Optimization of Cascaded D-Shaped Optical Fiber Sensors for Intensity-Modulated Ethanol Detection

This study presents the development and optimization of a multi-D-shaped optical fiber sensor designed for enhanced refractive index (RI) and ethanol detection. The sensor probes were fabricated using a precise side-polishing technique on standard single-mode fiber (SMF-28) to maximize the evanescent field interaction with the surrounding medium. We systematically investigated the influence of the number of D-shaped sensing zones (N = 1 to 5) on sensor performance using glycerin-water solutions (RI 1.357–1.428). The experimental results demonstrate a significant sensitivity enhancement as the number of sensing zones increases, achieving a peak sensitivity of 12.12 dB/RIU with a 4-point (N = 4) configuration. A subsequent performance decline at N = 5 indicated a saturation limit dominated by fundamental mode field distortion and cumulative insertion loss. When applied to ethanol detection (RI 1.333–1.365), the optimized 4-point sensor exhibited a highly linear response (R2 = 0.9828) with a sensitivity of 3.866 dB/RIU. Notably, the sensor demonstrated high wavelength stability with negligible spectral shift across the tested range, confirming its operation as a robust, intensity-modulated device suitable for cost-effective biochemical sensing applications.