More importantly, the effect of package thickness on bending sensitivity is justified in this project. Furthermore, the superior sensitivity of the fiber optic sensor in detecting a small scale deformation (strain and bending) of the material mode before fracture occurs, surpasses others in terms of failure inspection. The fiber optic sensor's pre-failure detection enables residents to take evacuation action immediately, before disaster strikes.
The most commonly implemented detection principles of optical fiber sensors are interferometry, grating and scattering.
Long Period Fibre Grating Sensor
Where 𝜆𝑟𝑒𝑠 is the resonance wavelength, 𝑛𝑐𝑜,𝑒𝑓𝑓01 is the effective index of refraction of the core mode and 𝑛𝑐𝑙,𝑒𝑓𝑓𝑚 is the effective index of the m-th cladding mode. In addition, extending the lattice period will introduce a similar effect on the sensor (the effect can be observed in Figure 1.3; (a) and (b)). Therefore, variations such as temperature, curvature, strain and external refractive indices can be observed by detecting the changes of the two parameters.
Fibre Bragg Grating Sensor
As the coupled mode propagates along the cladding, it is highly sensitive to the change of the surrounding refractive index, which in turn changes the propagation constant of cladding mode causing attenuation drops and phase shift. Any extension in the FBG will increase the grating period, and consequently this leads to a shift in the Bragg wavelength (as can be seen in eq. 2.2). Thus, by measuring the shift in the Bragg wavelength, structural deformation can be detected down to the scale of micron deformation (𝜇𝜀).
Mach-Zehnder Interferometer Sensor
When the cladding modes propagate along the interferometric length, it is very sensitive to the change of the curvature, where the bending will cause a huge signal loss. The adiabaticity of the MZI fiber, which indicates the degree of loss, depends on the dimension of the taper, such as the local length scale of the taper, 𝑧𝑡, and the angle of taper, Ω(𝑧). The fiber taper is considered adiabatic as 𝑧𝑡 ≫ 𝑧𝑏 , where the cladding propagation loss is negligible.
In most applications of the MZI sensor in strain detection, the non-adiabatic taper proves to be more preferable in terms of sensitivity, as a more pronounced bending effect can be observed through significant attenuation. As can be seen in Figure 2.8, the reduction in FSR is observed in the transmission spectra as the interferometric length varies from 20 mm to 40 mm. Calculating the criterion of the aforementioned fiber optic sensors, a comparison table consisting of the respective sensitivities, manufacturing methods, advantages and limitations is summarized as in Table 2.1.
Cross Sensitivity in Sensors
The mutually compensating transfer function matrix is the technique commonly used for most sensors. The tapered MZI sensor has thermal sensitivity competitive with LPG and FBG, and is highly sensitive to micro-bending. The temperature effect contributed to the MZI sensor can also be compensated for using the mutually compensating transfer function matrix, which as reported in a journal by Raji, et.
Fragility of Fabricated Segment in Sensors
On that note, brittleness is inevitably introduced into the fiber lattice during the fabrication process. As the fiber is illuminated under an intense UV laser light, the silicon-oxygen bonds are broken, resulting in a slight increase in the refractive index, at the same time causing damage to the fiber structure (Doyle & Crispin, 2003). With the combination of reasons of high curvature sensitivity, low cost and ease of manufacture, the MZI sensor is chosen to justify performing further curvature calibrations.
The design criterion of the MZI sensor in manufacturing and packaging will be discussed in the next section.
Fabrication Criteria
Packaging of Sensors
But not as prevalent as FBGs, the effect of packaging for conical MZI sensor is not popular in the research. The division of modes in the MZI sensor differs with its cone diameters and cone lengths. Practically speaking, MZI sensor with exactly identical taper diameters and taper lengths are difficult to duplicate.
Thus, each MZI sensor is unique in its sensitivity to curvature, which implies that the characterization must be performed using the same sensor. In the characterization phase, the optical output (power) of the sensor is calibrated to the physical curvature. A packaging design of MZI sensors is depicted as in Figure 3.4, which consists of two polypropylene sheets with dimension 12 cm × 2 cm × 0.1 cm.
The packaged MZI sensor is stored for several hours to ensure that the cyanoacrylate is fully cured before it is ready for testing. The packaged MZI sensor is input to a tunable optical source (single wavelength laser, with the wavelength initially set at 1310 nm) and output to an optical power meter, as shown in Figure 3.6(b). The MZI sensor was attached to the center of an 8-meter steel bar using cyanoacrylate glue.
