To meet the escalating demands for transmission capacity, spectral efficiency, energy efficiency, and integration, WDM optical fiber communication systems necessitate advanced laser sources beyond traditional modules. Microresonator-based optical frequency combs (microcombs) serve as high-coherence, equally spaced multi-wavelength sources. Their frequency spacing naturally matches the WDM channel spacing, offering a highly promising solution for high-capacity coherent communication [[1], [2], [3], [4], [5], [6]]. This technology not only boasts advantages such as a high carrier count and low phase noise, but also significantly reduces system volume, having demonstrated data transmission rates of up to 50 Tbit/s [7].

Currently, substantial advancements have been achieved in microcombs, and soliton mode-locking has been demonstrated on various material platforms such as SiO2 [8,9], MgF2 [10], Si3N4 [11], Hydex [12], and AlGaAs [13]. Beyond the common single bright solitons, microcombs have also developed dark pulse states [14] and soliton crystal states [15]. For mode-locking, various robust techniques have been achieved, including fast frequency tuning [16], auxiliary optical thermal compensation [17,18], and self-injection locking [[19], [20], [21]]. Overall, microcomb performance still requires further enhancement. Regarding comb line characteristics, the power of only a small fraction of the comb lines reaches the detection threshold of commercial photodetectors. Meanwhile, the power distribution across the comb spectrum is often uneven. This leads to a waste of input energy, as the power is not effectively transferred to the side comb lines [22]. Although perfect soliton crystals offer improved comb line power, their control is complex and typically requires a dual-pump scheme or avoided-mode-crossing. Furthermore, traditional passive microresonators are constrained by high cavity loss and low material nonlinearity, with quality factors (Q) for most microresonators typically ranging from 105 to 106 [23]. These factors contribute to high thresholds and low conversion efficiency, which is commonly below 5% [24]. While dark pulse microcomb schemes can improve conversion efficiency, their spectral range is generally limited to within hundreds of nanometers [25]. The dual-cavity configuration enhances pump-cavity coupling by controllably shifting the pump resonance, thereby relaxing the efficiency-detuning trade-off and achieving conversion efficiencies exceeding 50%. In contrast, a single soliton in a standard single-cavity configuration with anomalous dispersion typically remains within a 4–8% efficiency range [26]. Therefore, a microcomb light source that delivers sufficient comb teeth power without requiring external amplification, has high intrinsic flatness, and achieves high conversion efficiency is essential to meet the requirements of WDM systems. In recent years, active gain microresonators based on rare-earth ion doping have seen significant development [[27], [28], [29]]. Meanwhile, the formation of solitons in coherently driven active cavities also garners increasing attention. N. Englebert et al. generate optical solitons with both high peak power and high coherence in a coherently driven active resonator, and establish the generalized Lugiato-Lefever equation (LLE) incorporating gain saturation effects [30]. Yao et al. demonstrate the realization of fully dissipative Kerr solitons in active microcavities by sintering Er3+ ions on the surface of the cavity [31]. Notably, erbium ion (Er3+) can provide laser gain in the telecommunication band, and its fabrication process is compatible with current silicon nitride microresonator fabrication techniques [32]. This offers new possibilities for achieving low-threshold, high-comb-line-power, and high-flatness microcomb systems.

This study presents an Er3+-gain-assisted microcomb system that operates at the ITU-T G.694.1 standard 100 GHz repetition rate and delivers high comb-line power, while it investigates the performance enhancements and novel physical properties under gain-assisted conditions. Simulations indicate that the optical parametric oscillation threshold is reduced by a factor of 8 (from 24 mW to 3 mW). Concurrently, the effective comb bandwidth, defined by the sensitivity limitation of commercial photodetectors (typical requirement for > -30 dBm) [33], expands from about 35 nm to 59 nm (1521–1580 nm) and supports a channel count increase to 73. Furthermore, the enhancing effect of different gain levels on microcomb generation is investigated. As the gain increases from 0 to 20 dB/m, both the equivalent Q and the pump-to-comb conversion efficiency exhibit an improvement of about threefold (reaching 3.7 million and 14%, respectively), and the single-soliton formation probability increases to 47%, a rise of more than tenfold. Our work not only highlights the potential of active microresonators for low power consumption and system compactness, but also provides a path to high-performance communication light sources.