Applications of the optical fiber to the generation and to the measurement of low-phase-noise microwave signals

TL;DR

This paper explores the use of optical fiber as a low-noise, high-stability microwave delay line for generating and measuring low-phase-noise microwave signals, with applications in frequency referencing and oscillator stability measurement.

Contribution

It provides practical engineering insights, noise analysis, calibration methods, and experimental validation for optical fiber-based microwave signal applications.

Findings

Achieved parts-in-1E-12 short-term stability

Demonstrated measurement sensitivity using dual delay lines

Validated oscillator phase-noise models experimentally

Abstract

The optical fiber used as a microwave delay line exhibits high stability and low noise and makes accessible a long delay (>100 microseconds) in a wide bandwidth (about 40 GHz, limited by the optronic components). Hence, it finds applications as the frequency reference in microwave oscillators and as the reference discriminator for the measurement of phase noise. The fiber is suitable to measure the oscillator stability with a sensitivity of parts in 1E-12. Enhanced sensitivity is obtained with two independent delay lines, after correlating and averaging. Short-term stability of parts in 1E-12 is achieved inserting the delay line in an oscillator. The frequency can be set in steps multiple of the inverse delay, which is in the 10-100 kHz region. This article adds to the available references a considerable amount of engineering and practical knowledge, the understanding of 1/f noise, calibration, the analysis of the cross-spectrum technique to reduce the instrument background, the phase-noise model of the oscillator, and the experimental test of the oscillator model.