Level measurement for high-RI liquids based on the self-imaging effect in no-core fibers
Introduction
Liquid level measurement leads to lots of significant applications in the field of biochemical industries, public water supplies and fuel storage monitors. In special, it is urgent to measure oil consumption in engine (e.g. Oil appears in the cylinder, the valve guide rod, the crankcase or the turbocharger impeller shaft). Several methods such as level-weighing method, tracer element test method and sulfur element balance method have encountered many limits because of expensive equipment, large deviation of test accuracy or tedious measurement process. For instance, by weighting oil consumption in engine can provide another way to achieve level measurement. Although electrical level measurement based on an energy-trapping phenomenon in piezoelectric thickness-mode vibrators has been reported they can hardly be used as the measurand like oil is inflammable [1].
By contrast, level measurement based optical fibers presents inherent advantages such as miniature size, immunity to electromagnetic interference, remote sensing capability and high resistance to corrosion, making them suitable for various applications. In special, these well-established advantages (e.g. etched fiber Bragg gratings (FBGs) [2], long period gratings (LPGs) [3] and etched D- shaped fibers [4]) have been used to measure the liquid level. However, the etched FBGs often requiring extra chemical procedure make them more fragile while the LPGs suffer from the wide spectral bandwidth and high cross sensitivity to temperature. In recent years, level measurement based on multimode interference (MMI) effect has aroused various attentions [5], [6], [7], [8]. Most level measurements have been devoted to the fluid with the surrounding refractive index (RI) lower than that of silica ( 1.45) [9], [10], [11], [12], [13], [14]. Therefore, they are not applicable to the fluid with a higher RI like oil.
In this article, we propose and demonstrate a level sensor based on MMI effects that can be used to measure the level for both high-RI and low-RI liquids. The MMF employed in this experiment is a no-core fiber (NCF), which has a diameter of with a RI of 1.444 at m. When immersed in liquid with RI () lower than that of the NCF (), the level of the liquid can be correlated with the laser spectral shift with a maximum sensitivity of 0.457 nm/mm. For liquids ( > ), as the liquid level increases, the transmission at the peak wavelength has a linear response of −189.5 W/mm, corresponding to a level resolution of in the range of 1 mm to 5 mm. As a result, our proposed level sensor provides a real-time and highly sensitive approach to acquire the level measurement of oil.
Section snippets
Operation principle and numerical simulation
The proposed level sensor is comprised of a section of NCF with a precise length sandwiched between two single mode fibers (SMFs), which is defined as SNS. Thus, the NCF acts as new core and the external surrounding is now the new cladding. Owing to mode field mismatch, a series of high-order modes can be excited, as a result, self-imaging effects occur that the input field profile is reproduced in single or multiple images at periodic intervals along the propagation direction of the guide.
Experimental results
Firstly, the SNS structure in response to low RI liquid level measurement is schematically shown in Fig. 3. A section of erbium-doped fiber (EDF) with a length of 1.4 m which is pumped by a 980 nm laser diode (LD) with a power of 250 mw through a common wavelength-division-multiplexer (WDM) has been used as the gain medium. A 10/90 fiber optical coupler (OC) is used to tap out the laser light for wavelength monitoring. The 10% output port of the coupler is connected to an optical spectrum
Conclusion
In conclusion, we have comprehensively investigated the level measurement of the SNS structure. The sensor has approximately maximum responses of 0.475 nm/mm for the low-RI liquids. On the face of the level measurement for high-RI liquids, the way using the ASE source serves to the higher resolution (0.), while it is more practical for the latter method to monitor level change. Moreover, the high level resolution of can be applied in the high-RI surrounding medium like oil.
Funding
National Key Research and Development Project of China (2017YFC0306102), Natural Science Foundation of Guangdong Province(2016A030310305), National Postdoctoral Program for Innovative Talents (BX201600077, 2017M611877), the Outstanding Chinese and Foreign Youth Exchange Program of China Association of Science and Technology, the Jiangsu Association of science and Technology Youth Science and technology talent promotion project, the 1311 Talents Program of Nanjing University of Posts and
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.
References (22)
- et al.
Transmission characteristics of SMS fiber optic sensor structures
Opt. Commun.
(2003) - et al.
Self-imaging effect in liquid-filled hollow-core capillary waveguide for sensing applications
Sensors (Basel)
(2019) - et al.
Design and demonstration of a liquid level fiber sensor based on self-imaging effect
Sensors Actuators A
(2016) - et al.
Intensity modulated refractive index sensor based on optical fiber Michelson interferometer
Sensors Actuators B
(2015) - K. Yamada, S. Horiuchi, H. Honda, T. Kinai, A novel approach in liquid-level sensing by trapped-energy-mode thickness...
- et al.
Highly sensitive liquid-level sensor based on etched fiber Bragg grating
IEEE Photon. Technol. Lett.
(2007) - et al.
Implementation and characterization of liquid- level sensor based on a long-period fiber grating Mach- Zehnder interferometer
IEEE Sens. J.
(2011) - et al.
An optical liquid-level sensor based on D-shape fiber modal interferometer
IEEE Photon. Technol. Lett.
(2017) - et al.
All-fiber multimode-interference-based refractometer sensor: proposal and design
Opt. Lett.
(2006) - et al.
Tunable multimode- interference bandpass fiber filter
Opt. Lett.
(2010)
Fiber-optic sensor for liquid level measurement
Opt. Lett.
Cited by (6)
Simultaneous measurement of liquid level and refractive index based on a sandwich multimode optical fiber structure
2024, Optics and Laser TechnologyA liquid level optical fiber sensor with refractive index compensation through cascading double multi-mode interferences
2022, Infrared Physics and TechnologyCitation Excerpt :Researches showed that, the Li-Le sensors based on the MMI in the NCF were advantaged with relatively high sensitivity and simple structure. However, the liquid RI could interfere the Li-Le detection which were reported in ref. [39,41]. Thus, in order to get more accurate detection results, the RI compensation is necessary when measuring the Li-Le.
An optical fiber sensor for the detections of liquid level and strain through cascading Sagnac interference and modal interference
2022, Infrared Physics and TechnologyCitation Excerpt :Moreover, fiber liquid level sensors were easily influenced by external factors, such as the strain, refractive index, and temperature which could generate inaccurate detecting results. Compared with liquid level sensors based on the FBG and single mode fiber, the sensitivities of liquid level sensors based on the no core fiber were higher because of the coreless structure of no core fiber [12,16]. Moreover, in previous works, the influences of refractive index on the sensitivities of fiber liquid level sensors were investigated.
Second-order temporal Lau effect based on optical field intensity correlation
2021, Guangdianzi Jiguang/Journal of Optoelectronics Laser