fig6

Figure 6. Overview of spectral-based sensing principles. (A) Schematic diagram of the FBG sensing principle^[55]. Copyright 2019, MDPI; (B) Schematic diagram of the interferometric systems sensing principle^[37]. Copyright 2024, Elsevier; (C) Sensing for respiratory rate monitoring using reflectance spectroscopy^[146]. Copyright 2022, MDPI. FBG: Fiber Bragg grating.