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Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer
Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer Wenhua Wang a,n, Fang Li b a b
School of Science, Guangdong Ocean University, Zhanjiang 524088, China School of Physics and Optoelectronic Engineering, Dalian University of Technology, Dalian 116024, China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 January 2013 Received in revised form 12 June 2013 Accepted 13 June 2013
We propose an efficient approach to develop large-range liquid level sensors based on an extrinsic Fabry– Perot optical fibre interferometer with an all fused-silica structure and CO2 laser heating fusion bonding technology. The sensor exhibits signatures of a high sensitivity of 5.3 nm/kPa (36.6 nm/psi), a resolution of 6.8 Pa (9.9 10−4 psi) and an extreme low temperature dependence of 0.013 nm/1C. As a result, a high resolution of the water level measurement of approximately 0.7 mm on the length scale of 5 m and small errors of the water pressure measurement induced by the temperature dependence within 0.0025 kPa/1C (0.00036 psi/1C, water level 0.25 mm/1C) are achieved, thus providing useful applications for the detection of the large-range liquid level in harsh environments. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Optical fibre sensor Extrinsic Fabry–Perot interferometer Large-range liquid level measurement Laser heating fusion bonding Temperature cross-sensitivity
1. Introduction In modern industry and daily life, liquid level measurements play an important role in applications such as oil tanks, treatment plants, gasoline stations, fuel reservoirs for transport systems, and public water supplies. However, the traditional electric sensors cannot satisfy the demands of liquid level measurements in harsh environments, for example, conductive, explosive, flammable, and erosive environments. Recently, optical fibre sensors have been particularly attractive in these applications due to the extensively known intrinsic benefits such as immunity to electromagnetic interfaces and high sensitivity, which make them better for transducers than their electrical counterpart. In recent years, many optical fibre pressure sensors for liquid level measurements have been proposed. The optical fibre is the only section of the detection device immersed into the liquid tank (without an electrical section), and the optical fibre technology for liquid level measurements is safe and corrosion resistant. Liquid level monitoring sensor systems using fibre Bragg gratings (FBG) embedded in a cantilever rod were presented in Ref. [1]. The shift of the Bragg wavelength induced by the contraction or the expansion of the FBG due to the liquid level change is converted to an intensity signal corresponding to the liquid level. A high sensitivity differential
n
Corresponding author. Tel.: +86 1376 3085259. E-mail addresses: [email protected], [email protected] (W. Wang).
pressure sensor based on two FBGs for the detection of the liquid level or the liquid density was described in Ref. [2]. A non-intrusive method to measure liquid level variation with only a few centimetres of dynamic range was also proposed in Ref. [3]. An application incorporating a FBG and a Fabry–Perot (FP) pressure sensor to measure the liquid level and the specific gravity was introduced in Ref. [4], but these sensors had a measurement scale of only tens of centimetres. Thus, these sensors are characterised by a small-range measurement despite good sensitivities. Ref. [5] presented a fibre-optic liquid level sensor employing a long-period grating (LPG). However, the LPGs show a large temperature dependence, which results in a significant wavelength shift with the temperature change. Extrinsic Fabry–Perot interferometric (EFPI) sensors are one of the main optical fibre point sensors [6], and diaphragm-based EFPI (DEFPI) sensors are one of the best candidates among all optical fibre sensors for pressure measurements. DEFPI pressure sensors can be used to measure the liquid level variation and have attracted considerable research interest in many applications. Moreover, a variety of sensors based on polymer membranes [7], SiC [8], photoresist ferrules [9], and MEMS [10] have been proposed. The large coefficient of thermal expansion (CTE) mismatch will induce severe stress between different materials of the sensor head, which degrades the sensor performance or even leads to a failure of the sensor. To satisfy practical application requirements, Wang's research group has developed the CO2 laser heating fusion bonding technology to fabricate EFPI sensor [11–13], and the fabrication technology has improved sensor's performance. However, the fabrication processes are relatively complicated [12,13].
0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2013.06.009
Please cite this article as: Wang W, Li F. Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.optlaseng.2013.06.009i
2
W. Wang, F. Li / Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Refs. [11,12] do not solve the problem of the trapped air; that is, the thermal expansion of the air trapped by the FP cavity induces an undesirable pressure on the inner surface of diaphragm with an increase of the environment temperature, leading to a strong temperature dependence of the sensor. In this paper, a high performance DEFPI liquid level sensor based on an all fused-silica structure for large-range measurements is developed and demonstrated. The DEFPI liquid level sensor combines the optical fibre technology and the CO2 laser heating fusion bonding technology to realise liquid level measurements in harsh environments, and the FP cavity is not hermetically sealed during the fabrication of the sensor by laser heating fusion bonding, thus solving the problem of the trapped air in the FP cavity. Therefore, in addition to high sensitivity, high accuracy and large-range measurement, the thermal expansion of the air in FP cavity will not produce an undesirable pressure on the inner surface of diaphragm when the temperature of the environment increases, thus significantly reducing the measurement error of the liquid level due to temperature cross-sensitivity (a key problem of the DEFPI sensors). Because no chemical processes are involved, the fabrication of the DEFPI sensor is simple, costefficient and environmentally friendly.
