Fiber optic Fabry–Perot pressure sensor based on lensed fiber and polymeric diaphragm
Introduction
A variety of Fabry–Perot interferometric sensors based on optical fiber have recently been studied and applied in various industrial areas to measure temperature, refractive index, strain, pressure, and fluid dynamics [1], [2], [3], [4], [5], [6], [7], [8], [9]. The Fabry–Perot sensors have drawn much interest owing to their advantages, such as dimensional compactness, high sensitivity, applicability in harsh environments, low cost, easy handling and alignment, immunity to electromagnetic interference (EMI) and multiplexing capability [1], [2], [3], [4], [5], [6], [7], [8], [9]. In particular, the Fabry–Perot sensors for the pressure measurement have a remarkable attention in a many applications such as medical, automotive, aerospace, gas and oil industries [9], [10], [11], [12], [13], [14], [15], [16], [17]. Most of Fabry–Perot fiber interferometer for pressure measurement usually employs two reflecting flat-surfaces separated by a cavity, an end face of optical fiber and a reflecting diaphragm surface. For the pressure measurement, various types of diaphragms have been used, including those fabricated from polymer materials (silicon, SU8), metal and even optical fiber, according to their mechanical properties and applications [9], [10], [11], [12], [13], [14], [15], [16], [17]. The loaded pressure can be estimated by detecting the interference signal when pressure deforms the diaphragm. Specifically, the device measures the intensity variation of interference, and estimates the change in cavity length from the interference spectra [12]. To measure the effective interference signal, the cavity length of the pressure sensors mentioned in the previous reports was limited within 5–20 μm range for the reflecting beam light on the diaphragm can be effectively recoupled. However, this range of cavity length could pose restrictions in medical applications. The human blood pressure generally spans on the order of tens of kPa, and some parts of the human body, such as the bladder and intracranium, have a lower pressure range that lies within several kPa [18], [19]. Consequently, Fabry–Perot pressure sensor for medical applications should have higher sensitivity with wider range of cavity length for measuring bladder, intracranial pressure, and/or blood pressure than those for industrial applications.
The sensitivity of a Fabry–Perot pressure sensor depends on diaphragm thickness, material flexibility, and the distance the diaphragm deforms (or bends) in the cavity. When the diaphragm thickness is reduced or its material is very flexible, the deformation range increases. However, high flexibility also increases the possibility that the diaphragm may easily break or contact the opposite surface of the cavity, or crack the mirror [20]. So, to perform with high sensitivity, the polymeric diaphragm needs to tolerate large deformation without breaking or cracking and the cavity length requires to enough long without contacting the opposite surface.
In addition, when an external pressure induces a large deformation of the diaphragm of a Fabry–Perot sensor, it is not possible to acquire an effective interference signal because the intensity of the light back-reflected on the diaphragm decreases. In other words, the intensity of the back-reflected light coupled to the fiber is a fraction of the incident light intensity, and depends on the distance between the fiber and the reflecting surface, and therefore on the deformation of that surface: an increase or decrease of the distance causes a decrease in back-reflected intensity. So, to perform effectively, the Fabry–Perot interferometric pressure sensor must maintain low optical power loss even if the loaded pressure greatly deforms the diaphragm in long cavity.
In this paper, we propose an optical fiber Fabry–Perot pressure sensor with a lensed fiber and a polymeric diaphragm which is able to maintain high sensitivity, low optical power loss and long cavity length, and to prevent potential mechanical failure. First, the proposed Fabry–Perot pressure sensor is attached to a lensed fiber on the tip of a single mode fiber, and a cavity is formed between the end face of the lensed fiber and the polymeric diaphragm. This arrangement maintains a high coupling ratio of back-reflected light by collecting the light reflected from the diaphragm, and provides a longer cavity length than that of a conventional Fabry–Perot pressure sensor by focusing the incident light on the diaphragm. The lensed fiber is fabricated by forming a lens on a short piece of coreless silica fiber (CSF) that is spliced with a single mode fiber (SMF).
Second, to achieve high sensitivity and wide pressure measurement, we fabricated a three-layer diaphragm design: PDMS, parylene, and gold. As PDMS is very flexible and biocompatible material, it is chosen to face the external medium for pressure measurement. The innermost gold layer improves the reflectivity of light on the diaphragm, and the intermediate parylene layer is used to prevent crack of gold layer that often occurs over repeated bending. The polymeric diaphragm was fabricated by MEMS processing including spin-coating, vaporization, and sputtering. The feasibility of the proposed Fabry–Perot sensor was studied by numerical simulation of the pressure deformation of the polymeric diaphragm, and experimentally measured at pressures ranging from 0 kPa to 4 kPa, equivalent to the measurement of bladder and intracranial pressure.
Section snippets
Theoretical analysis and numerical modeling
The proposed Fabry–Perot pressure sensor is shown in Fig. 1. The fiber optic pressure sensor consists of a lensed fiber and a polymeric diaphragm. The lensed fiber is spliced with a single mode fiber. The polymeric diaphragm, fabricated by MEMS processing, is fixed at the tip of a glass ferrule. Two beams, which are reflected from the end face of the lensed fiber and the gold coated surface of the polymeric diaphragm respectively, are interfered through the single mode optical fiber.
In general,
Fabrication process of the proposed pressure sensor
The fabrication sequences of the lensed fiber and the thin diaphragm of the pressure sensor are shown in Fig. 4.
