Fiber optic refractive index sensing using an inline dual semi-distributed interferometer

In this work, we present a fiber optic refractive index (RI) sensor based on a dual inline semi-distributed interferometer (SDI) device. The SDI is formed within a telecom-grade optical fiber by simply splicing a short length of a high-scattering fiber; the presence of a high density of scattering centers induces a distributed cavity effect, forming a low-finesse interferometer. Here, we propose an approach based on two serial SDIs, with few millimeters spacing between the two cavities; the outer SDI encodes the changes of surrounding RI, while the inner probe is RI-insensitive and can be used as a reference. Experimental analysis shows that the dual-SDI device has RI sensitivity of 111.2-466.2dB/RIU with RI-sensitive mode, and 0.4-3.7dB/RIU with RI-insensitive mode, with a ratio up to 1165.5 between sensitive and insensitive modes. In addition, the Vernier effect can be interrogated by detecting the wavelength shift of the narrow spectral lines, with sensitivity of 0.303nm/RIU.The dual-SDI structure maintains the same fabrication ease of a single SDI, since all process is made through splicing and cleaving fibers with a telecom-grade fiber. The key advantage is the availability of modes having both high and near-zero RI sensitivity, which therefore can be used for a referenced RI detection as an in-situ probe.