A hybrid microwave heterodyne receiver design for use in distributed fibre sensing of spontaneous Brillouin backscatter

Optical fibre sensing has long made use of direct detection methods in order to detect a Brillouin backscatter signal which, due to its very nature, is able to divulge information pertaining to the temperature and strain being applied to an optical fibre over a distance of many tens of kilometres. As the resolution of both these measurands has improved, physical limits have been approached which have necessitated research into other forms of detection such as coherent detection. In this instance a weak backscatter signal is effectively amplified by an optical local oscillator signal generating a beat frequency at 11 GHz with all the information needed for temperature and strain sensing remaining ever present. This beat frequency is then detected before being subsequently analysed. At the University of Southampton previous work on this detection method has proved fruitful with the upshot being that research in this area is now being carried out both in an academic and industrial environment.

The research conducted in this thesis takes what has been demonstrated in previous coherent detection of Brillouin backscatter at the University of Southampton and builds upon its weakest point, namely that of having a spatial resolution of only 20m as determined by the resolution bandwidth of the electronic spectrum analyser incorporated as the previous sensor's receiver.

In order to improve this spatial resolution the electronic spectrum analyser has needed to be removed from the fibre sensor and to be replaced with purpose built microwave electronics in the form of a hybrid microwave heterodyne receiver. This has meant a stage of microwave heterodyning being incorporated via this new receiver in order to bring the 11GHz beat frequency down to a more amenable 1GHz second beat frequency, still with all the information necessary for temperature and strain sensing remaining on this signal. The subsequent incorporation of a bandwidth-tuneable bandpass filter centred at 1GHz has then allowed for improved spatial resolution to be observed. This 1GHz intermediate frequency was chosen as components at this frequency have better specifications in terms of noise figure and gain flatness than those at a higher frequency.

Theoretical models have been developed for noise and signal to noise evaluation of this new optical fibre sensor both before and during construction.

Results are highlighted in this thesis which demonstrate that the new hybrid receiver is efficient and powerful in its ability to improve the spatial resolution of the coherent optical fibre sensing technique. Simultaneous sensing of strain and temperature at a section 20km distant along a sensing fibre has demonstrated a temperature and strain resolution of <7.3K and <190µε when employing the sensor for 10m spatial resolution sensing together with <9.5K and <240µε when employing it for 4m spatial resolution sensing.

University of Southampton

Lawrence, Nicholas P.

09b33e69-50df-4392-985f-549992845f13

2002

Lawrence, Nicholas P.

09b33e69-50df-4392-985f-549992845f13

Lawrence, Nicholas P. (2002) A hybrid microwave heterodyne receiver design for use in distributed fibre sensing of spontaneous Brillouin backscatter. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)