Thermal Effects in Kilowatt Fiber Lasers - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 1, JANUARY 2004

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Thermal Effects in Kilowatt Fiber Lasers Yong Wang, Member, IEEE, Chang-Qing Xu, Senior Member, IEEE, and Hong Po

Abstract—Thermal effects and their influences on kilowatt ytterbium-doped double-clad (YDDC) fiber lasers are studied through numerical modeling. Solutions to suppress the thermal effects in the YDDC fiber lasers under bidirectional end pump and distributed pump are presented and compared for the first time. It is shown that lower operating temperature and more uniform heat dissipation in fibers can be obtained by optimizing the arrangement of pump powers, pump absorption coefficients, and fiber lengths. Index Terms—Fiber lasers, heat dissipation, thermal effects, ytterbium-doped.

I. INTRODUCTION

H

IGH-POWER rare-earth-doped double-clad fiber lasers have attracted considerable attention recently in commercial and military applications due to their high efficiency, compactness, and high beam quality, compared to traditional gas and solid-state lasers [1]–[3]. In continuous-wave (CW) regime, a ytterbium (Yb)-doped single-mode fiber laser with an output [4], and a Yb-neodymium power of 400 W and (Nd) codoped fiber laser with an output power of 500 W and [5] have been reported recently. Although the thermal effects can be ignored in low-power fiber lasers, the heat dissipation is an important feature and affects laser performance in kilowatt power domain [2], [6], [7]. In this letter, we propose some solutions to facilitate heat dissipation and reduce the operating temperature in CW kilowatt double-clad fiber lasers through numerical modeling for the first time. The model and results are important to the design and development of kilowatt fiber lasers.

Fig. 1. Schematic configurations of YDDC fiber laser under (a) end pump and a uniform pump absorption coefficient, (b) end pump and nonuniform pump absorption coefficients, (c) distributed pump and a uniform pump absorption coefficient.

with nonuniform pump absorption coefficients and distributed pump, as shown in Fig. 1(b) and (c), respectively, are discussed in the next section. A set of simplified steady-state rate equations describing these lasers are given by [9]

(1) (2)

(3)

II. LASER CONFIGURATION AND MODELING The configuration of a typical Yb-doped double-clad and ) is (YDDC) fiber laser under CW end pump ( schematically shown in Fig. 1(a). The YDDC fiber has a length and a uniform pump absorption coefficient of . The of laser has a high-reflectivity (HR) mirror of 99% at the left end and a cleaved facet (4%) as the output coupler (OC). The YDDC fiber, used for this work, has a core diameter of 30 m , an inner cladding diameter of 250 m , an outer . Based on a mode filtering cladding diameter of 400 m technique [8], this YDDC fiber can be appropriately coiled to realize a single-mode operation without a significant bending loss. A simple air-cooling method is considered to facilitate the heat dissipation for the laser [2]. The proposed laser schemes Manuscript received July 9, 2003; revised August 7, 2003. Y. Wang and C.-Q. Xu are with the Department of Engineering Physics, McMaster University, Hamilton, ON L8S 4L7, Canada. H. Po is with the Lasersharp Corporation, Hopkinton, MA 01748 USA. Digital Object Identifier 10.1109/LPT.2003.818913

(4) is the Yb ion concentration, and are ground where is the pump and upper-level populations respectively. power, and is the signal power ( correspond to forward and backward propagations, respectively). The pump and signal wavelengths ( and ) are 915 and 1065 nm. is the light velocity in the vacuum. and are the absorption and emission cross-sections of Yb ions, respectively. is the Planck constant. is the doped area of the YDDC fiber, is the spontaneous lifeis the overlapping factor between the pump (signal) time. is the fiber attenuation coefand the fiber doped area. ficient. In the modeling, we take , , , . and are from [3]. of 2 nm and pump Furthermore, a signal bandwidth coupling efficiency of 90% are considered. It is worth noting that two-point boundary conditions associated with the above differential equations (1) to (4) are applied to Fig. 1(a), and multipoint boundary conditions are considered for Fig. 1(b) and (c). The heat dissipation as well as transverse and longitudinal

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 1, JANUARY 2004

Fig. 2. Temperature and power distributions in 20-m YDDC fiber under end pump and with a uniform pump absorption coefficient of 1.0 dB/m.

