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Given the increasing demands for quality assurance in the food industry, a significant challenge emerges in the form of expensive integration of food sensors into packaging. This integration is crucial for strengthening food safety measures and ensuring the impeccable quality of food products. Official laboratory food safety testing heavily relies on expensive and bulky equipment. This article presents a new chemical sensing platform and a comparative study of in-house built novel designs for a robust multimode chemical sensor head probed by highly sensitive light-sheet skew rays for addressing cost and footprint issues. The sensing mechanism is the interaction between evanescent field mediated by refined skew rays propagating through a structured coreless multimode fiber and external chemicals, resulting in probe light absorption. The sensitivity is enhanced by the controlled excitation of skew rays using a light sheet and four specially engineered coreless multimode fiber structure, including uniform, tapered, microstub and microbubble designs. The sensitivity was demonstrated to be as high as 0.046 (dB/cm) / dB(1 ng/ml) and the limit of detection as low as 1.028 ng/ml for the microbubble structure. The results of our research pave the groundwork for a new range of chemical sensors suitable for food safety monitoring.

Optical humidity sensors have evolved through decades of research and development, constantly adapting to new demands and challenges. The continuous growth is supported by the emergence of a variety of optical fibers and functional materials, in addition to the adaptation of different sensing mechanisms and optical techniques. This review attempts to cover the majority of optical humidity sensors reported to date, highlight trends in design and performance, and discuss the challenges of different applications.