Reversible Switching of Microcomb Soliton Crystal States

Excitation of microcomb solitons requires optimization of a number of pump laser parameters such as wavelength scan speed, pump detuning from the cavity resonance, and optical power, making their efficient excitation, and reversible switching between a N-soliton crystal and the single-soliton comb highly challenging. Furthermore, limited wavelength tunability and low-power of on-chip lasers make controlled excitation of different soliton comb states in an integrated platform, where both the pump laser and microresonator are on the same chip, difficult. Here we address all these issues and demonstrate efficient excitation of soliton combs, and reversible switching between a two-soliton crystal and single-soliton state using polarization control of a fixed wavelength pump. We directly pump a silica microsphere with ~35 mW power from a continuous-wave laser at a fixed wavelength and tune its polarization to generate Brillouin laser-pumped soliton combs and achieve deterministic switching from a two-soliton crystal to a single-soliton comb and back to two-soliton crystal. While switching from a higher-order soliton crystal to single-soliton state has been demonstrated earlier, to the best of our knowledge, deterministic switching from a single-soliton to a higher-order soliton crystal has not been observed. The observed comb lines for the three- and two-soliton crystals are only 4 and 6 dB lower, respectively, than the Brillouin Stokes signal, resulting in an enhancement factor of up to ~2000 compared to similar works, where the comb lines are 20-40 dB lower than the center comb line. The obtained comb was immune to variations in polarization and could be re-excited and reproduced at the same wavelength just by controlling the polarization. Demonstration of a Brillouin laser-pumped microcomb soliton through the polarization control of a low-power laser at a fixed frequency paves the way for an efficient fully integrated microcomb soliton using on-chip lasers, microresonators and electro-optic polarization controllers for applications ranging from on-chip optical atomic clocks to quantum computing.