2. Operation principle and sensor design Fig. 1 depicts the basic structure of the DEFPI pressure sensor for liquid level measurement with a lead-in silica single mode optical fibre (SMF) – SM-28, a fused-silica ferrule with a length of 7 mm and an outer diameter of 1.8 mm, and an ultra-thin fusedsilica diaphragm with a thickness of 30 μm. There is a through hole with a diameter of 127 μm in the axis of the fused-silica ferrule, and one end of the fused-silica ferrule has a taper-shape cup with an inside diameter of 1 mm and a deep pit of 2 mm, as formed during preparation. As shown in Fig. 1, the air gap between the end face of the lead-in silica optical fibre and the inner surface of the silica diaphragm forms the FP cavity of the DEFPI sensor. The light signal couples into the silica lead-in optical fibre and then propagates along the optical fibre. Finally, it is partially (approximately 4%) reflected by the fibre end face and the inner surface of the diaphragm due to Fresnel reflection at the glass-air interface. The beam reflected by the diaphragm couples into the lead-in optical fibre, then propagates back along it and produces interference fringes of the cosine intensity variations with the beam reflected by the end face of the optical fibre. When the DEFPI sensor is immersed into a liquid, the liquid pressure is applied to the diaphragm. The FP cavity length will vary with the deflection of the diaphragm as a result of the liquid pressure change with the liquid level change. Therefore, the interference fringes modulated by the liquid pressure are determined by the demodulation of the FP cavity length. Accordingly, the liquid pressure can be derived inversely. For a clamped, rigidly round diaphragm of uniform thickness, the out-of-plane deflection
of the centre point of the diaphragm resulting from the change of the liquid pressure can be expressed as [14] Y¼
3ð1−μ2 ÞP 16Eh
3
ða2 −r 2 Þ2
ð1Þ
where μ and E are Poisson's ratio and Young's modulus of the diaphragm material, respectively, a is the inside radius of the ferrule taper-shape cup, r is the radial distance to the diaphragm centre, and h is the diaphragm thickness. For the liquid pressure measurement of the DEFPI sensor, the key problem is that the temperature cross-sensitivity influences the measurement accuracy of the senor. In spite of the all-fusedsilica structure of the DEFPI sensor, the thermal expansion coefficient of the SMF (5.6 10−7 1C−1) is not equal to those of the ferrule and the diaphragm (5.5 10−7 1C−1) because of germanium doping in the optical fibre core. Generally, the greater the germanium doping in the optical fibre, the larger the thermal expansion coefficient of the optical fibre is. The difference between the thermal expansion coefficients of the materials will generate the temperature pressure cross-sensitivity (causing the temperature dependence) of the DEFPI sensor, which increases the measurement error of the liquid pressure. Furthermore, if the FP cavity is hermetically sealed, the air trapped by FP cavity will lead to a larger temperature dependence due to the air pressure at different temperatures. The thermal expansion of the trapped air in the FP cavity induces the unwanted pressure on the surface of the diaphragm when the temperature of the environment increases, which increases the measurement error of the liquid pressure significantly. As illustrated in Fig. 1, Ls is the length from the SMF end face to the fibre heating fusion bonding point, and L is the length of the FP cavity. The total temperature dependence of the DEFPI sensor is given by SP e ΔL ¼ αf ðLs þ LÞ þ ð2Þ −αs Ls ΔT Te where αf and αs are the thermal expansion coefficients of the fused-silica ferrule and the SMF, respectively, S is the pressure sensitivity of the sensor, and Pe is the pressure of the trapped air at the temperature of the environment Te. The thermal expansion coefficient of the optical fibre is very close to that of the fusedsilica ferrule; therefore, the second term on the right hand side of the equation is the primary factor of the temperature dependence. In addition, the thermal expansion of the trapped air in the FP cavity is the greatest source of measurement error of the liquid pressure. The value of the second term of Eq. (2) is zero if the FP cavity is not thoroughly sealed and is connected to the ambient environment. We designed a vent hole between the SMF and the ferrule, as shown in Fig. 1, to ensure that the FP cavity is open to reduce the temperature dependence of the DEFPI sensor. The vent hole formed during the heating fusion bonding connects the FP cavity to the ambient environment. Consequently, the DEFPI sensor has an extremely low temperature dependence.
3. Sensor fabrication
Fig. 1. Configuration of a DEFPI liquid level sensor.
