高功率光纤中传输光模式与其波长相关性研究

Analysis of wavelength dependence of mode in high power fiber laser

针对高功率光纤激光模式诊断和光谱诊断的需求,研究了光纤中传输模式的波长相关性,数值计算了光纤中各个模式的模场分布随波长的变化曲线及相应的光束质量,采用双傅里叶变换F^2法实际测量了光纤模式成分与波长的关系曲线.结果表明,光纤中各个模式的模场分布随波长变化,波长越长,模场面积越大;模式的光束质量随波长变化不大,但在截止频率附近明显变差;光纤中各个模式的功率占比与波长有关.

High power fiber lasers and amplifiers are widely used in the scientific and industrial field. In order to meet the requirements for high output powers the effective area of fibers becomes larger and larger to reduce optical nonlinearities.With the increase of effective area, the number of high-order modes will increase. In the case of high output power,the spectral shift and broadening of the optical fiber will also affect the modal number and content. The number and content of fiber modes affect the pointing stablity and quality of the laser beam. The M2-parameter is commonly used to define the quality of the laser beam, but a small M^2 number is not guaranteed for single mode operation. Therefore,the relationship between wavelength and transmission mode in fiber transmission is studied in this paper. We use the spatial and spectral Fourier transform（F^2） method to establish a theoretical-experimental method of describing the relationship between wavelength and mode. This method can directly give out the modal content of optical fibers without any priori parameter such as the properties of fiber and requirement for setup accuracy. On the one hand, the theoretical modeling of wavelength affects modal content. In the simulation, the sources with the same wavelength bandwidth and different central wavelengths are used to test the fiber. The results show that the modal content and number of the fiber change with the wavelength bandwidth and center wavelength. The mode components of the corresponding optical fiber will change after changing the central wavelength. As the spectral width of the light source increases, the number of high-order modes increases. On the other hand, in order to further verify the relationship between wavelength and mode of fiber, the F^2 method is used to measure the optical fiber modal content with different wavelengths. The final experimental results are in agreement with the theoretical results. The experimental and simulation results show that the mode field distribution of each mode varies with wavelength： the longer the wavelength, the larger the mode field is. The beam quality has little change with the wavelength except for those positions with frequency near the cutoff frequency, and the power ratio of each mode relates to the wavelength.