Direct fiber-coupled soliton microcomb system with enhanced stability and reproducibility via high numerical-aperture polarization-maintaining single-mode fibers and temperature control

We propose a relatively simple and robust system architecture for soliton microcomb generation, based on two key techniques: direct fiber coupling using high numerical-aperture polarization-maintaining single-mode fibers (high-NA PMFs) and active temperature control of the microresonator. These complementary strategies address two major challenges in microcomb implementation: environmental sensitivity and resonance instability. Building on prior work using single-mode fiber (SMF)-based direct coupling, which demonstrated device miniaturization and partial suppression of thermal drift in total fiber-to-fiber insertion loss, our PMF-based approach offers enhanced thermal stability and significantly greater robustness to environmental disturbances such as temperature fluctuations and vibration. In our system, the need for precision alignment using microscopes or multi-axis stages is alleviated, enabling a less complex optical setup and stable long-term operation. The direct coupling scheme achieved a total fiber-to-fiber insertion loss of 2.39 dB and maintained stable soliton operation for over 24 hours under external perturbations. In parallel, active temperature control of the microresonator was quantitatively evaluated, reducing the wavelength variation of the pump and auxiliary lasers by 79% and 97%, respectively. This stability enables reproducible soliton generation even in thermally dynamic environments. Comparative experiments with SMF-based direct coupling and lensed-SMF-based free-space coupling systems confirmed the superior performance of the PMF-based design in terms of coupling stability, soliton lifetime, and immunity to environmental noise. The architecture developed in this study lays a strong foundation for future integration into compact modules, with ongoing efforts aimed at reducing the reliance on laboratory-scale laser systems and optical components.