Soliton microwave oscillators using oversized billion Q optical microresonators

Microresonator soliton frequency combs offer unique flexibility in synthesizing microwaves over a wide range of frequencies, while their phase noise is currently limited by thermal noise. Enlarging the mode volume would mitigate thermal noise but also raise power consumption. Here, we fabricate optical microresonators with large mode volumes by lathe machining high-purity fiber preforms. Quality factors greater than 4 billion result in a record-low threshold power of 110 µW to initiate comb operation in millimeter-sized devices. The synthesized X-band microwaves with an absolute phase noise level of − 107 ( − 133 ) d B c / H z at 1(10) kHz offset frequency feature considerable noise reduction compared to other silica-microresonator-based oscillators, which are further used to cope with high-speed data links as carrier waves. Our work illuminates a pathway toward low-noise photonic microwave generation as well as the quantum regime of soliton microcombs.