Analysis of Ytterbium Doped Seven-core Fiber Laser for Materials

Processing and Particle Image Velocimetry Applications

Ali.Nassiri

, H. Idrissi-Saba

and Abdelkader Boulezhar

Department of physiques, Metrology and Information Processing Laboratory, Faculty of Sciences, Ibn Zohr University,

Agadir-BP 8106, Morocco

Department of Physiques, Laboratory of Theoretical and Applied Physics, Faculty of Sciences-Ain Chock, Hassan II

Casablanca University, Casablanca-BP 9167, Morocco

Keywords: Ytterbium ion, seven-core fiber, fiber laser, materials processing, Particle Image Velocimetry (PIV).

Abstract: In this work, we have developed a theoretical model of a new type of pulsed fiber laser used a new generation

of fiber used as an amplifying medium. This latter is called seven-core fiber. After performing a parametric

study of this kind of fiber, we have successfully modeled our fiber laser cavity. Based on the simulations of

the theoretical model, we have successfully demonstrated that the modelled laser can be generating two genres

of nanoseconds pulses. The first one is to generate a large pulse with high energy applicate to materials

processing (cutting, drilling and deep engraving of materials…). The second is to generate a pair of

nanosecond pulses identical in energy for Particle Imagery Velocimetry (PIV) application. This seven-core

cavity laser modelled is a good candidate to replace the other classical lasers for these applications.

1 INTRODUCTION

Over the past decade, rare earth fiber laser sources

have become one of the most popular and fast

developing laser technologies. Q-switched laser

pulses with nanosecond durations are required for

applications such as range finding, laser surgical,

materials processing, high peak- power pulses,

OTDR, Lidar and for pumping of nonlinear devices

such as optical parametric oscillators

(Majumdar, 2013),

(Adachi, 2002), (Pan, 2009)

. This is mainly because fiber

lasers possess several advantages including high

conversion efficiency, high beam quality,

maintenance, beam delivery and therefore are poised

for a great leap in their popularity and reduction in

their cost of mass production

(Majumdar, 2013),

(Limpert, 2007), (Wang, 2007)

. The conventional double

clad fibers suffer from the limitations of nonlinear

phenomena like Stimulated Brillouin Scattering (SBS)

and mode quality degradation at high power levels. To

have high power and high energy fiber laser, a new

fiber generation has been achieved like seven-core

fiber laser

(Michaille, 2009), (Huo, 2004), (Shirakawa,

2011). A pumped multicore cladding is important,

because the cores can be pumped by a common pump

source

(Abedin, 2012), (Huo, 2004). These lasers also

have an advantage of being a large area what leads to

a high doping concentration and a good beam quality

(Kochanowicz, 2011). Due to the nature of the

distributed cores, the thermal effects of the mechanical

laser of multicore fiber (MCF) are attenuated

compared to fiber laser

(Huo, 2005).

In the one hand,

several applications of materials

processing need a large nanosecond pulse laser with

pulse duration of 150 ns to 1000 ns with a high peak

power. For example, the process of the texturing

surface

(Jouvard, 2007) and the process of surface

coloring (Jouvard, 2007). These applications require

long pulse laser varying from 200 to 500 ns, with pulse

energy between 3 and 6 mJ

(Jouvard, 2007). A large

pulse laser with energy varying between 5 to 10 mJ is

required to apply it to coherent anti-Stokes Raman

spectroscopy (CARS)

(Beyrau, 2004). Large

nanosecond pulse with energy of 7 mJ and peak power

about 25 kW is also recommended for metal drilling

as Aluminum and Titanium and deep engraving

applications

(Kremser, 2014). Several efforts have been

made to obtain this genre of pulse by using different

types of fiber lasers cavity.

(Mgharaz, 2009) report an

Yb-doped fiber laser using a ring cavity actively Q-

switched fiber laser by using an Acousto-optic

modulator. They demonstrated an Ytterbium doped Q-

switched fiber laser with which generated 4.8 mJ and

150 ns width pulses

(Mgharaz, 2009). Their research

clearly demonstrated the feasibility of achieving long

Q-switched pulses in all-fiber configuration. However

these lasers, so far, produced only relatively small

pulse energy and peak power and far from the

requirements for all laser material processing. Also, a

large mode area fiber with a 30 mm core diameter is

260

Nassiri, A., Idrissi-Saba, H. and Boulezhar, A.

Analysis of Ytterbium Doped Seven-core Fiber Laser for MaterialsProcessing and Particle Image Velocimetry Applications.

