<p>In this paper, we propose a two-dimensional (2D) photonic crystal (PC) based salinity sensor using a hexagonal air-hole lattice structure. The design comprises a primary Hexagonal Ring Resonator (HRR), a Micro Ring Resonator (MRRs), and an input and output bus waveguides. Initially, a dense wavelength division multiplexing (DWDM) filter is designed to obtain the narrow spectral linewidth of 0.2&#xa0;nm in the conventional band (C band) for optical communication systems. Subsequently, a refractive index-based salinity sensor is proposed, in which variations in salinity are detected through changes in the analyte’s effective RI. The sensor is realized by coupling the selected modes of the Hexagonal Ring Resonator to the Micro Ring Resonators in response to changes in the analyte RI, thereby improving the resonance response and allows accurate detection of salt concentration in seawater. The sensing region identify the analyte by measuring variations in refractive index associated with different salinity levels, resulting in distinct wavelength shifts. The sensor achieves a narrow spectral width of 0.19&#xa0;nm, a high-quality factor of 8161.263, a sensitivity of 30.42&#xa0;nm/RIU, a resolution of 10.73&#xa0;nm, a figure of merit of 160 RIU^− 1, and a low detection Limit of 6.2 × 10^− 4 RIU. The design operates within the standard telecommunication range and is compatible with available active components. The device footprint is 25 µm², making it compact and well-suited to future nanoscale sensing systems.</p> Graphical abstract <p></p>