A pulse propagating along a length of optical fibre generates an echo that contains its fingerprint. This fingerprint can be made sharper depending on the pulse width, whereas the fibre length determines how frequently it can be accessed. The echo is caused by Rayleigh backscattering and has already been widely exploited in optical time domain reflectometry, offering information on the features of optical fibres in a distributed fashion.

This paper by Murray et al. demonstrates how the ability to manipulate and process Rayleigh backscattering can reveal information on the spectral content of the forward-propagating signal with very high resolution. Normally, a trade-off exists between the extent of the bandwidth that can be measured and the spectral resolution of the measurement. To overcome this limitation, this paper introduces a technique based on the successive use of pulses of two different durations, the combined analysis of which offers the final result, while at the same time it removes any spurious points from the measurement.

This paper is a prime example of how optics can offer solutions to the processing of signals by means that have no obvious electronic equivalent. In the specific case studied in the paper, this is applied to the RF spectral analysis of signals over a broad bandwidth, with high resolution and at a fast refresh rate, these combined attributes would be both difficult and expensive to be implemented with the electronic analogue-to-digital converters available today.

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