The fabrication process of the DEFPI sensor involves only CO2 laser heating fusion bonding, and the CO2 laser is a GEM-60 (Coherent Inc.). The schematic diagram of the CO2 laser heating fusion bonding is shown in Fig. 2. First, an ultra-thin fused-silica diaphragm with a thickness of 30 μm is directly welded onto the taper-shape cup end face of the ferrule via the CO2 laser. The diaphragm is locally and quickly heated by the CO2 laser at the edge; consequently, its surface near the centre is not influenced. The microscopic image of bonding between the diaphragm and the ferrule is illustrated in Fig. 3. Second, the lead-in silica SMF
Please cite this article as: Wang W, Li F. Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.optlaseng.2013.06.009i
W. Wang, F. Li / Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎
(SM-28) connected to a sm125 Optical Sensing Interrogator (Micron Optics Inc.) is cleaved and inserted into the through hole of the ferrule fixed on a V-groove to form a FP cavity between the optical fibre end face and the inner surface of the diaphragm. The length of the FP cavity is pre-adjusted to approximately 84 μm by a three-dimensional translation stage holding the fibre tail and monitored in real-time by sm125. Then, the fibre is bonded with the inside sidewall of the ferrule with a CO2 laser. The setup is the same as in Fig. 2 except for the holder of the ferrule, as shown Fig. 4(a). Fig. 4(b) illustrates the microscopic image of the bonding between the fibre and the inner sidewall of the ferrule. Finally, the welded DEFPI sensor is annealed with an out-of-focus laser with 2 mm of defocusing. Fig. 4(b) shows that the heating fusion bonding leaves a vent hole, that is, the FP cavity is not hermetically sealed. The
Fig. 2. Schematic diagram of a CO2 laser heating fusion bonding system.
3
fabrication process is epoxy-free; therefore, the sensors are very reliable and can tolerate extremely high temperature close the operational limit of the fibre. Compared with the previous fabrication technologies researched, the fabrication process with laser heating fusion bonding is simple, clean, environmentally friendly and inexpensive.
4. Experimental results and discussion The structure parameters of the sensor head is 0.5 mm for the ferrule taper-shape cup inside radius and 30 μm for the diaphragm thickness; therefore, the theoretical value of the pressure sensitivity of the sensor is 5.77 nm/kPa (39.78 nm/psi) according to Eq. (1). Fig. 5 shows the schematic diagram of the liquid level test setup. The light from a commercial interrogator (Micron Optics Inc. Model: sm125) propagates to the FP cavity of the DEFPI sensor along the lead-in optical fibre, and then its interference signal shown in Fig. 6 is routed back to the detector built in the sm125 along the same optical fibre. The commercial interrogator that is composed of a spectrometer with a tunable laser light source and a detector is a type of programmable interrogator, and the laser light source has a spectral range of 1510–1590 nm with a 1 pm accuracy. A cross-correlation signal processing method is used to process the interference signal to demodulate the FP cavity length, L. The DEFPI sensor head after packaging is submerged in water, and the pressure applied on diaphragm is proportional to the water level. The length L corresponding to the different water levels is calculated by the crosscorrelation algorithm based on Labview according to the FP cavity reflection interference fringes. After filtering the direct current term,
Fig. 3. Taper-shape cup end face of a ferrule (a) before and (b) after welded by use of a CO2 laser.
Fig. 4. CO2 laser heating fusion bonding between the fibre and the inner sidewall of the ferrule.
Please cite this article as: Wang W, Li F. Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.optlaseng.2013.06.009i
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W. Wang, F. Li / Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 5. Setup for liquid level test of a DEFPI sensor based on Labview platform.
Fig. 7. Response of the FP cavity length versus the liquid pressure.
Fig. 6. Interference fringes achieved by a sm125 interrogator.
the optical intensity of the FP cavity interference spectrum is expressed as 4πL þπ ð3Þ IðλÞ ¼ 2I s ðλÞγ cos λ where Is(λ) is the spectrum intensity distribution of the optical source, γ is the contrast of the interference fringe, and π is the additional phase of the half-wave loss. The absolute FP cavity length, L, can be determined by calculating the cross-correlation coefficient. The cross-correlation coefficient of a given L is evaluated by N 4πL CðLÞ ¼ ∑ xðnÞ cos þπ ð4Þ λn i¼1 where x(n) is the spectrum intensity data sequence acquired by the interrogator, λ(n) is the wavelength corresponding to element x(n), and N is the total number of the intensity data sequence. For a given series of acquired data, the effective FP cavity length corresponds to the L value where the cross-correlation coefficient C(L) is maximal. Fig. 7 shows the pressure response curves of the DEFPI sensor from 0 to 50 kPa corresponding to the water level from 0 to 5 m at
room temperature, and (b) is an enlarged figure of the blue dashed rectangle in (a). The pressure sensitivity is approximately 5.3 nm/ kPa (36.6 nm/psi) according to the change of the cavity length, and the repeatability of the DEFPI liquid level sensor is good within 50 kPa. The good repeatability and the standard deviation shown in Fig. 8 indicate that the DEFPI liquid level sensor has high measurement accuracy. Fig. 8 gives the measurement result of the cavity length in response to the pressure resolution, and the standard deviation is approximately 0.018 nm; that is, the resolution of the cavity length is 0.036 nm, and the pressure resolution is approximately 6.8 Pa (9.9 10−4 psi). Therefore, the resolution of the DEFPI liquid level sensor is 0.7 mm (water). However, the pressure response in Fig. 7 is nonlinear, and the fitting function is written as P ¼ 3:3 108 −1:17 104 L þ 0:14L2 −5:56 10−7 L3
ð5Þ
which does not agree with Eq. (1) for the following reasons. (a) The silica lead-in fibre is not aimed specifically at the central position of the diaphragm (r≠0 in Eq. (1)). In addition, the value of r increases gradually with the increase of the water pressure. When the liquid pressure is greater, the change of the r value with the liquid pressure measurement can be demonstrated in the liquid pressure test. (b) The residual stress inside the diaphragm during heating fusion bonding easily causes the ultra-thin diaphragm to deform; therefore, the diaphragm is not an ideal plane. For a diaphragm of an ideal plane, the Eq. (1) should be rewritten as P¼
16Eh
3
3ð1−μÞ2 a4
Yþ
ð7−μÞh 3 Y 3ð1−μÞa4
ð6Þ
Please cite this article as: Wang W, Li F. Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.optlaseng.2013.06.009i
W. Wang, F. Li / Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 8. Experimental results of the DEFPI sensor resolution (about 5 min time span).
