Fiber optic pH sensors for in-situ planetary exploration

The main goal of planetary exploration missions is the search for life and the study of the habitability. For this purpose, the development of instrumentation that can analyze parameters as pH, which is an indicator that provides astrobiological information, is of great interest. The study is carried out with two kind of fiber optic sensors to demonstrate the feasibility for planetary exploration since they are sensitive to the surrounding refractive index: Long Period Fiber Gratings (LPFG) fabricated with two different methods and Tilted Fiber Bragg Gratings (TFBG). In order to develop a fiber optic pH sensor, a polyvinyl alcohol (PVA)/polyacrylic Acid (PAA) hydrogel film (~ 2 μm thickness) has been deposited on the gratings and the first measurements have been performed. The refractive index of the polymer changes depending on the pH value, and this change is measured by monitoring the Dip Wavelength Shift (DWS) and the transmission power. The experimental results obtained with the coated fiber optic sensors in a range from pH2 to pH8 show that the LPFGs have a higher sensitivity in terms of nm/pH than the TFBG as expected. It was observed that the longer the grating length of the sensors, the greater the hysteresis. The aim of the future work is to submit the pH sensors to an ionizing radiation test to verify that the polymer does not suffer degradation in a harsh environment similar to the one encountered in future in-situ explorations for astrobiology studies.