利用半导体激光器（LD）抽运大模场增益光纤实现了输出功率大于4 kW的主振荡功率放大结构全光纤激光器。实验研究了增益光纤纤芯直径和抽运波长不同情况下激光器的受激拉曼散射（SRS）和横向模式不稳定（TMI）特性。为了抑制SRS，选择纤芯为30 μm的大模场掺镱光纤作为增益介质；为了抑制光纤放大器中的TMI，利用增益光纤吸收系数较低波段对应的915 nm LD作为抽运源，将增益光纤弯曲半径降低到10 cm以提高高阶模的损耗。在种子功率为100 W、最高注入抽运功率为5.3 kW时获得了4.1 kW的功率输出，光束质量M2为2.2，输出激光中无SRS和TMI现象。

A laser diode (LD) pumped all-fiber laser with master oscillator power amplification configuration is demonstrated, of which output power is more than 4 kW. The stimulated Raman scattering (SRS) and transverse mode instability (TMI) charateristics of lasers are experimentally studied under different core-diameter gain fibers and different pump wavelengths. In order to mitigate SRS, a large mode area Yb3+ doped fiber with core diameter of 30 μm is selected as gain media. In order to suppress TMI, LD with center wavelength of 915 nm is used as pump source, so the absorption coefficient of the gain fiber is lower at 915 nm, and the bending radius of the gain fiber is reduced to 10 cm to increase the loss of the higher modes. When the seed power is 100 W, the highest pump power is 5.3 kW, the laser power of 4.1 kW is achieved, and the beam quality M2 is 2.2. There are no SRS and TMI effects observed in the output laser.