The temperature dependence of the DEFPI liquid level sensor should be very low according to the discussion in Section 2. The thermal expansion coefficients of the SMF and the fusedsilica ferrule are 5.6 10−7 1C−1 and 5.5 10−7 1C−1, respectively. In addition, in eq. (2), the value of the second term is zero as a result of the FP cavity without a thorough seal. Therefore, for a value of Ls of approximately 3.1 mm and a value of L of 84 μm, the theoretical value of the temperature dependence of the DEFPI sensor is approximately 0.015 nm/1C. Its temperature dependence was tested after the sensor welded by heating fusion bonding technology was annealed. The test setup is illustrated in Fig. 5 except for the sensor head. The sensor head was inserted into a temperature control case together with a platinum resistance thermometer (Hart Scientific 1502A, 70.012 1C). The measurement results shown in Fig. 9 indicate that the temperature dependence is 0.013 nm/1C from room temperature to 77 1C. The temperature dependence of the DEFPI sensor is consistent with the theoretical value. Therefore, the liquid pressure measurement error induced by the temperature dependence is approximately 0.0025 kPa/1C (0.00036 psi/1C); that is, the water level measurement error is below 0.25 mm/1C. 5. Conclusions In conclusion, a large-range continuous liquid level measurement optical fibre DEFPI sensor incorporating both an all fusedsilica structure and CO2 laser heating fusion bonding technology was developed. The FP cavity without a seal solves the problem of the thermal expansion of the trapped air in the FP cavity, guaranteeing that the DEFPI liquid level sensor possesses an extremely low temperature dependence of 0.013 nm/1C. A high sensitivity of 5.3 nm/kPa (36.6 nm/psi) and a resolution of 6.8 Pa (9.9 10−4 psi) were obtained. Therefore, the resolution of the water level measurement is approximately 0.7 mm at the length scale of 5 m. In addition, the measurement error of the water pressure induced by the temperature dependence is limited
5
Fig. 9. Temperature dependence of a DEFPI liquid level sensor.
within 0.0025 kPa/1C (0.00036 psi/1C), corresponding to the water level measurement error of 0.25 mm/1C. The DEFPI sensor has significant potential for large-range liquid level measurements in applications such as oil reservoirs.
References [1] Sohn KR, Shim JH. Liquid-level monitoring sensor systems using fiber Bragg grating embedded in cantilever. Sens Actuator A—Phys 2009;152:248–51. [2] Sheng HJ, Liu WF, Lin KR, Bor SS, Fu MY. High-sensitivity temperatureindependent differential pressure sensor using fiber Bragg gratings. Opt Express 2008;16:16013–8. [3] Singh HK, Chakroborty SK, Talukdar H, Singh NM, Bezboruah T. A New nonintrusive optical technique to measure transparent liquid level and volume. IEEE Sens J 2011;11:391–8. [4] Lai CW, Lo YL, Yur JP, Chuang CH. Application of Fibre bragg grating level sensor and Fabry–Perot pressure sensor to simultaneous measurement of liquid level and specific gravity. IEEE Sens J 2012;12:827–31. [5] Khaliq S, James SW, Tatam RP. Fiber-opitc liquid-level sensor using a longperiod grating. Opt Lett 2001;26:1224–6. [6] Bhatia V, Murphy KA, Claus RO, Tran TA, Greene JA. Recent developments in optical-fiber-based extrinsic Fabry–Perot interferometric strain sensing technology. Smart Mater Struct 1995;4:246–51. [7] Cibula E, Donlagic D. Miniature fiber-optic pressure sensor with a polymer diaphragm. Appl Opt 2005;44:2736–44. [8] Pulliam W. Micromachined, SiC fiber optic pressure sensors for high temperature aerospace applications. Proc SPIE 2000;4202:21–30. [9] Bae H, Zhang XM, Liu H, Yu M. Miniature surface-mountable Fabry–Perot pressure sensor constructed with a 45 degrees angled fiber. Opt Lett 2010;35:1701–3. [10] Wang X, Li B, Xiao Z, Lee S, Roman H, Russo OL, et al. An ultra-sensitive optical MEMS sensor for partial discharge detection. J Micromech and Microeng 2005;15:521–7. [11] Xu J, Wang X, Cooper KL, Wang A. Miniature all-silica fiber optic pressure and acoustic sensors. Opt Lett 2005;30:3269–71. [12] Xu J, Pickrell GR, Wang X, Yu B, Cooper KL, Wang A. Vacuum-sealed high temperature high bandwidth fiber optic pressure and acoustic sensors. Proc SPIE 2005;5998:5998091–6. [13] Han M, Wang X, Xu J, Cooper KL, Wang A. Diaphragm-based extrinsic Fabry– Perot interferometric optical fiber sensor for acoustic wave detection under high background pressure. Opt Eng 2005;44:0605061–2. [14] Giovanni MD. Flat and Corrugated Diaphragm Design Handbook. New York: Mercel Dekker; 1982.
