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Thermal effects in kilowatt all-fiber MOPA

Yuanyuan Fan,1,2,3 Bing He,1,2,4 Jun Zhou,1,2,5 Jituo Zheng,1,2,3 Houkang Liu,1,2,3 Yunrong

Wei,1,2 Jingxing Dong,1,2 and Qihong Lou1,2

1Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China

2Shanghai Key Laboratory of All solid-state Laser and Applied Techniques, Shanghai 201800, China

3Graduate University of the Chinese Academy of Sciences, Beijing 100049, China

4bryanho@siom.ac.cn

5junzhousd@siom.ac.cn

Abstract: Thermal effects and output power characteristics of kilowatt all-

fiber master-oscillator power amplifier (MOPA) are investigated. Proper

designs for cooling apparatus are proposed and demonstrated

experimentally, for the purpose of minimizing splice heating which is

critical for the reliability of high power operation. By using these optimized

methods, a thermal damage-free, highly efficient ytterbium-doped double-

clad fiber MOPA operating at 1080 nm with 1.17 kW output was obtained.

The maximum surface temperature at the pump light launching end splice

of the booster amplifier was 345 K, and the temperature rise for this key

splice was 0.052 K/W.

©2011 Optical Society of America

OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (140.3510) Lasers, fiber;

(140.6810) Thermal effects; (140.3615) Lasers, ytterbium; (140.3480) Lasers, diode-pumped.

References and links

1. S. Yin, P. Yan, and M. Gong, “End-pumped 300 W continuous-wave ytterbium-doped all-fiber laser with master

oscillator multi-stage power amplifiers configuration,” Opt. Express 16(22), 17864–17869 (2008),

http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-22-17864.

2. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives

[Invited],” J. Opt. Soc. Am. B 27(11), B63–B92 (2010), http://www.opticsinfobase.org/abstract.cfm?URI=josab-

27-11-B63.

3. Y. Jeong, J. Sahu, D. Payne, and J. Nilsson, “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-

wave output power,” Opt. Express 12(25), 6088–6092 (2004),

http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-25-6088.

4. E. Stiles, “New developments in IPG fiber laser technology,” in Proceedings of the 5th International Workshop

on Fiber Lasers (2009).

5. B. He, J. Zhou, Q. Lou, Y. Xue, Z. Li, W. Wang, J. Dong, Y. Wei, and W. Chen, “1.75 killowatt continuous-

wave output fiber laser using homemade ytterbium-doped large-core fiber,” Microw. Opt. Technol. Lett. 52(7),

1668–1671 (2010).

6. C. Wirth, O. Schmidt, I. Tsybin, T. Schreiber, T. Peschel, F. Brückner, T. Clausnitzer, J. Limpert, R. Eberhardt,

A. Tünnermann, M. Gowin, E. ten Have, K. Ludewigt, and M. Jung, “2 kW incoherent beam combining of four

narrow-linewidth photonic crystal fiber amplifiers,” Opt. Express 17(3), 1178–1183 (2009),

http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-3-1178.

7. D. C. Brown and H. J. Hoffman, “Thermal, stress, and thermal-optic effects in high average power double-clad

silica fiber lasers,” IEEE J. Sel. Top. Quantum Electron. 37(2), 207–217 (2001).

8. Y. Wang, C. Q. Xu, and H. Po, “Thermal effects in kilowatt fiber lasers,” IEEE Photon. Technol. Lett. 16(1), 63–

65 (2004).

9. N. A. Brilliant and K. Lagonik, “Thermal effects in a dual-clad ytterbium fiber laser,” Opt. Lett. 26(21), 1669–

1671 (2001), http://www.opticsinfobase.org/abstract.cfm?URI=ol-26-21-1669.

10. M.-A. Lapointe, S. Chatigny, M. Piché, M. Cain-Skaff, and J.-N. Maran, “Thermal effects in high power CW

fiber lasers,” Proc. SPIE 7195, 719511 (2009).

11. Y. Wang, “Heat dissipation in Kilowatt fiber power amplifiers,” IEEE J. Quantum Electron. 40(6), 731–740

(2004).

12. L. Li, H. Li, T. Qiu, V. L. Temyanko, M. M. Morrell, A. Schülzgen, A. Mafi, J. V. Moloney, and N.

Peyghambarian, “3-Dimensional thermal analysis and active cooling of short-length high-power fiber lasers,”

Opt. Express 13(9), 3420–3428 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-9-3420.

13. P. Li, C. Zhu, S. Zou, H. Zhao, D. Jiang, G. Li, and M. Chen, “Theoretical and experimental investigation of

thermal effects in a high power Yb3+-doped double-clad fiber laser,” Opt. Laser Technol. 40(2), 360–364 (2008).

14. A. Hardy and R. Oron, “Signal amplification in strongly pumped fiber amplifiers,” IEEE J. Quantum Electron.

33(3), 307–313 (1997).

