Spatiotemporal mode-locked (STML) fiber lasers have become an excellent platform in nonlinear optics research due to the rich nonlinear evolution process. In order to overcome modal walk-off and realize phase locking of different transverse modes, it is usually crucial to reduce the modal group delay difference in the cavity. In this paper, we use long-period fiber grating (LPFG) to compensate the large modal dispersion and differential modal gain in the cavity, realizing the spatiotemporal mode-locking in step-index fibers cavity. The LPFG inscribed in few-mode fiber could induce strong mode coupling, which has wide operation bandwidth based on dual-resonance coupling mechanism. By using dispersive Fourier transform involved intermodal interference, we show that there is a stable phase difference between the transverse modes constituting the spatiotemporal soliton. These results would be beneficial for the study of spatiotemporal mode-locked fiber lasers.

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