Microelectromechanical system-based, high-finesse, optical fiber Fabry–Perot interferometric pressure sensors

Journal Pre-proof Microelectromechanical system-based, high-finesse, optical fiber Fabry–Perot interferometric pressure sensors Weiyi Ma (Conceptualization) (Methodology) (Software) (Validation) (Formal analysis) (Investigation) (Data curation) (Writing - original draft) (Writing - review and editing) (Visualization), Yi Jiang (Conceptualization) (Formal analysis) (Investigation) (Resources) (Supervision) (Project administration) (Funding acquisition), Jie Hu (Funding acquisition), Lan Jiang (Resources) (Project administration)

PII:

S0924-4247(19)31787-X

DOI:

https://doi.org/10.1016/j.sna.2019.111795

Reference:

SNA 111795

To appear in:

Sensors and Actuators: A. Physical

Received Date:

26 September 2019

Revised Date:

29 November 2019

Accepted Date:

13 December 2019

Please cite this article as: Ma W, Yi J, Hu J, Jiang L, Microelectromechanical system-based, high-finesse, optical fiber Fabry–Perot interferometric pressure sensors, Sensors and Actuators: A. Physical (2019), doi: https://doi.org/10.1016/j.sna.2019.111795

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Sensors and Actuators A: Physical

Microelectromechanical system-based, high-finesse, optical fiber Fabry–Perot interferometric pressure sensors Weiyi Maa, Yi Jianga,*, Jie Hub, Lan Jiangb

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a School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China; b Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing, 100081, China

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Corresponding authors at: School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China. Tel.: +86-0086-6891-3586 E-mail addresses: [email protected] (W. Ma), [email protected]; (Y. Jiang), [email protected] (J. Hu), [email protected] (L. Jiang).

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Accepted: date; Published: date

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Graphical Abstract

Highlights  We design the high-finesse EFPI cavity that consists of a Pyrex glass wafer, a micromachined silicon wafer, and highly reflective dielectric films.

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We combine the high-finesse EFPI cavity with MEMS-based technique to yield an improved pressure sensing resolution (0.002% of the full scale). In this paper, the full scale is 1MPa, the resolution is 20Pa.This resolution is very accurate in atmospheric pressure applications. But in many other MEMS based research, the resolution of pressure is rarely mentioned and they just show the pressure characteristic. Our custom-made white-light interferometric interrogator has a large wavelength bandwidth of the order of 125 nm that can realize short-cavity length interrogation (less than 30um). We use the optical fiber collimator that facilitates the realization of the alignment of the incident light.

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

Abstract:

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A high-finesse, optical fiber, extrinsic Fabry–Perot interferometric (EFPI) pressure sensor based on a microelectromechanical system (MEMS) technique is proposed and experimentally demonstrated. The essential element in the pressure sensor is the high-finesse EFPI cavity that consists of a Pyrex glass wafer, a micromachined silicon wafer, and highly reflective dielectric films. Another Pyrex glass is used for fixing an optical fiber collimator, which allows the realization of the alignment of the incident light. Experimental results show that the proposed sensor exhibits a pressure sensitivity of 1.598 μm/MPa and a high-pressure sensing resolution of 0.002% of the full scale. This sensor is expected to benefit many applications that require high-accuracy pressure measurements, and especially atmospheric pressure applications.

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Keywords: optical fiber sensor; extrinsic Fabry–Perot interferometer; microelectromechanical system; pressure measurement

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1. Introduction

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Optical fiber sensors possess distinct advantages compared to conventional electrical sensors, including their small dimensions, immunity to electromagnetic interference, and resistance to adverse environments [1–5]. Various types of optical fiber sensors have been explored for different physical, chemical and biological parameters that need to be monitored [6–10]. For example, the optical fiber pressure sensors which are based on the Fabry–Perot interferometer (FPI) have yielded promising results for pressure sensing owing to their robust structures and increased sensitivities The sensors that are usually formed directly on a fiber end-face consist of a cleaved optical fiber, a sensing diaphragm, and a housing structure to hold the diaphragm. The fiber end-face and the diaphragm, which are separated by an air gap, effectively form the FP cavity that senses the pressure. Many different types of optical fiber FPI pressure sensors have been reported, including those fabricated from polymeric [3, 6], metallic [11, 12], silicon/silica [13–17], and ceramic materials [18], according to their mechanical properties and applications. However, all of these types have their inherent drawbacks. Some examples include their increased cross-sensitivity to temperature (polymeric materials), complex fabrication process (ceramic materials), and increased sensitivity to electromagnetic interference (metallic materials). Technologies based on micro-electro-mechanical systems (MEMS) have provided desirable solutions. By employing MEMS technologies, a variety of optical pressure sensors based on FPI have been proposed, including fibers that use glass plates and etched silicon diaphragm structures or alumina ceramic housing structures to form the sensing element [19–22], fibers that utilize the total internal reflection at a 45° angled fiber end-face to steer the optical axis by 90° [23, 24], and fibers which employ the SU–8 diaphragm which deflects under

