  In this paper we propose a simple method to reject the high-frequency noise
in the evaluation of statistical uncertainty of coherent optical fiber links.
Specifically, we propose a preliminary data filtering, separated from the
frequency stability computation. In this way, it is possible to use the Allan
deviation as estimator of stability, to get unbiased data, which are
representative of the noise process affecting the delivered signal. Our
approach is alternative to the use of the modified Allan deviation, which is
largely adopted in this field. We apply this processing to the experimental
data we obtained on a 1284 km coherent optical link for frequency
dissemination, which we realized in Italy. We also show how the so-called
Lambda-type commercial phase/frequency counters can be used to this purpose.
