Range-resolved optical interferometric signal processing

The ability to identify the range of an interferometric signal is very useful in interferometry, allowing the suppression of parasitic signal components or permitting several signal sources to be multiplexed. Two novel range-resolved optical interferometric signal processing techniques, employing very different working principles, are theoretically described and experimentally demonstrated in this thesis. The first technique is based on code-division multiplexing (CDM), which is combined with single-sideband signal processing, resulting in a technique that, unlike prior work, only uses a single, regular electro-optic phase modulator to perform both range-based signal identification and interferometric phase evaluation. The second approach uses sinusoidal optical frequency modulation (SFM), induced by injection current modulation of a diode laser, to introduce range-dependent carriers to determine phase signals in interferometers of non-zero optical path difference. Here, a key innovation is the application of a smooth window function, which, when used together with a time-variant demodulation approach, allows optical path lengths of constituent interferometers to be continuously and independently variable, subject to a minimum separation, greatly increasing the practicality of the approach. Both techniques are applied to fibre segment interferometry, where fibre segments that act as long-gauge length interferometric sensors are formed between pairs of partial in-fibre reflectors. Using a regular single-mode laser diode, six fibre segments of length 12.5 cm are multiplexed with a quadrature bandwidth of 43 kHz and a phase noise floor of 0.19 mrad · Hz -0.5 using the SFM technique. In contrast, the 16.5 m spatial resolution achieved with the CDM technique points towards its applicability in medium-to-long range sensing. The SFM technique also allows high linearity, with cyclic errors as low as 1 mrad demonstrated, and with modelling indicating further room for improvement. Additionally, in an industrial measurement, the SFM technique is applied to single-beam, multi-surface vibrometry, allowing simultaneous differential measurements between two vibrating surfaces

