Fiber optic coupler displacement sensor for detection of glucose concentration in distilled water

A simple intensity modulated displacement sensor is demonstrated using a fused coupler as a probe for measuring glucose concentration in distilled water. It uses a plastic optical fiber (POF) based multimode fused coupler as a probe and a flat mirror as a reflector is captured by the same coupler. The light source is a He-Ne red laser (peak wavelength of 633 nm) which is chopped using a mechanical chopper. The reflected optical signal is detected by the silicon photodiode and the electrical signal is then fed into the lock-in amplifier together with the reference signal of the mechanical chopper. The fiber optic probe is first immersed in de-ionized water to measure the output voltage of a 0 % glucose concentration, followed by liquid with glucose concentrations from 5 % to 25 %. It is found that the peak voltage or the collected light intensity linearly related to the glucose concentration. The sensitivity is measured to be around 0.012 mV/(%) when the glucose concentration is varied from 0 to 25%. It is also observe that the slope of the displacement curve increases linearly with the glucose concentration at a rate of 0.0035 mV/(%)².