Multi-Wavelength Analysis of Intensity-Modulated U-Shaped Plastic Optical Fiber Humidity Sensors

Abstract

This paper presents a comprehensive multi-wavelength characterization of a humidity sensor based on an intensity-modulated U-shaped tapered plastic optical fiber (POF). The sensor exploits evanescent-wave interaction between guided light and the surrounding environment, enabling variations in relative humidity (RH) to modulate the transmitted optical power. A Mitsubishi SH4001 POF was manually tapered using fine-grade abrasive polishing to produce waist diameters of 500 µm and 600 µm, followed by bending into U-shaped structures with radii of 3 cm, 4 cm, and 5 cm. Light from light-emitting diodes (LEDs) at 470 nm, 530 nm, and 645 nm was launched into the fiber, and changes in output intensity were measured using a phototransistor and microcontroller-based signal acquisition system. Experimental results, obtained over 35–90 %RH, reveal a consistent inverse relationship between humidity and output voltage for all wavelengths. Among all configurations, the 645 nm wavelength paired with a 500 µm waist and 5 cm bend radius yielded the highest sensitivity of 0.0385 V/%RH and linearity of 98.74%. Comparative analysis demonstrates the significant influence of wavelength on evanescent-wave penetration depth and sensing performance. The findings confirm the suitability of tapered POF sensors as low-cost and robust alternatives for environmental humidity monitoring.