Setup and application of a combined Brillouin-Raman system

Die Schwingungsspektroskopie ist ein etabliertes Verfahren in der Materialwissenschaft sowie in der biomedizinischen Forschung zur Untersuchung von probenspezifischen Eigenschaften. Sie nutzt die Wechselwirkung von Licht (Photonen) mit Schwingungsquanten (Phononen) aus, um materialspezifische Informationen zu erlangen. Die Ramanspektroskopie ist sensitiv gegenüber optischen Phononen und findet seit der Erfindung des Lasers rege Verwendung. Im Gegensatz dazu untersucht die Brillouinspektroskopie die akustischen Phononen. Sie kam lange Zeit jedoch nur vereinzelt in der Materialwissenschaft zum Einsatz, da sie auf Grund der sequentiellen Spektrenaufnahme sehr zeitaufwendig war. Erst kürzlich konnte durch die Verwendung von einem virtually imaged phased array (VIPA) als dispersives Element im Brillouinspektrometer die Messzeit drastisch verkürzt werden, wodurch auch ein Einsatz in der biomedizinischen Forschung möglich ist. Im Rahmen dieser Dissertation wurde ein kombiniertes Brillouin-Raman-System aufgebaut, welches eine zeitgleiche und ko-lokalisierte Aufnahme von Brillouin und Ramanspektren ermöglicht. Im Vergleich zu anderen Systemen, profitiert dieser Aufbau von der Verwendung zweier VIPAs mit unterschiedlichen freien Spektralbereichen was zu einer Entkopplung der spektralen Achsen und damit zu sensitiveren Messungen führt. Außerdem ermöglicht dies eine eineindeutige Bestimmung von Brillouin-Verschiebungen. Darüber hinaus zeichnen die nahinfrarote Anregung bei 780 nm und die kontinuierliche Kalibrierung des Brillouinspektrums dieses System aus. In einer ersten Anwendung wurden mit diesem kombinierten System ferroelektrische Domänenwände in periodisch gepoltem Lithiumniobat untersucht. Dabei stellte sich heraus, dass neben dem aus der Literatur bekannten Raman-Kontrast (Intensitätsvariation bei 635 cm−1) auch das Brillouinspektrum einen Unterschied zwischen der Domänenwand und einer flächigen Domäne aufweist (Verringerung der Brillouin-Verschiebung). Es konnte gezeigt werden, dass bestehende Theorien für den Raman-Kontrast auch verwendet werden können, um den Brillouin-Kontrast zu erklären. Kombinierte Brillouin-Raman Messungen verdeutlichten, dass beide Kontraste zu Bildgebungszwecken genutzt werden können. In einer zweiten Anwendung, wurde das System dazu genutzt, die Tumorbiologie von Glioblastomzellen zu charakterisieren. Die Kombination der beiden spektroskopischen Methoden erlaubte es, die biochemischen mit den biomechanischen Eigenschaften zu korrelieren. So konnte ermittelt werden, dass der Zellkern die höchste Steifigkeit innerhalb einer adhärenten Zelle aufweist. Ein Vergleich zwischen adhärent und als Sphäroid gewachsenen Zellen offenbarte, dass letztere eine signifikant höhere Steifigkeit aufweisen, was bei der Wahl eines geeigneten Tumormodells berücksichtigt werden sollte. Darüber hinaus konnte anhand der klinisch bedeutsamen IDH1-mutation gezeigt werden, dass sich auch der Genotyp einer Zelle auf die Biomechanik auswirkt. Kombinierte Messungen an Sphäroiden wiesen darauf hin, dass sowohl Proteine sowie indirekt auch Lipide maßgeblich die biomechanischen Eigenschaften beeinflussen. Die beiden Anwendungen verdeutlichen, welche Vorteile eine Kombination dieser beiden spektroskopischen Verfahren mit sich bringt. Ihre nicht-invasive, zerstörungs- und präparationsfreie Arbeitsweise bietet dabei die Grundlage für weitere Untersuchungen auch in anderen Anwendungsfelder.Vibrational spectroscopy is an established technique in materials science as well as in biomedical research for the investigation of sample-specific properties. It exploits the interaction of light (photons) with vibrational quanta (phonons) to obtain material-characteristic information. Raman spectroscopy is sensitive to optical phonons and has been used extensively since the invention of lasers. In contrast, Brillouin spectroscopy investigates acoustic phonons. However, for a long time it was only used sporadically in materials science as it was very time-consuming due to the sequential spectrum acquisition. Only recently, the use of a virtually imaged phased array (VIPA) as dispersive element in the Brillouin spectrometer has drastically reduced the measurement time, thus facilitating its application in biomedical research. In this dissertation, a combined Brillouin-Raman system was built, which allows simultaneous and co-localized acquisition of Brillouin and Raman spectra. Compared to other systems, this setup benefits from the use of two VIPAs with different free spectral ranges, which leads to a decoupling of the spectral axes and thus to more sensitive measurements. It further allows for an unambiguous determination of Brillouin shifts. Moreover, the near-infrared excitation at 780 nm and the continuous calibration of the Brillouin spectrum characterize this system. In a first application, ferroelectric domain walls in periodically poled lithium niobate were studied with this combined system. It was found that, in addition to the Raman contrast known from literature (intensity variation at 635 cm−1), the Brillouin spectrum also shows a difference between the domain wall and a bulk domain (decrease of the Brillouin shift). It could be shown that existing theories for the Raman contrast can also be applied to explain the Brillouin contrast. Combined Brillouin-Raman measurements demonstrated that both contrasts can be used for imaging purposes. In a second application, the system was used to characterize the tumor biology of glioblastoma cells. The combination of the two spectroscopic methods allowed the biochemical properties to be correlated with the biomechanical properties. Thus, it could be determined that the nucleus has the highest stiffness within an adherent cell. A comparison between adherent cells and cells grown as spheroid revealed that the latter exhibit significantly higher stiffness, which should be taken into account when choosing a suitable tumor model. In addition, clinically relevant IDH1 mutation was used to show that the genotype of a cell also affects biomechanics. Combined measurements indicated that proteins as well as in an indirect way also lipids significantly influence biomechanical properties. These two applications illustrate the advantages of combining the two spectroscopic techniques. Their non-invasive, non-destructive and preparation-free operation provides the basis for further investigations also in other fields of application

