Single-Multi-Single Mode Fiber Optic Structure-Based Water Depth Sensor

We propose and demonstrate a novel, easy to use water depth pressure sensor based on selective mode excitation using a single-multi-single mode structure. The multi-mode fiber of the single-multi-single mode structure was coiled in a loop to introduce losses and reduce the size of the sensor. The change in ellipticity of the multimode fiber loop caused a spectral shift in the transmission spectrum of the structure, which was used for sensing application. A sensitivity of ~ 0.63 nm/cm and an extinction ratio of ~ 7 dB over a wavelength range of 1550-1600 nm has been reported. The high extinction ratio was achieved by exciting higher order modes of the multi-mode fiber by optimizing the core offset at both the input/output splices of the single-multi-single mode structure. We have also theoretically studied the transmission characteristics of the structure, which shows an excellent agreement with the experimental results.