Please cite this article as: Wang W, Li F. Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.optlaseng.2013.06.009i
Contents lists available at SciVerse ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer Wenhua Wang a,n, Fang Li b a b
School of Science, Guangdong Ocean University, Zhanjiang 524088, China School of Physics and Optoelectronic Engineering, Dalian University of Technology, Dalian 116024, China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 January 2013 Received in revised form 12 June 2013 Accepted 13 June 2013
We propose an efficient approach to develop large-range liquid level sensors based on an extrinsic Fabry– Perot optical fibre interferometer with an all fused-silica structure and CO2 laser heating fusion bonding technology. The sensor exhibits signatures of a high sensitivity of 5.3 nm/kPa (36.6 nm/psi), a resolution of 6.8 Pa (9.9 10−4 psi) and an extreme low temperature dependence of 0.013 nm/1C. As a result, a high resolution of the water level measurement of approximately 0.7 mm on the length scale of 5 m and small errors of the water pressure measurement induced by the temperature dependence within 0.0025 kPa/1C (0.00036 psi/1C, water level 0.25 mm/1C) are achieved, thus providing useful applications for the detection of the large-range liquid level in harsh environments. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Optical fibre sensor Extrinsic Fabry–Perot interferometer Large-range liquid level measurement Laser heating fusion bonding Temperature cross-sensitivity
1. Introduction In modern industry and daily life, liquid level measurements play an important role in applications such as oil tanks, treatment plants, gasoline stations, fuel reservoirs for transport systems, and public water supplies. However, the traditional electric sensors cannot satisfy the demands of liquid level measurements in harsh environments, for example, conductive, explosive, flammable, and erosive environments. Recently, optical fibre sensors have been particularly attractive in these applications due to the extensively known intrinsic benefits such as immunity to electromagnetic interfaces and high sensitivity, which make them better for transducers than their electrical counterpart. In recent years, many optical fibre pressure sensors for liquid level measurements have been proposed. The optical fibre is the only section of the detection device immersed into the liquid tank (without an electrical section), and the optical fibre technology for liquid level measurements is safe and corrosion resistant. Liquid level monitoring sensor systems using fibre Bragg gratings (FBG) embedded in a cantilever rod were presented in Ref. [1]. The shift of the Bragg wavelength induced by the contraction or the expansion of the FBG due to the liquid level change is converted to an intensity signal corresponding to the liquid level. A high sensitivity differential
n
Corresponding author. Tel.: +86 1376 3085259. E-mail addresses: [email protected], [email protected] (W. Wang).
pressure sensor based on two FBGs for the detection of the liquid level or the liquid density was described in Ref. [2]. A non-intrusive method to measure liquid level variation with only a few centimetres of dynamic range was also proposed in Ref. [3]. An application incorporating a FBG and a Fabry–Perot (FP) pressure sensor to measure the liquid level and the specific gravity was introduced in Ref. [4], but these sensors had a measurement scale of only tens of centimetres. Thus, these sensors are characterised by a small-range measurement despite good sensitivities. Ref. [5] presented a fibre-optic liquid level sensor employing a long-period grating (LPG). However, the LPGs show a large temperature dependence, which results in a significant wavelength shift with the temperature change. Extrinsic Fabry–Perot interferometric (EFPI) sensors are one of the main optical fibre point sensors [6], and diaphragm-based EFPI (DEFPI) sensors are one of the best candidates among all optical fibre sensors for pressure measurements. DEFPI pressure sensors can be used to measure the liquid level variation and have attracted considerable research interest in many applications. Moreover, a variety of sensors based on polymer membranes [7], SiC [8], photoresist ferrules [9], and MEMS [10] have been proposed. The large coefficient of thermal expansion (CTE) mismatch will induce severe stress between different materials of the sensor head, which degrades the sensor performance or even leads to a failure of the sensor. To satisfy practical application requirements, Wang's research group has developed the CO2 laser heating fusion bonding technology to fabricate EFPI sensor [11–13], and the fabrication technology has improved sensor's performance. However, the fabrication processes are relatively complicated [12,13].