DOI: 10.5220/0009774202600263

In Proceedings of the 1st International Conference of Computer Science and Renewable Energies (ICCSRE 2018), pages 260-263

ISBN: 978-989-758-431-2

used and this large core enhances the number of modes

guiding in a laser cavity and the constraints of

choosing a small numerical aperture.

In the other hand, several research items were

studied using the Q-switched fiber laser for emission

of millijoules and microjoules nanosecond pulses used

for several applications. The Particle Image

Velocimetry (PIV) is one of these applications. The

basic principle involves photographic recording of the

motion of microscopic particles that follow the fluid

or gas flow. Image processing methods are then used

to determine the particle motion, and hence the flow

velocity, from the photographic recordings. Provided

there are enough particles within the area of flow

under investigation, the entire velocity field of the

flow can be determined

(Http, 2017). This application

requires a pair of nanosecond pulses separated by

more than 500 ns. A seven-core fiber laser offers this

alternative and is one of the best lasers for these

applications such as their high efficiency,

compactness size low cost, and good beam quality.

In this paper, after studying the characteristics

of the multicore fiber, we are going to describe the

numerical modeling the seven-core fiber laser. From

the simulated results of the analytical model, we

confirm that the modeled laser is a good candidate to

emit two genres of pulses. The first is to generate a

two identical nanosecond pulses for PIV application.

The second is to product a large pulse width with pulse

duration of 200 ns to 1000 ns and high peak power

possible used for some materials processing.

2SIMULATED RESULTS AND

MODEL VALIDATION

The fiber cavity laser that we are considering is

based on the typical linear Fabry-Perot cavity

presented schematically in Figure 1. It consists

mainly of two mirrors M

and M

an acousto-optic

modulator (AOM) which is operating at the first

diffraction order and the laser diode for pumping the

cavity. The pump and the power and the output power

are separated at the left end by a beam splitter. The

ytterbium doped fiber has seven identical cores that

are arranged in a hexagonal array.

After studying the characteristics of our seven-core

fiber used in our simulation, we have validate the

simulation that we carry out, using the same

parameters as considered by

(Huo, 2004). We have

calculated the pulses predicted with our simulator.

Figure 1: Fabry-Perot seven-core fiber laser cavity

design.

We could then compare them to those observed

experimentally and predicted with the simulator of the

corresponding reference

(Huo, 2004). The two output

pulses calculated by our model and those obtained

experimentally by Huo et al

(Huo, 2004). are in good

agreement: the same pulse shape, the same peak

power of about 3.47 kW, the same pulse duration

nearly 47 ns and even the energy which is equal to

0.22 mJ

(Nassiri, 2017). Therefore, the output pulse

energy obtained by simulated results of Huo et al. are

is about 0.24 mJ

(Huo, 2004). Consequently, it shows

that our model is well validated to describe this type

of multicore Q-switched fiber laser

(Nassiri, 2017).

This case corresponds to a 3m fiber length doped

ytterbium with a concentration of 8.10

−3

. The

acousto-optic modulator has a repetition rate equal to

5 kHz. The M1 mirror has low reflectivity of 4% and

mirror has high reflectivity of 40%. The pump

power is 15W. The pump and signal wavelengths of

940 µm. The other parameters are described in Table

Table 1: Parameters used in our simulation

(Huo, 2004).

Paramete

Value

(

)1,4.10

-27

−3

(

)2,25.10

-25

−3

(

)3. 10

-25

−3

(

)5. 10

-26

−3

Δ(

) 20 n

h 6.626068.10

-27

m/s

1ms

5.10

3.10

n1.45

c3.10

m/s

Analysis of Ytterbium Doped Seven-core Fiber Laser for MaterialsProcessing and Particle Image Velocimetry Applications

261

3 SEVEN-CORE FIBER LASER

FOR MATERIALS PROCESSING

AND PIV APPLICATIONS

In this section, we will study the two pulses obtained

by our simulated model. The first is for PIV

application and the second is for materials processing

application.

3.1 Seven-core Fiber Laser for Materials

Processing Application

The laser configuration proposed in fig.1 with the

following modification has been used and already

studied

(Nassiri, 2017). The output power is extracted

in the right hand of the cavity and the reflectivity of

the two mirrors M

and M

are respectively 0.99 and

0.4. The pump power is 10 W, and the length of fiber

is limited to 4m, and the Yb concentration is equal to

4.10

-3

. The core radius of each one is limited only

a 6µm. The laser provides 7 mJ pulse energy with

estimated peak power of 26 KW and the full width at

half maximum (FWHM) is 165 ns. This fiber laser

configuration is a good candidate to replace Q-

switched thin disc laser used to drilling and deep

engraving of Aluminum and titanium in a laser

wavelength emission equal to 1030 nm

(Kremser,

2014). Therefore, in this paper, an enhanced pulse with

200 ns FWHM, 23 mJ and 58.24 kW at 1 kHz can be

emitted by this nanosecond fiber configuration at 1064

nm by enlarging core radius to 8 μm only and reduced

rise time to 280 ns is obtained as described in fig.2.

