Distributed pH sensing based on optical frequency domain reflectometry

A distributed pH sensor was proposed by using the optical frequency domain reflectometry and a hydrogel coated single mode fiber (SMF). The swelling of the hydrogel will convert the pH value to the axial strain in the fiber. Taking capacity of distributed strain measurement with high spatial resolution, the pH value of the external medium surrounding the hydrogel-based optical fibers is distributed measured by the optical frequency shifts of the local back-reflection spectra in optical frequency domain reflectometry. In the experiment, the range of the pH value from 2 to 5 was measured with a spatial resolution of a 3.1 mm and a sensitivity of 60 pm/pH when the hydrogel coating diameter is 0.8 mm.