We demonstrate a highly sensitive whispering gallery mode (WGM) relative humidity (RH) sensor based on a glycerol microdroplet. WGMs were excited using a 760 nm tunable semiconductor laser. We used free space coupling, which is effective when using a liquid resonator. A detailed analysis of different parameters influencing the sensor’s characteristics (sensitivity, hysteresis, resolution, stability, and temperature) is presented. The sensitivity of the sensor is one of the highest reported (2.85 nm/% RH in the range 50–70% RH with the resolution 1 × 10−4% RH). This type of humidity sensor has several advantages, such as high sensitivity, extended lifetime, good repeatability, and low cost, as well as the use of a non-toxic and environmentally friendly liquid.

We demonstrate a wavelength sensor based on whispering gallery mode (WGM) resonators. For the first time, multiple polymethyl methacrylate (PMMA) microspheres were simultaneously attached to a tapered fiber. WGM resonances from these commercially available PMMA microspheres were observed with a NIR camera, monitoring the scattered light. Circulating light in the WGMs was scattered on the outer layer of the microspheres and appeared as bright spots due to scattering defects. For each laser wavelength fed into the tapered fiber, the light interfered differently for the various sizes of PMMA microspheres. We measured scattered light intensity for different wavelengths and created a barcode for each microsphere. Combining these barcodes into a mode map allowed for unknown wavelength detection. A tunable laser around 1550 nm was used for measurements. As a result, a laser wavelength sensor system with a detection limit of 5 pm was demonstrated. The principles of increasing selectivity, as well as creating a compact device, were discussed.

We demonstrate an integrated on-chip whispering gallery mode (WGM) ring resonator tested for humidity sensing. When developed, SU-8 is chemically and mechanically stable, as well as optically transparent above 400 nm wavelength and it has a high refractive index. Therefore, it is a suitable material for optical WGM resonators. When light is coupled in the resonator, it circulates along the surface for prolonged periods of time interacting with the surrounding environment. Resonance wavelength depends on the refractive index and/or the radius of the resonator. Polymers, including SU-8, are sensitive to gas and temperature changes in the environment. We tested the ring resonators in changing relative humidity (RH). Due to changes in RH, the refractive index of SU-8 changes, and we observed a shift in the resonance wavelength. While the sensitivity was average compared to similar studies using other materials or geometries, the ring geometry showed excellent response and recovery times. This property is important in many areas, such as industrial production and environmental monitoring.

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