Advanced Coding Techniques for Long-Range Raman/BOTDA Distributed Strain and Temperature Measurements

We provide an overview of the use of optical pulse coding to enhance the performances of long-range distributed optical fiber sensors for strain and temperature measurements. First, pulse coding techniques are introduced for distributed sensing, emphasizing in particular the advantages of advanced cyclic coding for fast distributed strain and simultaneous temperature measurements. Pulse coding techniques in Raman-based distributed temperature sensors employing multimode graded index fibers are introduced, also outlining their potential to achieve meter-scale long-distance sensing in standard single-mode fibers (SMF). The use of Simplex coding in Brillouin optical time-domain analysis (BOTDA) sensors is then described, showing accurate ultralong-distance sensing with meter-scale spatial resolution. Finally, the implementation of advanced cyclic coding in a hybrid Raman/BOTDA distributed sensor is presented, allowing fast long-distance strain and simultaneous temperature measurements over the same SMF, using a single narrowband laser with meter-scale spatial resolution. We show that advanced cyclic Simplex coding enables long-distance strain sensing with subsecond measurement times, also overcoming the typical temperature and strain cross-sensitivity issue affecting standard BOTDA sensors.