0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2013.06.009
Please cite this article as: Wang W, Li F. Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.optlaseng.2013.06.009i
2
W. Wang, F. Li / Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Refs. [11,12] do not solve the problem of the trapped air; that is, the thermal expansion of the air trapped by the FP cavity induces an undesirable pressure on the inner surface of diaphragm with an increase of the environment temperature, leading to a strong temperature dependence of the sensor. In this paper, a high performance DEFPI liquid level sensor based on an all fused-silica structure for large-range measurements is developed and demonstrated. The DEFPI liquid level sensor combines the optical fibre technology and the CO2 laser heating fusion bonding technology to realise liquid level measurements in harsh environments, and the FP cavity is not hermetically sealed during the fabrication of the sensor by laser heating fusion bonding, thus solving the problem of the trapped air in the FP cavity. Therefore, in addition to high sensitivity, high accuracy and large-range measurement, the thermal expansion of the air in FP cavity will not produce an undesirable pressure on the inner surface of diaphragm when the temperature of the environment increases, thus significantly reducing the measurement error of the liquid level due to temperature cross-sensitivity (a key problem of the DEFPI sensors). Because no chemical processes are involved, the fabrication of the DEFPI sensor is simple, costefficient and environmentally friendly.
2. Operation principle and sensor design Fig. 1 depicts the basic structure of the DEFPI pressure sensor for liquid level measurement with a lead-in silica single mode optical fibre (SMF) – SM-28, a fused-silica ferrule with a length of 7 mm and an outer diameter of 1.8 mm, and an ultra-thin fusedsilica diaphragm with a thickness of 30 μm. There is a through hole with a diameter of 127 μm in the axis of the fused-silica ferrule, and one end of the fused-silica ferrule has a taper-shape cup with an inside diameter of 1 mm and a deep pit of 2 mm, as formed during preparation. As shown in Fig. 1, the air gap between the end face of the lead-in silica optical fibre and the inner surface of the silica diaphragm forms the FP cavity of the DEFPI sensor. The light signal couples into the silica lead-in optical fibre and then propagates along the optical fibre. Finally, it is partially (approximately 4%) reflected by the fibre end face and the inner surface of the diaphragm due to Fresnel reflection at the glass-air interface. The beam reflected by the diaphragm couples into the lead-in optical fibre, then propagates back along it and produces interference fringes of the cosine intensity variations with the beam reflected by the end face of the optical fibre. When the DEFPI sensor is immersed into a liquid, the liquid pressure is applied to the diaphragm. The FP cavity length will vary with the deflection of the diaphragm as a result of the liquid pressure change with the liquid level change. Therefore, the interference fringes modulated by the liquid pressure are determined by the demodulation of the FP cavity length. Accordingly, the liquid pressure can be derived inversely. For a clamped, rigidly round diaphragm of uniform thickness, the out-of-plane deflection
of the centre point of the diaphragm resulting from the change of the liquid pressure can be expressed as [14] Y¼
3ð1−μ2 ÞP 16Eh
3
ða2 −r 2 Þ2
ð1Þ
where μ and E are Poisson's ratio and Young's modulus of the diaphragm material, respectively, a is the inside radius of the ferrule taper-shape cup, r is the radial distance to the diaphragm centre, and h is the diaphragm thickness. For the liquid pressure measurement of the DEFPI sensor, the key problem is that the temperature cross-sensitivity influences the measurement accuracy of the senor. In spite of the all-fusedsilica structure of the DEFPI sensor, the thermal expansion coefficient of the SMF (5.6 10−7 1C−1) is not equal to those of the ferrule and the diaphragm (5.5 10−7 1C−1) because of germanium doping in the optical fibre core. Generally, the greater the germanium doping in the optical fibre, the larger the thermal expansion coefficient of the optical fibre is. The difference between the thermal expansion coefficients of the materials will generate the temperature pressure cross-sensitivity (causing the temperature dependence) of the DEFPI sensor, which increases the measurement error of the liquid pressure. Furthermore, if the FP cavity is hermetically sealed, the air trapped by FP cavity will lead to a larger temperature dependence due to the air pressure at different temperatures. The thermal expansion of the trapped air in the FP cavity induces the unwanted pressure on the surface of the diaphragm when the temperature of the environment increases, which increases the measurement error of the liquid pressure significantly. As illustrated in Fig. 1, Ls is the length from the SMF end face to the fibre heating fusion bonding point, and L is the length of the FP cavity. The total temperature dependence of the DEFPI sensor is given by SP e ΔL ¼ αf ðLs þ LÞ þ ð2Þ −αs Ls ΔT Te where αf and αs are the thermal expansion coefficients of the fused-silica ferrule and the SMF, respectively, S is the pressure sensitivity of the sensor, and Pe is the pressure of the trapped air at the temperature of the environment Te. The thermal expansion coefficient of the optical fibre is very close to that of the fusedsilica ferrule; therefore, the second term on the right hand side of the equation is the primary factor of the temperature dependence. In addition, the thermal expansion of the trapped air in the FP cavity is the greatest source of measurement error of the liquid pressure. The value of the second term of Eq. (2) is zero if the FP cavity is not thoroughly sealed and is connected to the ambient environment. We designed a vent hole between the SMF and the ferrule, as shown in Fig. 1, to ensure that the FP cavity is open to reduce the temperature dependence of the DEFPI sensor. The vent hole formed during the heating fusion bonding connects the FP cavity to the ambient environment. Consequently, the DEFPI sensor has an extremely low temperature dependence.
