Orbital angular momentum mode femtosecond fiber laser with over 100 MHz repetition rate

Orbital Angular Momentum (OAM) lasers have potential demand in many applications such as large capacity communication systems, laser processing, particle manipulation and quantum optics. OAM mode femtosecond fiber laser has become the research focus with the advantages of simple structure, low cost and high peak power. The current OAM mode femtosecond fiber lasers have made breakthroughs in the repetition frequency, pulse width, spectrum width and other key parameters, but it is difficult to achieve good overall performance. Besides, the repetition rate is currently in tens of MHz. In this paper, a large-bandwidth mode coupler is made based on the mode phase matching principle. Among them, the first order mode coupler with 3dB polarization dependent loss is made by the technology of strong fused biconical taper, and the second order mode coupler with 0.3dB polarization dependent loss is made by the technology of weak fused biconical taper. Combined with the nonlinear polarization rotation mode-locking mechanism, OAM mode femtosecond fiber lasers with over 100 MHZ repetition rate are built. The achievement of the key parameters is attributed to the selection of dispersion shifted fibers that can accurately adjust intracavity dispersion. Compared to traditional dispersion compensation fibers (DCF), the group velocity dispersion is reduced by an order of magnitude, so it can better adjust intracavity dispersion to achieve the indicators of large spectral bandwidth and narrow pulse width. In addition, the diameter of the fiber is 8μm, which is the same as that of a single mode fiber. Compared to DCF, the fusion loss can be ignored, so only a shorter gain Erbium-doped fiber is required that ensure a shorter overall cavity length and achieve high repetition frequency. The experimental results show that the first order OAM mode fiber laser has 113.6 MHz repetition rate, 98 fs half-height full pulse width, and 101nm 10-dB bandwidth. Second-order OAM mode fiber laser has 114.9 MHz repetition rate, 60 fs half-height full pulse width, and 100nm 10-dB bandwidth. Compared with the reported schemes, our scheme has better performance in key parameters such as repetition rate, pulse width and spectral width. We believe that the OAM mode fiber laser with good over performance is expected to be more widely used in OAM communication, particle manipulation and other research fields.