High-order mode fiber lasers are gathering the rising attentions from fundamental researches to practical applications due to their distinctive spatial and temporal properties. Here, we demonstrate a carbon nanotube mode-locked two-mode fiber laser and investigate the formation and evolution process of the soliton based on the time-stretch dispersive Fourier transform method. Stable single pulse and bound-state pulses are observed at pump powers of 32.0 and 41.3 mW, respectively. Attributing to the coexistence of fundamental mode and first group high-order modes, the optical spectrum exhibits slight intensity modulations and the autocorrelation trace shows additional small side lobes. Similar to that of single-mode fiber lasers, the buildup process of soliton includes the relaxation oscillation, beating behavior, and stable mode-locking state. However, owing to the interplay between two transverse modes in the two-mode fiber laser, the duration and fluctuation intensity of the relaxation oscillation are much larger than that of single-mode fiber lasers.

中文翻译：

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双模光纤激光器中孤子的形成与演化

高阶模式光纤激光器由于其独特的空间和时间特性，正引起从基础研究到实际应用的日益关注。在这里，我们展示了一种碳纳米管锁模双模光纤激光器，并基于时间拉伸色散傅立叶变换方法研究了孤子的形成和演化过程。分别在 32.0 和 41.3 mW 的泵浦功率下观察到稳定的单脉冲和束缚态脉冲。由于基模和第一组高阶模共存，光谱表现出轻微的强度调制，自相关轨迹显示出额外的小旁瓣。与单模光纤激光器类似，孤子的形成过程包括弛豫振荡、跳动行为和稳定的锁模状态。