Probing linewidth dynamics with short-time delayed linear interferometer in integrated frequency microcombs.

Optical frequency microcombs generate a series of equidistant, coherent frequency references over a broad spectrum, advancing spectroscopy, communications, metrology, and astronomy. Here, we design and fabricate a silicon nitride adiabatic ring microresonator that achieves low average dispersion with negligible avoided mode crossings. We explore the soliton microcomb bifurcation diagram for various mode-locking states in the microresonator, and investigate the stochastic linewidth through both numerical simulations and experimental measurements. The fabricated microresonator exhibits a loaded quality factor of 1.8 million, a measured group velocity dispersion of -3 ± 1.1 fs/mm, and a free spectral range of 88 GHz, characterized using swept-wavelength interferometry. We experimentally demonstrate thermally stabilized microcomb formation in the device using a dual-polarization-driven method, enabling access to single soliton, double soliton, and soliton crystal microcombs with forward and backward pump wavelength tuning. A short-time delayed linear interferometer is developed to examine the linewidths of individual comb teeth and the soliton microcomb linewidth distributions. The measured interferometric envelopes for the single-soliton, double-soliton, and soliton crystal microcombs show linewidths of 2.3 kHz, 3.0 kHz, and 2.4 kHz, respectively. The linewidth distribution broadens slightly away from the pump towards shorter wavelengths, exhibiting increased fluctuations due to the structured optical spectrum. Understanding the fundamental linewidths in high-clock-rate frequency microcombs is essential for unlocking new applications.