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Received 25 May 2011; revised 3 Jul 2011; accepted 4 Jul 2011; published 21 Jul 2011

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15. D. E. Gray, American Institute of Physics Handbook, 3rd ed. (McGraw-Hill, 1972).

16. P. Yan, A. Xu, and M. Gong, “Numerical analysis of temperature distributions in Yb-doped double-clad fiber

lasers with consideration of radiative heat transfer,” Opt. Eng. 45(12), 124201 (2006).

17. B. Zintzen, T. Langer, J. Geiger, D. Hoffmann, and P. Loosen, “Heat transport in solid and air-clad fibers for

high-power fiber lasers,” Opt. Express 15(25), 16787–16793 (2007),

http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-25-16787.

18. J. P. Gwinn and R. L. Webb, “Performance and testing of thermal interface materials,” Microelectron. J. 34(3),

215–222 (2003).

1. Introduction

Due to high efficiency, compactness, and outstanding beam quality, fiber lasers are now

competing with their bulk solid-sate counterparts in various scientific and industrial

applications [1–3], such as material processing, defense, remote sensing, free-space

communication, display, etc. The rising in output powers from ytterbium-doped double-clad

fiber (YDCF) sources, via the use of cladding-pumped fiber architectures in combination with

high-power and high-brightness diode pump sources, has been dramatic recently and they

have matured to the point where the average power reaches the kilowatt (kW) level and

beyond [4–6]. Thanks to the long and thin fiber geometries, stress fracture and beam

distortion, which are major problems for bulk solid-state lasers, can both be alleviated

strongly in fiber lasers [6]. But thermal management is still one of the most critical issues for

scaling higher output powers from YDCF sources. Although the intrinsic large surface-to-

active-volume ratio of fibers brings many direct and indirect advantages to their heat

dissipations, the use of shorter fibers with higher thermal loading densities in kW fiber lasers

indeed require careful thermal management [7–9]. In practice, low index polymer coatings of

conventional double-clad fibers are always so sensitive to a high thermal load that it will

cause thermal damage when the temperature is approaching 150~200 °C (long-term reliability

may require operation below 80 °C) [10], so it need to be controlled not to extend the safe

range though the core temperature is always below the melting point of quartz (1982 K).

Transverse and longitudinal temperature distributions in fibers have been calculated by

solving thermal conduction equations [11–13]. Based on the convective heat transfer

coefficient h, heat dissipations of fiber lasers or amplifiers are analyzed and discussed in most

of these studies. However, it’s difficult to analyze in this way for a single point on the fiber

under different conduction conditions. In this paper, thermal contact resistance is introduced,

and the thermal effects of active fibers and fiber-to-fiber splices of an all-fiber master-

oscillator power amplifier (MOPA) are discussed in detail. For the purpose of figuring out

heat dissipation requirements for different parts of the fibers and providing effective cooling

ways, the temperature distribution of the active fiber in booster amplifier stage is analyzed

through a numerical modeling. Three kinds of cooling apparatus were discussed theoretically

and verified experimentally. At last, a 1.17 kW output, thermal damage-free, YDCF MOPA

operating at 1080 nm with a total optical-to-optical conversion efficiency of 82.4% was

developed.

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Received 25 May 2011; revised 3 Jul 2011; accepted 4 Jul 2011; published 21 Jul 2011

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2. Thermal Analysis

Fig. 1. Model for solving the temperature distribution of DCF.

The model for solving the radial thermal distribution for the double-clad fiber (DCF) is shown

in Fig. 1. It can be given by the thermal conduction equations [7]

1

r r

( )

r

[],

T r

Q

k

r

(1)

where a, b, and c denote the core, inner cladding, and outer cladding radii, respectively, r the

radial coordinate, Q the heat power density, and k the thermal conductivity. The temperatures

and their derivatives must be continuous across the boundary. In addition, they should also

satisfy the following boundary conditions

0

3

33

0,

[( )],

r

r c

c

T

r

T

r

k h TT rc

(2)

where h is the convective heat transfer coefficient, and Tc the coolant temperature.

Straightforward solution of Eq. (1), subject to Eq. (2) and other boundary conditions, results

in the following expressions for the temperature in Regions I, II and III

2

1

10

1

22

11

20

1

2

2

22

111

30

123

2222

1111

0

123

( ),

4

( )ln( ),

42

( )ln( )ln( ),

422

ln( ) ln( ),

b

2422

c

q r

T rT

k

q a q a

r

a

T rT

kk

q aq aq a

b

a

r

b

T rT

kkk

q aq aq aq a

b

a

c

TT

hckkk

(3)

where q1 is the heat power density of the core, k1, k2, and k3 the thermal conductivities in

Regions I, II and III, respectively, and T0 the center temperature of the core.

