Towards Simplified Optical Sensing: Enhancement of Interrogation Techniques based on Intensity Modulation


Publication date: 24/03/2023

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Examining board:

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MARIA JOSE PONTES Internal Examiner *

Summary: Optical fiber sensors can be straightforwardly utilized in places WHERE electrical instrumentation is dangerous and requires meticulous packaging for explosion insulation. For most of applications, cost is a key concern and choosing the best sensing system depends on making a trade-off between price and performance. Many low-cost optical sensors meet the demands of several applications, from personal healthcare to small-scale industrial operation. In this context, this Thesis describes two proposals to enhance low-cost interrogation techniques that require only an unmodulated broadband source and a few low-complexity optical components. Distributed interrogation technologies have explored different solutions to enhance the backscattered power and, as a consequence, increase accuracy in short-range sensing. The first technology herein presented is a distributed interrogation system based on
Transmission Reflection Analysis (TRA) with enhanced spatial resolution due to the use of a high-scattering optical fiber. During experiments, bending losses were induced at several points of the fiber, and a spatial resolution of 15 𝑐𝑚 was achieved over a 5.4-m fiber. The localization of a strong disturbance (normalized transmitted power of 0.035) with an estimated accuracy of up to 4.01 𝑚𝑚 near the source end and 4.69 𝑐𝑚 near the fiber end was analytically demonstrated. With regard to weak disturbances (normalized transmitted power of 0.8), the sensor accuracy has been estimated up to 3.19 𝑐𝑚 near the
source and 19.44 𝑐𝑚 near the fiber end. Passive edge filtering is one of the most inexpensive interrogation techniques, however it is prone to errors induced by the cross-sensitivity between temperature and strain. Another
technique herein proposed is a self-referenced solution for mitigation of temperature cross-sensitivity in strain measurements. The proposed technique consists of matching the reference and sensing fiber Bragg gratings (FBGs) at the ascending and descending slopes of a Fabry-P´erot filter. The Grey Wolf Opmitizer is utilized for spectral tuning. It has been shown that the proposed solution does not affect the system’s dynamic range and sensitivity, being compatible with different edge filters, including ultra-low-cost interferometers produced by straightforward manufacture processes. Temperature signals
with sinusoidal, random, linear and constant profiles were analyzed under simulation. For a strain-free condition, the normalized optical power remained essentially constant, exhibiting a maximum standard deviation of 2.1 × 10−3
. In a variable strain condition, strain measurements presented an average error of 3.4 % due temperature variations up to 30 ∘𝐶. Simulation and experimental results presented a maximum relative error of 5.79 %. Both aforementioned technologies integrate small and lightweight optical components, being easily incorporated into fully portable prototypes or embedded into smart textiles. Hybrid systems that merge both solutions utilizing the same broadband source can be further explored for accuracy enhancement. In addition, IoT modules can be easily connected to the acquisition hardware for applications based on Internet of Things (IoT).

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