A model based on propagation-rate equations with consideration of transverse gain distribution is built up to describe the transverse mode competition in strongly pumped multimode fiber lasers and amplifiers. An approximate practical numerical algorithm by multilayer method is presented. Based on the model and the numerical algorithm, the behaviors of multitransverse mode competition are demonstrated and individual transverse modes power distributions of output are simulated numerically for both fiber lasers and amplifiers under various conditions.

This paper presents a 3.89 kW 1123 nm Raman all-fiber laser with an overall optical-to-optical efficiency of 70.9%. The system consists of a single-wavelength (1070nm) seed and one-stage bidirectional 976 nm non-wavelength-stabilized laser diodes (LDs) pumped Yb-doped fiber amplifier. The unique part of this system is the application of non-wavelength-stabilized LDs in high power bidirectional pumping configuration fiber amplifier via refractive index valley fiber combiners. This approach not only increases the pump power, but also shortens the length of fiber by avoiding the usage of multi-stage amplifier. Through both theoretical research and experiment, the bidirectional pumping configuration presented in this paper proves to be able to convert 976 nm pump laser to 1070 nm laser via Yb³⁺ transfer, which is then converted into 1123 nm Raman laser via the first-order Raman effect without the appearance of any higher-order Raman laser.

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A comprehensive observation of the stimulated Brillouin scattering (SBS) phenomena in a high power nanosecond linearly polarized Yb-doped double-clad fiber amplifier is presented. According to the behavior of SBS, the amplification process is divided into four regions. All the temporal, spectral and power characteristics of SBS in both forward and backward directions in each region are described in detail. Cascaded SBS with up to 30 orders of Stokes waves and 10 orders of anti-Stokes waves is observed. The fiber damage caused by the giant SBS pulses is reported. And the influence of the linewidth, pulse duration, feedback and polarization on SBS is discussed.

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