In the field of rare-earth doped fiber applications there has been a growing interest in high-power CW and pulsed fiber lasers and amplifiers using Er3+, Yb3+, Nd3+ and Tm3+ ions as active dopants [1-5]. A specialist doped-fiber model has been developed to support the accurate simulation and optimization of doped fiber devices. It allows designing arbitrary architectures utilizing various amplification stages, pumping schemes, and supplementary components for coupling and filtering. To simulate the doped-fiber sections, a bidirectional model is applied solving the propagation equations of equivalent multi-level laser media. The spectral characterization is performed via cross-sections of each dopant, or via Giles parameters (for single doped fibers). For accurate modeling under realistic conditions, the model allows to consider cross-relaxation between dopants, Rayleigh backscattering, background loss, concentration quenching, excited-state absorption, spectral-hole burning, and others. Fig. 1 shows design results for a high-power (1.36kW) cladding-pumped large-core Yb fiber laser . The 12m fiber with Yb3+-concentration of ~6000ppm (wt) is simulated using realistic data for cross-sections and lifetime . The double cladding is modeled by separate overlap factors for signals and pumps considering a fiber structure with D-shaped inner cladding. Multiple iterations (~30) are required to model the feedback in the laser cavity and to achieve a steady-state lasing condition. The laser radiation builds up from ASE noise and its spectrum is formed solely by the gain profile of the YDF (Fig. 1a).The power characteristic of the YDF laser is shown in Fig. 1b. Simulation results show excellent agreement with experimental data without performing any data fitting. Parameters relevant for the power conversion efficiency have been set in accordance with : the pump coupling factor to 90%, the fiber end reflections to 4%, the bend-induced loss to 0.04dB/m. The longitudinal distributions of the excited Yb ions and the signal power are shown in Fig. 2c,d. Due to complete signal reflection at the mirror on the left side of the laser cavity, the forward- and backward-propagating signals have the same intensity. On the right side of the laser the power ratio is ~14dB, which corresponds to 96% output coupling. The level of the excited ions in YDF lasers is low compared to the level in YDF amplifiers, because of the low gain value (14dB) required to compensate the signal outcoupling.In the full paper we discuss efficient design methods for high-power CW and pulsed fiber lasers and amplifiers using single or co-doped fibers that are core or cladding pumped. Simulation results will be compared with published experimental data.References Y. Jeong et al., Optics Express, vol. 12, no. 25, p. 6088 (2004). R. Paschotta et al., IEEE J. Quantum Electr., vol. 33, p. 1050 (1997). J. Nilsson, et al., in Advances in Fiber Lasers, Proc. SPIE vol. 4974, p. 50 (2003). J. Nilsson, et al., Journal of Quantum Electron., vol. 39, p. 987 (2003). P. Dupriez, et al., OFC 2005, Postdeadline paper PD3 (2005). A. Richter et al., NFOEC 2002, paper p503 (2002).
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