In this section, a comparison between the three techniques for measuring temperature and strain simultaneously is presented. Table 4.3 summarises temperature and strain resolutions at 23km for all three techniques.
Table 4.3: Comparison of temperature and strain resolutions estimated in all three sensing techniques.
Measurands Brillouin LEAF Spontaneous
Frequency & Power Peak 1 & 2 Raman & Brillouin
Temperature(◦C) 4 29 6
Strain(µ²) 120 630 150
Analysis of the temperature and strain resolutions in all three techniques reveals that the Brillouin frequency and power based technique gives the best resolution for both measurands, whilst measurands resolution obtained using the spontaneous Raman and Brillouin based technique is better than those obtained with the multiple Brillouin peaks based technique. The temperature and strain resolutions using the LEAF fibre and the two frequency measurements were∼7 and∼5 times worse than that achieved using the Brillouin frequency and power measurements. The temperature and strain resolution using the spontaneous Raman and Brillouin signal were∼1.5 and∼1.2 times worse than those achieved using Brillouin frequency and power measurements. It is concluded that distributed fibre sensors based on LEAF or other fibres with multiple Brillouin peaks provide an exciting new possibility for extending range and accuracy. However, this preliminary investigation to compare the performance of the dual frequency analysis of the Brillouin spectrum to the peak frequency and power analysis of Brillouin spectra in SMF proved that there was no advantage of using such a technique at present. The feasibility of the spontaneous Raman and Brillouin based technique was demonstrated, and it may prove suitable, especially if combined with commercial Raman DTS. In fact, the large input power and high temperature sensitivity make the Raman technique
Chapter 4 Simultaneous Temperature and Strain Measurement Techniques 86 worth consideration for combined sensor applications at medium ranges. However, for a combined long range sensor >30km, the Brillouin frequency and power technique is considered to be the best candidate so far.
4.6
Conclusions
Three different techniques for measuring distributed temperature and strain simultane- ously were investigated and compared. Analysis of the two frequency components of the LEAF fibre to determine temperature and strain produced almost seven fold dete- rioration in the temperature resolution from 4◦C to ∼29◦C, and more than a five fold
deterioration in the strain resolution compared to the Brillouin frequency and power based technique in SMF. Raman anti-Stokes power was used to measure the tempera- ture and is independent of the strain, and this, combined with the Brillouin frequency shift measurement, allows the strain information to be determined. The results of this technique are encouraging and demonstrate its feasibility, with Raman sensitivity and higher input power making this technique worth considering, for example, combined with commercially available DTS. The best measurand resolutions obtained were with the Brillouin frequency and power based technique measured in SMF. This technique may be used in longer sensing range, provided that the accuracy of the power measure- ment can be improved. One obvious way is to launch a higher pulse power, as the signal strength is directly proportional to the pulse power. The next chapter investigates in de- tail the nonlinear effects that limit the launched pulse power and investigates a possible solution.
Bibliography
[1] S. M. Maughan, H. H. Kee and T. P. Newson, “Simultaneous Distributed Fibre Tem- perature and Strain Sensor using Microwave Coherent Detection of Spontaneous Bril- louin Backscatter,” Measurement Science and Technology, vol. 12, p. 834, February 2001.
[2] J. D. C. Jones, “Review of Fibre Sensor Techniques for Temperature-Strain Discrim- ination,” 12th International Conference on Optical Fiber Sensors Technical Digest
(OFS), vol. 16, p. 36, October 1997.
[3] C. C. Lee, P. W. Chiang, and S. Shi, “Utilization of a Dispersion-shifted Fiber for Simultaneous Measurement of Distributed Strain and Temperature Through Bril- louin Frequency Shift,” IEEE Photonics Technology Letters,, vol. 13, no. 10, pp. 1094–1096, October 2001.
[4] N. Shibata, R. G. Waarts and R. P. Braun, “Brillouin-gain Spectra for Single- mode Fibers Having Pure-silica GeO2-doped and P2O2-doped Cores,”Optics Letters, vol. 12, no. 4, p. 269, April 1987.
[5] A. H. Hartog, A. P. Leach, and M.P. Gold, “Distributed temperature sensing in solid-core fibres,” Electronics Letters, vol. 21, no. 23, p. 1061, November 1985.
[6] Distributed Temperature Sensor-DTS800, SENSA, www.sensa.org.
The Influence of Modulation
Instability on Spontaneous
Brillouin Based Sensors
5.1
Introduction
In the previous chapter, it was concluded that the Brillouin frequency and power based technique provides the best sensing results when compared to other techniques using standard SMF and its performance is expected to improve further provided that accurate measurement of the Brillouin power can be achieved. The Brillouin power accuracy is greatly dependent on the strength of Brillouin backscattered signal which in turns is proportional to the pulse energy within the sensing fibre. The pulse energy depends on the pulse peak power and width. However, the pulse width is limited by the spatial resolution requirement and the pulse power is limited by the need to avoid nonlinear effects. Typical nonlinear effects that affect spontaneous Brillouin based sensors in SMF are stimulated Raman scattering, stimulated Brillouin scattering, self-phase modulation, four wave mixing and modulation instability. This chapter initially reviews the nonlinear effects, and, since modulation instability was found to have the lowest threshold, this is then investigated in greater detail.
Chapter 5 The Influence of Modulation Instability on Spontaneous Brillouin Based
Sensors 90