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.
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.Go To OPTICS EXPRESS