Dr Sally Maughan's PhD Thesis: Distributed Fibre Sensing Using Microwave Heterodyne Detection of Spontaneous Brillouin Backscatter
First submission: September 2001
Revised following viva: March 2002
Copyright © Sally Maughan 2000-02
Brillouin scattering has been used for many years in optical fibre temperature and strain sensing applications, due to the dependence of both the power and the frequency shift of the scattered radiation on these two quantities. Simultaneous temperature and strain sensing is possible if both the power and frequency shift are measured. Much work has been published which presents either power or frequency shift measurements alone. Simultaneous measurement, however, has been less well documented, with all recent cases using direct detection of the backscattered power.
This thesis discusses heterodyne detection of spontaneous Brillouin backscatter, presenting the first simultaneous temperature and strain sensing results using this technique. Microwave beat frequencies of ~11GHz were used, with the use of a high bandwidth (20GHz) detector. Since the approximate value of the Brillouin frequency shift is ~11GHz, the backscatter may be mixed directly with the heterodyne local oscillator, avoiding the need for the complicated optical shifting used in previous heterodyne sensors. The high beat frequency also means that the signal is shifted out of the self-beat noise of the local oscillator and that independent measurement of either Stokes or anti-Stokes Brillouin power is possible.
A 25km microwave distributed heterodyne sensor was constructed with a spatial resolution of 20m, which demonstrated a temperature resolution of <4K and a strain resolution of <100µε. Frequency shift measurements alone were also obtained for a 57km sensing length, capable of <2.5K resolution at the distal end of the fibre in the absence of strain. Theoretical modelling of the noise properties of this sensor agreed well with experiment and provided a comparison of the sensor’s performance with those of direct detection and low beat frequency heterodyne detection.
As the input pulse power is increased, nonlinear processes ensue, compromising the accuracy of the sensor. These processes were investigated both experimentally and theoretically for a range of input pulse durations, focussing on their effect on Brillouin power and frequency shift measurement.
In this first section, the thesis is split up into manageable(ish) chunks for easier download..
- Title Page ( 20K )
- Abstract ( 23K )
- Acknowledgements ( 24K )
- Glossary ( 20K )
- Contents ( 39K )
- Chapter 1 - Introduction ( 41K )
- Chapter 2 - Distributed Optical Fibre Sensing: Overview ( 698K )
- Chapter 3 - Constituents and Design of a Coherent Brillouin Sensor ( 1.2M )
- Chapter 4 - Theoretical Analysis of the Sensor ( 451K )
- Chapter 5 - Temperature and Strain Sensing: Experiments ( 1.4M )
- Chapter 6 - Nonlinear Effects and Threshold Powers ( 718K )
- Chapter 7 - Conclusions ( 63K )
- Appendices/Publications ( 324K )
Here you can download the entire thesis in a compressed format, if you prefer.