Dual-Wavelength Brillouin Lasers as compact Opto-Terahertz References for Low-Noise Microwave Synthesis

Abstract: Compact, ultra-low phase noise 10 GHz signals are essential for modern radar, coherent communications, and time-frequency metrology, especially with rising demands for additional spectral purity and portability. Optical frequency division (OFD) of ultra-stable optical references produce the lowest noise microwaves, but typically rely on ultra-low-expansion cavities, self-referenced frequency combs, pulse interleaving, and high-end photodetectors. In contrast, electro-optic frequency division (eOFD) offers a streamlined alternative in which an electro-optic (EO) comb is generated from a microwave source and stabilized to an optically carried terahertz (opto-terahertz) reference. eOFD has already demonstrated comparable phase noise to OFD at 10, 20, and 40 GHz when using division ratios from references spanning over 1 THz, requiring broad EO comb spectra to bridge between optical signals. Generating such broad spectra often demands complex techniques such as cascaded modulators, pulse compression, and nonlinear fiber. We demonstrate a compact, low-noise 300 GHz opto-terahertz reference utilizing a dual-wavelength Brillouin laser within a total system volume of 20 liters. The 300 GHz phase noise of this system is transferred to a 10 GHz dielectric resonant oscillator via eOFD using a simple architecture that could be miniaturized. The resulting microwave signal achieves phase noise levels of -130 dBc/Hz at 1 kHz, -150 dBc/Hz at 10 kHz, and -170 dBc/Hz at 10 MHz. This architecture drastically simplifies eOFD while maintaining state-of-the-art phase noise.