Simultaneous OFDR spatial multiplexing by means of nanoparticle doped optical fibers, theory and sensing applications

Optical Backscatter Reflectometry (OBR) can transform a single mode fiber (SMF) in a distributed sensor. The operation principle is based on Rayleigh scattering occurring in optical fibers. Rayleigh scattering is a fiber deterministic property and its spectral signature is affected by temperature and strain changes in terms of wavelength shift. The OBR operation is limited to a single sensing fiber, while a parallel of multiple fibers results problematic for detection, since the parallel backscattered power cannot be discriminated. Critical applications, mainly in medical field, require both distributing sensing and high spatial sensor density. The current implemented solutions, exploiting optical switches, present a large temporal offset, not suitable for real-time operation. Here, an innovative paradigm of simultaneous spatial distributed multiplexing based on Scattering Level (SLMux) is presented. To achieve this operation, it is possible to exploit the properties of a high scattering MgO nanoparticles doped fiber (NPDF). This fiber is characterized by a random pattern of nanoparticles in the core. Its high backscattering, more than 40 dB larger than a SMF, can be used to discriminate the sensing point in the NPDF when inserted in a fiber parallel with other SMFs. This novel paradigm is suitable for critical applications of biomedical engineering, some of them successfully demonstrated in our laboratories. One of them consists in 2D/3D thermal mapping during hyperthermia tumor treatment, which is obtained by applying a radiofrequencies (RF) in the tumoral area to raise the temperature over 60 degrees C, thus inducing cellular mortality. The local temperature monitoring is fundamental to stop the RF dosing and avoid tissue carbonization. A second application consists of 3D shape sensing of medical tools, like needles. The shape reconstruction of needles during penetration in human body is fundamental in high precision applications. The shape of a needle equipped with optical fibers, can be reconstructed by sensing the strain along the fiber. A parallel of four fibers can reconstruct both 3D shape and compensate temperature.