In recent years, ultra-fast fiber lasers have attracted extensive attention in the applications such as ultra-precision manufacturing [1], optical communication [2], and optical frequency combs [3], owing to the advantages of excellent beam quality and high stability. Typically, pulsed fiber lasers can be realized via techniques including Q-switching, active mode-locking, and passive mode-locking. Among these, passive mode-locked fiber lasers have become a research hotspot due to their simple structure and flexible operation. Commonly, the effective methods of passive mode-locking include the nonlinear amplifying loop mirror (NALM) [4], nonlinear polarization rotation (NPR) [5], nonlinear multi-mode interference [6], semiconductor saturable absorber mirrors (SESAM) [7], and various saturable absorber (SA) materials [8]. However, the schemes based on real SAs are usually limited by the operating bandwidth and low damage threshold. Although the schemes relying on nonlinear effects offer greater flexibility, still result in high mode-locking thresholds and complex structures due to the weak nonlinearity and long interaction lengths of standard fibers.

Benefiting from the extremely high quality (Q) factors and small mode volumes of the microsphere resonators (MRs), which enable strong optical field confinement and enhanced light-matter interaction, substantially strengthen nonlinear effects and facilitate mode-locking. Consequently, MRs have been widely applied in low-threshold lasers [[9], [10], [11]], sensor detection [12,13], microwave photonics [14,15], nonlinear optics [[16], [17], [18]]. To date, several studies have integrated high Q microcavity into fiber laser systems to enhance intracavity nonlinear and achieve pulse lasers output. In 2017, Kues et al. integrated an on-chip microring resonator into the NALM structure, achieving stable nanosecond pulse output with a pulse duration of 4.3 ns and a repeat frequency of 104.9 MHz [19]. In 2024, A. Aadhi reported a fiber ring laser incorporating an integrated on-chip microring resonator. Based on an active frequency modulation mode-locking mechanism, the system generated pulse trains with durations of 5.1 ns and 3.1 ps, the repetition rates of 4.4 MHz and 48.7 GHz, respectively [20]. These studies collectively verify the critical role of high Q microcavities in pulsed laser generation and modulation, yet most schemes still rely on additional nonlinear elements or frequency selector components, which not only increase system complexity but also elevate the mode-locked threshold. Due to the inherent narrowband frequency selectivity and nonlinear modulation capabilities, MRs serve as an ideal medium for realizing mode-locking lasers (MLLs). The compact structure and simple manufacturing process of the MR facilitate the development of low-cost, integrated pulse sources, providing an effective solution to address the complexity and high threshold issues of existing MLL systems.

In this work, we demonstrate a compact dual-cavity composite system consisting of a high Q MR embedded in an erbium-doped fiber ring cavity. Specifically, the MR functions as both a nonlinear modulation element and a narrowband frequency selector, enabling simultaneous mode selection and intensity modulation. Furthermore, by adjusting the coupling between the MR and a fiber taper (FT), we identify Kerr-induced detuning compensation that is essential for stable mode-locking operation. Our approach provides an effective route to realizing high signal-to-noise ratio, low-threshold MLL output by utilizing the dual functionality of the MR, opening a novel pathway for developing miniaturized and highly stable passive mode-locked fiber lasers.