摘要： Micro-resonator-based frequency combs, or micro-combs, have gained considerable interest in recent years due to their many potential applications such as high-speed communication systems, spectroscopy and ultrafast optical clocks. Most micro-combs systems are based on laser pumped optical parametric oscillation and are typically non-self-starting, requiring a well-defined warm-up strategy involving smart control. An alternative approach to micro-combs is represented by the Filter-Driven Four-Wave Mixing (FD-FWM) laser, based on a nonlinear micro-resonator nested in a main amplifying fibre cavity. Although this system has demonstrated self-starting regimes, stable operation typically imposes a strict relation between the minimum free-spectral range (FSR) of the main-cavity and the Q-factor of the micro-resonator. The use of longer main-cavity fibre lengths (highly desirable for several positive features, such as a larger gain) results in unrecoverable unstable regimes, i.e. in super-mode instability, which arises from the existence of many oscillating main-cavity modes within each micro-resonator resonance. Here, we report a novel micro-comb scheme based on a three-cavitiy design shown in Fig. 1 (a). Our scheme exhibits the ability to spontaneously recover a set of unstable regimes in the FD-FWM laser. The basic concept is to introduce an intracavity periodic phase change via an additional short-loop fibre cavity, which effectively works as a low-Q, all-pass linear filter. Such a filter creates an irregular frequency-spacing among adjacent modes of the main-cavity loop, weakening the typical four-wave-mixing coupling between them. In sharp contrast to standard FD-FWM scheme, the system is tolerant to a significantly narrower spacing of the main-cavity modes. In the example, the total fibre length of the main-cavity is fixed to 20 metres, corresponding to a main-cavity FSR of 7.5 MHz - more than an order of magnitude lower than the micro-resonator bandwidth (~120MHz). This main-cavity FSR is generally very unstable at any pumping regime in the absence of the additional short-loop fibre cavity. An example of a stable mode-locking state with the properly chosen parameters is shown in Fig. 1(b)-(j). The experimental optical spectrum along with the corresponding intensity autocorrelation traces and RF spectrum are shown in Fig. 1 (b)-(d). A low background in the autocorrelation (AC) trace and a flat RF spectrum indicate the high stability of the generated micro-combs. A frequency comb-assisted spectroscopy is performed to accurately characterize resonances in the micro-cavity and the additional all-pass resonator filter, as well as the position of the oscillating comb lines (Fig. 1 (e)-(g)). The intra-cavity spectrum reveals that there is only a single longitudinal mode lasing within each micro-cavity resonance, regardless of the high modal density of the main cavity. Our analysis reveals that the introduced periodical spectral-phase modulation dominates the emergence of the oscillating modes.