  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.