Study and application of spectral monitoring techniques for optical network optimization

One of the possible ways to address the constantly increasing amount of heterogeneous and variable internet traffic is the evolution of the current optical networks towards a more flexible, open, and disaggregated paradigm. In such scenarios, the role played by Optical Performance Monitoring (OPM) is fundamental. In fact, OPM allows to balance performance and specification mismatches resulting from the disaggregation adoption and provides the control plane with the necessary feedback to grant the optical networks an adequate automation level. Therefore, new flexible and cost-effective OPM solutions are needed, as well as novel techniques to extract the desired information from the monitored data and process and apply them. In this dissertation, we focus on three aspects related to OPM. We first study a monitoring data plane scheme to acquire the high resolution signal optical spectra in a nonintrusive way. In particular, we propose a coherent detection based Optical Spectrum Analyzer (OSA) enhanced with specific Digital Signal Processing (DSP) to detect spectral slices of the considered optical signals. Then, we identify two main placement strategies for such monitoring solutions, enhancing them using two spectral processing techniques to estimate signal- and optical filter-related parameters. Specifically, we propose a way to estimate the Amplified Spontaneous Emission (ASE) noise or its related Optical Signal-to-Noise (OSNR) using optical spectra acquired at the egress ports of the network nodes and the filter central frequency and 3/6 dB bandwidth, using spectra captured at the ingress ports of the network nodes. To do so, we leverage Machine Learning (ML) algorithms and the function fitting principle, according to the considered scenario. We validate both the monitoring strategies and their related processing techniques through simulations and experiments. The obtained results confirm the validity of the two proposed estimation approaches. In particular, we are able to estimate in-band the OSNR/ASE noise within an egress monitor placement scenario, with a Maximum Absolute Error (MAE) lower than 0.4 dB. Moreover, we are able to estimate the filter central frequency and 3/6 dB bandwidth, within an ingress optical monitor placement scenario, with a MAE lower than 0.5 GHz and 0.98 GHz, respectively. Based on such evaluations, we also compare the two placement scenarios and provide guidelines on their implementation. According to the analysis of specific figures of merit, such as the estimation of the Signal-to-Noise Ratio (SNR) penalty introduced by an optical filter, we identify the ingress monitoring strategy as the most promising. In fact, when compared to scenarios where no monitoring strategy is adopted, the ingress one reduced the SNR penalty estimation by 92%. Finally, we identify a potential application for the monitored information. Specifically, we propose a solution for the optimization of the subchannel spectral spacing in a superchannel. Leveraging convex optimization methods, we implement a closed control loop process for the dynamical reconfiguration of the subchannel central frequencies to optimize specific Quality of Transmission (QoT)-related metrics. Such a solution is based on the information monitored at the superchannel receiver side. In particular, to make all the subchannels feasible, we consider the maximization of the total superchannel capacity and the maximization of the minimum superchannel subchannel SNR value. We validate the proposed approach using simulations, assuming scenarios with different subchannel numbers, signal characteristics, and starting frequency values. The obtained results confirm the effectiveness of our solution. Specifically, compared with the equally spaced subchannel scenario, we are able to improve the total and the minimum subchannel SNR values of a four subchannel