Single-ring suspended fiber for Bragg grating based hydrostatic pressure sensing


Essentially, microstructured optical fibers (MOF) are optical fiber waveguides where light guidance is obtained through manipulation of waveguide structure rather than its index of refraction. As such, they present an excellent platform for fiber sensors due to their flexibility in design and fabrication. Ideally, most MOFs are constructed with a fiber core surrounded by one or more layers of air-holes in the cladding, thus most research focus usually revolves on the arrangement of air-holes in the MOFs to achieve high sensitivity or birefringence. Recent publications have however suggested that pressure sensitivity can be improved by increasing the size of the air-holes or by placing the air-holes in close proximity to the core of the fiber. To achieve this, interferometric techniques have been employed, regardless, they show high pressure sensitivities thus imposing a limitation on the length of the sensor head. Alternatively, fiber Bragg gratings (FBGs) based sensors are capable of being multiplexed and compact and thus more reliable than other sensing schemes. Nonetheless, FBGs fabricated in conventional single-mode fibers (SMFs) have low pressure sensitivities. All factors considered, pressure sensitivity can be enhanced through effective modification of the structure of the MOF.

Recently, researchers at The Hong Kong Polytechnic University: Dr. Lin Htein, Dr. Zhengyong Liu, Dr. Dinusha Gunawardena and Professor Hwa-Yaw Tam developed a novel sensor design which contains a central core, referred to as a suspended core, supported by a single silica ring. Their goal was to enhance the sensitivity of FBG based sensors through modification of the MOF structure. Their work is currently published in the research journal, Optics Express.

In brief, the research team designed and fabricated an optical fiber composed of a suspended core, a supporting ring and an outer ring. So as to establish a large holey region, a germanium-doped core was suspended by a silica ring and the entire structure was enclosed by another silica ring. Per se, the researchers performed a numerical simulation with the aim being that it would enable them to optimize the best possible pressure sensitivity.

The authors reported that by monitoring the Bragg wavelength shift of an FBG written in such a fiber with an air filling fraction of 65%, an intrinsic hydrostatic pressure sensitivity of –43.6 pm/MPa was achieved experimentally. Additionally, the team highlighted that due to the significant impact of the fiber core suspended in the large holey region inside the fiber, the pressure sensitivity improved by approximately eleven times compared to a Bragg grating inscribed in a standard single-mode fiber.

In a nutshell, The Hong Kong Polytechnic University research team successfully presented two kinds of single-ring suspended fibers in which the cores were suspended by a single silica ring and the entire structure enclosed by an annular ring. The pressure sensing characteristics of Bragg gratings inscribed in these specially designed fibers were investigated numerically and experimentally. Overall, the high-pressure sensitivity achieved was in good agreement with the numerically calculated value. Remarkably, they were able to obtain the highest-pressure sensitivity obtained for an FBG-based sensor experimentally, when compared to other FBG-based pressure sensors reported up to date.

Single-ring suspended fiber for Bragg grating based hydrostatic pressure sensing - Advances in Engineering

About the author

Dr. Lin Htein, received his M.S. degree from the Department of Physics, Mandalay University, Mandalay, Myanmar in 2005 and Ph.D degree from the Department of Photonics and Applied Physics, Gwangju Institute of Science and Technology, Gwangju, South Korea in 2014. He is currently working as a Research Associate at the Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong SAR.


About the author

Dr. Zhengyong Liu, received his B.Sc. degree in information engineering from Zhejiang University, Zhejiang, China, in 2010, and the Ph.D. degree from The Hong Kong Polytechnic University, Hong Kong SAR, in 2015. As part of the study during Ph.D., he spent half a year at Clemson University, Clemson, SC, USA, working on specialty optical fibers and high-power fiber lasers. His Ph.D. thesis was on the fabrication of specialty optical fibers and the development of their sensing applications. Currently, he is a Senior Research Fellow at The Hong Kong Polytechnic University.

His current research includes the design and fabrication of specialty optical fibers, fiber Bragg gratings, fiber-optic sensors especially for biomedical applications. He has published over 60 technical papers in peer-reviewed international journals and proceedings, including two book chapters. He also presented invited talks at international conferences like ICOCN, OFS-China, POEM.

About the author

Dr. Dinusha Serandi Gunawardena, received the B.Eng. degree from the Department of Electrical and Electronic Engineering, University of Nottingham Malaysia Campus, in 2013, and the Ph.D. degree from the Photonics Research Centre, University of Malaya, Malaysia, in 2017. She is currently a Postdoctoral Fellow at the Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong SAR.

Her research interests include photosensitivity in optical fibers, fiber Bragg grating sensors and applications, thermal sustainability, and related technologies.

About the author

Prof. Hwa-Yaw Tam, received his primary and secondary education in Sabah, Malaysia. He studied B.Sc. and Ph.D. in Electrical and Electronic Engineering at The University of Manchester (UK). From 1989 to 1993 he was with Hirst Research Center, GEC-Marconi Ltd. (London), working on optical components and systems, and erbium optical fiber amplifiers. He conducted pioneering works in optical fibre amplifiers, and in 1992 built two of the first batch of optical amplifiers for Italian PTT.

Prof. Tam joined The Hong Kong Polytechnic in 1993 and is currently the Head of Department, Chair Professor of Photonics at the Department of Electrical Engineering and Director of the Photonic Research Centre at The Hong Kong Polytechnic University. Prof. Tam established several world-class research facilities at PolyU, including two fibre-drawing towers for fabrication of photonics crystal fibres and polymer optical fibres, an ultra high-speed communication laboratory, and laser platforms for the fabrication of advanced fibre gratings.

His current research interests include fabrication of special optical silica fibres and polymer fibres, optical fibre communications, and fibre sensor systems based on fibre Bragg gratings and photonic crystal fibres. Prof. Tam published more than 500 technical papers and awarded/applied about 20 patents, has extensive international research collaborations with many universities around the world and is a keynote/invited speaker at more than 40 international conferences.

Prof. Tam has strong R&D collaboration with industry and his team installed many FBG sensing systems, including an FBG-based SHM system for the 610-m Canton Tower in Guangzhou, China and several condition-monitoring systems for railways in Hong Kong, China mainland, Taiwan, and India.

Currently, Prof. Tam’s R&D team is building the world’s first city-wide fibre-optic sensing network for condition-based monitoring of metro systems in Hong Kong. Prof. Tam won numerous international awards for his inventions, and is the Third Prize Winner of the Berthold Leibinger Innovationspreis 2014. Berthold Leibinger Innovationspreis is a biennial event and it is one of the highest remunerated international innovation prizes for laser technology


Lin Htein, Zhengyong Liu, Dinusha Gunawardena, Hwa-Yaw Tam. Single-ring suspended fiber for Bragg grating based hydrostatic pressure sensing. Volume 27, Number 7 | 2019 | OPTICS EXPRESS 9655.


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