Air pressure measurement of circular thin plate using optical fiber multimode interferometer
Introduction
Optical fiber based pressure sensors have gained good acceptance due to their superior performance characteristics such as small size, immunity to electromagnetic interference and corrosion resistance. Various optical fiber sensors have been reported and employed for pressure measurement based on different mechanisms [1]. Optical fiber gratings are capable to measure pressure, such as normal FBG [2], chirped fiber grating [3] and long-period fiber grating (LPG) [4]. By adopting the wavelength difference between a pair of FBGs, Zhao et al. [2] developed a pressure sensor on the basis of diaphragm-cantilever with an accuracy of 258.28 pm/MPa. The tapered LPGs have been proven to monitor pressure in [4], where, this device exhibited a pressure sensitivity of 51 pm/MPa. However, the fabrication of grating sensors requires a complex process or expensive masks, and the length of the chirped grating and LPG is relatively long.
Optical pressure sensors based on concepts of special fibers are proposed, for instance, high-birefringence (Hi-Bi) fiber sensor [5] and photonic crystal fiber (PCF) sensor [6] are also good choices for pressure measurement. Chen et al. [5] reported a novel grating sensor inscribed on a Hi-Bi optical fiber, which can measure air pressure and temperature simultaneously. However, it requires a special fiber and complex drawing technique, which further increases the production cost. Since the PCF has the unique advantage of low dependence on temperature, PCF based pressure sensor has also been demonstrated in [6], but the cost of PCFs is expensive. Some structures fabricated based on multiple beam interference have been reported [7], [8]. The Fabry-Perot (FP) interferometric sensors have been also investigated in [9], [10], and the pressure sensitivities were obtained at − 1.598 nm/kPa [9] and 1.336 nm/kPa [10], respectively. However, the configuration of FP sensor is relatively complex. Compared with the above sensors, multimode interference (MMI) based SMS sensors offer the advantages of low cost, simple structure, extreme flexibility and easy fabrication.
At present, the MMI occurs in the SMS structure has been investigated and developed to act as various sensors [11], such as temperature [12], refractive index (RI) [13], strain [14], curvature [15], etc. A sensitivity-enhanced strain gauge based on the model interference of a step-index fiber has been described, and its sensitivity is twice that of normal FBG [14]. Huang et al. [16] developed an SMS fiber structure based on the polymer multimode fiber (MMF), which the large detection range can reach 20000με and the sensitivity is − 1.72 pm/με. However, due to the high thermal expansion coefficient and thermo-optic coefficient of the polymer fiber, it has a relatively high temperature-strain crosstalk of 33 με/°C. Gong et al. [15] proposed a curvature sensor by measuring the wavelength and intensity of the interference notch in its transmitted spectrum, and the maximum sensitivities are about − 10.38 nm/m−1 and 130.37 dB/m−1 respectively. In addition, the novel curvature sensors using no-core fiber (NCF) [17], [18], three-core fiber [19], and seven-core fiber [20] instead of the traditional MMF section have been reported. Their curvature sensitivities can be further improved.
There are few researchers focusing on measuring air pressure using SMS structure. May-Arrioja et al. [21] investigated a pressure sensor based on polydimethilsiloxane (PDMS) polymer and NCF. The NCF directly contacted with PDMS layer which was previously attached to pressure sensitive membrane. The measurement sensitivity is − 0.145 × 10−3 mW/kPa in the pressure range of 0–960 kPa. Although this sensor has been demonstrated to be feasible for measuring pressure, the sensitivity and resolution of pressure measurement are small. Besides, the thermal optics and thermal expansion effects of the polymer material lead to a strong dependence on temperature. The previous investigations have shown that the output spectrum of an SMS fiber structure depends on the length, the effective RI and the bending radius of the MMF section. The strain and bending applied to the MMF changes the length of the MMF and effective RI distribution along with MMF [22], [23], respectively. This induces the transmission loss and wavelength shift of the spectrum of the SMS fiber structure. By utilizing the characteristics that the SMS fiber structure is highly sensitive to bending, this paper provides a solution to pressure measurement.
In this paper, an SMS sensor is used to measure the air pressure of a small-size circular thin plate. This paper is organized as follows: in Section 2, the principle of strain and curvature measurement of sensors based on multimode interference is illustrated; in Section 3, the design parameters of the sensor are determined by numerical analysis and finite element method (FEM); in Section 4, the construction of the test system is introduced, which is used to measure the transmission of sensors spectrum under different air pressure; and finally, conclusions of this paper are summarized in Section 5.
Section snippets
Sensing principle
A traditional SMS fiber structure is fabricated by splicing an MMF segment between two segments of singlemode fiber (SMF) [12], as shown in Fig. 1. The MMF section has a step-index profile. The high-order eigenmodes are excited in the MMF section when the input light field that propagating along the SMF coupled into the MMF. Then the interference between different modes occurs in the MMF, and recoupled into the output SMF [24]. Since the transmission spectrum of the SMS sensor depends on the
Theory of bending and strain of thin plate
The schematic diagram of the thin plate configuration is shown in Fig. 4. In Fig. 4 (a), the red dotted box shows the cutaway picture of the structure. It consists of a thin plate, a pressure casing, fixed screw thread, pressure chamber, and SMS sensor, their geometrical dimensions are annotated in Fig. 4 (a). In order to give a universal internal screw thread application for the air inlet, the fixed M 20 × 1.5 screw thread was cut outside of the pressure chamber. The SMS sensor with a length
Air pressure experimental setup
The entire air pressure measurement system is presented in Fig. 12, all measurements were carried out under the condition of a stabilized room temperature. In the experiment, the SMS sensor was uniformly adhered to the surface of the thin plate using ethyl cyanoacrylate adhesive (LOCTITE-401), which is curable at room temperature. The sensor was located on an axis across the center of the circular thin plate. Thus, the SMS sensor bend synchronously with the thin plate as air pressure is applied
Conclusions
This paper reports the measurement of the pressure of circular thin plate by an SMS optical fiber sensor, and the theoretical analysis and experimental verification are carried out. The bend applied to the MMF can effectively cause significant change of the output transmission power of the SMS fiber sensor. The sensor works on the basis of monitoring the wavelength and intensity changes of the SMS fiber sensor. The experimental results show that it is feasible to track the light intensity and
CRediT authorship contribution statement
Yufang Bai: Writing - original draft. Jie Zeng: Funding acquisition, Project administration. Jiwei Huang: Data curation. Zhenfeng Yan: Software. Yaxing Wu: . Kaiyu Li: Methodology. Qiang Wu: Writing - review & editing. Dakai Liang: Supervision.
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.
Acknowledgments
This research was supported by the Fund of Aeronautics Science (Grant Nos.: 20170252004, 20185644006, 20200009023017) and the Key Research and Development Plan of Jiangsu Province-Industrial Foresight and Common Key Technology (Competition Project) (Grant No.: BE2018047).
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