  Microwaves generated by optical techniques have demonstrated unprecedentedly
low noise and hold significance in various applications such as communication,
radar, instrumentation, and metrology. To date, the purest microwave signals
are generated using optical frequency division with femtosecond mode-locked
lasers. However, many femtosecond laser combs have a radio frequency (RF)
repetition rate in the hundreds of megahertz range, necessitating methods to
translate the generated low-noise RF signal to the microwave domain. Benchtop
pulse interleavers can multiply the pulse repetition rate, avoid saturation of
photodetectors, and facilitate the generation of high-power low-noise microwave
signals, which have to date only been demonstrated using optical fibers or free
space optics. Here, we introduce a large-scale photonic integrated
circuit-based interleaver, offering size reduction and enhanced stability. The
all-on-chip interleaver attains a 64-fold multiplication of the repetition
rate, directly translated from 216 MHz to 14 GHz in microwave Ku-Band. By
overcoming photodetector saturation, the generated microwave power was improved
by 36 dB, with a phase noise floor reduced by more than 10 folds to -160 dBc/Hz
on the 14 GHz carrier. The device is based on a low-loss and high-density
photonic integrated circuit fabricated by the photonic Damascene process. Six
cascaded stages of Mach-Zehnder interferometers with optical delay lines up to
33 centimeters long are fully integrated into a compact footprint of 8.5 mmx1.7
mm. The lithographically defined precision of the optical waveguide path length
enables the scaling up of the interleaved frequency to millimeter-wave bands,
which is challenging the fiber-based counterparts. This interleaver has the
potential to reduce the cost and footprint of mode-locked-laser-based microwave
generation, allowing for field deployment.
