Individual Mode Characterization of 3D Solitons in a Spatiotemporal Mode-Locked Fiber Laser

3D solitons in spatiotemporal mode-locked (STML) fiber lasers have garnered significant attention due to their distinctive mode characteristics. However, the detailed characterization of individual modes remains challenging, limited by current measurement equipment and techniques. Here, a straightforward real-time method leveraging both modal dispersion and chromatic dispersion, called the spatiotemporal dispersive Fourier transform (ST-DFT) technique, is proposed for the individual mode characterization of 3D solitons within STML fiber lasers. By utilizing a segment of two-mode fiber, the ST-DFT technique is effectively employed for the individual mode characterization of 3D single, multiple, and harmonic solitons, with each mode being precisely calibrated. For single solitons, the solitons with identical or distinct peak wavelengths for different modes are observed. For multiple solitons and harmonic solitons, the peak wavelengths of different modes within the same soliton can be either identical or distinct as well, as observed in the single solitons. However, different solitons exhibit identical peak wavelengths for the same modes, indicating that the mode characteristics of different solitons within multiple solitons and harmonic solitons are identical. The findings reveal the individual mode characteristics of 3D solitons and deepen the understanding of their formation mechanisms, advancing the investigations and applications of STML fiber lasers.