Transforming a Noisy RF Oscillator Into an Ultralow Phase Noise Signal Source With Photonic Delay-Lines

We introduce a novel scheme to transform a low-cost noisy voltage-controlled oscillator (VCO) into a high-performance radio frequency (RF) source by deploying photonic delay-line (PhDL) enabled carrier suppression interferometry (CSI), assisted by self-injection locking (SIL), to achieve an unprecedented noise reduction beyond the limit imposed by the noises in the PhDL. With 2.15 and 0.2 km optical fibers in the PhDL, we successfully reduced the phase noises of self-built low-cost X-band VCOs by up to 65 and 60 dB, achieving ultralow phase noises down to −123 and −144 dBc/Hz at 1 and 10 kHz, respectively, from the carrier, with a spur of only −110 dB. Further phase noise reduction using longer optical fibers without increasing side mode spurs is expected. We also developed and experimentally validated a Laplace domain phase noise analysis model for better understanding the phase noise reduction system and optimizing system parameters. Our work points to an attractive pathway for transforming low-cost noisy VCOs, including on-chip optoelectronic oscillators (OEOs), into high-performance RF sources, and hence revamps the design approaches of high-frequency oscillators up to 220 GHz and beyond.