In response to the critical need to better understand the cycle of CH4, the 2nd greenhouse gas (GHG) after CO2, in aquatic environments, the OASIS team, through the ANR FRAME grant, aims to develop an innovative fiber refractometric sensor integrating two technologies based on multimode fibers and photonic crystals. A resolution of 5 nmol/L of dissolved CH4 over a range of 10 nmol/L to 100 µmol/L with a relatively fast response of less than 2 minutes in the natural environment is envisaged by exploiting the combined performance of MMF and PCF sensors, designed with the help of AI and innovative accelerating numerical methods. Finally, without any loss of generality, the versatile nature of the fibered sensor developed may also open up future avenues for detecting other gas species, as well as complex molecules present at low concentration in water and recognized as aquatic pollutants (wastewater, endocrine pesticide molecules, etc.), using molecularly imprinted polymer techniques. Identifying, observing and locating these toxic entities can therefore have a major impact on ensuring safe access to healthy water in a broader context of sustainable development.

In parallel, the OASIS team is developing a range of geophysical equipment to study, detect and collect data linked to seismic activity (ANR XSTRAIN).

Within the ANR FRAME project (Fiber Refractometers for in situ detection of Aquatic MEthane), the OASIS team is developing an innovative fiber-optic refractometric sensor for in situ detection of dissolved methane in aquatic environments. The goal is to better monitor the CH4 cycle, the second most important greenhouse gas after CO2, using a compact, sensitive optical solution compatible with real environmental measurements.

The approach relies on microstructured and capillary optical-fiber architectures, illustrated here by an immersed fiber refractometer designed to interact directly with the surrounding aqueous medium. Combined with methane-sensitive functional coatings, these structures convert the presence of dissolved CH4 into measurable optical-property changes. In the longer term, this platform may contribute to real-time monitoring of gas-water exchanges in the environment and open perspectives for the detection of other dissolved species of interest.

Related publications. These developments are supported by three recent publications covering microstructured, differential multimode, and capillary-fiber-based refractometers:

1. A. Rahman, F. Beffara, H. Apriyanto, O. D. Bernal, F. Surre, G. Humbert, J.L. Auguste, H.C. Seat, “Experimental Investigation of Microstructured and Capillary Optical Fibers for Refractive Index Measurement from 1.316 to 1.425 RIU,” 2024 IEEE SENSORS, doi: 10.1109/SENSORS60989.2024.10784668,
2. O. Bernal, A. Rahman, H. Apriyanto, F. Surre, S. Pullteap, and H. C. Seat, “A High-Resolution Phase Shift Detection System for a Differential Multimode Fiber Refractometer,” 2024 IEEE SENSORS, doi: 10.1109/SENSORS60989.2024.10784672, and
3. A. Rahman, F. Beffara, H. Apriyanto, O. D. Bernal, F. Surre, G. Humbert, J.L. Auguste, H.C. Seat, “Capillary Fiber-Based Refractometer with Scalable Performance,” Photonics Research, vol. 14, no. 5, May 2026, doi: 10.1364/PRJ.580264.

Illustration of a capillary fiber refractometer used to detect the presence of methane dissolved in water