This thesis studies frequency-shifted interferometry (FSI), a useful and versatile technique for fiber-optic sensing. I first present FSI theory by describing practical FSI configurations and discussing the parameters that affect system performance. Then, I demonstrate the capabilities of FSI in fiber-optic sensor multiplexing and high sensitivity chemical analysis. We implemented a cryogenic liquid level sensing system in which an array of 3 fiber Bragg grating (FBG) based sensors was interrogated by FSI. Despite sensors' spectral overlap, FSI is able to separate sensor signals according to their spatial locations and to measure their spectra, from which whether a sensor is in liquid or air can be unambiguously determined. I showed that a broadband source paired with a fast tunable filter can be used in FSI systems as the light source. An array of 9 spectrally overlapping FBGs was successfully measured by such a system, indicating the potential of system cost reduction as well as measurement speed improvement. I invented the the FSI-CRD technique, a highly sensitive FSI-based fiber cavity ring-down (CRD) method capable of deducing minuscule loss change in a fiber cavity from the intensity decay rate of continuous-wave light circulating in the cavity. As a proof-of-principle experiment, I successfully measured the fiber bend loss introduced in the fiber cavity with FSI-CRD, which was found to be 0.172 dB/m at a bend radius of 12.5 mm. We then applied FSI-CRD to evanescent-field sensing. We incorporated fiber tapers as the sensor head in the system and measured the concentration of 1-octyne solutions. A minimum detectable 1-octyne concentration of 0.29% was achieved with measurement sensitivity of 0.0094 dB/% 1-octyne. The same system also accurately detected the concentration change of sodium chloride (NaCl) and glucose solutions. Refractive index sensitivity of 1 dB/RIU with a measurement error of 1*10^-4 dB was attined for NaCl solutions. Finally, I proposed a theoretical model to study the polarization effects in FSI systems. Preliminary results show that the model can already explain the experimental observations. It not only provides insight into how to improve system performance but also suggests potential new applications of the technique.

This thesis studies frequency-shifted interferometry (FSI), a useful and versatile technique for fiber-optic sensing. I first present FSI theory by describing practical FSI configurations and discussing the parameters that affect system performance. Then, I demonstrate the capabilities of FSI in fiber-optic sensor multiplexing and high sensitivity chemical analysis. We implemented a cryogenic liquid level sensing system in which an array of 3 fiber Bragg grating (FBG) based sensors was interrogated by FSI. Despite sensors' spectral overlap, FSI is able to separate sensor signals according to their spatial locations and to measure their spectra, from which whether a sensor is in liquid or air can be unambiguously determined. I showed that a broadband source paired with a fast tunable filter can be used in FSI systems as the light source. An array of 9 spectrally overlapping FBGs was successfully measured by such a system, indicating the potential of system cost reduction as well as measurement speed improvement. I invented the the FSI-CRD technique, a highly sensitive FSI-based fiber cavity ring-down (CRD) method capable of deducing minuscule loss change in a fiber cavity from the intensity decay rate of continuous-wave light circulating in the cavity. As a proof-of-principle experiment, I successfully measured the fiber bend loss introduced in the fiber cavity with FSI-CRD, which was found to be 0.172 dB/m at a bend radius of 12.5 mm. We then applied FSI-CRD to evanescent-field sensing. We incorporated fiber tapers as the sensor head in the system and measured the concentration of 1-octyne solutions. A minimum detectable 1-octyne concentration of 0.29% was achieved with measurement sensitivity of 0.0094 dB/% 1-octyne. The same system also accurately detected the concentration change of sodium chloride (NaCl) and glucose solutions. Refractive index sensitivity of 1 dB/RIU with a measurement error of 1*10^-4 dB was attined for NaCl solutions. Finally, I proposed a theoretical model to study the polarization effects in FSI systems. Preliminary results show that the model can already explain the experimental observations. It not only provides insight into how to improve system performance but also suggests potential new applications of the technique.

