  Millimeter-wave (mmWave) technology continues to draw large interest due to
its broad applications in wireless communications, radar, and spectroscopy.
Compared to pure electronic solutions, photonic-based mmWave generation
provides wide bandwidth, low power dissipation, and remoting through low-loss
fiber. However, at high frequencies, two major challenges exist for the
photonic system: the power roll-off of the photodiode, and the large signal
linewidth derived directly from the lasers. Here, we demonstrate a new photonic
mmWave platform by combining integrated microresonator solitons and high-speed
photodiodes to address the challenges in both power and coherence. The
solitons, being inherently mode-locked, are measured to provide 5.8 dB
additional gain through constructive interference among mmWave beatnotes, and
the absolute mmWave power approaches the theoretical limit of conventional
heterodyne detection at 100 GHz. In our free-running system, the soliton is
capable of reducing the mmWave linewidth by two orders of magnitude from that
of the pump laser. Our work leverages microresonator solitons and high-speed
modified uni-traveling carrier photodiodes to provide a viable path to
chip-scale high-power, low-noise, high-frequency sources for mmWave
applications.
