Fig. 1. Application areas of multimodal fiber antenna sensor including environmental monitoring, biomedical applications, human–robot interactions, and cyber–physical systems. Figure created with permission from Ref. 17.

Fig. 2. (a) Thermal drawing of the multimodal fiber antenna sensor. (b) The preform fabrication steps. (c)-i. Cross-sectional image of the fiber sensor under an optical microscope (4× magnification, $\text{scale bar}=500\mu \mathrm{m}$). (c)-ii. SEM image of the fiber sensor (FEI Quanta 600, $\text{scale bar}=500\mu \mathrm{m}$). (c)-iii. Close-up illustration capturing the drawn fiber, showcasing its maintained cross-sectional geometry and enclosed parallel copper wires. (c)-iv. Photo displaying $600\text{-}\mu \mathrm{m}$ diameter, and 65 m of drawn fiber coiled around a hand. 2.42 cm quarter coin shows the scale of the coiled fiber (scale bar = 5 cm).

Fig. 3. (a) Electromagnetic coupling scheme of multimodal fiber antenna sensor and TDR. (b) Principle of pressure sensing of fiber sensor by change in the capacitance due to change in the distance between metal wires. (c). Principle of proximity sensing of fiber sensor by change in the dielectric environment of the fiber.

Fig. 4. Electromagnetic field FEM simulation using COMSOL Multiphysics. (a)-i. Field surrounding the fiber sensor at fiber mode 0 (symmetric). (a)-ii. Field surrounding the fiber sensor at fiber mode 1 (antisymmetric). (b)-i. Field surrounding the fiber sensor at fiber mode 0 with the presence of a foreign object within the fiber’s diffraction limit. (b)-ii. Field surrounding the fiber sensor at fiber mode 1 with a foreign object within the fiber’s diffraction limit. The simulations are conducted at a frequency of 18 GHz, a characteristic median frequency for pulses generated by electronics operating in a frequency band of up to 35 GHz. The fiber device cladding has a filleted-corner square cross section with a side of $600\mu \mathrm{m}$. The dielectric constant is 2 for both the SEBS fiber cladding and the foreign object brought into the fiber proximity.

Fig. 5. TDR measurements of the multimodal fiber antenna under different configurations. (a) The sensitivity curve when the metal wires are placed horizontally (horizontal configuration). Inset, an example of a measurement with 100 gr. (b) The sensitivity curve when the metal wires are placed vertically (vertical configuration). Inset, an example of a measurement with 50 gr. (c) Sensitivity measurements in the vertical configuration, using light weights of 1, 2, and 5 gr. (d) Spatial resolution in the vertical configuration. (e) Distributed pressure measurement using 50 gr at 9.5, 17, 27.5, 37.5, and 47 cm in the vertical configuration. (f) Proximity measurement without the plastic pedestal used in the pressure measurements. The weights used in this case have a footprint diameter of 7 and 18 mm for the 2 and 50 gr weights, respectively. In addition, an index finger with a width of $\sim 13\mathrm{mm}$ is used in the measurements.

Fig. 6. Signal analysis of fiber antenna evaluating dispersion and sensitivity. (a) The refractive indices of the fiber at antisymmetric and symmetric modes are simulated at the frequency range of 0.8 to 35 GHz by COMSOL Multiphysics simulations. 0.8 and 1 GHz results of mode refractive index have error bars indicating increasing sensitivity to the mesh fineness. (b) Signal-to-noise graphic of the fiber antenna with vertical wire configuration using 1 to 50 gr weights indicates an increase in noise relevant to an increase in signal. The power function fitting shows the sensitivity limit of the fiber antenna as 0.26 kPa.

Table 1. Changes in the fiber sensor’s refractive index and impedance at mode 0 (symmetric mode) and mode 1 (antisymmetric mode) for deformation and proximity studies. Notations: O (10−x) is “order of 10−x.”