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pressure as one end of the FPI [25]. These MEMS-based sensors have high sensitivity and highpressure ranges, and some of them can even survive at high temperatures [22]. However, the resolution of pressure measurements has rarely been reported explicitly for all the aforementioned optical MEMS-based FPI pressure sensors. Their construction methodologies have been based on the utilization of the back reflection from the fiber end, or the sensing diaphragmatic surface whose reflectivity is typically low (∼4%). This can form a two-beam interference pattern, and the spectrum from the FPI is a quasisine curve that has a flat peak and a broad bandwidth. It is difficult to precisely determine the peak wavelength when peak-detection-based methods are used for the interrogation [26, 27]. Therefore, the precision of the cavity length readings is limited, which would result in the reduction of pressure resolution. In this study, we proposed a high-finesse, optical fiber extrinsic Fabry–Perot interferometric (EFPI) pressure sensor based on the MEMS technique. The sensing element was produced by anodically bonding the Pyrex glass wafer and the silicon wafer with grooves etched on both sides. Both the upper surface of the Pyrex glass and the bottom of the downside groove were coated with a highly reflective dielectric film to form the high-finesse EFPI cavity. Compared to the reported MEMS-based pressure sensors with similar structures, our proposed pressure sensor yielded an increased pressure sensing resolution (0.002% of the full scale). 2. Sensor Design and Principle

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A schematic of the MEMS-based EFPI sensor is shown in Fig. 1. The sensor head can be divided into three parts: a sensing structure, a Pyrex glass substrate, and an optical fiber collimator. The sensing structure employs a high-finesse EFPI cavity which is composed of a Pyrex glass wafer, highly reflective dielectric films, and a silicon wafer with grooves etched on both sides. Highly reflective dielectric films ( SiO2 / Si3 N 4 ) were sputtered on the upper surface of the Pyrex glass and on the bottom of the downside

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groove to improve the reflectivity of the reflective surfaces (up to 95%). In addition, an antireflection film ( SiO2 / Si3 N 4 ) was sputtered on the lower surface of the Pyrex glass wafer to suppress unwanted

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back-reflections from the end of the Pyrex glass. This similar process was used on the fiber side of the optical fiber collimator as well. The optical fiber collimator contained a single-mode fiber (SMF) and was inserted into the Pyrex glass substrate that was used for alignment and fixing. Incident light from the SMF was aligned by the optical fiber collimator, propagated to the high-finesse EFPI cavity, and formed a multiple-beam interference pattern. The back-reflected spectrum formed by the multiplebeam interference pattern can be acquired by the white-light interferometric (WLI) interrogator for absolute EFPI cavity length calculations.

Figure 1. Schematic of the proposed high-finesse optical fiber extrinsic Fabry–Perot interferometric (EFPI) pressure sensor (SMF: single-mode fiber)

The cavity length is equal to L. Hence, the applied perturbation can be obtained in real-time by using the equation:

L=

12 2(1  2 )

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where 𝜆1 and 𝜆2 are two wavelengths between two adjacent apexes in the white-light optical spectrum that are out of phase by 2𝜋. According to (1), the measurement precision of the cavity length is determined by the interrogation precisions of these two wavelengths. In this study, the design of the EFPI cavity yields spectra with sharp peaks whose peak positions can be detected precisely. Thus, the calculated value of the cavity length is more accurate with the use of the peak-to-peak method [28]. When the pressure is applied to the MEMS-based optical fiber EFPI pressure sensor, the silicon diaphragm deforms, and thus changes the cavity length of EFPI. The cavity length change ∆ 𝐿𝑝 owing to the applied pressure can be expressed as [29]

Lp 

3(1   2 )  R 4 P  S p  P 16 Eh3

(2)

2.8793 109  P 1  0.003661 T

(3)