The pulse produced in this wavelength can be used to

process of texturing and coloring surface of titanium

in place of Nd-Yag laser.

Figure 2: Large nanosecond pulse produced by a

seven-core fiber laser cavity for drilling and deep

engraving of aluminium and titanium.

3.2 Seven-core Fiber Laser for Partical

Image Velocimetry Application

The laser classically used for PIV application is

the Nd-Yag laser based on the two identical cavities.

The first cavity emits the first pulse after sometimes

the second cavity emits the second pulse and the

pulses are finally superposed at the output of the two

cavities. This genre of cavities suffer in several

limitations such as the alignment of the two cavities

and the problem of synchronization, moreover, the

less beam quality. The fiber laser is an alternative key

to avoid these limitations in terms of attributes

including efficiency, reliability, beam quality,

maintenance, and beam delivery.

Several research items were studied using the

Q-switched fiber laser for emission of millijoules and

microjoules nanosecond pulses. It is possible to emit a

pair of nanoseconds pulses using only one fiber laser

cavity with a good alignment of two pulses at the

output of the system.

(Mgharaz, 2009) report an Yb-

doped fiber laser using a Fabry-Pérot Q-switched fiber

laser cavity to produce two identical pulses for PIV.

They demonstrate that fiber laser cavity that is able to

emit a pair of nanosecond pulses separated by more

than 626 ns for a 42 kHz repetition rate. The energy

the two pulses are respectively 36.5µJ and 35 µJ while

their pulse-durations at FWHM are 8.15 ns and 8.1 ns

respectively. The same authors report an Yb-doped

fiber laser using a ring cavity which generated two

identical pulses separated by a temporal spacing

exceeding 532 ns

(Mgharaz, 2011). The FWHM and the

energy of each pulse are (185 ps; 0.15 mJ) and (115

ps; 0.14 mJ), respectively. The two pulses are not

rigorously identical. But the main parameter for PIV

experiments is there concerning energy between both

pulses

(Mgharaz, 2011). These researchers have clearly

demonstrated the feasibility of achieving two identical

pulses for PIV in all double clad fiber laser

configurations. However these lasers, so far, produced

only relatively small pulse energy and peak power, far

from the requirements for all laser applicate to PIV.

In this paper, we have successfully demonstrated

an actively Q-switched seven-core fiber laser which

produces two identical pulses of high energy with

large nanosecond time duration to be used for PIV

application. These pulses are obtained by used the

parameters described previously. The reflectivity of

the two mirrors M

and M

were respectively 0.95 and

0.99. The pump power was 10.75 W, and the length of

fiber was 25m. The Ytterbium concentration was

equal to 1, 19.10

−3

. The core radius of each one

is limited a 8 µm and the rise time of AOM is 8 ns. We

insert also an undoped fiber with length of 40m to

increase the temporal interval between the multiple

peaks.

ICCSRE 2018 - International Conference of Computer Science and Renewable Energies

262

Figure 3: Pair of Nanoseconds Pulses Produced by a Seven-

Core Fiber Laser Cavity for PIV Application.

The produced pulses energy and FWHM of the first

pulse and the second pulse are respectively (1.71 mJ,

13.6 ns) and (1.70 mJ, 10.2ns) (Fig. 3). The separation

of the neighboring peaks is actually equal to the round

trip time for our 25 m doped fiber and 40m for

undoped fiber. This multicore fiber laser configuration

is a good candidate to replace the classical Nd-Yag

laser and the one core fiber laser cavities for this

application.

4 CONCLUSION

In this paper, we have presented the design of seven

core fiber laser cavity that is able to emit a pair of

nanoseconds pulses separated by more than 500 ns

applied to PIV. The produced pulses energy and

FWHM of the first pulse and the second pulse can be

exceed the some millijoules and would satisfy PIV

requirements. This seven-core fiber laser cavity is a

very good candidate to replace the Nd-YAG laser used

classically in term of compact, low cost, beam quality,

and spatial alignment, and the one core fiber laser

cavity in term of emitted energy.

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