Development of a highly sensitive fibre optic O 2 /DOsensor based on a phase modulation technique

A fibre-optic sensor for oxygen gas and dissolved oxygen is reported. The sensor employs the quenching by oxygen of the fluorescence from a ruthenium complex trapped in a cage-like structure of a sol-gel derived porous thin film coated on a multimode optical fibre. The light from a blue LED is modulated and used as the excitation source. The phase difference between the extremes of the measurement range (0 and 100% O2 in gas and ~0 ppm and water saturated with O2 (9 ppm) in liquid) is measured as a function of modulation frequency. The optimum frequency was experimentally found to be 75 kHz using a standard lock-in amplifier for phase detection. Using the optimum pulse frequency, the sensor calibration curves in gas and liquid toward oxygen were determined. The sensor exhibited excellent sensitivity and reversibility.