Polymer optical fiber twisted macro-bend coupling system for liquid level detection

Abstract

The liquid level detection principle of cladding mode frustrated total internal reflection (CMFTIR) effect is proposed. The significant enhancement of CMFTIR effect is realized through macro-bend coupling system in which the dark-field coupling phenomenon between two multimode polymer optic fibers is observed through experiment. Especially twisted macro-bend coupling structure (TMBCS) is adopted to achieve stable coupling of two naked POF. The testing result showed that the dark-filed forward coupling efficiency reached 2‰ and the extinction ratio of the liquid level probe reached 4.18 dB. Compared with existing optical fiber liquid level sensors, the TMBCS probe is simpler, robuster, and cheaper. In addition, the TMBCS has the potential for displacement or stress sensing.

Liquid level sensor based on CMFTIR effect in polymer optical fiber

“…This phenomenon which is called "first bath" is due to the remaining liquids on the sensor surface after the sensor coming out of the liquids. In the previous study [17], since gaps existed between the two twisted fibers of the TMBCS structure, water droplets easily remained in the cracks, resulting in more significant "first bath" phenomenon. The output power difference of TMBCS between the dry and wet states reached 30%.…”

“…Another more important reason is that the POF fibers are flexible and can be bent to a small loop with no breakage. In the previous report [17], we used the TMBCS to acquire the dark-filed coupling signal and achieved the enhancement of the CMFTIR effect. In this paper, we find that as long as the macro-bending radius decreases to a certain extent, the CMFTIR effect can also be enhanced to a satisfactory degree.…”

“…And the extinction ratio is relatively low, so erroneous judgment easily appears at noisy environments. Hou et al [17] using plastic optical fibers with the twisted macro-bend coupling structure inspired cladding modes which had cladding modes frustrated total internal reflection (CMFTIR) effect and realized a liquid level probe with an extinction ratio of 4.18 dB. On one hand, because the coupling ratio is low and the sensing signal of sensor is weak, the signal will attenuate continuously in the long-distance propagation and hardly be detected in this situation.…”

A Multipoint Liquid Level Sensor Based on Two Twisted Polymer Optical Fibers in a Race-Track Helical Structure

“…Additional elements are always applied to eliminate the influence of the undesired signal, increasing the complexity of detection. Hence, a macrobend coupler based on POFs is introduced [18]. The active fiber is connected to the light source, of which the macrobend losses are coupled partly to the passive fiber and called the side-coupling power.…”

An Optical Fiber Liquid Level Sensor Based on Side Coupling Induction Technology

“…The variations of output power is adjusted by the liquid level. The twisted macrobend coupling structure (TMBCS) improves the signal-to-noise ratio (SNR) substantially, overcoming the drawback of intensity-based sensors [12,13]. The fabrication process of the proposed sensor is simple and cheap.…”