Optical Fibre Sensing

We research and develop techniques that utilise optical fibres, with applications as sensors for industry.

Optical fibres are thin strands of glass, typically around 0.1 mm in diameter, with a  narrow core (around 8 µm in diameter) running along their length. The core is doped to have a refractive index slightly higher than the surrounding cladding. Optical fibres guide light along their core and are widely used for telecommunications applications such as the internet. However, we use optical fibres as sensors for the remote monitoring of infrastructure, where they have many advantages over electrical cables: they are resistant to corrosion, immune to electromagnetic interference, able to access confined spaces, and they are not an ignition hazard.

A femtosecond laser is used to write a core along a sapphire optical fibre.

Fibre Bragg Gratings

We create sensors in optical fibres by focussing a femtosecond laser through the side to selectively expose regions of the glass on a micrometre scale, permanently modifying its refractive index. By writing a periodic structure along the length of the fibre core we form a Fibre Bragg Grating (FBG). An FBG reflects a specific wavelength of light which is determined by its period. Parameters such as strain and temperature change this period and hence the wavelength of the reflected light, enabling the fibre to act as a sensor. We can create many FBGs along a single fibre, enabling sensing at multiple points.

Microscope image of an optical fibre incorporating a fibre Bragg grating.

Publications

Femtosecond fiber Bragg grating fabrication with adaptive optics aberration compensation 
Patrick S. Salter, Matthew J. Woolley, Stephen M. Morris, Martin J. Booth & Julian A. J. Fells. Optics Letters 43, 24, 5993-5996 (2018).

Optical fibre temperature and strain sensors

Fibre Bragg Gratings have the ability to measure strain or temperature, but it is generally not possible to distinguish between them. We have pioneered a method of changing the temperature dependence of an FBG by injecting novel materials into the glass fibre. The optical properties of the injected material are 50 times more sensitive to temperature changes than the silica glass and respond in the opposite direction. Using this technique, we have managed to fabricate FBGs that are temperature insensitive, and others that are extremely temperature sensitive.

Optical Fibre Bragg Grating with fluid-filled microchannels
in the cladding to alter temperature sensitivity.

Our work includes the installation of FBG temperature sensors on gas turbine blades. This allows the measurement of temperature distribution over the blade to determine the effectiveness of blade cooling schemes.

A gas turbine blade instrumented with Fibre Bragg Grating temperature sensors.

Publications

Fiber Bragg grating sensors with ultrahigh or ultralow temperature sensitivity
Zipei Song, Mohan Wang, Patrick S. Salter, Tongyu Liu, Steve J. Elston, Martin J. Booth, Stephen M. Morris & Julian A. J. Fells. Technical Digest Series (Optica Publishing Group, 2023) paper W2.2

Measurement of Internal Temperatures in Engine-Scale Gas Turbine Blade with Fiber Optic Sensors
Emily K. Berexa, Ben Coulton, Julian A. Fells & Peter T. Ireland. AIAA 2024-2574. AIAA SCITECH 2024 Forum (2024) 

Optical fibre pressure sensors

We create pressure sensors by using laser-assisted etching to make voids in the fibre cladding along the optical fibre on one cross-sectional axis. We place a Fibre Bragg Grating in the core between the voids. Pressure causes a distortion in the fibre, resulting in a stress-induced change in its refractive index. This change is different for the two orthogonal polarizations of light, causing the FBG to reflect light at two different wavelengths. By measuring the wavelength separation between the two reflection peaks, we can measure the pressure experienced by the fibre.

Microscope image of an optical fibre pressure sensor.

Publications

3D Structured Optical Fiber Pressure Sensors
Tongyu Liu, Zipei Song, Richard J. Reeves, Mohan Wang, Crispin T.M. Doyle, Martin J. Booth & Julian A.J. Fells. Journal of Lightwave Technology 42, 18, 6375–6380 (2024)

Optical Fibre Gas Sensors

The group has developed gas sensors using hollow-core fibres filled with gases, using spectroscopy to detect the concentration of different molecules. The long path length of optical fibres improves the sensitivity of absorption measurements but gives a poor time response. We have solved this problem by using a femtosecond laser to machine holes through the side of the fibre, spaced along its length.

Scanning electron microscope image of a hollow core fibre gas sensor
incorporating laser-micromachined side access channels.

Publications

A High Sensitivity, Fast Response Optical Fiber Gas Sensor using Micro-drilled Anti-Resonant Fiber 
Eleanor A. Warrington, Robert Peverall, Patrick S. Salter, Gus Hancock, Martin J. Booth, Grant A. D. Ritchie & Julian A. J. Fells. Technical Digest Series (Optica Publishing Group, 2023), paper Tu3.10

Sapphire Optical Fibre Sensors

Conventional optical fibres can withstand temperatures of up to around 1000 °C, but this is insufficient for some applications. We are fabricating sensors in sapphire optical fibre which can withstand temperatures of up to 2000 °C. These also have the advantage of being radiation hard and extremely chemically resistant. They have potential applications for aeroengine monitoring as well as for space and nuclear fusion applications.

A problem with sapphire fibre is that, unlike conventional optical fibre, it does not have a core. Light is guided by the air-sapphire interface. This means that, instead of observing a single wavelength peak from an FBG, there are many reflection peaks across a broad spectrum, severely compromising measurement precision. We have solved this problem by using a femtosecond laser to write a core along the length of the fibre, such that only a single optical mode can propagate. Within this core there is a Bragg grating, which gives a single wavelength peak and very high precision. 

Single-mode sapphire optical fibre with a Bragg grating.

Publications

Single-mode sapphire fiber Bragg grating
Mohan Wang, Patrick S. Salter, Frank P. Payne, Adrian Shipley, Stephen M. Morris, Martin J. Booth & Julian A. J. Fells. Optics Express 30, 9, 15482–15494 (2022) https://doi.org/10.1364/OE.446664

Single-Mode Sapphire Fiber Temperature Sensor 
Mohan Wang, Patrick S. Salter, Frank P. Payne, Adrian Shipley, Igor N. Dyson, Tongyu Liu, Tao Wang, Kaihui Zhang, Jian Zhang, Zhitai Jia, Stephen M. Morris, Martin J. Booth & Julian A. J. Fells. Journal of Lightwave Technology 42, 18, 6409–6416 (2024).