Wearable and Skin-Mountable Fiber-Optic Strain Sensors Interrogated by a Free-Running, Dual-Comb Fiber Laser

Wearable and skin-mountable strain sensors are highly demanding for monitoring skin deformations induced by human activities. Realization of such sensor devices based on fiber-optic approaches offers attractive advantages such as electromagnetic immunity and inherent electric safety in comparison to their electronic counterparts. However, fiber-optic sensors, conventionally made of stiff silica fibers, are not mechanically compliant with the soft human skins and have limited strain range (<1%) for measuring large deformations. Here, a stretchable fiber Bragg grating (FBG)-based optical (SFO) strain sensor with skin-like compliance for human activity monitoring is presented. The SFO sensor is fabricated by a sinuous-shaped FBG incorporated with a stretchable substrate that responds to strain deformations by shifting of the Bragg wavelength. This structural design enables measurement of various dynamic strains associated with tension, bending and torsion in a large sensing range up to 50%. To facilitate wearable integrations, a novel free-running fiber laser with coherent dual-comb output is developed to interrogate the SFO sensors by dual-comb spectroscopy, which enables fast spectral acquisition with a single photodiode. It is shown that the SFO strain sensors can be used for wearable and skin-mountable detection of diverse human activities including breathing, phonation, facial expression, and joint movements in real time.