Investigation of long period grating imprinted on a plastic optical fiber for liquid level sensing

Abstract A corrugated surface long period grating (LPG) imprinted on a plastic optical fiber (POF) was proposed as a liquid level sensor. The LPG was fabricated by a simple mechanical die-press-print method. The operation principle of the sensor was analyzed by the method of geometrical optics. The liquid-level sensing performances of the sensor with different structural parameters were evaluated. The results showed that the diameter of POF and the structure parameters of LPG would affect the liquid level sensing performance. When the LPG with a period of 300μm, a groove depth of 75μm, and a tilted angle of 30° was fabricated on a POF with a diameter of 0.25 mm, the highest sensitivity of -0.4381 dB/mm was obtained in the water-level range of 20 mm. The calculated results nearly coincided with the measured results. The hysteresis of the proposed sensor performed well and the sensitivity of the sensor was increased when the liquid RI was increased. Moreover, the water absorption would influence the liquid level sensing performance when the LPG was immersed in the water for a long time.