Fiber-based Terahertz Time-Domain Spectroscopy Systems Operated in the Telecom Band

The aim of the doctoral thesis is the study of Terahertz time domain spectrometers relying on telecommunication fiber technology. Optical fiber offers low losses, high stability and compactness, features that ease the deployment of this kind of sensing instruments in industrial scenarios. The development of terahertz signal sources working at telecom wavelengths has enabled the employment of mature, telecom-related photonic components that allowed a transition within THz research from being mainly object of scientific interest to an application-oriented technology. In this thesis, fiber terahertz systems utilizing ultrafast photoconductors with integrated antenna structures have been investigated at different levels, including the control of the photoconductor structure, as well as at instrument and system levels. The carrier transport in InGaAs-InAlAs multilayer hetero-structures, present in the employed photoconductive antennas, has been investigated under the additional injection of a continuous optical wave. By varying the amplitude level of the respective optical signal injected into either the emitter or the receiver, it has been shown that the amplitude of the detected photocurrent could be controlled without affecting its bandwidth. Unlike increasing the optical power of the pulsed signal, raising the continuous optical power results in a reduction of the measured photocurrent. This lowering of the conductivity is related to changes in the instantaneous carrier momentum relaxation time in the photoactive material, rather than to variations of the free carrier density level. This behavior affects systems including continuous-wave optical components, as, for instance, optical amplifiers. The effect has been further exploited to modulate the operation conditions of photoconductive antennas, enabling an all-optical control of the THz amplitude. This represents a method to implement a signal modulation, necessary, for instance, for lock-in signal detection. Different industrial applications and THz imaging systems require fast data acquisition. Slow, stepwise working mechanical optical delay lines are about to be replaced by faster schemes. A fast THz-time-domain spectroscopy system using a coil-based rapid mechanic delay line has been set up and analyzed. A convenience of usage of optical fibers is the simplicity of signal multiplication and distribution. It can be exploited to allow centralized operation of a set of parallel terahertz sensing units. A centralized architecture with optical source sharing simplifies the implementation as well as the cost of nondestructive inspection platforms, where several sensing units would have to work in the same facility, for example at quality control in factories or security checkpoints. The cost of such a distribution system is evaluated, its feasibility experimentally demonstrated, and key features relevant to the system performance are discussed. The present document is formally structured in a brief introduction, Chapter 2, which review common terahertz technology as a whole, with the focus on optoelectronic schemes and respective technology in the telecom band. Chapter 3 includes work carried out dealing with the carrier dynamics under continuous optical wave irradiation of the photoconductive antenna modules and the application of the effect as modulation method. Chapter 4 deals with the implementation of the fast delay in the system and Chapter 5 describes and analyses architecture for parallel, remotely controlled sensing. Finally, Chapter 6 provides conclusion and future work perspectives.El objetivo de la presente Tesis Doctoral es el estudio de espectroscopios temporales de Terahercios basados en tecnología de fibra óptica para telecomunicaciones. La fibra óptica ofrece bajas pérdidas de propagación, alta estabilidad y la capacidad de implementar sistemas robustos y compactos, características que facilitan el despliegue de este tipo de instrumentos de sensado en escenarios industriales. El desarrollo de fuentes de THz que operan en la banda infrarroja empleada en telecomunicaciones permite el uso de componentes maduros de la industria de las comunicaciones ópticas, lo que a su vez se ha traducido en una transición desde el uso de la banda de THz básicamente para intereses científicos al desarrollo de sistemas para aplicaciones industriales. En la presente tesis se investigan sistemas de THz basados en antenas fotoconductivas y fibra óptica a distintos niveles: control de la estructura fotoconductiva, instrumento y sistema. El transporte de portadores en heteroestructuras multicapa InGaAs-InAlAs, empleadas actualmente en antenas fotoconductivas, se ha investigado bajo la inyección de una onda óptica continua. Se ha observado que variando el nivel de amplitud de esta onda continua tanto en el emisor como en el receptor es posible controlar la fotocorriente detectada sin afectar a su ancho de banda. A diferencia de un incremento en la potencia óptica de la señal pulsada, elevar el nivel de continua resulta en una reducción de la fotocorriente medida. Esta reducción de la conductividad se relaciona con cambios en el tiempo de relajación del momento de los portadores en el material fotoactivo en lugar de variaciones de la densidad de portadores libres. Este comportamiento puede tener un efecto en sistemas que introduzcan componentes ópticos continuos como por ejemplo sistemas de sensado que empleen amplificadores ópticos. Este efecto puede ser usado para modular las condiciones de operación de las antenas fotoconductivas permitiendo el control todo-óptico del sistema. Este método permite modular la señal, lo que resulta necesario por ejemplo para realizar detección lock-in. Tanto