Detection of magnesium ion concentration was successfully demonstrated using a fiber coupler based displacement sensor probe in conjunction with a concave mirror target. Analysis of peak voltage result, which was obtained by moving a sensing port displacement from a fixed concave mirror (CM) was found to be useful to detect a magnesium ion concentration, which is filled in between the probe and target. The proposed sensor is capable for measuring magnesium ion with variation of concentration 0–5%. The resolution of the sensor can be achieved at 0.2%. It was also observed that the use of CM with a longer focal length improves the sensitivity of the sensor. The high sensitivity, the small size, the ease fabrication process, and the bio-compatibility of the proposed device are appealing characteristics that makes it ideal for practical bio-sensing applications
