Sensing of multiple parameters with whispering gallery mode optical fiber micro-resonators

Monitoring of multiple physical parameters, such as humidity, temperature, strain, concentrations of certain chemicals or gases in various environments is of great importance in many industrial applications both for minimizing adverse effects on human health as well as for maintaining production levels and quality of products. In this paper we demonstrate two different approaches to the design of multi-parametric sensors using coupled whispering gallery mode (WGM) optical fiber micro-resonators. In the first approach, a small array of micro-resonators is coupled to a single fiber taper, while in the second approach each of the micro-resonators within an array is coupled to a different tapered fiber section fabricated along the same fiber length. Simultaneous measurement of relative humidity and ammonia concentration in air is demonstrated with an array of two microspheres with different functional coatings coupled to a single fiber taper. Sensitivity to ammonia of 19.07 pm/ppm ammonia molecules and sensitivity to relative humidity of 1.07 pm/% RH have been demonstrated experimentally. In the second approach, an inline cascade of two cylindrical micro-resonators fabricated by coupling to multiple tapered sections along a single optical fiber is demonstrated for measurement of strain and temperature simultaneously. A strain sensitivity of 1.4 pm/με and temperature sensitivity of 330 pm/ºC have been demonstrated experimentally. Both the proposed sensing systems have the potential for increase of the number of microresonators within an array for sensing of a larger number of parameters allowing for reduction of the overall cost of sensing system.