Wide range pH fiber sensor with congo-red- and methyl-red-doped poly(methyl methacrylate) cladding

We introduce an evanescent-wave-spectroscopic fiber optic pH sensor. This sensor has part of the polymer cladding doped with either congo red (CR): which responds to pH=3 to 5: or methyl red (MR), which responds to pH=5 to 7: as the pH sensor film, In this study we succeeded in expanding the measurable pH range by using two pH sensor probes with the different pH sensitivities. In addition making the pH fiber sensor into a probe structure enabled us to achieve high pH sensitivity and to monitor the pH value easily. Reducing the quantity of doped dye made it possible to use the sensor for objects highly sensitive to pH chemical equilibrium.