Refractive Index Sensing Free From Critical Wavelength Referencing Using Fiber-Optic Directional Coupler

Two unique wavelengths ( critical and cross-over ) observed in the transmission spectra of fiber optic directional couplers exhibit a common (conjugate) characteristic feature, that the nature of spectral shifts flip around them. To free the transmission spectra from the opposite spectral shifts we demonstrate a judicious overlap of these wavelengths using optimized coupler parameters and dispersion management. Theoretical analysis explaining the optimization steps, dispersion management and the influence of these on the transmission and sensing properties of the considered directional coupler are presented. An ultra-high experimental refractive-index sensitivity ( $\sim$ 20,550 nm/RIU) has been demonstrated by operating the directional coupler close to the critical wavelength (nearest to it largest spectral shifts are observed). This study should find applications in designing ultra sensitive refractometers which are free from critical wavelength referencing.