Introduction

Flexible devices exhibit excellent foldable and wearable performances, have important applications in the fields of physiological monitoring\citet{Takei_2014},\citet{Liu_2017},soft robotics\citet{Majidi_2014},\citet{Kim_2013},\citet{Wu_2017},bioelectronics\citet{Kim_2018},\citet{Chen_2020},flexible display\citet{Sekitani_2009},\cite{Chen_2013},\citet{Kim_2011},and smart clothing\citet{Choi_2019},\citet{Yang_2019} .Flexible strain sensors that can measure the environmental strains and mechanical deformations for real-time in situ during the bending movements, have been the essential flexible devices in human health monitoring and other fields. In order to track the extremely weak strain signals in cardiovascular diseases\citet{Fang_2021}, blood flow\citet{Gu_2020}, skin tissue healing\citet{Tang_2021}, and acoustic vibration\citet{Yan_2022}, improving the sensitivity and detection limit of the flexible strain sensor has become an urgent affair. These advanced sensors will capture the change details of human physiological indicators more easily and provide more effective data for drug development, clinical diagnosis, and disease treatment\citet{Li_2018},\citet{Zhao_2020},\cite{Li_2021},\citet{McNerney_2020}.
The electronic flexible sensors achieve the outstanding performances for identifying the extremely weak strain sensing signals by measuring the variations of capacitance\citet{Xu_2019},\citet{Mannsfeld_2010},resistance\citet{Jason_2017},\citet{Kim_2018a},piezoelectricity\citet{Zhao_2018},\citet{Zhu_2019}, and triboelectricity\cite{Ha_2018},\citet{Wu_2018}. For example, Gao et al.\citet{Gao_2022} embedded carbon nanotubes into new polyurethane fibers, which was highly sensitive to mechanical signals such as micro strain and vibration, and the detection limit was 0.0075%. Zhao et al.\citet{Zhao_2021}have achieved the detection limit of 0.001% by using MXene/CNT to construct the microcrack structure. However, parasitic effects, insufficient insulation, and electromagnetic disturbances restrict the practical applications of electronic sensors, to some extent\citet{Roriz_2014},\citet{Wang_2013}.
Optical sensors, especially fiber-optic sensors, provide a promising alternative to electronic sensors due to their distinct advantages of high sensitivity, high precision, fast response, inherent electric safety and electromagnetic interference immunity. At present, flexible optical fiber strain sensors mainly realize the measurement based on optical power detection scheme, which could be classified into three types, mechanoluminescent effect, optical loss effect and optical interference effect. The mechanoluminescent effect type is to generate and convert mechanical stimuli related strain into fluorescence signals based on intelligent mechanoluminescent materials. Liang\citet{Liang_2021}proposed a self-powered stretchable strain sensor based on the integration of mechanoluminescent phosphors with an elastomer optical fiber, and demonstrated a relative high detection limit (10%) due to the high threshold pressure of ZnS:Cu mechanoluminescent materials. The second optical loss type is to measure the changes of transmitted light intensity as the fiber is deformed by strain, where the sensitivity is mainly determined by the compromise between attenuation coefficient and transmission characteristics of optical sensing fiber. Zhu et al.\citet{Zhu_2021}designed a self-assembled wavy optical microfiber strain sensor by detecting the micro bending loss of the microfibers, and achieved a detection limit of 0.5%. Pan et al.\citet{Pan_2020} proposed a pre-bent microfiber sensor for controlling the transition of the radiation modes into guided modes in the bending microfiber and achieved a detection limit of 0.25%. Guo et al.\citet{Guo_2019} proposed a flexible polymer fiber strain sensor based on the localized surface plasmon resonance absorption and scattering effect of gold nanoparticles, which showed a detection limit of 0.09%. The third optical interference type is to detect the optical power variation based on strain-induced optical interference effect. Peng et al.\citet{Li_2018a} proposed a hybrid plasmonic microfiber knot resonator that weak strain would shift resonant wavelength resulting in output light power change, and achieved a detection limit of 0.01%. It is worth emphasizing that the interference effect in traditional silica optical fiber has been proved to measure strain with a sensitivity limit at sub-pε level,\citet{Gagliardi_2010}which offers the great enormous potentialities for improving the sensitivity and detection limit of flexible optical fiber strain sensors.
In this paper, we propose a flexible optical fiber strain sensor with an ultralow detection limit of 0.001% strain. The fabricated microfiber coupler is encapsulated into PDMS films to develop flexible strain sensors. The strong coupling effect of this microfiber coupler sensor is highly sensitive to the strain-induced coupling length change, so the coupling ratio (CR) between the two output fibers has been adopted to realize weak strain demodulation. The proposed CR demodulation can effectively resist the interference of hybrid variables, such as optical loss and input light intensity fluctuation. Based on these advantages, this flexible strain sensor exhibits ultrahigh strain sensitivity (gauge factor, GF=900), low detection limit of 0.001%, ultrafast response time (<0.167 ms), and superior durability and stability (>10 000 cycles). The optical loss parameter has been adopted as the second measurand with CR simultaneously, so the strain sensing range has been expanded from 0.034% to 0.45%, which greatly break the sensing range limit within a quarter cycle of strain-CR cosine curve. These superior sensing properties make this flexible microfiber coupler sensor feasible for wearable photonics healthcare applications, such as demonstrations of monitoring ultra-weak strain signals arising from a human’s arterial pulse, detecting the dynamic impacts associated with crawling of an ant, and restoring the sound signals caused by weak vibrations of human voice. In addition, the simultaneous monitoring of arterial pulse signals and human voice have been achieved based on frequency division multiplexing demodulation technique. The advantages of high sensitivity and fast response of this proposed flexible microfiber coupler sensor shows great potential for the human-machine interfaces, artificial intelligence and soft robotics.