diferentes aplicaciones industriales como los sistemas de imagen en THz requieren sistemas rápidos de captura. Para ello es necesario sustituir las líneas de retardo ópticas tradicionales basadas en motores paso-a-paso por otros sistemas de mayor velocidad. Se ha implementado y caracterizado un sistema THz-TDS usando una línea de retardo rápida basada en bobinas de voz. Una característica fundamental de la fibra óptica es su extraordinaria simplicidad para realizar la distribución de señales ópticas. Esta característica puede ser explotada para permitir la operación centralizada de un conjunto paralelo de sensores de THz. Una arquitectura centralizada en la que la fuente óptica se comparte entre muchos sensores simplifica la implementación y reduce el coste de sistemas de inspección no destructiva que requieran de múltiples sensores en paralelo, como, por ejemplo, en control de calidad industrial o en controles de seguridad. Se ha evaluado el coste de estos sistemas distribuidos, se ha validado experimentalmente su viabilidad y se han identificado y estudiado sus prestaciones. El documento de la tesis doctoral se estructura formalmente en una breve introducción, el capítulo 2, en el que se revisa la tecnología de THz en su conjunto, los esquemas optoelectrónicos y el uso de tecnologías ópticas basadas en la banda de las telecomunicaciones. El capítulo 3 incluye el estudio realizado sobre la dinámica de los portadores bajo la irradiación dela antena fotoconductiva con una onda óptica continua y su uso como técnica de modulación. El capítulo 4 trata con la implementación de un sistema THz-TDS rápido mientras que el capítulo 5 describe y analiza una arquitectura de sensado paralela para reducir costes. Finalmente el capítulo 6 recoge las conclusiones y futuras líneas de actuación.L'objectiu de la present Tesi Doctoral és l'estudi d'espectroscopis temporals de terahertzs basats en tecnologia de fibra òptica per a telecomunicacions. La fibra òptica ofereix baixes pèrdues de propagació, alta estabilitat i la capacitat d'implementar sistemes robustos i compactes, característiques que faciliten el desplegament d'aquest tipus d'instruments de sensat en escenaris industrials. El desenvolupament de fonts de THz que operen a la banda infraroja emprada en telecomunicacions permet l'ús de components madurs de la indústria de les comunicacions òptiques, el que al seu torn s'ha traduït en una transició des de l'ús de la banda de THz bàsicament per interessos científics al desenvolupament de sistemes per a aplicacions industrials. En la present tesi s'investiguen sistemes de THz basats en antenes fotoconductivas i fibra òptica a diferents nivells: control de l'estructura fotoconductiva, instrument i sistema. El transport de portadors en heteroestructures multicapa InGaAs-InAlAs, emprades actualment en antenes fotoconductivas, s'ha investigat sota la injecció d'una ona òptica contínua. S'ha observat que variant el nivell d'amplitud d'aquesta ona contínua tant en l'emissor com en el receptor és possible controlar la fotocorriente detectada sense afectar el seu ample de banda. A diferència d'un increment en la potència òptica del senyal polsada, elevar el nivell de contínua resulta en una reducció de la fotocorrent mesurada. Aquesta reducció de la conductivitat es relaciona amb canvis en el temps de relaxació del moment dels portadors en el material fotoactiu en lloc de variacions de la densitat de portadors lliures. Aquest comportament pot tenir un efecte en sistemes que introdueixin components òptics continus com ara sistemes de sensat que utilitzen amplificadors òptics. Aquest efecte pot ser usat per modular les condicions d'operació de les antenes fotoconductivas permetent el control tot-òptic del sistema. Aquest mètode permet modular el senyal, el que resulta necessari per exemple per realitzar detecció lock-in. Tant diferents aplicacions industrials com els sistemes d'imatge en THz requereixen sistemes ràpids de captura. Per a això és necessari substituir les línies de retard òptiques tradicionals basades en motors pas-a-pas per altres sistemes de major velocitat. S'ha implementat i caracteritzat un sistema THz-TDS usant una línia de retard ràpida basada en bobines de veu. Una característica fonamental de la fibra òptica és la seua extraordinària simplicitat per realitzar la distribució de senyals òptiques. Aquesta característica pot ser explotada per a permetre l'operació centralitzada d'un conjunt paral·lel de sensors de THz. Una arquitectura centralitzada en la qual la font òptica es comparteix entre molts sensors simplifica la implementació i redueix el cost de sistemes d'inspecció no destructiva que requereixin de múltiples sensors en paral·lel, com, per exemple, en control de qualitat industrial o en controls de seguretat . S'ha avaluat el cost d'aquests sistemes distribuïts, s'ha validat experimentalment la seua viabilitat i s'han identificat i estudiat les seues prestacions. El document de la tesi doctoral s'estructura formalment en una breu introducció, capítol 2, en el qual es revisa la tecnologia de THz en el seu conjunt, els esquemes optoelectrònics i l'ús de tecnologies òptiques basades en la banda de les telecomunicacions. El capítol 4 inclou l'estudi realitzat sobre la dinàmica dels portadors sota la irradiació de la antena fotoconductiva amb una ona òptica contínua i el seu ús com a tècnica de modulació. El capítol 5 tracta la implementació d'un sistema THz-TDS ràpid mentre que el capítol 6 descriu i analitza una arquitectura de sensat paral·lela per reduir costos. Finalment, el capítol 7 recull les conclusions i futures línies d'actuació.Bockelt, AS. (2017). Fiber-based Terahertz Time-Domain Spectroscopy Systems Operated in the Telecom Band [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/86148TESI

