https://123dok.net/document/1y92novz-frequency-discriminators-for-the-characterization-of-narrow-spectrum-heterodyne-beat-signals-application-to-the-measurement-of-a-sub-hertz-carrier-envelope-offset-beat-in-an-optical-frequency-comb-stephane-schilt-nikola-bucalovic-lionel-tombez-vladimir-do.html