Low-phase-noise microwave generation with a free-running dual-pumped Si3N4 soliton microcomb.

Microwave signals can be generated by photodetecting the repetition frequencies of the soliton microcombs. In comparison to other methods, the dual-pumped method allows for the stable generation of the soliton microcombs even with resonators having lower Q-factors. However, introducing an additional pump laser may affect the phase noise of the generated microwave signals when using these dual-pumped soliton microcombs. Here, we investigate the factors that could influence the phase noise of microwave signals generated with dual-pumped soliton microcombs, including the polarization, amplitude noise, and phase noise of the two pumps. We demonstrate a 25.25 (12.63) GHz microwave with phase noise reaching -112(-118) dBc/Hz at a 10 kHz offset frequency, surpassing the performance of previous reports on microwave generation using free-running Si3N4 soliton microcombs, even those generated with higher Q microresonators. We analyze the noise floor of the generated microwave signals and establish a phase noise simulation model to study the limiting factors in our system. Our work highlights the potential of generating low-phase-noise microwave signals using free-running dual-pumped soliton microcombs.

Air-gap Fabry-Pérot cavity filtered 30 nm broadband electro-optic frequency combs for high-order coherent communications.

Broadband electro-optic (EO) frequency combs, which have flexible and high repetition frequencies, are prospective light sources for dense-wavelength-division-multiplexed coherent optical communications. In most cases, nonlinear spectral broadening and amplification procedures are needed to achieve broadband and high-power EO frequency combs. This leads to a low optical carrier-to-noise ratio (OCNR) for comb lines, limiting the transmission capacity. Here, we propose to use an air-gap Fabry-Pérot (FP) cavity to improve the OCNR for all the comb lines covering a 30 nm broadband spectrum. A 12 dB OCNR (0.1 nm bandwidth) improvement is obtained experimentally via using an FP cavity with ∼790 MHz bandwidth. We apply a 150-channel filtered EO comb with 25 GHz channel spacing and load 20 GBaud signals on each comb line to demonstrate the effect of OCNR improvement. The 137/150 channels have a bit error rate below the threshold of soft-decision forward error correction when using the 128 quadrature amplitude modulation (QAM) format. However, none of these channels can support this modulation format without cavity filtering. We also investigate dispersion tolerance and the long-term stability when using an air-gap FP cavity, highlighting its advantages. Our results show a practical solution to boost the transmission capacity when applying broadband EO combs in optical communications.

Large-scale true-time-delay remote beamforming with EO frequency combs and multicore fiber.

Optical true-time-delay (OTTD) beamforming is a promising solution to support the ultra-broadband radio access network. However, large-scale antenna arrays set at remote radio units require the OTTD counterpart to have corresponding larger-scale channel numbers. Here, we demonstrate an OTTD remote beamforming network with a record 287 channel number using electro-optic frequency combs and multicore fiber. Our proposed scheme can generate beams for both one-dimensional and two-dimensional antenna arrays. We highlight that using multicore fiber not only increases the channel numbers but also supports remote beamforming. We estimate the long-term stability of this remote beamforming network, and 1-ps-level relative time delay variation in 2 h is obtained when using multicore fiber. It is one order of magnitude better than using parallel single-mode fibers. Thus, highly stable beamforming is achieved. These results pave the way for the application of OTTD beamforming in 5G and beyond networks.