The main objective of this research is to explore, investigate and develop the new data analysis techniques along with novel sensing technologies for structural health monitoring applications. The study has three main parts. First, a systematic comparative evaluation of some of the most common and promising methods is carried out along with a combined method proposed in this study for mitigating drawbacks of some of the techniques. Secondly, nonparametric methods are evaluated on a real life movable bridge. Finally, a hybrid approach for non-parametric and parametric method is proposed and demonstrated for more in depth understanding of the structural performance. In view of that, it is shown in the literature that four efficient non-parametric algorithms including, Cross Correlation Analysis (CCA), Robust Regression Analysis (RRA), Moving Cross Correlation Analysis (MCCA) and Moving Principal Component Analysis (MPCA) have shown promise with respect to the conducted numerical studies. As a result, these methods are selected for further systematic and comparative evaluation using experimental data. A comprehensive experimental test is designed utilizing Fiber Bragg Grating (FBG) sensors simulating some of the most critical and common damage scenarios on a unique experimental structure in the laboratory. Subsequently the SHM data, that is generated and collected under different damage scenarios, are employed for comparative study of the selected techniques based on critical criteria such as detectability, time to detection, effect of noise, computational time and size of the window. The observations indicate that while MPCA has the best detectability, it does not perform very reliable results in terms of time to detection. As a result, a machine-learning based algorithm is explored that not only reduces the associated delay with MPCA but further iii improves the detectability performance. Accordingly, the MPCA and MCCA are combined to introduce an improved algorithm named MPCA-CCA. The new algorithm is evaluated through both experimental and real-life studies. It is realized that while the methods identified above have failed to detect the simulated damage on a movable bridge, the MPCA-CCA algorithm successfully identified the induced damage. An investigative study for automated data processing method is developed using nonparametric data analysis methods for real-time condition maintenance monitoring of critical mechanical components of a movable bridge. A maintenance condition index is defined for identifying and tracking the critical maintenance issues. The efficiency of the maintenance condition index is then investigated and demonstrated against some of the corresponding maintenance problems that have been visually and independently identified for the bridge. Finally, a hybrid data interpretation framework is designed taking advantage of the benefits of both parametric and non-parametric approaches and mitigating their shortcomings. The proposed approach can then be employed not only to detect the damage but also to assess the identified abnormal behavior. This approach is also employed for optimized sensor number and locations on the structure.

The article describes measurements of strains of concrete, steel and textile reinforcement with distributed fiber optic sensors (DFOS). The technology of distributed strain measurements gains currently increasing attention within the civil engineering field and indeed the DFOS can be applied in various measurement scenarios providing results and insights which were not possible before. Within this article, the fibers and adhesives that are most commonly used are compared and several measurement scenarios and their results are described, including precise strain measurements with high resolution as well as measurements on large-scale specimens. Concrete strains were measured in a multiaxial compression stress state and also during setting and hardening and in flexural tests. Strains of the steel and textile reinforcement were monitored along the bond zone and also in flexural tests. Finally, cracking patterns were observed and compared with digital image correlation methods. Validated examples of applications of DFOS in laboratory work are described.

Distributed fibre optic sensing (DFOS) is essential for structural health monitoring (SHM) of strain changes induced during the lifetime of a structure. Among different DFOS systems, the Brillouin Optical Time Domain Reflectometry (BOTDR) takes the advantages of obtaining full frequency spectrum to provide strain and temperature information along the optic fibre. The key parameters of distributed fibre optic sensors, spatial and frequency resolution, are strongly linked with the time-frequency (T-F) localisation in the system in three parts: pulse, hardware design and optical fibre. T-F localization is fundamentally important for the communication system, whereas in this study the importance of the T-F localisation to the spatial and frequency resolution, repeatability and the measurement speed are introduced in BOTDR. In this dissertation, the development of DFOS is first introduced, including both traditional methods and new developed designs. The literature review shows the signal to noise ratio (SNR) of BOTDR can be improved by investigating its T-F localisation. In the hardware design, in order to improve the T-F localisation in hardware architecture, a Short-Time Fourier Transform-Brillouin Optical Time-Domain Reflectometry (STFT-BOTDR), which implements STFT over the full frequency spectrum to measure the distributed temperature and strain along the optic fibre, is applied so that the conventional frequency sweeping method can be replaced for high resolution and fast speed measurement, providing new research advances in dynamic distributed sensing. The STFT based BOTDR has better T-F localisation, which in turn provides an opportunity for off-line post signal processing that is more adaptable for fast speed measurements. The spatial and frequency resolution of dynamic BOTDR sensing is limited by the Signal to Noise Ratio (SNR) and the T-F localization of the input pulse shape. The T-F localized input pulse shape can enhance the SNR and the spatial and frequency resolution in STFT-BOTDR. In this study, simulation and experiments of T-F localized different pulses shapes are conducted to examine the limitation of the system resolution. The result indicates that a rectangular pulse should be selected to optimize the spatial resolution and a Lorentzian pulse could be chosen to optimize the frequency resolution, while a Gaussian shape pulse can be used in general applications for its balanced performance in both spatial and frequency resolution. Meanwhile, T-F localization is used for pulse T-F localisation optimisation. A set of Kaiser-Bessel functions is used to simulate different pulse shapes and to compare their parameters in terms of T-F localisation and their Brillouin scattering spectrum. A method using an iterative filtering algorithm to achieve the optimised pulse in terms of T-F localisation is introduced to converge the Effective-pulse Width (TEW) in the time-domain and Effective-pulse Linewidth (FEL) in the frequency domain to identify the fundamental limitations. The optimised pulse can be fitted with a 7th order Gaussian (super-Gaussian) shape and it offers the best experimental performance compared to a Rectangular pulse. The sensitivity of a sensor to strain or temperature variations due to distributed Brillouin scattering is closely related to the power distribution on the Brillouin scattering spectrum which is related to the property of the optic fibre. The performance of a highly nonlinear fibre that can generate a higher Brillouin scattering signal is compared to that of a standard single mode fibre. The results show that much higher SNR of the Brillouin scattering spectrum and smaller frequency uncertainties in the sensing measurement can be achieved by using a highly nonlinear fibre for comparable launched powers. With a measurement speed of 4 Hz, the frequency uncertainty can be 0.43 MHz, corresponding to 10 με in strain or 0.43°C in temperature uncertainty for the tested highly nonlinear fibre. In contrast, for a standard single mode fibre, the value would increase to about 1.02 MHz (25 με or 1.02°C), demonstrating the advantage of the tested highly nonlinear fibre for distributed strain/temperature sensing. Results show that, by using a small effective area highly nonlinear fibre, the strain or temperature resolution can be improved because it generates stronger Brillouin scattering signal with high SNR and high Q factor spectrum, both of which determine the optimal averaging time in a single measurement. In general, the STFT-BOTDR can achieve 1 m spatial resolution, 10 με frequency resolution on a 10 km fibre with measurement speed at about 2.5 kHz.