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n=1+

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where μ is the Poisson’s ratio of the diaphragm, E is the Young’s modulus, h is the thickness of the diaphragm, R is the effective diaphragmatic radius, ∆P is the pressure difference between the inner and outer surfaces of the diaphragm, and 𝑆𝑝 is defined as the pressure sensitivity. However, this EFPI pressure sensor also exhibits a temperature dependence that can be attributed to the thermal expansion of the silicon material, and yields air refractive index (RI) and pressure changes within the EFPI cavity that may result in errors during the pressure measurement process. The RI of air is a function of pressure P and temperature T [30]

L0   silicon Pb  )  (T  T0 ) Sp Tb

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Perr  (

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Herein, the pressure P is equal to a standard atmospheric pressure (for a sealed EFPI cavity), and the temperature T is expressed in degrees Celsius. Given that the temperature range is low (less than 100 °C), the effect of RI of air (approximately equal to unity) can be ignored. Thus, the temperature dependence of the EFPI pressure sensor is mainly determined by the thermal expansion of the silicon material and the air pressure change in the EFPI cavity. The temperature-induced pressure error ∆ 𝑃𝑒𝑟𝑟 can be described for a sealed EFPI pressure sensor as [13], (4)

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where 𝛼silicon is the coefficient of thermal expansion (CTE) of silicon, 𝐿0 and 𝑇0 are the initial cavity length and initial temperature of the EFPI cavity, respectively, 𝑃𝑏 and 𝑇𝑏 are the pressure and absolute temperature in the EFPI cavity during sealing, respectively, and T is the applied temperature. From (4), it follows that the error depends linearly on the applied temperature because these proportionality factors 𝐿0 ∙ 𝛼silicon /𝑆𝑝 (caused by the thermal expansion of the silicon material) and 𝑃𝑏 /𝑇𝑏 (caused by the air pressure change in the EFPI cavity) are both constant volumes. 3. Sensor Fabrication The fabrication process of the MEMS-based EFPI pressure sensor is shown in Fig. 2. After a 4 in silicon wafer with a thickness of 300 μm was cleaned and polished, multiple photoresist layers were deposited on the bottom surface with the use of lithography technology (coating and baking) with the assistance of a mask [Fig. 2(a)]. The reactive ion etching (RIE) was then used for the etching of the bottom surface to obtain a shallower cavity [Fig. 2(b)]. This was followed by the dissolution of the photoresist with the acetone solution. To enhance the reflectivity of the reflecting surface, multiple highly reflective dielectric layers were deposited onto the surface of the etched groove [Fig. 2(c)]. The dielectric layers were formed by depositing 5-10 300-nm-thick SiO2 and

Si3 N 4 membrane layers alternately, which can achieve a reflectance of 90%-95%. Similar etching process was also performed on the upper surface of the silicon wafer to obtain a silicon diaphragm [Fig. 2(d) - Fig. 2(e)]. Note that the size and thickness of the sensing diaphragm can be adjusted to meet different pressure sensing

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needs. Subsequently, a 4 in Pyrex 7740 # glass wafer with a thickness of 500 μm was used to fabricate another reflecting EFPI surface cavity. Similarly, the multiple highly reflective dielectric layers were also deposited on its upper surface followed by the coating of the multiple antireflection dielectric layers on its lower surface [Fig. 2(f)]. Subsequently, the silicon wafer was anodically bonded to the Pyrex glass wafer [Fig. 2(g)]. During the anodic bonding, the temperature, voltage, pressure, and time period, were respectively controlled within the ranges of 300–500°C, 200–1000 V, 0.05–1 MPa, and 5–10 min. The bonded sample was then split into small pieces with dimensions equal to 5 × 5 mm. Finally, another Pyrex glass (5 × 5 mm), which served as the substrate, was mechanically drilled to construct a through-hole. The Pyrex glass substrate had a thickness of approximately 3 mm and its through-hole diameter was controlled to ~2.5 mm. The Pyrex glass substrate and the anodically bonded sample with the EFPI cavity were attached together with ultraviolet (UV) glue. This was also used to fix the optical fiber collimator (C-lens, 2.4×10) that was inserted in the through-hole of the Pyrex glass substrate [Fig. 2(h)]. Similarly, the antireflection membrane layers were also deposited on the fiber side of the collimator. During the alignment of the incident light, the sample was connected to a homemade WLI interrogator to obtain the white-light interference spectrum in a continuous manner. The microscopic image and a photograph of the fabricated sensor are shown in Fig. 3.

