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.

We experimentally demonstrate a stable and compact high pulse energy mid-infrared (MIR) 6.45 μm laser, which is produced by an optical parametric oscillator (OPO) based on the non-oxide BaGa4Se7 (BGSe) nonlinear crystal pumped by 1064 nm Nd:YAG laser oscillator. With optimizing the parameters of the OPO system, a record high idler energy of 6.76 mJ at 6.45 μm was obtained with 18.85 ns of pulse duration (FWHM) at the repetition rate of 10 Hz under 93.5 mJ of pump energy, corresponding to an optical-to-optical conversion efficiency up to 7.23% from 1064 nm to 6.45 μm. The experimental results were in close approximation with the theoretical calculations. To date, it is both the highest MIR pulse energy and the highest conversion efficiency at 6.45 μm for any 1 μm pumped OPOs. Further, excellent output energy stability is demonstrated, with an energy fluctuation of less than ±0.5% over 165 min. Such a laser is interesting for both scientific and medical applications, including, for instance, for trace gas analysis and tissue ablation.