3. Sensor fabrication
Fig. 1. Configuration of a DEFPI liquid level sensor.
The fabrication process of the DEFPI sensor involves only CO2 laser heating fusion bonding, and the CO2 laser is a GEM-60 (Coherent Inc.). The schematic diagram of the CO2 laser heating fusion bonding is shown in Fig. 2. First, an ultra-thin fused-silica diaphragm with a thickness of 30 μm is directly welded onto the taper-shape cup end face of the ferrule via the CO2 laser. The diaphragm is locally and quickly heated by the CO2 laser at the edge; consequently, its surface near the centre is not influenced. The microscopic image of bonding between the diaphragm and the ferrule is illustrated in Fig. 3. Second, the lead-in silica SMF
Please cite this article as: Wang W, Li F. Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.optlaseng.2013.06.009i
W. Wang, F. Li / Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎
(SM-28) connected to a sm125 Optical Sensing Interrogator (Micron Optics Inc.) is cleaved and inserted into the through hole of the ferrule fixed on a V-groove to form a FP cavity between the optical fibre end face and the inner surface of the diaphragm. The length of the FP cavity is pre-adjusted to approximately 84 μm by a three-dimensional translation stage holding the fibre tail and monitored in real-time by sm125. Then, the fibre is bonded with the inside sidewall of the ferrule with a CO2 laser. The setup is the same as in Fig. 2 except for the holder of the ferrule, as shown Fig. 4(a). Fig. 4(b) illustrates the microscopic image of the bonding between the fibre and the inner sidewall of the ferrule. Finally, the welded DEFPI sensor is annealed with an out-of-focus laser with 2 mm of defocusing. Fig. 4(b) shows that the heating fusion bonding leaves a vent hole, that is, the FP cavity is not hermetically sealed. The
Fig. 2. Schematic diagram of a CO2 laser heating fusion bonding system.
3
fabrication process is epoxy-free; therefore, the sensors are very reliable and can tolerate extremely high temperature close the operational limit of the fibre. Compared with the previous fabrication technologies researched, the fabrication process with laser heating fusion bonding is simple, clean, environmentally friendly and inexpensive.
4. Experimental results and discussion The structure parameters of the sensor head is 0.5 mm for the ferrule taper-shape cup inside radius and 30 μm for the diaphragm thickness; therefore, the theoretical value of the pressure sensitivity of the sensor is 5.77 nm/kPa (39.78 nm/psi) according to Eq. (1). Fig. 5 shows the schematic diagram of the liquid level test setup. The light from a commercial interrogator (Micron Optics Inc. Model: sm125) propagates to the FP cavity of the DEFPI sensor along the lead-in optical fibre, and then its interference signal shown in Fig. 6 is routed back to the detector built in the sm125 along the same optical fibre. The commercial interrogator that is composed of a spectrometer with a tunable laser light source and a detector is a type of programmable interrogator, and the laser light source has a spectral range of 1510–1590 nm with a 1 pm accuracy. A cross-correlation signal processing method is used to process the interference signal to demodulate the FP cavity length, L. The DEFPI sensor head after packaging is submerged in water, and the pressure applied on diaphragm is proportional to the water level. The length L corresponding to the different water levels is calculated by the crosscorrelation algorithm based on Labview according to the FP cavity reflection interference fringes. After filtering the direct current term,
Fig. 3. Taper-shape cup end face of a ferrule (a) before and (b) after welded by use of a CO2 laser.
Fig. 4. CO2 laser heating fusion bonding between the fibre and the inner sidewall of the ferrule.
Please cite this article as: Wang W, Li F. Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.optlaseng.2013.06.009i
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W. Wang, F. Li / Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 5. Setup for liquid level test of a DEFPI sensor based on Labview platform.
Fig. 7. Response of the FP cavity length versus the liquid pressure.