Terahertz PHASR Scanner with 2 kHz, 100 picosecond Time-Domain Trace Acquisition Rate and an Extended Field-of-View Based on a Heliostat Design

Recently, we introduced a Portable HAndheld Spectral Reflection (PHASR) Scanner to allow THz Time-Domain Spectroscopic (THz-TDS) imaging in clinical and industrial settings using a fiber-coupled and alignment-free telecentric beam steering design. The key limitations of the version 1.0 of the PHASR Scanner were its field-of-view and speed of time-domain trace acquisition. In this paper, we address these limitations by introducing a heliostat geometry for beam scanning to achieve an extended field-of-view, and by reconfiguring the ASynchronous OPtical Sampling (ASOPS) system to perform Electronically Controlled OPtical Sampling (ECOPS) measurements. The former change improved the deflection range of the beam, while also drastically reducing the coupling of the two scanning axes, the combination of which resulted in a larger than four-fold increase in the FOV area. The latter change significantly improves the acquisition speed and frequency domain performance simultaneously by improving measurement efficiency. To accomplish this, we characterized the non-linear time-axis sampling behavior of the electro-mechanical system in the ECOPS mode. We proposed methods to model and correct the non-linear time-axis distortions and tested the performance of the high-speed ECOPS trace acquisition. Therefore, here we introduce the PHASR Scanner version 2.0, which is capable of imaging a 40$\times$27 mm$^2$ FOV with 2000 traces per second over a 100 picosecond TDS range. This new scanner represents a significant leap towards translating the THz-TDS technology from the lab bench to the bedside for real-time clinical imaging applications.Comment: 15 pages; submitted to IEEE Transactions on Terahertz Science and Technolog

POTENTIAL OF TERAHERTZ PULSED REFLECTOMETRY AND IMAGING FOR THE EARLY DIAGNOSIS OF CUTANEOUS MELANOMA

In the last two decades the incidence rate of cutaneous malignant melanoma have been risen faster than any other form of cancer worldwide in the white-Caucasian population. The mortality rates over time show that an early diagnosis is the key point for quick treatment, which increase survival rates. As a standard procedure dermatologists use a dermascope or the naked eye for evaluation of possible lesions, where experts have a higher chance of spotting infiltrated tissue than untrained persons. Multiple investigations on diagnostic imaging for the detection of melanoma have been conducted in the past, like Ultrasound, Near-Infrared spectroscopy or Optical Coherence Tomography, with mixed, but not sufficient results to date. In recent years terahertz radiation has shown to be a promising technology for the early detection of various types of cancers, i.e., colon ex-vivo}, breast ex-vivo and non-melanoma skin cancers ex-vivo andmin-vivo as terahertz radiation is able to penetrate slightly into the bio-tissue but also deemed to be a non-ionising and therefore safe method for diagnosis of lesions in-vivo. Investigations into the practicality and benefits of using terahertz reflectometry for the early diagnosis of melanoma has never been performed. Therefore, as a pilot study, an investigation into the modalities of utilising terahertz technology on freshly excised human cutaneous melanoma is anticipated, which includes a comparison of the collected 3D terahertz images with visuals, comparison of histopathologists findings but also investigations about modelling skin and abnormalities of the skin using terahertz radiation. Diverse and manifold results can be reported based on the study conducted, which show that there is a good potential of terahertz detecting abnormalities on a per patient basis of up to 78% sensitivity and 95% specificity respectively. However, skin is a very diverse medium and results of the modelling approach have to be seen very critically. As modality for a diagnostic tool, this investigation suggests that there is potential in detecting margins and active regions of cancerous region spreading, which may help to support the dermatologists to determine better margins for the excision of the lesion.This work has been sponsored by the Hope Againast Cancer Foundation, Leiceste

