Coexistence of multiple microcombs in monochromatically pumped Si₃N₄ microresonators

Area of Science:

• Photonics
• Nonlinear Optics
• Materials Science

Background:

• Microcombs are powerful tools for frequency comb generation.
• Simultaneous generation of multiple microcombs is challenging.
• Silicon nitride microresonators offer unique nonlinear properties.

Purpose of the Study:

• To experimentally demonstrate the coexistence of multiple microcombs in a single microresonator.
• To investigate the generation mechanisms of different microcomb types.
• To analyze the coherence properties of simultaneously generated microcombs.

Main Methods:

• Monochromatic pumping of silicon nitride microresonators.
• Utilizing a continuous-wave (CW) laser with mixed polarization.
• Operating near mode crossing conditions.
• Heterodyning of transverse electric (TE) and transverse magnetic (TM) comb lines.

Main Results:

• Simultaneous generation of three types of coherent microcombs: TE-polarized soliton, TM-polarized Turing rolls, and cross-phase-modulation-induced TM-polarized microcombs.
• Demonstration that the cross-phase-modulation-induced microcomb shares the same comb line spacing as the soliton microcomb, despite different free spectral ranges for TE and TM modes.
• Extraction of a 22.95-GHz signal from a ~100-GHz microresonator.
• Phase noise analysis confirming coherence characteristics.

Conclusions:

• Multiple microcombs can be simultaneously generated and coexist within a single silicon nitride microresonator.
• The polarization and pumping conditions play a crucial role in determining the type and characteristics of the generated microcombs.
• This work paves the way for novel applications in optical signal processing and metrology.