Subharmonic injection‐locked photonic integrated thin‐film lithium niobate optoelectronic oscillator

Integrated optoelectronic oscillators (OEOs) have emerged as pivotal enablers of compact, energy‐efficient solutions for generating high‐frequency radio‐frequency (RF) signals with exceptional spectral purity – an essential demand in the advancement of radar and communication technologies. Yet, the quest for ultra‐low phase noise near the carrier remains hampered by laser frequency instability and environmental fluctuations. In this work, we unveil the first photonic‐integrated, high‐order subharmonic injection‐locked OEO realized on a thin‐film lithium niobate (TFLN) platform, seamlessly uniting a Mach–Zehnder modulator (MZM) and an add‐drop microring resonator (MRR) in a monolithic architecture. By harnessing an external RF source operating at a fractional subharmonic (1/2 N , with N = 1, 3, 5…) of the OEO’s free‐running frequency, our system achieves robust locking to the 2 N ‐th harmonic of the injected signal, made possible through the beating of ± N ‐th order modulation sidebands – precisely selected by the dual resonances of the MRR – at the photodetector. We experimentally demonstrate the generation of 28.7 GHz signals via second‐ and sixth‐order subharmonic injection locking, employing external RF injections at 14.35 GHz and 4.78 GHz, respectively. This yields an outstanding side‐mode suppression ratio (SMSR) exceeding 78 dB and remarkably low spurious emissions. Furthermore, the measured phase noise achieves values below −80 dBc/Hz at 100 Hz and below −115 dBc/Hz at 10 kHz offsets from the 28.7 GHz carrier, delineating a new standard for integrated OEO performance.