Measuring sensitivity of optical frequency transfers to acoustic vibrations in photonic networks

The fiber infrastructure in buildings can pose a security risk. Using interferometric measurement techniques, a fibre installed in a room can be turned into a sensitive microphone and thus misused for eavesdropping. In our contribution we are investigating the transfer of mechanical vibrations to the fiber in the audible frequency band. An installation of optical fiber connecting two buildings of the institute was measured using a heterodyne interferometric system. The phase changes of an ultra-narrow C-band laser working at 1540.5nm wavelength transmitted through the fiber were detected and converted to an audio signal. The power spectral density of the interference phase for audible Fourier frequencies was analyzed over a period of several working and non-working days. A practical example of post-processing the measured data for speech eavesdropping is discussed.