Robust and miniature optical fiber pH sensor based on methylene blue dye adsorption

An optical fiber pH sensor based on the surface absorption of methylene blue dye is reported. Previously obtained results for this sensor have indicated a large linear operating range (3-10 pH) coupled with a relatively short response time and immunity to ionic concentration. Further investigations are reported which include testing reproducibility, accuracy, and sensitivity. In addition to this, a two wavelength referencing technique is introduced where the sensor is illuminated by two wavelengths in sequence in a time domain multiplexed manner. The performance of the sensor in terms of temperature stability and immunity to small perturbations to the launch conditions is improved by the application of the multiplexing technique. Particular attention is given to the ability of this sensor to measure pH at very low ionic strength and the geometry of the sensing element has been refined to reduce the response time to below 45 seconds for solutions of ionic strengths of 100 μs. These measurements are significant since many applications for pH measurement require operation at such low ionic concentrations, e.g. underground water monitoring. Results are also included for higher ionic concentrations such as those encountered in blood pH measurement and other medical applications.