- The paper demonstrates an innovative optical frequency comb technique that produces microwave signals with a fractional stability of 6.5E-16 and a timing noise floor below 41 zs/√Hz.
- It employs an ultra-stable CW laser reference and a high-linearity InGaAs/InP photodiode to mitigate noise and limit flicker phase noise to below -140 dBc/√Hz.
- This technique has significant implications for applications in advanced radar systems, atomic clocks, and terabit communications through its exceptional phase noise performance.
Analysis of Photonic Microwave Signals with Zeptosecond-Level Absolute Timing Noise
The paper presents a comprehensive paper on the synthesis of photonic microwave signals, boasting unprecedented levels of phase purity and fractional frequency stability. This work successfully demonstrates the translation of optical frequency reference stability to microwave signals through an innovative optical frequency comb and photodetection approach. The resulting microwave signals are distinguished by their fractional frequency stability of 6.5 x 10-16 at 1 second and phase noise performance with a timing noise floor below 41 zs/√Hz for a 12 GHz carrier. Such signals represent a significant advancement over previously existing microwave generation technologies.
The central innovation relies on the use of a low-noise, erbium-doped fiber-based optical frequency comb with a 250 MHz repetition rate. The optical frequency comb is phase-locked to an ultra-stable CW laser at 1542 nm, which exhibits fractional frequency stability as low as 5.5 x 10-16 at 1 second. The photonic microwave generation process benefits from various sophisticated methods to mitigate noise, such as pulse compression and modulation of laser working state to control amplitude-to-phase noise conversion in the photodiode.
A notable aspect of the microwave generation system is the implementation of a high-linearity, low-noise InGaAs/InP photodiode optimized for flicker phase noise below -140 dBc/√Hz. This photodiode efficiently translates the optical pulse train into a microwave signal, ensuring that the phase noise observed in the microwave domain accurately reflects the stability of the optical reference.
The characterization approach utilizes a heterodyne cross-correlation scheme employing three similar yet independent optoelectronic microwave generation systems. This method allows precise assessment of phase noise with a measurement noise floor below -180 dBc/Hz for Fourier frequencies beyond a 1 kHz offset.
The results are significant within the broader context of photonic microwave systems, as they show very low phase noise levels close and far from the carrier frequency. Particularly, at low offset Fourier frequencies, the phase noise is constrained primarily by the laser reference, confirming an effective transfer of frequency stability from the optical reference to the microwave signals.
The implications of these findings are broad and impactful. The demonstrated low phase noise microwave sources have potential applications in high-precession systems, such as advanced radar systems, high-stability fountain atomic clocks, and time-frequency metrology. The research also opens possibilities for mobile microwave sources that could provide unprecedented resolution for defense radar systems and enhance data transmission rates in terabit communication systems.
Regarding future developments, advancements in ultra-stable laser frequency stability—potentially through the use of longer or cryogenic reference cavities with improved coatings—could further reduce phase noise. Improvements could also stem from enhanced photodiode technology capable of reducing shot noise limits.
Overall, the paper presents an impressive synthesis of both conceptual advances and experimental rigor, marking a significant contribution to the field of microwave photonics and its future technological applications.