Fig. 6. Interference fringes achieved by a sm125 interrogator.
the optical intensity of the FP cavity interference spectrum is expressed as 4πL þπ ð3Þ IðλÞ ¼ 2I s ðλÞγ cos λ where Is(λ) is the spectrum intensity distribution of the optical source, γ is the contrast of the interference fringe, and π is the additional phase of the half-wave loss. The absolute FP cavity length, L, can be determined by calculating the cross-correlation coefficient. The cross-correlation coefficient of a given L is evaluated by N 4πL CðLÞ ¼ ∑ xðnÞ cos þπ ð4Þ λn i¼1 where x(n) is the spectrum intensity data sequence acquired by the interrogator, λ(n) is the wavelength corresponding to element x(n), and N is the total number of the intensity data sequence. For a given series of acquired data, the effective FP cavity length corresponds to the L value where the cross-correlation coefficient C(L) is maximal. Fig. 7 shows the pressure response curves of the DEFPI sensor from 0 to 50 kPa corresponding to the water level from 0 to 5 m at
room temperature, and (b) is an enlarged figure of the blue dashed rectangle in (a). The pressure sensitivity is approximately 5.3 nm/ kPa (36.6 nm/psi) according to the change of the cavity length, and the repeatability of the DEFPI liquid level sensor is good within 50 kPa. The good repeatability and the standard deviation shown in Fig. 8 indicate that the DEFPI liquid level sensor has high measurement accuracy. Fig. 8 gives the measurement result of the cavity length in response to the pressure resolution, and the standard deviation is approximately 0.018 nm; that is, the resolution of the cavity length is 0.036 nm, and the pressure resolution is approximately 6.8 Pa (9.9 10−4 psi). Therefore, the resolution of the DEFPI liquid level sensor is 0.7 mm (water). However, the pressure response in Fig. 7 is nonlinear, and the fitting function is written as P ¼ 3:3 108 −1:17 104 L þ 0:14L2 −5:56 10−7 L3
ð5Þ
which does not agree with Eq. (1) for the following reasons. (a) The silica lead-in fibre is not aimed specifically at the central position of the diaphragm (r≠0 in Eq. (1)). In addition, the value of r increases gradually with the increase of the water pressure. When the liquid pressure is greater, the change of the r value with the liquid pressure measurement can be demonstrated in the liquid pressure test. (b) The residual stress inside the diaphragm during heating fusion bonding easily causes the ultra-thin diaphragm to deform; therefore, the diaphragm is not an ideal plane. For a diaphragm of an ideal plane, the Eq. (1) should be rewritten as P¼
16Eh
3
3ð1−μÞ2 a4
Yþ
ð7−μÞh 3 Y 3ð1−μÞa4
ð6Þ
Please cite this article as: Wang W, Li F. Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.optlaseng.2013.06.009i
W. Wang, F. Li / Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 8. Experimental results of the DEFPI sensor resolution (about 5 min time span).
The temperature dependence of the DEFPI liquid level sensor should be very low according to the discussion in Section 2. The thermal expansion coefficients of the SMF and the fusedsilica ferrule are 5.6 10−7 1C−1 and 5.5 10−7 1C−1, respectively. In addition, in eq. (2), the value of the second term is zero as a result of the FP cavity without a thorough seal. Therefore, for a value of Ls of approximately 3.1 mm and a value of L of 84 μm, the theoretical value of the temperature dependence of the DEFPI sensor is approximately 0.015 nm/1C. Its temperature dependence was tested after the sensor welded by heating fusion bonding technology was annealed. The test setup is illustrated in Fig. 5 except for the sensor head. The sensor head was inserted into a temperature control case together with a platinum resistance thermometer (Hart Scientific 1502A, 70.012 1C). The measurement results shown in Fig. 9 indicate that the temperature dependence is 0.013 nm/1C from room temperature to 77 1C. The temperature dependence of the DEFPI sensor is consistent with the theoretical value. Therefore, the liquid pressure measurement error induced by the temperature dependence is approximately 0.0025 kPa/1C (0.00036 psi/1C); that is, the water level measurement error is below 0.25 mm/1C. 5. Conclusions In conclusion, a large-range continuous liquid level measurement optical fibre DEFPI sensor incorporating both an all fusedsilica structure and CO2 laser heating fusion bonding technology was developed. The FP cavity without a seal solves the problem of the thermal expansion of the trapped air in the FP cavity, guaranteeing that the DEFPI liquid level sensor possesses an extremely low temperature dependence of 0.013 nm/1C. A high sensitivity of 5.3 nm/kPa (36.6 nm/psi) and a resolution of 6.8 Pa (9.9 10−4 psi) were obtained. Therefore, the resolution of the water level measurement is approximately 0.7 mm at the length scale of 5 m. In addition, the measurement error of the water pressure induced by the temperature dependence is limited
5
Fig. 9. Temperature dependence of a DEFPI liquid level sensor.
within 0.0025 kPa/1C (0.00036 psi/1C), corresponding to the water level measurement error of 0.25 mm/1C. The DEFPI sensor has significant potential for large-range liquid level measurements in applications such as oil reservoirs.
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Please cite this article as: Wang W, Li F. Large-range liquid level sensor based on an optical fibre extrinsic Fabry–Perot interferometer. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.optlaseng.2013.06.009i