Detection of electric current pulses by a fibre-optic sensor using spun fibre

We have studied distinctive features of the response of a fibre-optic current sensor with a spun-fibre sensing loop to short current pulses. It has been shown theoretically that, in a reflective interferometer, the polarisation mode dispersion (PMD) in the sensing spun fibre has no effect on the pulsed output signal of the sensor, in contrast to the PMD in a connecting fibre line. The response of the current sensor to a rectangular pulse has linear edges and its duration is determined by the relationship between the current pulse duration and the light propagation time in the spun fibre. The transition from a linear edge to the maximum sensor response is not accompanied by any transient process. The maximum response amplitude corresponds to the response to an equivalent direct current at a current pulse duration exceeding the light propagation time in the spun fibre. The present calculation results and experimental data agree well for current pulses longer than 300 ns at a light propagation time in the spun fibre of similar to 1 mu s.