The photodetection of ultrashort optical pulse trains for low noise microwave signal generation

TL;DR

This paper reviews how direct photodetection of ultrashort optical pulses can generate low noise microwave signals, emphasizing detector types, noise sources, and optimization strategies for high-fidelity optical-to-electrical conversion.

Contribution

It provides a comprehensive analysis of photodiode characteristics, noise impacts, and guidelines for achieving ultrastable low noise microwave signals from optical pulses.

Findings

Analysis of photodiode nonlinearities and their effects on noise

Comparison of photodiode types for microwave generation

Guidelines for optimizing photodetection for low phase noise

Abstract

Electrical signals derived from optical sources have achieved record-low levels of phase noise, and have demonstrated the highest frequency stability yet achieved in the microwave domain. Attaining such ultrastable phase and frequency performance requires high-fidelity optical-to-electrical conversion, typically performed via a high-speed photodiode. This paper reviews characteristics of the direct photodetection of optical pulses for the intent of generating high power, low phase noise microwave signals from optical sources. The two most popular types of photodiode detectors used for low noise microwave generation are discussed in terms of electrical pulse characteristics, achievable microwave power, and photodetector nonlinearities. Noise sources inherent to photodetection, such as shot noise, flicker noise, and photocarrier scattering are reviewed, and their impact on microwave phase fidelity is discussed. General guidelines for attaining the lowest noise possible from photodetection that balances power saturation, optical amplification, and amplitude-to-phase conversion, are also presented.