Fig. 3. Temperature and power distributions in 60-m YDDC fiber under end pump and with a uniform pump absorption coefficient of 0.25 dB/m.

temperature distributions in the YDDC fiber under conventional air-cooling is governed by the following thermal conductive [2], [7], equations in symmetric cylindrical coordinates (5) (6) (7) (8)

(9) where is the temperature in the fiber, and is the heat sink temperature. is the heat dissipated in unit volume, which is is the fiber thermal conchiefly generated in the fiber core. is the convective coefficient. , , and are ductivity, the Nusselt, Grashof, and Prandtl numbers, respectively. , and are the density, viscosity, and thermal conductivity of air. The air convection is selected to keep the heat sink temK , the perature constant. At the room temperature following parameters are used: m , , kg m , kg m , W m K , W m K . III. SIMULATION RESULTS First, we consider the above typical end-pumped scheme with an equal pump power of 500 W at both fiber ends as shown in Fig. 1(a) (namely scheme a). Fig. 2(a) shows the calculated temperature distributions at the fiber axis and the inner/outer claddings boundary m . The selection of m is based on a maximum output power dB/m. It can be seen that the temperature distribuwith tion is apparently uneven along the fiber, and the temperature difference in the radial direction is smaller than that in the axial direction. A maximum temperature of 340 C is reached at the fiber output side, while a minimum one of 94 C is located at the mid-span of the cavity. Fig. 2(b) shows the pump and signal power distributions in the cavity. The laser output power is 725 W. It indicates that the uneven temperature distribution results

Fig. 4. Temperature and power distributions in 32-m YDDC fiber under end pump and with nonuniform pump absorption coefficients in five segments.

from nonuniform pump absorption in the fiber, and a lot of heat is generated near the fiber ends due to higher pump absorption. Since the laser operation under this temperature is impractical for most double-clad fibers, especially fiber coatings, we have to adopt a lower pump absorption coefficient and a longer fiber to reduce the operating temperature for this scheme. dB/m and m (the optimal length for When this ) are used in this end-pumped scheme, the temperature and power distributions are plotted in Fig. 3. We can see that the operating temperature is in the range of 70 C to 137 C, and the output power is 630 W. Compared to the previous result, the maximum temperature in the fiber decreases by at least 200 C, whereas the laser output power drops by 13%. Hence, lowering pump absorption together with lengthening the fiber to suppress the thermal effects is not a good solution to some extent. One can see in scheme a that most of the pump power is absorbed near the two fiber ends, which causes a heat dissipation issue. On the other hand, the adoption of a relatively low for the whole fiber deteriorates the laser efficiency. Therefore for an end-pumped scheme, an uneven distribution of , e.g., a lower arranged near the pump launching points, as shown in Fig. 1(b) (namely scheme b), is expected to compromise heat dissipation and laser efficiency. The calculated results are depicted in Fig. 4, where pump absorption coefficients of 0.33, 0.57, 0.81, 0.38, 0.24 dB/m and fiber lengths of 6.0, 2.7, 15.0,

WANG et al.: THERMAL EFFECTS IN KILOWATT FIBER LASERS

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m power of more than 690 W can be obtained with m( to 7). It also indicates that an increase of and the number of pump segments can further improve the uniformity of the axial temperature distribution. However, the output power is lower due to more pump losses. Meanwhile, the laser suffers a higher manufacturing cost. This tradeoff as well as the requests for system volume, component availability, operating conditions, etc., should be comprehensively considered in the practical design and optimization of kilowatt fiber lasers. IV. CONCLUSION

Fig. 5. Temperature and power distributions in 55-m YDDC fiber under distributed pump and with a uniform pump absorption coefficient of 1.2 dB/m.

