300 km-ultralong Raman fiber lasers using a distributed mirror for sensing applications

Several configurations of ultralong Raman fiber lasers (URFL) based on a distributed mirror combined with Bragg gratings or fiber loop mirrors are studied. Two continuous-wave URFL configurations, with single and cascaded cavities using fiber Bragg gratings as mirrors are explored for a 300 km long fiber. For optical sensing, the cavity length was optimized for 250 km using one of the gratings an intensity sensor. Another URFL configuration based in a fiber loop mirror is also reported. For optical sensing using a 300 km long fiber it is shown that the best choice is a hybrid configuration. The sensitivity of the FBG laser sensor range was from (76 ± 2) × 10⁻⁶ με⁻¹ (for lower strain) to (9.0 ± 0.4) × 10⁻⁶ με⁻¹ (for higher strain).