Abstract: An optical fiber sensing apparatus, system, and method capable of in operando and/or in situ monitoring a state of charge (SOC) of an energy storage device such as a capacitor or a battery is provided. The apparatus comprises an optical fiber having a surface plasmon resonance (SPR)-stimulating structure, exemplarily including a tilted grating in a core, and a SPR-active layer coating a cladding, of the optical fiber. The apparatus is configured such that when arranged in a close proximity with an electrode of the energy storage device, SPR waves are stimulated upon receiving an actuating light. Through analysis of signals of the SPR waves, the SOC of the energy storage device can be determined. The apparatus can also be utilized to capture non-SPR optical waves, analysis of which can further derive information such as temperature, pressure, strain, etc. of the energy storage device and/or be used for calibration.