Real-time transition dynamics and stability of chip-scale dispersion-managed frequency microcombs

摘要

(1) SEM image of the dispersion-managed microcavity. (2) Conceptual schematic of the tapered Si N microring and the breathing pulse evolution along the cavity length. The varying widths of the cavity waveguide provide an oscillating group velocity dispersion (GVD) and varying nonlinear coefficient. (3) The waveguide width changes at different locations of the microcavity. (4) The GVD (blue curve) and nonlinear coefficient (red curve) at the pump wavelength (1598.5 nm) change at different locations of the microcavity. Cold cavity transmission of the tapered Si N microring, measured with a high-resolution coherent swept wavelength interferometer (SWI) (see Supplementary Section ). The existence of higher-order transverse modes is not observed across the wavelength region of interest. The factors and wavelength-dependent free spectral range (FSR) are determined from the transmission measurement. COMSOL-modeled GVD and third-order dispersion (TOD) of the Si N waveguide with respect to the waveguide width, taking into consideration both the waveguide dimensions and the material dispersion. At the pump wavelength of 1598.5 nm, the path-averaged GVD and TOD are −2.6 fs  mm and −397 fs mm , respectively. The red dots are the measured GVD for waveguides with widths of 1.2 μm, 1.5 μm, and 1.6 μm, showing good agreement with the simulation results (see Supplementary Section ). Wavelength dependence of the FSR, determining the residual non-equidistance of the modes, , of 54 ± 3 kHz. The extracted GVD is anomalous at −6.4 ± 0.4 fs  mm , in good agreement with the simulation result. RF amplitude noise of the Kerr frequency microcomb in different states, showing the transition into the low-phase noise state with amplitude noise reaching the detector background. The 5 GHz scan range is more than 50 times the cavity linewidth. The measured optical spectrum of the dispersion-managed dissipative soliton, which fits better to a Gaussian profile (red curve) than a sech profile (blue curve). The 3 dB bandwidth of the measured spectrum is 4.78 THz, and the corresponding FWHM of the transformed-limited pulse is 92 fs. Inset: simulated comb spectrum, also showing a better match with a Gaussian profile than a sech profile. Pulse shape (black line) and temporal phase (blue line) retrieved from the FROG measurement. The FWHM pulse duration is measured to be 167 fs