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Figure 2. Fabrication process of the pressure sensor (a)–(h)

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Figure 3. Images of the fabricated sensor: (a) microscopic image and (b) photograph

Using this method, a number of MEMS-based EFPI pressure sensors with different cavity lengths and diaphragmatic thicknesses were fabricated. Fig. 4 shows the reflection spectrum of a representative sensor that has a cavity length of approximately 30 μm, an effective diaphragmatic radius of 1.5 mm, and a diaphragmatic thickness of 145 μm. It can be observed from the reflection spectrum that the EFPI has a free spectral range (FSR) of 45 nm, a 3 dB bandwidth of 0.7 nm, and thus

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a finesse of 64. In addition, substituting these actual parameter values and constants (E = 190 GPa, μ = 0.22, and 𝛼silicon = 2.5 × 10−6 ) in (2), the pressure sensitivity 𝑆𝑝 can be estimated to be equal to 1.559 μm/MPa. According to Eq. 4 from [13], the pressure 𝑃𝑏 and absolute temperature 𝑇𝑏 are constant volumes that can be measured during sealing. In this study, the sealing was achieved with anodic bonding with a bonding pressure and an absolute temperature of 0.1 MPa and 773 K, respectively. Accordingly, for our proposed EFPI pressure sensor with the sealed air cavity, lowtemperature dependence ∆ 𝑃𝑒𝑟𝑟 ⁄∆𝑇 = 176.476 Pa/K can be obtained.

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Figure 4. Interference spectrum of a typical pressure sensor

4.1. Experimental Setup

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4. Experiments and Evaluation

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The experimental setup used for the sensor calibration tests is illustrated in Fig. 5. The MEMSbased EFPI pressure sensor was connected to the homemade WLI interrogator, which was consisted of an isolator, an etalon, 2 × 2 couplers, and a tunable wavelength-scanning laser based on a fiber FPtunable filter (FFP–TF) . The wavelength-scanning laser operated within a wavelength range from 1500 to 1625 nm. This provided a good bandwidth range in the calculation of the cavity length using the peak-to-peak method. Measurements of the initial cavity length and pressure were all carried out in real-time and the measurement frequency is 1Hz. The WLI interrogator had a spectral resolution which was less than 2 pm, which led to a cavity length resolution that was less than 36 pm [26, 28].

Figure 5. Experimental setup used for pressure measurements

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Figure 6 shows the cavity length deflections ΔL 100 consecutive times (value 0 is the mean value of the measurement results) measured at a temperature of 27°C and a pressure of 0 MPa (relative pressure to atmospheric pressure). This was accomplished by identifying the two adjacent peaks of the reflection spectrum. The mean value of the measurement results is equal to 28035.914 nm and the variation range is about 30 pm. The resolution of the cavity length was much higher than that measured by the low-finesse EFPI sensor. The error of the initial cavity length (30 μm) is believed to be correlated with the fabrication process and the error due to the demodulation algorithm.

4.2. Experimental Results

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Figure 6. Cavity length deflections ΔL measured continually at 100 different times at room temperature and a pressure of 0 MPa

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Calibration tests of the sensor were carried out by calculating the pressure sensitivity and by plotting the cavity length as a function of the applied pressure. As shown in Figure 5, the pressure sensor was mounted in a sealed corundum ceramic tube (with a length of 750 mm and a diameter of 10 × 6 mm) which was pressurized with the use of an air tank (maximum pressure of 10 MPa) and was controlled with the use of a pressure regulator. In addition, the stainless steel capillaries (diameters = 4 mm) were used for the connection of the corundum ceramic tube to the air tank, and the connecting parts were glued by epoxy resin (353ND, Epoxy Technology). During pressurization, an air pressure gauge (XSE–CHIB1M2V0) with a resolution of 0.1% was used to measure the actual pressure. The pressure in the corundum ceramic tube was increased from 0 to 1 MPa at 0.1 MPa steps at a temperature of 27°C and cavity length readings were recorded in real time. Fig. 7 shows the cavity length as a function of the applied pressure. As it can be observed from the calibration results, the sensor yielded a linear response (𝑅2 =0.99993) over the entire tested pressure range. Based on the linear fitting of the measured data, the pressure sensitivity was determined and was equal to 1.598 μm/MPa. This value is slightly larger than the predicted value. An error was expected regarding the effective radius and thickness of the sensing diaphragm owing to the fabrication process. In addition, the pressure resolution can also be determined to be 18.77 Pa according to the obtained cavity length resolution (Fig. 6) and pressure sensitivity (Fig. 7), which was less than 0.002% of the full scale (FS) within the measurement range of 1 MPa. To evaluate the pressure resolution experimentally, the sensor was pressurized to a fixed pressure of 1 MPa at room temperature, and pressure values were measured with the use of the calibration curve plotted in Fig. 7. Fig. 8 shows the measured results of the pressure sensor. A

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resolution of 20 Pa can be clearly observed in the presented figure. It is noted that the obtained pressure exhibits a decreasing trend during the measurements. This small drift is attributed to the minor air leakage from the experimental setup. The decreasing trend demonstrates that our proposed pressure sensor yields a good performance with increased sensitivity and increased resolution.

