Development of a Double-Clad fiber laser simulator for the design of laser cavities with specific applications

Development of a Double-clad Fiber Laser

Simulator for the Design of Laser Cavities with Specific Applications

Conference Paper · March 2011

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Development of a Double-clad Fiber Laser Simulator for the Design of Laser Cavities with Specific Applications

D. Mgharaz

, A. Boulezhar

, and M. Brunel

1

UMR 6614 CORIA CNRS, Universit´e de Rouen, France

2

Universit´e Hassan II Ain Chock, Casablanca, Morocco

Abstract— We have developed a numerical simulator to design actively Q-switched Yb-doped Double Clad fiber lasers. Based on our simulator, we can design specific fiber laser cavities for various applications: a cavity able to emit a pair of sub-nanosecond pulses separated by more than 500 ns for Particle Imagery Velocimetry applications; a second cavity that can emit long 150 ns pulses exceeding a few millijoules per pulse; a third system composed of coupled-cavities to increase the pulse-energy.

1. INTRODUCTION

In the last years, double-clad (DC) fibers have shown their potentiality for the development of low cost, compact, high power fiber lasers [1]. Many operating regimes have been demonstrated experimentally from Continuous Wave operation to Q-switched, or mode-locked regimes [2]. In parallel, numerical simulators have been developed to describe quantitatively the behaviors observed experimentally [3–5]. It is now possible to simulate precisely DC fiber laser cavities and to design cavities for specific applications. We are particularly interested in the development of Q-switched DC fiber lasers that can emit a pair of nanosecond pulses separated by more than 500 ns. The application of such lasers is the Particle Image Velocimetry (PIV) technique. PIV is commonly involved in flow measurements. A second domain of application concerns the emission of long nanosecond pulses. The peak power of such pulses remains relatively low while their energy is important. Industrial and scientific applications are coherent anti-Stokes Raman spectroscopy, emission spectroscopy of laser ablation, texturing and coloring surface in the domain of metal surface treatments.

We have developed a numerical simulator to design actively Q-switched Yb-doped Double Clad fiber lasers [6, 7]. Based on our simulator, we can design specific fiber laser cavities for various ap- plications: the first example that will be presented is a cavity able to emit a pair of sub-nanosecond pulses separated by more than 500 ns for Particle Imagery Velocimetry applications. The second cavity designed allows to emit long 150 ns pulses exceeding a few millijoules per pulse. Applications concern in this case materials science and combustion. In both cases, the rise time of the Q-switch modulator is an essential parameter.

In a second part, we show that our simulator can be used to describe laser combining. High efficiency coherent combining of CW fiber lasers has been demonstrated in the last years [8]. The combining method can be based on the use of a multi-arm resonator in an interferometric config- uration. Michelson and Mach-Zehnder type resonators have been successfully used to reach nearly 100% combining efficiency with two fiber lasers. This concept, which is adapted to the use of double clad doped fibers, has brought some novel perspectives for scaling the output power of the CW monomode fiber lasers. In a similar way, the power rising of an actively Q-switched erbium- doped fiber laser by using two coupled cavities with amplifying fibers could be demonstrated. The pulse peak power obtained could be 1.7 higher than in the case of a unique laser. This concept brings some novel perspectives for scaling the output peak power of single mode Q-switched fiber lasers, where the number of industrial applications is particularly important. In the last section, we present the development of a numerical simulator that can predict precisely the output pulses emitted by a laser system composed of two coupled Q-switched fiber laser cavities.

2. THEORETICAL MODEL

The fiber laser that will be considered is described schematically in Figure 1. The laser medium

is an Yb-doped Double-Clad fiber, noted YDCF. The YDCF is pumped with a laser diode. Other

elements are an electro-optic modulator (EOM), an optical isolator, a 90/10 output coupler and

a wavelength division multiplexer (WDM). The laser diode pump light is injected into the cavity

through the WDM. Q-switching is ensured by the EOM. Unidirectional oscillation is obtained with

Figure 1: Set-up of the fiber laser cavity.

the optical isolator. This configuration eliminates backward Stimulated Brillouin Scattering that could modify dramatically the pulse emission [2]. An un-doped optical fiber is inserted to adjust the ISL of the cavity. The output pulses are extracted with the 90/10 coupler.

The modeling of Q-switched lasers can be done using the traveling-wave model [4]. It has been applied to different pulsed solid-state lasers and amplifiers, and led to great success in explanations of experimental observations, and optimization of systems. The amplifying medium and the pump and signal powers are described versus time, along the fiber, using the rate equations. The Yb ions are described by two level-atoms. The total ytterbium density is assumed to be uniform in the doped fiber. We resolve numerically the equations governing the evolution of the level populations and of the intensities of the pump and signal lights along the fiber. We neglect the chromatic dispersion effect. The doped and undoped fibers are further characterized by attenuation constants for the pump and signal lights. At time t < 0, the pump power is applied to the laser but the EOM is off. The EOM is then turned on and Q-switching can occur. When the EOM is switched on, its transmission passes linearly from 0 to 95%. The EOM opening time is called τ

. In a next step, the EOM is switched off. This procedure is then repeated at a given repetition rate to obtain the final form of the pulse emitted [4, 6, 7].

