  Millimeter-wave (mmWave,>30 GHz) radars are the key enabler in the coming 6G
era for high-resolution sensing and detection of targets. Photonic radar
provides an effective approach to overcome the limitations of electronic radars
thanks to the high frequency, broad bandwidth, and excellent reconfigurability
of photonic systems. However, conventional photonic radars are mostly realized
in tabletop systems composed of bulky discrete components, whereas the more
compact integrated photonic radars are difficult to reach the mmWave bands due
to the unsatisfactory bandwidths and signal integrity of the underlining
electro-optic modulators. Here, we overcome these challenges and demonstrate a
centimeter-resolution integrated photonic radar operating in the mmWave V band
(40-50 GHz) based on a 4-inch wafer-scale thin-film lithium niobate (TFLN)
technology. The fabricated TFLN mmWave photonic integrated circuit consists of
a first electro-optic modulator capable of generating a broadband linear
frequency modulated mmWave radar waveform through optical frequency
multiplication of a low-frequency input signal, and a second electro-optic
modulator responsible for frequency de-chirp of the received reflected echo
wave, therefore greatly relieving the bandwidth requirements for the
analog-to-digital converter in the receiver. Thanks to the absence of optical
and electrical filters in the system, our integrated photonic mmWave radar
features continuous on-demand tunability of the center frequency and bandwidth,
currently only limited by the bandwidths of electrical amplifiers. We achieve
multi-target ranging with a resolution of 1.50 cm and velocity measurement with
a resolution of 0.067 m/s. Furthermore, we construct an inverse synthetic
aperture radar (ISAR) and successfully demonstrate the imaging of targets with
various shapes and postures with a two-dimensional resolution of 1.50 cm * 1.06
cm.