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Figure 7. Pressure calibration curve measured at room temperature

Figure 8. Pressure measurements as a function of time for an applied pressure of 1 MPa

As mentioned previously, owing to the CTE of the silicon material and the air pressure change in the EFPI cavity, the pressure sensor was expected to have a specific temperature sensitivity response. Therefore, the temperature effect should be evaluated to ensure accurate pressure measurements. The pressure sensor was then placed in a thermostat to control the temperature locally, and the temperature sensitivity of the pressure sensor was measured by monitoring the cavity length with respect to changes in the temperature. The thermostat was heated from 32 °C (the ambient temperature was 31 °C) to 72 °C at 2 °C increments at atmospheric pressure, and the cavity lengths

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were recorded. The temperature calibration outcomes of the pressure sensor are shown in Fig. 9. It is noted that the linear relationship between the EFPI cavity length and temperature can be observed to be associated with an increased regression coefficient ( 𝑅2 = 0.99954) and a low-temperature sensitivity of 269.33 pm/°C. The temperature dependence was 168.542 Pa/°C. The measurement was achieved with the use of the obtained pressure sensitivity from Fig. 7. This value was in a reasonable agreement with the theoretical temperature dependence of 176.476 Pa/°C.

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Figure 9. Characteristic curve of cavity length as a function of temperature for the tested pressure sensor

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5. Conclusions

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In conclusion, we have demonstrated a high-finesse EFPI pressure sensor, which was based on the MEMS technique. The sensor was divided into three parts: a pressure sensing structure, a Pyrex glass substrate, and an optical fiber collimator. A high-finesse EFPI cavity served as the core component in the pressure sensing structure, and was composed of a Pyrex glass wafer, highly reflective dielectric films, and a micromachined silicon wafer. This high-finesse EFPI cavity design can achieve an increased resolution. The use of anodic bonding and RIE technique were employed for the fabrication of the sensor. The thickness of the silicon diaphragm can be tuned for different pressure sensing applications. A representative pressure sensor with a diaphragmatic thickness of 145 μm was achieved which demonstrated a high-pressure sensitivity of 1.598 μm/MPa (measured in room temperature) and a good pressure sensing resolution of 0.002% FS in a pressure range of 1 MPa. These accurate pressure measurements exhibit tremendous promise for the sensor’s use in atmospheric pressure applications. Conflicts of Interest: The authors declare no conflict of interest.

Author statement: Manuscript title: Microelectromechanical system-based, high-finesse, optical fiber Fabry– Perot interferometric pressure sensors

Weiyi Ma: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing—original draft preparation, Writing—review and editing, Visualization; Yi Jiang: Conceptualization, Formal analysis, Investigation, Resources, Supervision, Project administration, Funding acquisition; Jie Hu: Methodology, Funding acquisition; Lan Jiang: Resources, Project administration;

Acknowledgments

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This research was funded by the National Key R&D Program of China under Grant 2018YFB1107200, and the Natural National Science Foundation of China (NSFC) [grant numbers 61775020 and 61575021].

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Biographies Weiyi Ma is currently working toward the Ph.D. degree in electronic science and technology, with the School of Opto-Electronics, Beijing Institute of Technology. His current research interest is focused on the optical fiber sensors and high-speed fiber-optic white-light interferometry. Yi Jiang received the B.A. degree in 1987 and Ph.D. degree in 1996, respectively, from Chongqing University, Chongqing, China. He is a Professor with the School of Opto-Electronics, Beijing Institute of technology, Beijing, China. His research interests include fiber optical sensors, smart structures, and measurement instruments. Jie Hu received the Ph.D. degree from the University of Illinois at Urbana-Champaign in 2011. She is currently a Professor with the Laser Micro/Nano Fabrication Laboratory, School of Mechanical

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Engineering, Beijing Institute of Technology. Her research interests include micro-nano machining and micro-nano electrochemical growth.

Lan Jiang received the Ph.D. degree in 2001 from the Beijing Institute of technology, Beijing, China. He is a Professor with the Laser Micro/Nano Fabrication Laboratory, School of Mechanical

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Engineering, Beijing Institute of technology. His research interests include optical fiber sensors,

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surface enhanced Raman scattering, and micromachining.