superchannel, of 1.45 dB and 1.19 dB, respectively.Una de las posibles formas de hacer frente a la creciente cantidad de tráfico heterogéneo y variable de Internet es la evolución de las actuales redes ópticas hacia un paradigma más flexible, abierto y desagregado. En estos escenarios, el papel que desempeña el modulo óptico de monitorización de prestaciones (OPM) es fundamental. De hecho, el OPM permite equilibrar los desajustes de rendimiento y especificación, los cuales surgen con la adopción de la desagregación; del mismo modo el OPM también proporciona al plano de control la realimentación necesaria para otorgar un nivel de automatización adecuado a las redes ópticas. En esta tesis, nos centramos en tres aspectos relacionados con el OPM. En primer lugar, estudiamos un esquema de monitorización para adquirir, de forma no intrusiva, los espectros ópticos de señales de alta resolución. En concreto, proponemos un analizador de espectro óptico (OSA) basado en detección coherente y mejorado con un específico procesado digital de señal (DSP) para detectar cortes espectrales de las señales ópticas consideradas. A continuación, presentamos dos técnicas de colocación para dichas soluciones de monitorización, mejorándolas mediante dos técnicas de procesamiento espectral para estimar los parámetros relacionados con la señal y el filtro óptico. Específicamente, proponemos un método para estimar el ruido de emisión espontánea amplificada (ASE), o la relación de señal-ruido óptica (OSNR), utilizando espectros ópticos adquiridos en los puertos de salida de los nodos de la red. Del mismo modo, estimamos la frecuencia central del filtro y el ancho de banda de 3/6 dB, utilizando espectros capturados en los puertos de entrada de los nodos de la red. Para ello, aprovechamos los algoritmos de Machine Learning (ML) y el principio de function fitting, según el escenario considerado. Validamos tanto las estrategias de monitorización como las técnicas de procesamiento mediante simulaciones y experimentos. Se puede estimar en banda el ruido ASE/OSNR en un escenario de colocación de monitores de salida, con un Maximum Absolute Error (MAE) inferior a 0.4 dB. Además, se puede estimar la frecuencia central del filtro y el ancho de banda de 3/6 dB, dentro de un escenario de colocación de monitores ópticos de entrada, con un MAE inferior a 0.5 GHz y 0.98 GHz, respectivamente. A partir de estas evaluaciones, también comparamos los dos escenarios de colocación y proporcionamos directrices sobre su aplicación. Según el análisis de específicas figuras de mérito, como la estimación de la penalización de la relación señal-ruido (SNR) introducida por un filtro óptico, demostramos que la estrategia de monitorización de entrada es la más prometedora. De hecho, utilizar un sistema de monitorización de entrada redujo la estimación de la penalización del SNR en un 92%. Por último, identificamos una posible aplicación para la información monitorizada. En concreto, proponemos una solución para la optimización del espaciado espectral de los subcanales en un supercanal. Aprovechando los métodos de optimización convexa, implementamos un proceso cíclico de control cerrado para la reconfiguración dinámica de las frecuencias centrales de los subcanales con el fin de optimizar métricas específicas relacionadas con la calidad de la transmisión (QoT). Esta solución se basa en la información monitorizada en el lado del receptor del supercanal. Validamos el enfoque propuesto mediante simulaciones, asumiendo escenarios con un diferente número de subcanales, distintas características de la señal, y diversos valores de la frecuencia inicial. Los resultados obtenidos confirman la eficacia de nuestra solución. Más específicatamente, en comparación con el escenario de subcanales igualmente espaciados, se pueden mejorar los valores totales y minimos de SNR de los subcanales de un supercanal de cuatro subcanales, de 1.45 dB y 1.19 dB, respectivamentePostprint (published version