In the practical situation of a strong pumping, one can assume that either the pump or the

signal is large compared to the ASE. In this limit, the time-dependent rate equations can be

solved analytically [14], and then the pump power distributions along the fiber can be

calculated. If the quantum defect heating is assumed to be the only factor to cause heating, we

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(C) 2011 OSA

Received 25 May 2011; revised 3 Jul 2011; accepted 4 Jul 2011; published 21 Jul 2011

1 August 2011 / Vol. 19, No. 16 / OPTICS EXPRESS 15164

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could then get how the heat power density varies along the fiber. As shown in Fig. 2, we

assume a single-stage MOPA system with 1.22 kW of pump power and 1.8 dB/m of pump

absorption at 975nm in YDCF, which is equivalent to a heat load of 40 W/m [10]. By taking

k1=k2= 1.38 W/mK [15], k3= 0.2 W/mK, b= 400 μm, c= 500 μm, and h= 100 W/m2K which is

appropriate to a fan-blowing air cooling condition, the temperature distribution in the fiber

center along a 9 m-long YDCF is achieved and shown in Fig. 3. It varies along the fiber,

decaying approximately exponentially. We can see directly that the position with the highest

temperature is at z=0 m, which is just the fusion splice point generating the maximum heat of

the MOPA, and the high-temperature region mainly focuses on the first two meters.

LD

975 nm

YDCF

pump

coupler

main

oscillator

Fig. 2. Schematic of a single-stage MOPA.

02468

200

300

400

500

600

700

z(m)

Fiber center temperature (K)

Fig. 3. Center temperatures along the fiber.

00.511.522.5

x 10

-4

570

580

590

600

610

620

Radial coordinate (m)

Temperature (K)

h=100 W/m2K

h=105 W/m2K

h=110 W/m2K

Fig. 4. Temperature distribution as a function of r for three different h values at z=0 m.

Without considering fusion splice losses, the calculated temperature distribution as a

function of r for three different h values at z=0 m is shown in Fig. 4. The temperature depends

strongly on the convective heat transfer coefficient, and for a certain value of r, a higher h

value leads to a lower T value. Then, for a certain value of h, the total temperature difference

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Received 25 May 2011; revised 3 Jul 2011; accepted 4 Jul 2011; published 21 Jul 2011

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in the fiber radial coordinate is about 25 K. When the heat-sinking temperature is taken as 290

K with h=100 W/m2K, the corresponding coating surface temperature is nearly 600 K which

is much higher than the coatings’ long-term reliability temperature of 353 K (80 °C). Figure 5

shows the temperatures of fiber center and coating surface as a function of h at z=0 m. The

fiber center temperature should be below 378 K to ensure the coatings’ safety (below 353 K),

and this is corresponding to h 450 W/m2K. If the heating caused by fusion splice losses is

considered, much higher h value is needed. That is to say, some more effective cooling

methods at this splice point of fiber other than merely air cooling must be taken.

0200400 600 8001000

300

350

400

450

500

550

600

650

h (W/m2K)

Temperature (K)

Tcenter

Tsurface

Fig. 5. Fiber center and coating surface temperatures as a function of h.

As free convection is an order of magnitude less efficient than conduction for dissipating

heat from an optical fiber [16], we introduce a copper heat sink with V-grooves to cool the

fiber. The thermal contact resistance per unit surface

heat sink can be expressed as [10]

"

tc

R (m2K/W) between the fiber and the

"

tc

,

"

s

TT

R

q

(4)

where Ts (K) is the fiber surface temperature, T (K) the heat sink temperature, and "

(W/m2) the heat flux. Equation (4) suggests the concept for the treatment of the heat flow

through a fiber layer in analogy to the diffusion of electrical charge [17], where the

temperature difference is analog to the electrical voltage which drives the heat flow through a

thermal resistance. For an active fiber, the heat generation q0 (W/m3), the heat load q (W/m)

and the pump absorption α (dB/m) are related to each other

q

/10

0

22

(1)

(1 10

),

p

dL

s

P

q dL

a

q

a

(5)

where P is the pump power through a section of length dL,

s and

P

the signal and pump

wavelengths, respectively. "

q is defined by

'

"

q

q

perimeter

.

Take one point on the fiber for example, Fig. 6 shows the two contact ways between the

fiber and the heat sink. Due to the limited machining precision, the grooves cannot perfectly

match the fibers within a period of length, and so the interstitial air between them may have a

significant thickness in some place as shown in Fig. 6(a). In order to improve the contact

closeness, the fiber could be pressured and fixed properly by thin copper tape, as shown in

Fig. 6(b). Since copper has good ductility and conductivity, the copper tape could contact well

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Received 25 May 2011; revised 3 Jul 2011; accepted 4 Jul 2011; published 21 Jul 2011

1 August 2011 / Vol. 19, No. 16 / OPTICS EXPRESS 15166