Applications of the optical fiber to the generation and to the measurement of low-phase-noise microwave signal
Résumé
The optical fiber used as a microwave delay line exhibits high stability and low noise and makes accessible a long delay (≳100 μs) in a wide bandwidth (≈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 10−12. Enhanced sensitivity is obtained with two independent delay lines, after correlating and averaging. Short-term stability of parts in 10−12 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. We add 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.