Fiber Bragg grating applied pulsed photoacoustic detection technique for online monitoring concentration of liquid

The need for real-time monitoring of the concentration of raw liquids and wastewater flowing in ducts is one of the significant process controls in chemical plants. A concentration sensor applicable to acidic fluids, however, has not been practically developed because of the maintenance required to sustain performance due to the metal erosion of the electrode from the acidity. To overcome this problem, we propose the fiber Bragg grating applied pulsed photoacoustic (PA) detection technique for online monitoring concentration of liquid. In our technique, optical fiber made of quartz is used for the PA signal generation and detection, for which a 0.6-mm-diameter step index fiber and Bragg grating composed of single mode fiber are used as an object, respectively, so that our metal-free sensor head is not only of small size but is also maintenance-free. The concentration of dye (Rhodamine 6G) dissolved into distilled water was measured to estimate the capability. A Q-switched frequency doubled Nd:YAG laser was used as the PA exiting beam, which is fed to the PA sensor through the fiber. It was experimentally confirmed that the detection linearity was significantly obtained in a concentration range from a few ppm to approximately 200 ppm.