  We performed a two-way remote optical phase comparison on optical fiber. Two
optical frequency signals were launched in opposite directions in an optical
fiber and their phases were simultaneously measured at the other end. In this
technique, the fiber noise was passively cancelled, and we compared two optical
frequencies at the ultimate 1E-21 stability level. The experiment was performed
on a 47 km fiber that is part of the metropolitan network for Internet traffic.
The technique relies on the synchronous measurement of the optical phases at
the two ends of the link, that is made possible by the use of digital
electronics. This scheme offers several advantages with respect to active noise
cancellation, and can be upgraded to perform more complex tasks.
