Theoretical and experimental study of fiber-optic displacement sensor using multimode fiber coupler

This paper studies the displacement sensor using multimode fiber coupler based on intensity modulation. Fiber coupler used is handmade from plastic optical fiber 1 mm diameter; it has coupling ratio 0.25, excess loss 1.37 dB, and directivity 25 dB. He-Ne laser (632.8 nm) and OPT 101 (Burr Brown) detector is used to detect the change in power-output due to object displacement. The correlation function between power-output and object displacement is analyzed theoretically by Gaussian electromagnetic beam approximation and characterize the dynamic range sensor 4 mm (with linear region 1 mm) and sensitivity is 55.4 55.5 μW/mm. The experiment shows that the sensor is able to detect object displacement (in front of silvered mirror) accurately in order of 5 μm.