by adjusting the reflectivity of
the Sagnac loop mirrors, SLM
1
and SLM
2,
while
keeping the former constant. The latter approach
involves the adjustment of the reflectivity of both
Sagnac loop mirrors, SLM
1
and SLM
2
while keeping
the gain of both SOAs constant (i.e., I
B1
and I
B2
set at
200 and 500 mA, respectively). Thus, the proposed
fiber compound-ring laser can be operated in single
or dual-output configuration depending on the
reflectivity of the SLM1 and SLM
2
. In single output
configuration, one of the Sagnac loop mirror, SLM
1
or SLM
2
, should be kept at high reflectivity (i.e.,
99.9%) by adjusting its polarization controller, PC
4
or PC
5
, respectively, while keeping the other Sagnac
loop mirror at its lowest reflectivity of 0.1%.
The tunable filter was set at 1550 nm wavelength
in order to characterize the power tunability
performance of both output ports of the fiber laser.
Then, we initialized the reflectivity settings of the
SLM
1
and SLM
2
to 0.1% and 99.9%,
respectively. The initial measured output power
from both output ports, OUT1 and OUT2 was
+11.85 dBm and -28.9 dBm, respectively. Moreover,
we gradually adjusted the reflectivity of the Sagnac
loop mirror, SLM
1
, by slowly changing the
polarization state of the counter-propagating light
beams into the SLM
1
by adjusting the polarization
controller, PC
4
, while recording the power meter
readings and the output signal spectrum at both
output ports, OUT1 and the FWHM at output port,
OUT1. We were able to control the output power
from the output port, OUT1, from +11.85 dBm to -
28.5 dBm while keeping OUT2, at -28.9 dBm by
also optimizing the polarization controller, PC
5
, of
SLM
2
. Similarly, we also set the reflectivity of
SLM
1
and SLM
2
to 99.9% and 0.1%,
respectively, and checked both output ports’
performance in the similar manner as stated above,
where the measured output power from output port,
OUT
2
was adjusted from +11.87 dBm to -28.9 dBm
while keeping the output port, OUT1, at -28.9 dBm.
Fig.6 illustrates the output power from both output
ports, OUT1 (unfilled squares) and OUT2 (unfilled
circles) as a function of the reflectivity of the SLM
1
and SLM
2
, respectively. Note that both output port
behave similarly and the 3dB-bandwidth of the light
beam from OUT1 (filled triangles) and OUT2 (filled
circles) increased as the reflectivity of the Sagnac
loop mirrors increased while the output power
decreased as shown in Fig.3, due to the strong
feedback (i.e., reflected light beam) from each
SLMs.
Figure 3: Shows the output power and the 3-dB-bandwidth
from both output ports, OUT1 (unfilled squares) and
OUT2 (unfilled circles) as a function of different
reflectivity values of the Sagnac loop mirrors for single
output port operation.
PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology
234
Fig. 4 shows the output power from the two
output ports of the proposed fiber laser. In dual-
output port configuration, both output ports can be
fixed and adjusted to any output power between
+11.9 and -28.9 dBm. We firstly set both output
ports, OUT1 and OUT2, to +8.94 and +8.95dBm by
adjusting the reflectivity of both Sagnac loop
mirrors, SLM
1
and SLM
2
at 0.1%, as shown in
Fig.4. Then, we gradually tuned the output power
from the output port, OUT1, from +8.94 to -28.9
dBm by adjusting the reflectivity of the SLM
1
from
0.1% to more than 99.9% while optimizing the
reflectivity of the SLM
2
in order to keep the output
power at OUT2 constant at +8.95 dBm. The
reflectivity of the SLM
2
was around 50% when the
one of the SLM
1
was around 99.9% in order to
maintain the output power at OUT2 constant at
+8.95 dBm.
Figure 4: Illustrates the output power from both output
ports, OUT1 (filled circles) and OUT2 (unfilled squares)
for different reflectivity values of the Sagnac loop mirror,
SLM
1
for dual-output port operation while keeping output
power at OUT2 constant at +8.95 dBm.
4.5 Wavelength Tunability and Power
Stability
The wavelength tuning range of the optical filter that
was used in the proposed fiber laser has 50 nm. Its
maximum IL is 5.5 dB at 1520 nm and its minimum
IL is 2.2 dB at 1570 nm. Fig.5 shows the fiber laser
wavelength-tuning spectrum.
We first set the bias current for both SOAs at
200 and 500 mA, for SOA
1
and SOA
2
, respectively.
The reflectivity of the SLM
1
and SLM
2
were set and
kept constant at 0.1% and 99.9%, respectively.
Then, the wavelength of the output light beam was
measured with an OSA, and was tuned by manually
adjusting the tunable filter, from 1535 to 1565 nm
while optimizing the polarization controllers, PC
1
,
PC
2
and PC
3
, at each wavelength of 1535, 1540,
1545, 1550, 1555, 1560 and 1565 nm as illustrated
in Fig.8. The peak signals from the measured output
wavelength spectra by using an optical spectrum
analyzer were used to determine the optical signal-
to-noise ratio (OSNR) of the proposed fiber
compound-ring laser. We subtracted the peak power
value at each center wavelength from the
background noise level of each wavelength
spectrum. The OSNR remained well above +39 dB
over the whole wavelength tuning range, where the
maximum OSNR of +44.6dB was obtained at
1565nm.
Figure 5: Shows the wavelength spectrum of the fiber
compound-ring laser where PC
1
, PC
2
and PC
3
, were
optimized at each wavelength.
We finally performed short-term optical power
stability test at room temperature with SOA
1
, and
SOA
2
set at the standard bias current levels of 200
and 500 mA, respectively, while the tunable filter
was set and kept fixed at 1550 nm central
wavelength. The OSA was also used to monitor and
acquire the data. The optical stability test was
carried out over a course of 180 minutes with time
interval of 1 minute, and OSA resolution bandwidth
of 0.01 nm without additional data averaging. The
power stability measurements with fluctuations of
less than ±0.02 dB were measured, which indicates
that the proposed fiber compound-ring laser is very
stable. The power fluctuations during the stability
measurement can be minimized by properly
packaging the proposed fiber compound-ring laser
system.
5 CONCLUSION
We successfully demonstrated very high coherent
beam combining efficiency of two SOAs used as
gain media in all-single-mode fiber compound-ring
cavities with two switchable and power adjustable
output couplers formed by using two Sagnac loop
Beam Combining of SOA-based Bidirectional Tunable Fiber Compound-ring Lasers with External Reflectors
235
mirrors. Due to the advanced technologies of
semiconductor optical amplifiers and tunable filters
covering wide range of wavelength spectrum in
different electromagnetic spectrum bands, the
proposed fiber compound-ring laser can be used to
build compact laser systems covering different
optical wavelength-bands.
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