在基于大模场面积的高功率光纤放大器中,随着放大激光功率的提升而出现的高阶模激发现象,将会导致放大激光的光束质量严重下降。以光纤模式的弯曲损耗特性为基础,研究了光纤弯曲半径对高功率全光纤放大器高阶模激发阈值特性的影响。搭建了掺镱全光纤窄线宽高功率主振荡放大系统,设计了不同弯曲半径的水冷光纤盘,进行了相应对比实验。通过优化光纤盘半径,成功抑制了高功率光纤放大器中的高阶模激发,得到了千瓦量级的近单模激光输出,在输出功率为1.31 k W时光束质量因子M2约为1.4。
We report a switchable dual-wavelength fiber grating laser in linear overlapping cavity. The laser features two overlapping cavities sharing a single Yb-doped gain medium fiber and two sets of fiber Bragg gratings. A coiling fiber setup is inserted into the 1035 nm laser cavity. Given that the bending loss is inversely related to the bending radius, the cavity loss of 1035-nm can be modulated. Modulating the bending loss facilitates the switching of the fiber grating laser to a single-or dual-wavelength output at 1030 or 1035 nm and convenient tuning of the power ratio of the two wavelengths. An approximately 152.6-mW output power and up to 38-dB polarization extinction ratio are observed. The simultaneous lasing at I030 and 1035 nm is a qualified seeder source for amplification to high power scale and can be applied to difference frequency generation of a terahertz signal. This dual-wavelength fiber grating laser is a potential pump source for generating terahertz radiation using a novel approach.
A low-threshold Raman effect in a kilowatt ytterbium-doped narrowband fiber amplifier system is reported. The Raman Stokes light at 1120 nm is achieved with the total output power of only N400 W, indicating that the Raman threshold of this kilowatt codireetional pumped continuous wave fiber amplifier is much lower than the predicted value estimated by the classic formula. To figure out the mechanism of this phenomenon, simulations based on the general stimulated Raman scattering （SRS） model are analyzed indicating that the key factor is the coupling between four-wave mixing （FWM） and SRS. The simulation results are in good agreement with our experiments.
