Smartphone-integrated portable liquid-level and RI sensor based on a coupling technique using POF

Current smartphone-based portable sensor designs are generally limited to single-point sensing, and extending them to multipoint parameter applications introduces challenges like signal decoupling and integration. This study presents a method for a portable dual-parameter optical fiber sensing system fully integrated with a smartphone platform. The proposed method is demonstrated for liquid-level and refractive index (RI) measurement. The sensing mechanism is based on light coupling modulation influenced by RI changes in the coupling region. Two sensors were fabricated on a single illuminating fiber by creating two independently twisted regions, each forming a distinct coupling zone. The smartphone's built-in flashlight serves as the light source, while its dual-camera system independently records optical responses from each sensor. Initially, the flashlight's spectral response was analyzed to ensure it provided sufficient power for multiple sensing units. In the proposed dual-parameter portable sensing method, the liquid level is demonstrated for a 0-100 mm sensing range, having a resolution of 0.007 mm and a sensitivity of 0.0102%/mm. For RI sensing, the system achieves a range of 1.333-1.361 RIU, a resolution of 1.03 × 10^-4 RIU, and a sensitivity of 102%/RIU. The proposed dual-camera system ensures high-resolution performance in a compact and field-deployable format.