Abstract
A Mach-Zehnder interferometer is formed in the transverse plane of an optical fibre. This is exploited to monitor the temperature of any given optical fibre in real time, without any predesigned temperature sensing elements engineered into the fibre and without a need to access the fibre ends. The interferometer is formed by the refraction at the air-glass interface and the subsequent internal reflections within the fibre. The temperature response of the fibre is then measured by quadrature phase shift detection of the interference pattern. For context, this is compared to finite element modelling and a previously studied Fabry-Pérot interferometer configuration. The temperature of an optical fibre was measured from room temperature up to 1174 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> C with an error of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm 2.6\%$</tex-math></inline-formula> for the Mach-Zehnder type interferometer. The measurement techniques presented offer a useful tool for precise temperature measurement for applications such as fibre post-processing.
Highlights
O PTICAL fibres have long been an invaluable tool to measure temperature themselves [1], for example, with fibre Bragg gratings [2], [3]
An external trigger opened the shutter of the CO2laser and simultaneously started the data acquisition of the two interferometers as well as the monitoring of the chemical composition grating (CCG)
With known calibration values for silica, both interferometers can be scaled for different fibre diameters and different probe wavelengths as described in Eq 7. This allows for the measurement of optical fibres of varying sizes with different probe beam wavelengths and for the measurement of any circular structure given a known optical path length (OPL) for each configuration. Both Mach-Zehnder and Fabry-Pérot interferometer configurations have been assessed with a CCG for measuring the temperature at a specific point on an optical fibre
Summary
O PTICAL fibres have long been an invaluable tool to measure temperature themselves [1], for example, with fibre Bragg gratings [2], [3]. Remote temperature sensing can be done using IR cameras or pyrometers for example, but these have limitations in precision and accuracy, for small, reflective or transparent objects such as optical fibres. These techniques are based on surface temperature only and require exact knowledge of the temperature-dependent emissivity of the material to be precise. The interferometric techniques discussed here are practical methods of remotely monitoring the temperature inside an optical fibre without disturbing it or accessing the ends of the fibre. These techniques can be utilised with only prior knowledge of the sample diameter and probe wavelength
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