Calibration Based on Various Wavelengths
The backtracking process is proposed at the end of the calibration to prove the real practicality of the sensor. Where in this section, the optical power is recorded by the optical power meter for each successive charge and discharge. In this section, the bending sensitivities of the MZI sensor in intact thickness packaging will be characterized based on three operating wavelengths (1310 nm, 1490 nm, and 1550 nm).
The operating region (also referred to as the linearity region) was selected from the raw data so that regions of nonlinearity were excluded in the analyses. The non-linearity region relates the optical power to the weakly imposed weight, so it should be excluded as it contains no analyzable information. Despite this, the region of linearity gives a clear and analyzable relationship between the optical power and the load, which is labeled as the region of operation of the sensor.
The curvature sensitivity of the MZI sensor can be characterized within the region by calibrating the optical power to the appropriate curvature. Differences in both trend and offset are observed among the response slopes of the three wavelengths. As a result, the sensitivities of the packaged MZI sensor at the wavelength of 1310 nm, 1490 nm, and 1550 nm are different, which are 0.461 𝜇Wm-1, 0.346 𝜇Wm-1, and -1.10 𝜇Wm-1, respectively. These phenomena can be explained by the wavelength and polarization dependent properties of the MZI sensor, which will be described in more detail in the following sections.
Wavelength Dependent Property of MZI Sensor
Polarization Dependent Property of MZI Sensor
Thereby we can deduce that the variation of the polarization state in the MZI sensor contributes to fluctuations in the offset power. The next section adds an additional 2mm plate to the sensor in two configurations (as discussed earlier). Therefore, the newly thickened sensor is expected to have a deviation in the offset power from the original sensor.
Packaging of the sensor is the highlight of this project, where the thickness of the packaging is an important criterion to be characterized. The thickness of the packaging was intuitively expected to affect the sensitivity of the sensor in terms of strain (the degree of elongation). The package center (location of MZI sensor) is defined using distance, y outward away from the natural axis.
Geometrically, the location of MZI sensor, y for thickness A is higher than the pristine thickness and thickness B. Thus, sensor with thickness A is expected to experience a higher stress than the pristine thickness and thickness B (i.e. it bends more compared to the other two). Therefore, it can be concluded that the sensitivity of sensor in thickness A is relatively better, while thickness B will not significantly affect the sensitivity.
Comparison of the Pristine Thickness, Thickness A and B
To compensate for the power deviation caused by the change of the transmission path, the polarization controller is added to the sensor system. Nevertheless, introducing the polarization controller into the system will result in signal attenuation up to -5 dBm. Curvature calibration for sensor with the untouched 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 optical power offset so that the offset power is the same as that obtained in the pristine thickness calibration. In Figure 4.13, 4.14 and 4.15, sensor for each thickness is calibrated to the equivalent offset power using polarization controller, for ease of comparison. The changing trends of the optical power are found to be identical for the three thicknesses when tested at wavelength 1310nm.
The result matches the expectations in Section 4.2, where the sensitivity of the sensor in thickness A is expected to be higher than that of the other two, as shown in the table 𝜇Wm-1 for thickness A, -1.33 𝜇Wm-1 for the original thickness and - 1.90 𝜇Wm -1 for thickness B. As for the sensor in thickness B, the sensitivities do not deviate much from the original thickness for all three wavelengths. However, the slope trend is reversed at thickness A for 1490 nm and 1550 nm wavelength.
Disparity in Trend of Optical Power Slope
As an aid to the figure, some intersection points (A, B, C and D) between the two spectra have been selected to show how the deviation in trend is possible. From the previous section, the MZI sensor was characterized in terms of package thickness, sensitivity (gradient) and offset power. In the last part of the project, the curvature was traced back with a thickness sensor A at a wavelength of 1310 nm.
Polypropylene packaging was introduced to the fiber-based inline Mach-Zehnder Interferometer sensor to protect the sensor under harsh conditions of the real-world sensing environment. A polarization controller was used to offset the deviation, so that the optical power of the three thicknesses could be compared based on the same offset value. Comparison of the effect of cyanoacrylate- and polyurethane-based adhesives on a longitudinal deformation solitary wave in layered PMMA waveguides.
In Proceedings of the SPIE, Sensors, and Command, Control, Communications, and Intelligence (C3I) Technologies for Homeland Security and Homeland Defense IV, 5778, p. A highly sensitive fiber optic refractive index sensor based on a long period edge-written fiber grating. Highly sensitive optical refractometer based on edge-written ultra-long-period fiber grating formed by periodic grooves.