Controlling Terahertz Radiation - Novel Fabrication Methods and Materials for Terahertz Components

The interaction between light and matter has been a field of research for centuries, from the days of Sir Isaac Newton in the 17th century up to today, where new effects, such as plasmonics open up new applications or the extension of the accessible electromagnetic spectrum, are still engaging scientists and engineers in this field of research. The understanding of the interaction between light, or more general: electromagnetic radiation and matter is a crucial step in the development of components which give the necessary control to gain access to the desired part of the electromagnetic spectrum. One of the less developed parts of the electromagnetic spectrum is terahertz (THz) radiation. THz radiation promises many applications, from spectroscopy for material and medical applications to communication technology. But, so far, most applications have not managed to overcome the experimental status, mostly because of missing materials and manufacturing methods suitable for the required length scales and material properties in the terahertz regime. This thesis focuses on structures for the control of THz radiation. To do so, and to overcome the natural limitations of many materials in the THz region, new materials and modern fabrication techniques are used to find new ways to overcome the shortage of readily available components for this part of the electromagnetic spectrum. As such, ceramics and polymers are used for various components, from lenses to spoof plasmonic waveguides, fabricated with a variety of techniques, including 3D printing and micro-milling. Finite-Difference Time-Domain simulations are used for the design of all structures. The ultimate goal is to demonstrate low-cost methods to produce THz components for future industrial implementation

Nondestructive Evaluation of Aircraft Composites Using Terahertz Time Domain Spectroscopy

Terahertz (THz) time domain spectroscopy (TDS) was assessed as a nondestructive evaluation technique for aircraft composites. Material properties of glass fiber composite were measured using both transmission and reflection configuration. The interaction of THz with a glass fiber composite was then analyzed, including the effects of scattering, absorption, and the index of refraction, as well as effective medium approximations. THz TDS, in both transmission and reflection configuration, was used to study composite damage, including voids, delaminations, mechanical damage, and heat damage. Measurement of the material properties on samples with localized heat damage showed that burning did not change the refractive index or absorption coefficient noticeably; however, material blistering was detected. Voids were located by THz TDS transmission and reflection imaging using amplitude and phase techniques. The depth of delaminations was measured via the timing of Fabry-Perot reflections after the mail pulse. Evidence of bending stress damage and simulated hidden cracks was also detected with terahertz imaging

Experimental and Analysis of Electromagnetic Characterization of Biological and Non-Biological Materials in Microwave, Millimeter-wave, and Terahertz Frequency Bands

The goal of this research is to characterize the electromagnetic properties of biological and non-biological materials at terahertz (THz), millimeter-wave, and microwave frequency bands. The biological specimens are measured using the THz imaging and spectroscopy system, whereas the non-biological materials are measured using the microwave and millimeter-wave free-space system. These facilities are located in the Engineering Research Center at the University of Arkansas. The THz imaging system (TPS 3000) uses a Ti-Sapphire laser directed on the photoconductive antennas to generate a THz time domain pulse. Upon using the Fourier Transform, the spectrum of the pulsed THz signal includes frequencies from 0.1 THz to 4 THz. On the other hand, the free space system uses a PNA network analyzer to produce a signal at frequencies ranging from 10 MHz to 110 GHz. For the biological specimens, the research focused on imaging the freshly excised breast tumors to detect the cancer on the margins using THz radiation. The tumor margin assessment depends on the THz contrast between cancer, collagen, and fat tissues in the tumor. Three models of breast tumors are investigated in this research: humans, mice (xenograft and transgenic), and Sprague Dawley rats. The results showed good differentiation between the cancerous and non-cancerous tissues in all three models. In addition, an excellent distinction was observed between cancer, collagen, and fat in the formalin-fixed paraffin-embedded (FFPE) block tissue with ~ 90-95% correlation with the pathology images. Furthermore, the FFPE ductal carcinoma in situ (DCIS) tumors are investigated, also using the THz imaging. The THz images of the DCIS samples are compared with those of the FFPE invasive ductal carcinoma (IDC) specimens. The results demonstrated that THz electric field reflection from the IDC were higher than that from the collagen, DCIS, and then the fat tissue region. Furthermore, a pilot study is conducted to investigate the effect of optical clearance (e.g., glycerol solution) on THz images of freshly excised tumors. The results showed that the glycerol reduced the absorption coefficients of pre-treated tissues compared with those of untreated tissues. For the non-biological materials, the research focuses on characterizing highly conductive non-magnetic radar absorbing materials (RAM) for the automotive industry. The ingredients of material components in the RAM samples are unrevealed under a non-disclosure agreement (NDA). The material characterization involves the extraction of the complex relative permittivity utilizing the transmission measurement data obtained at the K-band (18 GHz to 26.5 GHz) and the W-band (75 GHz to 110 GHz). The measurements are obtained using the free-space conical horn antenna system. A transmission line based extraction model is implemented, and the results are validated with the experimental measurements of the S-parameters. The maximum error reported between the measured and the calculated S-parameters was less than 1 dB. In conclusion, the THz imaging of breast cancer tumors presents a potential margin assessment of other solid tumors, and the microwave, millimeter-wave, and THz spectroscopy of materials demonstrate a potential application in the fifth and sixth generations of wireless communications