First, the lensed fiber is fabricated by using a fiber cleaving system, with a single mode fiber (SMF; core/cladding diameters of 9/125 μm), a coreless silica fiber (CSF; outer diameter of 150 μm), and a fusion splicing machine (S183PM, FITEL Co.). In the fabrication process, two fibers, the SMF and the CSF, are cleaved by the fiber cleaving system after their protective plastic jackets
Experimental results and discussions
Fig. 6 shows the optical coupling ratio of back-reflected light in two cases: one is for the lensed fiber and the other is for the cleaved fiber. The optical coupling ratio of back-reflected light on a mirror was monitored using a 1550 nm light source and a power meter as the mirror on a translation stage that is moving away (Fig. 6(a)). As shown in Fig. 6(a), the back-reflected light on the mirror converges into the SMF due to the lensed fiber. However, in the case of the cleaved fiber, the
Conclusion
A novel optical fiber-based Fabry–Perot pressure sensor is presented which employs a lensed fiber formed at a cleaved tip of a CSF spliced on a SMF, and a polymeric diaphragm fabricated by MEMS processing. The curvature of the end of the lensed fiber is formed by an arc discharge technique. The lensed fiber and polymeric diaphragm form the cavity of the Fabry–Perot interferometer. Due to the lensed fiber, the proposed sensor obtained an effective interference signal while maintaining a high
Acknowledgements
This research was supported by the Research Project Development (RPD) Program through the Institute of Medical System Engineering (iMSE) in the GIST, Korea.
Jonghyun Eom received his B.S. in Biomedical Engineering from Yonsei University and his M.S. in the Department of Medical System Engineering from Gwangju Institute of Science and Technology (GIST), Republic of Korea, in 2008 and 2010, respectively. He is currently a Ph.D. student in the Department of Medical System Engineering at GIST, Gwangju, Republic of Korea. He is pursuing this Ph.D. in noncontact photoacoustic imaging systems using an optical heterodyne interferometer and optical
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Jonghyun Eom received his B.S. in Biomedical Engineering from Yonsei University and his M.S. in the Department of Medical System Engineering from Gwangju Institute of Science and Technology (GIST), Republic of Korea, in 2008 and 2010, respectively. He is currently a Ph.D. student in the Department of Medical System Engineering at GIST, Gwangju, Republic of Korea. He is pursuing this Ph.D. in noncontact photoacoustic imaging systems using an optical heterodyne interferometer and optical measurement system for biomaterials. His research interests include optical coherence tomography and photoacoustic imaging systems based on optical fiber sensors, and optical measurements.
Chang-Ju Park received his B.S. degree in Biomedical Engineering from Jeonbuk National University and his M.S. degree in DMSE (Department of Medical System Engineering) from GIST (Gwangju Institute of Science and Technology), Korea, in 2009 and 2011, respectively. Currently, He is a Ph.D. candidate in DMSE from GIST, Korea. He is pursuing his PhD degree in MEMS technology. His research interests include bio and medical MEMS such as Microfluidic components (micropumps and valves) and medical devices (glaucoma drainage device, ureteral stent). He is also has significant interest in integrated systems such as Micro total analysis system and Lab on a chip.
Byeong Ha Lee received his B.S. and M.S. degrees in physics from Seoul National University, Korea, in 1984 and 1989, respectively. He obtained the Ph.D. degree in physics from University of Colorado at Boulder, USA. After working as STA in Osaka National Research Institute of Japan from 1997 to 1999, he joined Gwangju Institute of Science and Technology (GIST), Korea, where he is currently serving as a full-time professor. He is specialized in areas related to fiber optic sensors, fiber gratings, photonic crystal fibers, and optical coherence tomography. His current research interests are related to developing fiber optic systems for biomedical applications.
Jong-Hyun Lee received his B.S. degree in mechanical design from Seoul National University, Korea, in 1981, and his M.S. degree and Ph.D. degree (major: ultrasonic transducers) from KAIST (Korea Advanced Institute of Science and Technology) in 1983 and 1986, respectively. Joining ETRI (Electronics and Telecommunication Research Institute, Korea) in 1986, he worked on semiconductor processes and MEMS technologies. In 2000, he moved to GIST (Gwangju Institute of Science and Technology, Korea) and was a visiting scientist at the University of Cincinnati (bio-MEMS Lab), USA from August 2004 to July 2005. He had been a director of the DMSE (department of medical system engineering) and the iMSE (Institute of Medical System Engineering) from February 2008 to January 2014. He is currently a professor at the DMSE and the school of mechatronics. His research interests include biomedical micro/nano devices, optical MEMS, micro sensor/actuators, etc.
Il-Bum Kwon received his Ph.D. from Korea Advanced Institute of Science and Technology (KAIST) in 1997. His major field of study was aerospace engineering with specialization in structural health monitoring using fiber optic sensors. He has been employed by Pohang Research Institute of Industrial Science and Technology (RIST/POSCO) since February 1989 and conducted research in rolling research laboratory until August 1992. Currently, he is working for center for safety measurements in Korea Research Institute of Standards and Science since March 1997. His research interests include fiber optic sensors for structural health monitoring, optical fibers and optical communications for sensor networks. His societal memberships include SPIE, OSA.
Euiheon Chung, Ph.D., has been an Assistant Professor of the Department of Medical System Engineering and School of Mechatronics at the Gwangju Institute of Science and Technology (GIST) since 2011. After receiving B.S. and M.S. degrees from Korea Advanced Institute of Science and Technology, Dr. Chung gained his Ph.D. from the Harvard-MIT Division of Health Sciences and Technology, Medical Engineering and Medical Physics program in 2007. He helped a biotechnology start-up company (Nanopoint, Inc.) and a medical venture company (Cambridge Devices, Inc.) as a technical consultant. He was trained in the Steele Laboratory of Tumor Biology at Harvard Medical School as a post-doc prior to joining the GIST. His research focuses on the biomedical optical imaging and bioengineering for applications in translational medicine.
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These authors contributed equally to this work.