Light transmission mechanisms in a SMF-capillary fiber-SMF structure and its application to bi-directional liquid level measurement

Abstract

Capillary fiber (CF) has been extensively investigated in a singlemode fiber (SMF)-CF-SMF (SCS) sensing structure since multiple light guiding mechanisms can be easily excited by simply tuning the air core diameter (cladding diameter) and length of the CF. Understanding the light guiding principles in an SCS structure is essential for improved implementation of a CF based fiber sensor. In this work, light guiding principles in a relatively large air core diameter (≥ 20 µm) and long length of CF (> 1 mm) are investigated theoretically and experimentally. It is found that both multimode interference (MMI) and Anti-Resonant Reflecting Optical Waveguide (ARROW) light guiding mechanisms are excited in the SCS structure in the transmission configuration. However, MMI dips are not observed in the spectrum for the air core diameters of CF smaller than 50 µm in the experiment due to large transmission loss in small air core CFs. Further experimental results demonstrate that a CF with a bigger air core diameter shows a higher sensitivity to curvature, and the highest sensitivity of -16.15 nm/m-1 is achieved when an CF-100 was used. In addition, a SMF-CF-20-CF-30-SMF (SCCS) structure is proposed for high sensitivity bi-direction liquid level measurement for the first time, to the best of our knowledge. Two types of ARROW dips (Dip-20 and Dip-30) are simultaneously excited in transmission, hence both liquid level and liquid flow direction can be detected by tracing the dip strength changes of Dip-20 and Dip-30, respectively.

All-Fiber In-Line Twist Sensor Based on a Capillary Optical Fiber

“…The capillary fiber, being the simplest structure among hollow fibers (or microstructure fibers), has received a lot of attention in recent years because of its benefits, such as its simple structure, flexibility, low invasiveness, and low cost of manufacture [19]. Capillary fibers are now being studied for different sensing domains, including the refractive index [20,21], temperature [22], liquid level [23,24], strain [25], pressure [26], and curvature [27]. However, the aforementioned capillary fiber applications necessitate that all fibers be aligned and twist-insensitive.…”

Fiber ring cavity laser based on cascading two peanut-shape structures for liquid level measurement

“…In recent years, liquid level sensor has received extensive attention. Because of its wide application scenarios, such as the ice making machine in the refrigerator, the washing machine needs to prevent excessive water overflow when the liquid level is too high, the water heater needs to automatically replenish water when it detects that the liquid level in the internal water tank is too low, and the toilet needs to keep water in the water tank for leakage identification or liquid level identification [1][2][3][4]. However, the traditional electrical sensors have the disadvantages of slow response speed, high cost and electromagnetic interference.…”