10−20-level long-term relative frequency instability in a multi-branch optical frequency comb enabled by fiber noise cancellation

We demonstrated an all-polarization-maintaining (PM), multi-branch optical frequency comb based on an all-fiber configuration.To achieve fiber noise cancellation, we aligned the beat note of the out-of-loop branch with that of the in-loop branch by phase-locking both branches to a common 1550 nm ultra-stable laser.To further assess the out-of-loop frequency instability of the constructed fiber comb, we heterodyned it with another quasi-identical fiber comb.Thanks to effective fiber noise cancellation and a small difference in out-of-loop fiber lengths, the frequency instability of a single comb reached an exceptional level of 2.26 10 -17 at 1 s and 8.4 10 -20 at 4000 s.