Experimental Section

The central part of our flexible sensor is the microfiber coupler, where the slight deformations of the coupler induced by the external stimuli would cause the apparent variations of CR. The microfiber coupler was fabricated by tapering and fusing two standard telecommunication single-mode optical fibers (SMF-28, Corning) together at the same time using a method known as the flame brushing technique\citet{Brambilla_2004} as shown in Figure 1 a. An ideal microfiber coupler with a diameter of several hundred nanometers to several micrometers and a uniform waist with a length of several millimeters to several centimeters could be fabricated by carefully controlling the heating temperature, flame flow, sweeping of micro-heater, and moving of the translational stages. The optical loss of our fabricated microfiber coupler was measured as low as about 0.05 dB. The second step was to encapsulate the microfiber coupler into a flexible sandwich structure for improving its stability. The encapsulation layer was selected as PDMS film owing to the virtues of high flexibility, high transparency and biocompatibility. The PDMS precursor (mixing ratio of base polymer and curing agent) was poured into a homemade model groove (2 cm × 5 cm size with 100 µm thickness) and formed through thermal treatment at 80 ℃ for 30 min to obtain a flat and smooth film as the bottom layer. The prepared bottom PDMS film was transferred to the other deeper groove mold (2 cm × 5 cm size with 250 µm thickness) for the assembling process. After that, the fabricated microfiber coupler was transferred onto the bottom PDMS film in the deeper groove mold, and it could firmly adhere to the PDMS film due to van der Waals forces. Finally, some PDMS precursor was uniformly dispersed in the deeper groove mold to cover the microfiber coupler with a thickness of 150 µm, and the layered flexible microfiber coupler sensor was fully assembled and realized after 30 min of curing at 80 ℃. After encapsulation, a highly transparent sensing film with a thickness of about 250 µm was demonstrated to manipulate in a twisted state (Figure 1c) and a bent state (Figure 1d,e) which showed the outstanding structural flexibility. Figure 1b,e revealed that this encapsulated microfiber coupler kept a good light guiding ability even under giant deformations. This encapsulated microfiber coupler was stuck on the wrist by Van der Waals force with good contact which showed that the sensor had good wearability, as shown in Figure 1f. This prepared flexible sensor with skin-like mechanical characteristics and excellent optical transparency enabled it to be a powerful tool for wearable application.