The use of planar waveguides has recently shown great success in the field of optical sensors based on the Lossy Mode Resonance (LMR) phenomenon. The properties of Graphene Oxide (GO) have been widely exploited in various sectors of science and technology, with promising results for gas sensing applications. This work combines both, the LMR-based sensing technology on planar waveguides and the use of a GO thin film as a sensitive coating, to monitor ethanol, water, and acetone. Experimental results on the fabrication and performance of the sensor are presented. The obtained results showed a sensitivity of 3.1, 2.0, and 0.6 pm/ppm for ethanol, water, and acetone respectively, with a linearity factor R² > 0.95 in all cases.

The sensitivity of a liquid level sensor based on a single-mode-multimode-single-mode fiber structure has been increased by hydrofluoric acid etching. The etching process was analyzed and monitored both theoretical and experimentally, which permitted to observe that a sinusoidal spectrum can be obtained for low diameters. As an example, a 2.77 fold sensitivity increase was attained by etching from diameter 125 to 50 μm. Moreover, the sinusoidal shape of the optical spectrum permitted to monitor liquid level changes both in wavelength and phase. The cross sensitivity of the sensor to refractive index and temperature was also studied.

Ethylene monitoring has long been a method of controlling the ripening of climacteric fruits. But it turns out that this gas is an important biomarker in biomedical applications. This work presents an optical gas sensor based on the lossy mode resonance (LMR) effect for ethylene detection in planar waveguide configuration. Two different approaches have been explored: one in the visible (VIS) spectral region and the second one in the mid infrared (MIR) region. Optical resonances have been achieved, in all cases, by means of sputtered tin oxide thin films. Response and recovery times were 54 and 246 s respectively for the sensor with the resonance in the VIS region while the device operating in the MIR obtained response and recovery times of 19 s and 47 s respectively. The sensitivity during ethylene detection varied from 93.8 pm/ppm to 187.5 pm/ppm with the devices working in the VIS and MIR regions, respectively. According to the calibration curve, devices shown an ethylene limit of detection (LOD) of 4.0058 ppm and 0.6532 ppm in the VIS and MIR spectral regions respectively, which finds applications in climacteric fruit ripening assessment as well as hemodialysis control. Cross sensitivity with humidity was also characterized for both devices.

Gas sensors play a critical role in numerous human activities. Their necessity continues to grow across diverse fields as technological advancements drive demand for precision agriculture and bioengineering among other applications. Among existing sensor technologies, optical gas sensors stand out due to their ability to operate remotely in high-risk environments while remaining unaffected by electromagnetic interference. Resonance-based optical sensors offer targeted gas detection through the functionalization of their sensitive surfaces. This work focuses on reviewing the state of the art in resonance-based optical gas sensors (ROGS), addressing their fundamental principles, recent advances in fabrication processes, waveguide designs, and materials employed both for resonance generation and as sensitive coatings. Additionally, the review examines achieved sensitivity, emerging applications, and key developments in the field, including those efforts on improving ROGS performances by means of artificial intelligence (AI) techniques. The study encompasses optical sensors leveraging Surface Plasmon Resonance (SPR), Lossy Mode Resonance (LMR), and Hyperbolic Mode Resonance (HMR)—the latter representing a notable breakthrough in recent years as a particular case of Bloch Surface Waves (BSWs).