Fibre optic methods for measuring detonation velocity

Abstract

Throughout this project, a number of fibre optic systems have been developed in order to measure detonation velocity — the propagation speed of a shockwave through an explosive medium. The aim of this work has been to utilise the small size and high speed infrastructure of fibre optic systems to develop precise, high spatial resolution, embedded fibre optic velocity probes.

The three key developments, demonstrated through a combination of simulations, laboratory testing and explosive trials are: an optimised chirped fibre Bragg grating (CFBG) measurement system; a brand new uniform fibre Bragg grating (UFBG) measurement system, and; a simplified, low-cost active fibre measurement system.

It has been discovered that CFBG velocity measurements are prone to innate nonlinearities due to Fourier limitations on the grating bandwidth. These effects can be mitigated if the CFBGs are designed with a high chirp-rate and a low reflectivity. This is shown using transfer-matrix simulations and in explosive tests — where the longest continuous CFBG detonation velocity measurement (24 cm) is also demonstrated.

Explosive test results from a new UFBG velocity probe show a maximum noise level that is an order of magnitude lower than similar CFBG tests — demonstrating a noise-limited spatial uncertainty below 10 m.

Tests on Er and Er/Yb doped fibres demonstrate the potential for these fibres to be used as strain-insensitive detonation velocity probes. This is put into practice by implementing the fibres in a helical geometry — amplifying the spatial precision tenfold and resulting in a 2 mm uncertainty over a 100 mm measurement range.

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