IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 1, JANUARY 2004 63 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, yt- terbium-doped. I. INTRODUCTION H IGH-POWER rare-earth-doped double-clad fiber lasers have attracted considerable attention recently in commer- cial and military applications due to their high efficiency, com- pactness, 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 power of 400 W and [4], and a Yb-neodymium (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 dis- sipation is an important feature and affects laser performance in kilowatt power domain [2], [6], [7]. In this letter, we pro- pose 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. II. LASER CONFIGURATION AND MODELING The configuration of a typical Yb-doped double-clad (YDDC) fiber laser under CW end pump ( and ) is schematically shown in Fig. 1(a). The YDDC fiber has a length of and a uniform pump absorption coefficient of . The 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 cladding diameter of 400 m . Based on a mode filtering 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, Mc- Master 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 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 equa- tions describing these lasers are given by [9] (1) (2) (3) (4) where is the Yb ion concentration, and are ground and upper-level populations respectively. is the pump 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 ve- locity 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 life- time. is the overlapping factor between the pump (signal) and the fiber doped area. is the fiber attenuation coef- ficient. In the modeling, we take , , , . and are from [3]. Furthermore, a signal bandwidth of 2 nm and pump 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 1041-1135/04$20.00 © 2004 IEEE