Sensitivity optimization of tapered optical fiber humidity sensors by means of tuning the thickness of nanostructured sensitive coatings

Electrostatic self-assembly has been used in this work to fabricate a new optical fiber humidity sensor. Said sensor consists of a single-mode tapered fiber coated with a [PDDA/Poly R-478] nanostructured overlay, in such a way that the thickness can be controlled in order to optimize the sensor sensitivity, by stopping the deposition process at the maximum slope of the transmitted optical power. The same tapered optical fiber tested with an overlay coating at the optimal working point achieves 26.8 times better sensitivity than with a double thickness overlay. A variation of 16 dB in optical power is achieved with responses time of 300 ms for changes in relative humidity from 75% to 100%. The high dynamic performance and low temperature cross-sensitivity allows this sensor to be used for human breathing monitoring.