High-sensitive and millisecond-response all-nanofiber-optic humidity sensor using evanescent coupling

The reported humidity sensors usually rely on various sensitive materials to increase the humidity sensitivity, while leading to long response time (1 s similar to 10 s). By now, it is still challenging to obtain both high-sensitivity and ultrafast-response (similar to tens of ms), i.e., high figure of merit (FOM), for optical humidity sensors. Here, we demonstrate an all-nanofiber-optic strategy to break the limit by using evanescent coupling between a nanofiber (NF) and a substrate without any extra sensitive materials. To implement the sensor, a NF is suspended over a 450 nm deep pit etched in a glass-slide. With increase in the ambient humidity, a thin water film over the NF is formed and strengthens both the evanescent field outside the NF and evanescent coupling into the pit, thus leading to decrease in the transmission of the NF. Experiment shows that the sensor achieved a high sensitivity of - 1.162 dB/%RH in the high RH range of 66.5-83.8%RH with ultrafast response (40 ms) and recovery (100 ms) times. A sensor FOMs, defined as the absolute ratio of sensitivity to response time or recovery time, are also proposed to assess the comprehensive performance of the sensor. The response and recovery FOM of the demonstrated sensor here are up to 29.05 |(dB/%RH)/s| and 11.62 |(dB/%RH)/s|, respectively, which are 2-3 order of magnitude higher than the sensor reported previously. Interestingly, the NF sensor also exhibits a high RH resolution of 0.0189%RH in our experiment. The ultra-high FOMs and flexibility of the sensor allows to easily be integrated with a mask for continuously human fast breath monitoring with a 10 Hz bandwidth. Moreover, our sensor has potential applications in the non-contact switch, noninvasive diagnostic of skin conditions and micron location sensing. The proposed sensing mechanism could provide a novel all-fiber-optic strategy to improve both RH sensitivity and response of the optic humidity sensor simultaneously.