Progress on mode field distribution and characterization technology of the optical fiber laser

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In fields of practical applications such as the optical fiber communications, optical fiber lasers, and optical fiber sensinIn fields of practical applications such as the optical fiber communication, the optical fiber laser, and the optical fiber sensing, it is necessary to focus on the mode problems in optical fibers. Mode division multiplexing is an effective method to improve the information capacity of optical communication. Interference between modes is the basic method for the most of optical fiber sensing. Mode controlling technique is one of the key technologies for beam quality control of high-power fiber lasers. Therefore, research on the theory of optical fiber modes, modes’ generation, modes’ conversion, and mode characterization technology are of great significance and practical application value. In this paper, we discuss the mode and beam quality of the optical fiber, analyze the methods of multiple modes’ generation and conversion, and summarize the mode characterization methods by means of incoherent, coherent and low-coherence measurement. Currently, the fiber mode characterization is a hot research topic. Among a variety of characterization methods, the spatial and spectral imaging method (S2) and the spatial and spectral double Fourier transform method (F2) have significant advantages. Without knowing the geometric parameters of the fiber in advance, we can obtain characteristics such as the mode field distribution, the mode power ratio, and the group time delay. Results indicate that the F2 method is better for characterizing mode field distributions of high-power fiber lasers.