Elsevier

Optik

Volume 251, February 2022, 168367
Optik

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

https://doi.org/10.1016/j.ijleo.2021.168367Get rights and content

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 300 µm, a groove depth of 75 µm, 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.

Introduction

Liquid level measurement is widely used in fuel storage and transportation, leakage monitoring in chemical products and industrial production, sewage treatment, and so on. Over the years, liquid level sensors based on mechanical [1], electrical [2], ultrasonic [3], and fiber optic technology [4], [5], [6], [7] have been developed. Compared with the other techniques, fiber optic sensors have the advantages of compact size, immune to electromagnetic interference, resistance to corrosion, and available for remote distributed measurement, which can provide a safe scheme of measurement for conductive or corrosive liquids and suitable for working in the flammable or explosive environments. To date, different methods based on fiber optic sensors for liquid level measurement have been proposed, such as fiber Bragg grating (FBG) [8], fiber long period gratings (LPG) [9], multimode interferometers (MMI) [10], Mach-Zehnder interferometer (MZI) [11], Fabry-Perot cavity [12], and fiber surface plasmon resonance (SPR) [13]. Most of the sensors based on interferometry used wavelength modulations which require expensive sensing systems. Moreover, these sensors which are constructed with single-mode glass optical fiber (GOF) usually require complex manufacturing processes. For example, the fabrication of fiber gratings requires a photolithographic process, the liquid level sensors based on the Fabry-Perot cavity use optical film with high reflectivity, and a coating process is needed to be applied for the sensor based on SPR.

In recent years, liquid level sensors based on plastic optical fiber (POF) have attracted considerable attention. POF has inherited the advantages of regular optical fiber and also has the features of low cost, better flexibility, easy handling, and operation in the visible range, which makes them an ideal candidate for developing low-cost liquid level sensors. Nowadays, different types of POF-based level sensors have been proposed for liquid level sensing. M. Lomer et al. reported a multipoint liquid level measurement sensor based on lateral-polished multi-U-bent POF [14]. X. Lin et al. proposed a multipoint liquid level sensor using a group of POF segments, which are aligned coaxially and spaced equally [15]. J. Park et al. presented a POF sensor with in-fiber micro-holes for multipoint liquid level measurement [16]. P. Antunes et al. proposed a multipoint liquid level sensor based on a periodic V-Groove structure with a minimum groove spacing of 1 cm [17]. However, these sensors mentioned above are suitable for discrete measurements and always with a worse resolution. The resolution of the sensor is the ability of a sensor to perceive the smallest measured change, which is determined by the interval between two adjacent measurement points. The continuous liquid level measurement could realize a real-time dynamic monitoring of the liquid level change. Currently, continuous liquid level sensors based on POF have been developed, Y. Hou et al. proposed a twisted macro-bend coupling structure, by coiling the twisted POFs around a cylinder [18], they obtained a continuous liquid level sensor with an average resolution of less than 0.3 mm [19]. N. Jing et al. presented a multi-S-bend structure for continuous liquid level sensing, and the sensitivity of 0.04 dB/mm was obtained [20]. A. S. Rajamani et al. proposed a decladded POF probe as a liquid level sensor, and a resolution of 0.21 mm was obtained [21]. P. Xue et al. reported a screw-shaped POF for liquid level sensing, the screw-shaped was fabricated by a heat pressing and twisting method which is difficult to achieve mass production [22]. In addition, liquid level sensors based on polymer fiber FBGs have been reported, however, the fabrication and demodulation costs of the Bragg gratings are also very high.

The LPG fabricated on a POF has been used for refractive index (RI) measurement [23]. In this work, we are trying to realize a liquid level sensing based on this kind of LPG on POF. The corrugated surface LPG was fabricated by a simple mechanical die-press-print method, and the liquid level sensor which is based on the LPG was proposed. The influence of the POF diameter, structure parameters of the LPG, the RI of liquid, and hysteresis of the sensor over liquid level measurement was investigated. In addition, the effects of the water absorption are also studied.

Section snippets

Fabrication

The commercial step-index POFs with different diameters (Super Eska) were used to fabricate the LPGs. The parameters of the POFs are listed in Table 1.

The corrugated surface LPG was fabricated by a mechanical die-press-print method, as shown in Fig. 1(a). The POF was firstly fixed on a sample holding platform, then a metal mould with a periodic V-groove structure was pressed on the POF in the way that the mould was perpendicular to the fiber axial. Keeping the state for a few seconds, the

Experiment and discussions

The schematic of the experimental setup for liquid level measurement is shown in Fig. 4. The LPG was vertically placed into the liquid and the liquid level before the first V-groove was set as the position of zero level. One end of the LPG was connected with a laser source (TLS001–635, Thorlabs) with a wavelength of 635 nm and a launched power of 2 mW, and the other end was connected with a photodiode (S120C, Thorlabs) with a responsivity of 0.41 A/W and a resolution of 1nW. The liquid level

Conclusion

In this paper, a corrugated surface LPG imprinted on a commercial POF was proposed as a liquid level sensor. The LPG was fabricated on POF by a simple mechanical die-press-print method. The operation principle of the sensor was described and analyzed by the method of geometrical optics. The liquid level sensing performance for the LPG based sensor with different structural parameters was evaluated. The experimental results show that the diameter of POF and the structure parameters of LPG could

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by National Natural Science Foundation of China (NSFC) (Grant #61377058).

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