4. Conclusion
In summary, we have developed a flexible strain sensor with high sensitivity and fast response based on PDMS film encapsulated optical microfiber coupler structure. The CR between the two output fibers of the microfiber coupler has been adopted to realize weak strain demodulation. This flexible strain sensor exhibits an ultrahigh strain sensitivity (gauge factor, GF=900), a 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 as to expand the strain sensing range from 0.034% to 0.45%. These superior sensing properties make this flexible microfiber coupler sensor feasible for wearable photonics healthcare applications, such as the accurate detection of arterial pulse, ant crawling and human voice. This proposed flexible microfiber coupler sensor paves the way towards future wearable optical devices for highly sensitive sensors applications including personalized monitoring of cardiovascular system, human-machine interfaces, soft robotics and artificial intelligence.