6.8, 5.5 m are arranged for segments 1 to 5, respectively. The total fiber length is 36 m. It is assumed that these five segments are well spliced together, with a splicing loss of 0.05 dB for both the signal and pump lights. It is shown in Fig. 4 that the fiber temperature is in the range of 84 to 137 C, and the output power is 665 W. Compared to the previous result in Fig. 3, the output power increases by 35 W, the fiber length is shortened by 24 m, whereas the maximum temperature in the fiber is the same. It is found that further reducing the maximum temperature can degrade the laser efficiency significantly. To further decrease the fiber temperature and flatten its distribution, more uniform pump power distribution is necessary. Therefore, compared to the previous schemes a and b, a distributed pump (scheme c) as shown in Fig. 1(c), can be the best solution, where the pump power is separately launched into several segments of a uniform YDDC fiber through the side-coupling technique [10]. Furthermore, this scheme can effectively relax the requests for high brightness of pump laser diodes in those end-pumped schemes. For the th segment with a length , and stand for forward and backward of , pump powers. Pump coupling efficiency of 90% and a pump transmission loss of 10% at each internal launching point are assumed. Based on laser efficiency with respect to a maximal , , operating temperature, these parameters, i.e., , , , can be optimized. For example, with a maximal temperature of 98 C, the optimal solution with seven segments is given blow. The first and last segments are 10 m long, the rest ones are identically 7 m long, and the total fiber length is 55 m. The pump absorption coefficient is 1.2 dB/m. The pump powers, totally 1000 W, are 100, 85, 85, 75, 75, 70, 70, 65, 65, 61, 61, 61, 61, 66 W for segments 1 to 7, respectively. The pump and signal power distributions in the laser cavity are plotted in Fig. 5. We can see that the maximum and minimum temperatures at the fiber axis are 98 and 66 C, and the output power is 670 W. Compared to the results in Figs. 3 and 4, this scheme offers the best temperature uniformity and the highest output power. Moreover, under a maximum temperature of 128 C, an output

By solving a set of rate equations in conjunction with thermal conductive equations under air-cooling, we have obtained the temperature and power distributions in the YDDC fibers, with which we can investigate the thermal effects in these different schemes. To lower the operating temperature in fibers, a basic solution for a traditional end-pumped scheme is to simply decrease pump absorption and lengthen the cavity. However, the laser suffers relatively low efficiency. As a better solution, the arrangement of uneven pump absorption coefficients along the cavity can improve the laser efficiency and reduce fiber temperature to some extent. The best solution is the distributed pump, with which the acceptable laser efficiency and temperature distribution can be achieved by optimizing the arrangement of pump powers, pump absorption coefficients, and fiber lengths. We also conclude that due to apparent temperature nonuniformity in the axial direction, the uniform heat dissipation assumed in most of the previous work [2], [7] cannot be applied to kilowatt fiber lasers. REFERENCES [1] H. Po, J. Cao, B. M. Laliberte, R. A. Minns, R. F. Robinson, B. H. Rockney, R. R. Tricca, and Y. H. Zhang, “High power neodymiumdoped single transverse mode fiber laser,” Electron. Lett., vol. 29, pp. 1500–1501, 1993. [2] L. Zenteno, “High-power double-clad fiber lasers,” J. Lightwave Technol., vol. 11, pp. 1435–1446, Sept. 1993. [3] H. M. Pask, R. J. Carman, D. C. Hanna, A. C. Tropper, C. J. Mackechnie, P. R. Barber, and J. M. Dawes, “Ytterbium-doped silica fiber lasers: versatile sources for the 1–1.2 m region,” IEEE J. Select. Topics Quantum Electron., vol. 1, pp. 2–13, Jan. 1995. [4] V. P. Gapontsev, N. S. Platonov, O. Shkurihin, and I. Zaitsev, “400 W low-noise single-mode CW ytterbium fiber laser with an integrate,” in Proc. Conf. Lasers and Electro-Optics (CLEO) 2003, Baltimore, MD, postdeadline paper CThPDB9. [5] J. Limpert, A. Liem, H. Zellmer, and A. Tunnermann, “500 W continuous-wave fiber laser with excellent beam quality,” Electron. Lett., vol. 39, pp. 645–647, 2003. [6] M. K. Davis, M. J. F. Digonnet, and R. H. Pantell, “Thermal effects in doped fibers,” J. Lightwave Technol., vol. 16, pp. 1013–1023, June 1998. [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. Quantum Electron, vol. 37, pp. 207–217, Feb. 2001. [8] J. P. Koplow, D. A. V. Kliner, and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier,” Opt. Lett., vol. 25, pp. 442–444, 2000. [9] I. Kelson and A. Hardy, “Optimization of strongly pumped fiber lasers,” J. Lightwave Technol., vol. 17, pp. 891–897, May 1999. [10] D. J. Ripin and L. Goldberg, “High efficiency side-coupling of light into optical fibers using imbedded v-grooves,” Electron. Lett., vol. 31, pp. 2204–2205, 1995.