Optical fiber humidity sensor based on a tapered fiber asymmetrically coated with indium tin oxide

In this work a new humidity sensor based on lossy mode resonances (LMR) generated on a biconically tapered single mode optical fiber is presented. The tapered fiber was coated only on one side using indium tin oxide (ITO) by means of sputtering in order to generate the LMR. This asymmetrical structure (180 degrees of coating) makes the device sensitive to the polarization of the light so that the LMR can be separated in its TE and TM modes. The polarization dependence produces the effect of a smaller bandwidth in the LMR resonance with respect to the symmetrical coating, allowing a better resolution in the peak detection procedure. The sputtering process was carefully monitored in order to have the LMR placed in the range of measurement. Then the sensor was characterized using a climatic chamber with humidity in the range 20-80%RH. A sensitivity of 200 pm/%RH was obtained using a tapered fiber of 35 μm of waist diameter.