Figure 1
From: Long-range fiber-optic earthquake sensing by active phase noise cancellation
Measurement principle and experimental setup. (a) Schematics of the interferometric phase measurement for PNC. The signal source is an ultrastable laser. A beam splitter (BS) and two mirrors (M1 and M2) form a Michelson-type interferometer that measures the optical phase noise \(\varphi \) accumulated on the 123 km long interferometer arm connecting the laboratories in Bern and Basel. The optical phase is detected on a photodiode (PD) and processed to generate a correction frequency \(\Delta \nu (t)\) via a phase noise cancellation setup (PNC). This frequency correction is imposed on the optical frequency using an acousto-optic modulator (AOM), thereby compensating the phase noise. We record \(\Delta \nu \) with a sampling rate of 500 Hz. In Basel, part of the optical frequency is coupled out for local use in metrology applications. (b) Geometry of the fiber-optic cable connecting METAS in Bern to the University of Basel. The close-up shows the complex cable geometry within the city of Basel. The epicenter and source mechanism of the Mulhouse earthquake are marked by the beach ball. The event occurred at a depth of \(\sim \)13 km32. Black triangles indicate seismic stations that provided recordings for the validation of the seismic velocity model (see Fig. 3).