Advanced trends in nonlinear optics applied to distributed optical-fibre sensors

The distributed optical-fibre sensors based on the properties of Brillouin scattering is the central object of this thesis. In the past decade, optical fibres have gained a large interest as sensors: attractive solutions based on the non-linear stimulated Brillouin scattering have been proposed in the early 90s and the possibility to achieve long-range fully distributed strain measurements has been extensively demonstrated. The Brillouin interaction is responsible for the coupling between two optical waves and an acoustic wave when a resonance condition is fulfilled. Since the resonance condition is strain and temperaturedependent, by determining the resonance frequency we directly get a measure of temperature or strain. Local information about the acousto-optical resonance condition is typically obtained by using pulsed lightwaves and a classical time-of-flight technique (BOTDA technique). The main goal of this work has been the development of an innovative technique for the generation of optical signals, using a set of locked lasers – instead of the traditional techniques using external modulators. The utilisation of the injection-locking of semiconductor lasers is the key of the entire set-up and represents an entirely new and original approach, since it brings significant improvements in terms of SNR and costs. As long as intense pulses propagate along the fibre, the optical signals can be seriously degraded by several nonlinear interactions occurring inside the fibre; we show that the nonlinear effect exhibiting the lowest threshold power is the modulation instability (MI) process. From the study of the dynamic behaviour of MI we could observe the Fermi-Pasta-Ulam (FPU) recurrence over few periods in very comfortable conditions. One original application of Brillouin sensing has been the dosimetric measurement of ionising radiations in a nuclear environment. The measurement campaign has not only shown that distributed sensors based on Brillouin spectral analysis are radiation tolerant up to very high doses, but has also revealed the first observation – to our knowledge – of the negative compaction of silica in fibres. Distributed fibre sensors based on stimulated Brillouin scattering offer a unique capability for the analysis of optical signals and nonlinear phenomena in optical fibres. We present a generalised theoretical approach to the problem of localised sensing and report on the first distributed measurement – to our knowledge – of the parametric gain in a single-pump fibre-optics parametric amplifier (FOPA)

Matter-wave Atomic Gradiometer Interferometric Sensor (MAGIS-100)

MAGIS-100 is a next-generation quantum sensor under construction at Fermilab that aims to explore fundamental physics with atom interferometry over a 100-meter baseline. This novel detector will search for ultralight dark matter, test quantum mechanics in new regimes, and serve as a technology pathfinder for future gravitational wave detectors in a previously unexplored frequency band. It combines techniques demonstrated in state-of-the-art 10-meter-scale atom interferometers with the latest technological advances of the world's best atomic clocks. MAGIS-100 will provide a development platform for a future kilometer-scale detector that would be sufficiently sensitive to detect gravitational waves from known sources. Here we present the science case for the MAGIS concept, review the operating principles of the detector, describe the instrument design, and study the detector systematics.Comment: 65 pages, 18 figure

Structural Performance Evaluation of Bridges: Characterizing and Integrating Thermal Response

Bridge monitoring studies indicate that the quasi-static response of a bridge, while dependent on various input forces, is affected predominantly by variations in temperature. In many structures, the quasi-static response can even be approximated as equal to its thermal response. Consequently, interpretation of measurements from quasi-static monitoring requires accounting for the thermal response in measurements. Developing solutions to this challenge, which is critical to relate measurements to decision-making and thereby realize the full potential of SHM for bridge management, is the main focus of this research. This research proposes a data-driven approach referred to as temperature-based measurement interpretation (TB-MI) approach for structural performance evaluation of bridges based on continuous bridge monitoring. The approach characterizes and predicts thermal response of structures by exploiting the relationship between temperature distributions across a bridge and measured bridge response. The TB-MI approach has two components - (i) a regression-based thermal response prediction (RBTRP) methodology and (ii) an anomaly detection methodology. The RBTRP methodology generates models to predict real-time structural response from distributed temperature measurements. The anomaly detection methodology analyses prediction error signals, which are the differences between predicted and real-time response to detect the onset of anomaly events. In order to generate realistic data-sets for evaluating the proposed TB-MI approach, this research has built a small-scale truss structure in the laboratory as a test-bed. The truss is subject to accelerated diurnal temperature cycles using a system of heating lamps. Various damage scenarios are also simulated on this structure. This research further investigates if the underlying concept of using distributed temperature measurements to predict thermal response can be implemented using physics-based models. The case study of Cleddau Bridge is considered. This research also extends the general concept of predicting bridge response from knowledge of input loads to predict structural response due to traffic loads. Starting from the TB-MI approach, it creates an integrated approach for analyzing measured response due to both thermal and vehicular loads. The proposed approaches are evaluated on measurement time-histories from a number of case studies including numerical models, laboratory-scale truss and full-scale bridges. Results illustrate that the approaches accurately predicts thermal response, and that anomaly events are detectable using signal processing techniques such as signal subtraction method and cointegration. The study demonstrates that the proposed TB-MI approach is applicable for interpreting measurements from full-scale bridges, and can be integrated within a measurement interpretation platform for continuous bridge monitoring