Beam Combining of SOA-based Bidirectional Tunable Fiber

Compound-ring Lasers with External Reflectors

Muhammad A. Ummy

, Simeon Bikorimana

and Roger Dorsinville

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.

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

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

, (Kamelian, OPA-20-N-C-SU), a tunable

optical filter (TF-11-11-1520/1570), and a

polarization controller, PC

. Branch II contains an

SOA

(Thorlabs, S1013S), and a polarization

controller, PC

. Branch III contains only a

polarization controller, PC

. 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

and C

. Each 3dB fiber

coupler, C

and C

is also connected to a Sagnac

loop mirror, SLM

and SLM

, 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

or PC

), 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

of around 75

mA) is injected into the SOA

(branch I), the emitted

ASE emitted by the SOA

(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

after propagating through a

polarization controller, PC

. Then, the selected light

beam that arrives at port 1 of the 3dB fiber coupler

, 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

before it is amplified by

SOA

when its bias current level I

is around 180

mA. Then, the amplified light beam arrives at port 2

of the 3dB fiber coupler C

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

. A half of the

light beam coupled into port 1 of the 3dB fiber

coupler C

is further amplified by SOA

. 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

injected into the input port 4 (i.e., I

) of the Sagnac

loop mirror, SLM

. The polarization controller, PC

controls the reflectivity of the SLM

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

of SLM

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,

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

reaches the port 1 of the 3dB coupler

, 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

The amplified light beam

that takes the path of branch II passes through a

polarization controller PC

before it reaches port 2

of the 3dB fiber coupler C

, while the light beam

that propagates through the branch III passes

through the polarization controller PC

before it

reaches port 3 of the 3 dB fiber coupler C

Half of

the light beam at the 3 dB fiber coupler C

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

. Similarly, the light

beam that is fed into the SLM

exits at the output

port 1 of the 3dB fiber coupler C

(OUT2). The

polarization controller, PC

, 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

, and SOA

were characterized by using an

optical spectrum analyzer (OSA) where both SOAs

were set at the same bias current (I

) 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

has higher gain than SOA

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

) 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

and SLM

, 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

(mA) at 1550 nm wavelength with both Sagnac loop

mirror reflectivity set at <0.1%.

SOA

(mA)

SOA

(mA)

OUT1

(dBm)

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

(Kamelian model) and SOA

(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

and SLM

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

and SOA

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

(filled squares) and SOA

(unfilled circles). The leakage optical power

spectrum (unbroken line crosses) at the other output

port (OUT

) remained below -28.5 dBm. The

maximum output power delivered by the fiber laser

operating with a single SOA, SOA

(Kamelian

model) and SOA

(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

(filled squares), SOA

(unfilled circles), output

power summation spectrum of both SOAs (unfilled

triangles), and actual measured output power (crosses) at

the output port , OUT2 with SOA

and SOA

driven at 200

mA and 500 mA constant bias current. The PC

and PC

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

and SOA

, by controlling their bias current

levels (see Table 1) or by adjusting the reflectivity of

the Sagnac loop mirrors, SLM

and SLM

while

keeping the former constant. The latter approach

involves the adjustment of the reflectivity of both

Sagnac loop mirrors, SLM

and SLM

while keeping

the gain of both SOAs constant (i.e., I

and I

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

. In single output

configuration, one of the Sagnac loop mirror, SLM

or SLM

, should be kept at high reflectivity (i.e., ≥

99.9%) by adjusting its polarization controller, PC

or PC

, 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

and SLM

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

, by slowly changing the

polarization state of the counter-propagating light

beams into the SLM

by adjusting the polarization

controller, PC

, 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

, of

SLM

. Similarly, we also set the reflectivity of

SLM

and SLM

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

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

and SLM

, 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

and SLM

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

from

0.1% to more than 99.9% while optimizing the

reflectivity of the SLM

in order to keep the output

power at OUT2 constant at +8.95 dBm. The

reflectivity of the SLM

was around 50% when the

one of the SLM

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

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

and SOA

, respectively.

The reflectivity of the SLM

and SLM

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

and PC

, 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

, PC

and PC

, were

optimized at each wavelength.

We finally performed short-term optical power

stability test at room temperature with SOA

, and

SOA

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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