Contrast in Terahertz Images of Archival Documents—Part I: Influence of the Optical Parameters from the Ink and Support

This study aims to objectively inform curators when terahertz time-domain (TD) imaging set in reflection mode is likely to give well-contrasted images of inscriptions in a complex archival document and is a useful non-invasive alternative to current digitisation processes. To this end, the dispersive refractive indices and absorption coefficients from various archival materials are assessed and their influence on contrast in terahertz images from historical documents is explored. Sepia ink and inks produced with bistre or verdigris mixed with a solution of Arabic gum or rabbit skin glue are unlikely to lead to well-contrasted images. However, dispersions of bone black, ivory black, iron gall ink, malachite, lapis lazuli, minium and vermilion are likely to lead to well-contrasted images. Inscriptions written with lamp black, carbon black and graphite give the best imaging results. The characteristic spectral signatures from iron gall ink, minium and vermilion pellets between 5 and 100 cm−1 relate to a ringing effect at late collection times in TD waveforms transmitted through these pellets. The same ringing effect can be probed in waveforms reflected from iron gall, minium and vermilion ink deposits at the surface of a document. Since TD waveforms collected for each scanning pixel can be Fourier-transformed into spectral information, terahertz TD imaging in reflection mode can serve as a hyperspectral imaging tool. However, chemical recognition and mapping of the ink is currently limited by the fact that the morphology of the document influences more the terahertz spectral response of the document than the resonant behaviour of the ink

Generation of terahertz-modulated optical signals using AlGaAs/GaAs laser diodes

The Thesis reports on the research activities carried out under the Semiconductor-Laser Terahertz-Frequency Converters Project at the Department of Electronics and Electrical Engineering, University of Glasgow. The Thesis presents the work leading to the demonstration of reproducible harmonic modelocked operation from a novel design of monolithic semiconductor laser, comprising a compound cavity formed by a 1-D photonic-bandgap (PBG) mirror. Modelocking was achieved at a harmonic of the fundamental round-trip frequency with pulse repetition rates from 131 GHz up to a record-high frequency of 2.1 THz. The devices were fabricated from GaAs/AlGaAs material emitting at a wavelength of 860 nm and incorporated two gain sections with an etched PBG reflector between them, and a saturable absorber section. Autocorrelation studies are reported, which allow the device behaviour for different modelocking frequencies, compound cavity ratios, and type and number of intra-cavity reflectors to be analyzed. The highly reflective PBG microstructures are shown to be essential for subharmonic-free modelocking operation of the high-frequency devices. It was also demonstrated that the multi-slot PBG reflector can be replaced with two separate slots with smaller reflectivity. Some work was also done on the realisation of a dual-wavelength source using a broad-area laser diode in an external grating-loaded cavity. However, the source failed to deliver the spectrally-narrow lines required for optical heterodyning applications. Photomixer devices incorporating a terahertz antenna for optical-to microwave down-conversion were fabricated, however, no down-conversion experiments were attempted. Finally, novel device designs are proposed that exploit the remarkable spectral and modelocking properties of compound-cavity lasers. The ultrafast laser diodes demonstrated in this Project can be developed for applications in terahertz imaging, medicine, ultrafast optical links and atmospheric sensing