Whispering gallery modes on optical micro-bottle resonator for humidity sensor application

We investigate the use of a class of whispering gallery mode optical resonators, namely, optical microbottle resonators (MBR) as a relative humidity (RH) sensor. The MBR is fabricated via the so-called “soften-and-compress” method to create a bulge (bottle) structure with a bottle diameter D _b =190⁡μm, stem diameter of D _s =125⁡μm and bottle length of L _b =182⁡μm. The MBR is then optically excited by using a 2 μm optical microfiber and was found to have a Q-factor of >10 ⁴ . The MBR was then employed as a humidity sensor with a RH range of between 40%–80% and the performance is compared with a no-MBR microfiber. The MBR RH sensor was found to have a sensitivity 0.0487 dB/%, linearity >90% and P-value >10 ⁵ and is superior to the no-MBR microfiber in all measured parameters. The MBR RH sensor was also found to have good repeatability and stability over a period of 60 s. The wavelength shifted for the MBR is 0.03 nm greater than no-MBR.