Předložená disertační práce zkoumá možnosti použití nového typu polovodičového MGY- Laseru elektricky laditelného v širokém spektrálním rozsahu a zabývá se možnostmi jeho nasazení v optovláknové senzorové síti založené na metodě FBG (Fiber Bragg Grating). Výzkum byl započat komplexními dlouhodobými testy reálného měřícího scénáře z oblasti stavebnictví, sestaveného pro účely ověření limitujících aspektů současných technik. Inženýrské aplikace nabízejí velké množství vzájemně se vylučujících požadavků pro návrh strukturálních senzorových systémů. Tyto požadavky jsou sdíleny mnoha dalšími technologickými oblastmi, což přispívá k vysokému stupni univerzálnosti použití dosažených výsledků. Na základě posouzení stavu současné techniky a aplikačních požadavků byly v práci nejprve identifikovány aspekty, které mají být výzkumem zlepšeny. V dalším kroku byl detailně charakterizován MG-Y laser Syntune/Finisar S7500. Na základě dat získaných měřením byla zkoumána nová metoda spojitého řízená vlnové délky záření laseru. Provedené experimenty vedly nejen k návrhu nového způsobu spojité regulace vlnové délky ale také k vytvoření prostředků pro vlastní kalibraci systému na základě jeho vnitřních vlastností (podélných módů rezonátoru).

The purpose of this project is to explore the FBG (Fiber Bragg Grating) technology and create a force sensor. The result can be used as a basis for further projects.The project starts with force and strain measurements. The project then evolves to incorporate multiple FBG sensors. An uncommon method of writing the FBG withcoating is tested, which results in a FBG with most of the coating left.The result is a multi-FBG sensor. And even though the individual FBG is not linear the sum shows fantastic linearity with R-square of 0.99999. The change in wavelength is 1328pm/N. A common issue in the strain measurement is discussed and proof is provided. This shows that the reference value of the FBG is 1.12pm/μstrain instead of 1.21pm/μstrain. This is important if the FBG is mounted in a structure, because then the material proprieties will be dominating. Another result is that the peaks of Fabry Perot grating pair are linear but with different coecients.

The objective of the paper is to analyze the shrinkage and expansion strain development in thin slabs made of expansive concrete and reinforced with carbon textile reinforcement. The symmetrical textile reinforcement grid provided a biaxial restraint for the concrete shrinkage and expansion. Strains of the slabs were measured with distributed fiber optic sensors (DFOS) in both directions so that a 2D visualization of their distribution can be presented and analyzed. Parallel, standard restrained expansion tests (RET) were conducted to assess the expansive concrete mixture and large-scale beam specimens with uniaxial steel reinforcement were also equipped with DFOS and analyzed. This study aimed to compare the strains in uniaxially restrained elements with steel reinforcement and biaxially restrained textile reinforced concrete elements, in order to assess to what extent the results of the standard RET can be used for evaluation of textile reinforced concrete members.