Using coreless fiber Sensor (CLF) is Created and constructed to detect different concentrations of cobalt. Sensing is created by combining a coreless fiber Sensor (CLF) between two single-mode fibers (SMF). The CLF is then gradually reduced to 96.054 \(\:\mu\:\) m, utilizing specially designed tapering equipment, which significantly increases its sensitivity to ambient index. In the center of the sensor, a small portion (4 cm) of CLF is created tuned tapered diameters and lengths of CLF cladding were immersed in various concentrations of the cobalt sample to sense the effective refractive index (RI) variations. Single optical fibers were used in the development and implementation of SPR-based coreless optical fibers. Refractive indices and cobalt sample concentrations were also determined using it. The exposed sensing area of the sensor measured 4 cm in length, was 1500 nm/RIU, while the SNR was 0.18 the resolution was 6 \(\:{*10}^{-6}\) , and the figure of merit was 45.
