  We theoretically and experimentally investigate relevant noise processes
arising in optical fiber links, which fundamentally limit their relative
stability. We derive the unsuppressed delay noise for three configurations of
optical links: two-way method, Sagnac interferometry, and actively compensated
link, respectively designed for frequency comparison, rotation sensing, and
frequency transfer. We also consider an alternative two-way setup allowing
real-time frequency comparison and demonstrate its effectiveness on a
proof-of-principle experiment with a 25-km fiber spool. For these three
configurations, we analyze the noise arising from uncommon fiber paths in the
interferometric ensemble and design optimized interferometers. We demonstrate
interferometers with very low temperature sensitivity of respectively -2.2,
-0.03 and 1 fs/K. We use one of these optimized interferometers on a long haul
compensated fiber link of 540km. We obtain a relative frequency stability of
3E-20 after 10,000 s of integration time.