3. EMISSION OF A PAIR OF SUB-NANOSECOND PULSES FOR PIV APPLICATIONS Our first application concerns the emission of a pair of subnanosecond pulses for Particle Image Velocimetry applications. The emission of pulses composed of multiple peaks has been studied in detail in recent works (see for example [5]). Our work consists in the optimization of the different parameters of the cavity, to favour the emission of a pair of peaks. After several optimization steps, we consider the following configuration: the undoped fiber length is 80 m. This fiber allows to increase the temporal interval between the multiple peaks without increasing the pulse duration of each peak. The total cavity length is 110 m. The EOM has a short rise time of 10 ns to generate short pulses, and an opening time of 1.7 µs. Figure 2 shows the output pulses emitted by our fiber ring cavity at a 30 kHz-repetition frequency. Figure 2 shows clearly the emission of a pair of pulses separated by approximately 530 ns (i.e., the round trip time of our 110 m-ring cavity) [6]. The FWHM and the energy of each pulse are 190 ps and 0.15 mJ for the first pulse, 120 ps and 0.14 mJ for the second pulse. The two pulses are not rigorously identical. Note however that the energy difference between them is only 6%. This results show the possibility to design a Q-switched DC fiber laser for PIV applications. The main advantage of this system is that there is only one cavity (and not 2 as in Nd:YAG PIV systems). The two pulses are thus systematically spatially aligned.

Combined to the low-cost of our device, it offers an important range of applications to DC fiber lasers.

4. EMISSION OF LONG ENERGETIC PULSES

In this section, we consider another case, i.e., the emission of long energetic pulses for applications in spectroscopy and materials science. We consider the ring cavity of Figure 1. The length of the Yb-doped fiber is 3.5 m, and there is no undoped fiber in this case. The Yb-dopant concentration is 2 · 10

m

. The fiber is pumped by a 14 W laser diode at 940 nm. The EOM parameters are:

a rise time of 380 ns, and an opening time of 5 µs. 10% of the signal that propagates in the cavity

is extracted through a 90/10 coupler. Figure 3 shows the pulse predicted in this case. We obtain

a long smoothed envelope. The pulse length is 150 ns at full width half time, and the pulse energy

is 4.8 mJ. It is thus possible to develop Q-switched DC fiber lasers that can emit 150 ns pulses

Figure 2: Emission of a pair of sub-nanosecond pulses.

Figure 3: Emission of a long nanosecond pulse.

Figure 4: Set-up for coupled-cavities.

exceeding the millijoule per pulse.

5. MODELING OF COUPLED CAVITIES

In this last section we show that it is now possible to use our simulator to develop a modeling of combined cavities. High efficiency coherent combining of CW fiber lasers has been demonstrated in the last years [8]. The laser configuration that will be considered is presented in Figure 4. It is composed of two elementary cavities numbered 1 and 2. Each cavity consists of a laser medium (an YDCF), an acousto-optic modulator (noted AOM), and two mirrors of reflectivities R

on the left-hand side and R

on the right-hand side. The Yb-concentration into the fiber-core is 4 ∗ 10

m

. The core diameter of the YDF is 16 µm, while the inner-cladding diameter is 400 µm.

Other parameters describing the YDCF correspond to those of classical double-clad fibers. The pump power is injected on the left-hand side of the cavity through dichroic mirror. Both cavities are mutually injected through two identical unbalanced fiber couplers with a 70:30 splitting ratio (resp. 60/40) The 70% (resp. 60) ports are spliced to the two branches of the elementary lasers, whereas the 30% (resp. 40) ports of both couplers are connected to ensure the radiation transfer between lasers 1 and 2. The unused coupler ports are angle cleaved to avoid any parasitic feedback.

Pump powers for both cavities are P

= P

= 10.5 W. The length of the amplifying fibers are L

= L

= 5 m. The lengths of the intermediate fibers between the different couplers are L

= 3 m, L

= 1 m, L

= 3 m, L

= 1 m and L

= 1 m. We adjust the rise time of the modulators to the value τ

= 1 µs. This slow opening time of the AOMs allows to generate long smoothed pulses.

Figure 5 shows the pulses predicted using a sole cavity (solid line), or the bi-cavity system using

70/30 couplers (dotted line) or 60/40 couplers (dashed line). The different pulse shapes are very

similar. The FWHM pulse duration is approximately 120 ns is all three cases. However the pulse

energies are different. It is only 0.16 mJ using only one cavity wile it equals 0.22 mJ using the 60/40

couplers and reaches 0.24 mJ using 70/30 couplers.

Figure 5: Comparison between the pulses emitted by a sole cavity and a pair of coupled-cavities.

6. CONCLUSION

In conclusion, we have developed a numerical simulator to design actively Q-switched Yb-doped Double Clad fiber lasers. Based on our simulator, we can design specific fiber laser cavities for various applications: a cavity able to emit a pair of sub-nanosecond pulses separated by more than 500 ns for Particle Imagery Velocimetry applications; a second cavity that can emit long 150 ns pulses exceeding a few millijoules per pulse; a third system is based on coupled-cavities to increase the pulse-energy.

REFERENCES

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2. Hideur, A., T. Chartier, M. Brunel, M. Salhi, C. ¨ Ozkul, and F. Sanchez, “Mode-lock, Q-switch and CW operation of an Yb-doped double clad fiber ring laser,” Opt. Commun., Vol. 198, 141–146, 2001.

3. Wang, Y. and C. Q. Xu, “Modeling and optimization of Q-switched double-clad fiber lasers,”

Appl. Opt., Vol. 45, 2058–2071, 2006.

4. Huo, Y., R. T. Brown, G. G. King, and P. K. Cheo, “Kinetic modeling of Q-switched high- power ytterbium-doped fiber lasers,” Appl. Opt., Vol. 43, 1404–1411, 2003.

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