Brillouin optical time-domain analysis over a 240 km-long fiber loop with no repeater

In this paper we combine the use of optical pulse coding and seeded second-order Raman amplification to extend the sensing distance of Brillouin optical time-domain analysis (BOTDA) sensors. Using 255-bit Simplex coding, the power levels of the Raman pumps and the Brillouin pump and probe signals were adjusted in order to extend the real physical sensing distance of a BOTDA sensor up to 120 km away from the sensor interrogation unit, employing a 240-km long loop of standard single-mode fiber (SSMF) with no repeater. To the best of our knowledge, this is the first time that distributed measurements are carried out over such a long distance with no active device inserted into the entire sensing loop, constituting a considerable breakthrough in the field.