To offset the power deviation caused by the changing of transmission path, polarization controller is added into the sensing system. Nevertheless, introducing the polarization controller into the system will result signal attenuation up to -5 dBm.
Therefore, pristine power offset is inevitable changed. Curvature calibration for sensor with the pristine thickness is repeated, the result is taken as a new reference to compare the effect of the two packaging thicknesses. As the calibration proceeds to thickness A and B, the polarization controller is used to offset the deviation of optical power, so that the offset power is same as what obtained in the calibration of the pristine thickness.
In Figure 4.13, 4.14and 4.15, sensor for each thicknesses were calibrated to the equivalent offset power with aids of polarization controller, for ease of comparison.
The changing trends of the optical power is found identical for the three thicknesses when tested at wavelength 1310nm. The result tallies with the expectation in section 4.2, where the sensitivity of sensor in thickness A is expected higher than the another two, as reported in Table 4.1, -3.27 πWm-1 for thickness A, -1.33 πWm-1 for pristine thickness, and -1.90 πWm-1 for thickness B. As for sensor in thickness B, the sensitivities do not deviate a lot from pristine thickness for all the three wavelengths.
However, the trend of slope is inverted at thickness A for both wavelength 1490 nm and 1550 nm. The contradiction in trend of slope of will be explained in the following part.
Figure 4.16: Loading Spectra in Arbitrary State 1 and 2
Figure 4.16 shows two spectrum in responding to maximum variation of polarization states, by tuning the 2nd and 3rd paddle ( π
2 and π
4 paddle). In aids of the figure, few points of intersection (A, B, C and D) between the two spectrum were selected to demonstrated the how the deviations in trend is possible to happen. Among these points, the same value of optical power is observed for the two spectrum at the same operating wavelength, where the two spectrum are contributing to a different trend of slope (more obvious at point B, C and D). This implies that the identical offset power measured can come from different spectrum which might consequent to disparity in trend. This explained the contradiction in trend of optical power slope for the case of thickness A at both wavelength 1490 nm and 1550 nm.
Back-tracing of Curvature
From the previous section, MZI sensor was characterised according to the packaging thicknesses, in term of the sensitivity (gradient) and the offset power. At the last part of the project, the curvature was back-traced using the sensor with thickness A at wavelength 1310 nm. The optical power in responding to the loading and unloading was recorded, and back-traced using the characterised offset and gradient. As shown below is the back-tracing equation of curvature for MZI sensor with thickness A which operates at wavelength 1310 nm.
Arbitrary state 1
Arbitrary state 2 A
B
C
D
πΆππ π‘ππππ‘ππ = πππ‘ππππ πππ€ππ β πππ‘ππππ πππ€ππ ππππππππππ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ Μ
πΊπππππππ‘
The mean difference between each measured optical power and the respective characterised data is computed to offset the values, if there is any fluctuation in the optical power from the characterised data. Figure 4.17 shows the comparison of the back-traced curvature and the characterised curvature, with error bar of 14% and correlation values of 0.9928 (loading slope) and 0.9887 (unloading slope).
Figure 4.17: Correlation between the Back-traced Curvature and the Characterised Curvature (with error bar of 14%)
0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010
0 2 4 6 8 10 12
Curvature (m-1)
5 CONCLUSION AND RECOMMENDATIONS
Conclusion
Polypropylene packaging was introduced to the fibre-based in-line Mach-Zehnder Interferometer sensor to protect the sensor under harsh condition of the real sensing environment. At the first part of calibration, the packaged MZI sensor was characterised based on the imposed curvature at various wavelengths. Disparity in trend and offset power were observed among the three different operating wavelengths due to the wavelength dependent property of MZI sensor. Besides, polarization dependent property of MZI sensor also contributes to fluctuation of power. A polarization controller was used to offset the deviation, so that the optical power of the three thicknesses can be compared based on the same offset value. Packaging with thickness A was found to have the best curvature sensitivity than the other two, where the optimal sensitivity is up to is -3.27 πWm-1. The packaged MZI sensor is capable to detect minimum curvature of 0.25 km-1 and maximum curvature radius up to 4 km, which is considerably sensitive in monitoring the structural health.
Future Works
Proceeding to this project, the packaged sensor is suggested to embedded into the concrete reinforcement bar to implement the real sensing condition. Besides, to improve the accuracy of curvature sensing, temperature compensation is suggested to be carried out using the mutual-compensating technique which has been discussed in section 2.2.1.
Furthermore, the distributed sensing system is proposed to be implemented, to cater the multiple points curvature sensing. Wherein, several multiplexing techniques are suggested as follow for the distributed sensing module. The common found multiplexing technique in MZI sensor is TDM. Figure 5.1 illustrates MZI multiplexing technique using TDM in time domain. Single wavelength source is input into a modulator and an 1ΓN splitter before entering the sensor array. A modulator is used to manipulates N pulses vary with time, and split them accordingly into the arrays by a splitter. Fibre loops in each array cause delay to the input signal in different extent, therefore only allow pulse at certain time frame pass through.
Figure 5.1: Schematic Diagram of MZI Multiplexing Technique using TDM Figure 5.2 illustrates the MZI multiplexing technique using Subcarrier Multiplexing Method (SMM), which is a costlier but more complicated method. The configuration is slightly different at the modulator part, where the modulators are allocated in every arrays after the splitter, to produce the signal in distinct pulses at various frequencies. The signals with distinct frequency pass though the sensor and respond to the curvature change. (any additional points to add in?)
Figure 5.2: Schematic Diagram of MZI Multiplexing Technique using SDM
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APPENDICES
APPENDIX A: Derivation of the strain equation
Figure A.1: Dimension of packaging slab in curvature, C
(a) (b)
Figure A.2: Segment of packaging slab for derivation demonstration
From Figure 0.2 (b),
π
βπ₯ = π β π¦
βπ β²
From Figure 0.2 (a),
Nature axis, βπ =βπ₯ Hence,
βπ β² =π β π¦ π βπ By knowing that the strain is equated as,
π =βπ β²β βπ₯
βπ Therefore,
π = π¦ π