Academic literature on the topic 'Terahertz fiber'

A hollow core antiresonant photonic crystal fiber (HC-ARPCF) with metal inclusions is numerically analyzed for transmission of terahertz (THz) waves. The propagation of fundamental and higher order modes are investigated and the results are compared with conventional dielectric antiresonant (AR) fiber designs. Simulation results show that broadband terahertz radiation can be guided with six times lower loss in such hollow core fibers with metallic inclusions, compared to tube lattice fiber, covering a single mode bandwidth (BW) of 700 GHz.

Terahertz (THz) applications have emerged as one of the most new powerful nondestructive evaluation (NDE) techniques. A new T-ray time-domain spectroscopy system was utilized for detecting and evaluating orientation influence in carbon fiber-reinforced plastics (CFRPs) composite laminates. Investigation of terahertz time-domain spectroscopy (THz-TDS) was made, and reflection and transmission configurations were studied as a nondestructive evaluation technique. Here, the CFRP composites derived their excellent mechanical strength, stiffness, and electrical conductivity from carbon fibers. Especially, the electrical conductivity of the CFRP composites depends on the direction of unidirectional fibers since carbon fibers are electrically conducting while the epoxy matrix is not. In order to solve various material properties, the index of refraction (n) and the absorption coefficient (α) are derived in reflective and transmission configurations using the terahertz time-domain spectroscopy. Also, for a 48-ply thermoplastic polyphenylene-sulfide-(PPS-) based CFRP solid laminate and nonconducting materials, the terahertz scanning images were made at the angles ranged from0°to180°with respect to the nominal fiber axis. So, the images were mapped out based on the electrical field (E-field) direction in the CFRP solid laminates. It is found that the conductivity (σ) depends on the angles of the nominal axis in the unidirectional fiber.

In this letter, we numerically demonstrate a hybrid-core microstructure fiber for low-loss terahertz guidance. Finite element method with circular perfectly matched layer boundary condition is applied to characterize the guiding properties. It is shown that by using a triangular-core inside a square lattice microstructure exhibits ultra-low effective material loss (EML) of 0.169 dB/cm and low confinement loss of 0.087 dB/cm at the operating frequency of 0.75 THz. We also discuss how other guiding properties including power fraction, single mode propagation and dispersion vary with changing of core diameter and operating frequencies. This low-loss microstructure fiber can be effectively used in numerous applications in the THz regime. Full Text: PDF ReferencesJ. J. Bai, J. N. Li, H. Zhang, H. Fang, S. J. Chang, "A porous terahertz fiber with randomly distributed air holes", Appl. Phys. B 103, 2 (2011). CrossRef S. Atakaramians, S. Afshar, B. M. Fischer, D. Abbott, T. M. Monro, "Porous fibers: a novel approach to low loss THz waveguides", Opt. Express 16, 12 (2008). CrossRef K. Wang, D. M. Mittleman, "Metal wires for terahertz wave guiding", Nature 432, 7015 (2004). CrossRef R. Islam, G. K. M. Hasanuzzaman, M. S. Habib, S. Rana, M. A. G. Khan, "Low-loss rotated porous core hexagonal single-mode fiber in THz regime", Opt. Fiber Technol. 24, (2015). CrossRef M. I. Hasan, S. M. A. Razzak, G. K. M. Hasanuzzaman, M. S.Habib, "Ultra-Low Material Loss and Dispersion Flattened Fiber for THz Transmission", IEEE Photon. Technol. Lett. 26, 23 (2014). CrossRef S. F. Kaijage, Z. Ouyang, X. Jin, "Porous-Core Photonic Crystal Fiber for Low Loss Terahertz Wave Guiding", IEEE Photon. Technol. Lett. 25, 15 (2013). CrossRef M. R. Hasan, M. A. Islam, A. A. Rifat, "A single mode porous-core square lattice photonic crystal fiber for THz wave propagation", J. Eur. Opt. Soc. Rapid Publ. 12, 1 (2016). CrossRef M. R. Hasan, M. A. Islam, M. S. Anower, S. M. A. Razzak, "Low-loss and bend-insensitive terahertz fiber using a rhombic-shaped core", Appl. Opt. 55, 30 (2016). CrossRef S. Ali et al. "Ultra-low loss THz waveguide with flat EML and near zero flat dispersion properties", in 9th Int. Conf. on Elect. and Comp. Eng., IEEE, (2016). CrossRef K. Nielsen, H. K. Rasmussen, A. J. Adam, P. C. Planken, O. Bang, P. U. Jepsen, "Bendable, low-loss Topas fibers for the terahertz frequency range", Opt. Express 17, 10 (2009). CrossRef A. W. Snyder, J. D. Love, Optical waveguide theory (London, Chapman & Hall 1983). DirectLink L. Vincetti, A. Polemi, in Antennas and Propagation Society International Symposium, IEEE (2009)G. P. Agrawal, Nonlinear fiber optics (Boston, Academic Press 1989). CrossRef B. S. Williams, "Terahertz quantum-cascade lasers", Nat. Photon. 1, 9 (2007). CrossRef H. W. Hubers et al. "Terahertz quantum cascade laser as local oscillator in a heterodyne receiver", Opt. Express 13, 15 (2005). CrossRef

This paper examines the use of the Finite Element Method (FEM) in the field of optical waveguides and terahertz signals, with the main goal of explaining how this method aids in recent advances in this field. The basics of FEM are briefly reviewed, and the technique's application to waveguide discontinuity analysis is observed. Second-order and higher-order derivatives result from optical waveguide modeling, which is significant for information exchange and many other nonlinear phenomena. The use of FEM in the improvised design of hexagonal sort air hole porous core microstructure fibers, which produces hexagonal structure cladding and rectangular-shaped air holes in the fiber core for excellent terahertz signal transmission, was also observed. These modifications were intended to improve the fiber's properties in comparison to other structures. This approach verifies that the fiber has high birefringence, low material loss, a high-power fraction, and minimal dispersion varia-tion. The features of square-type microstructure fiber are investigated. A folded-shaped po-rous cladding design is recognized for sensing applications. This type of photonic crystal fiber is also known as FP-PCF since it features circular air holes. The most approximate findings of this application are obtained using FEM. In comparison to many other approach-es for various applications, it is evident that FEM is a powerful and numerically efficient tool. This work does a survey of optical waveguides and terahertz signals using the Finite Element Method. Terahertz signals can be used in conjunction with electromagnetic waves to identify viruses. Thus, Terahertz signals are employed in real-world applications such as fuel adulteration, liquid metal synthesis, and virus detection.

AbstractA new kind of square lattice porous core microstructure fiber is proposed for promising low loss terahertz applications. To analyze the guiding characteristics of proposed fiber Finite Element Method (FEM) based Comsol V4.2 software is used. The proposed fiber structure is very simple and easy to realize. The numerical results ensures that this proposed microstructure fiber exhibits low effective absorption loss of 0.06 cm^–1, low confinement loss of 9.2 × 10^–3 cm^–1, low bending loss of 8.8 × 10^–9 dB/m, and very high power fraction of 47% through the core at 1 THz simultaneously. Moreover, our proposed fiber offers very low dispersion variation of 0.85 ± 0.12 ps/THz/cm over a wide range of frequency from 0.7 to 1.15 THz. The investigated results indicates that this fiber will be a good candidate in terahertz signal transmission as well as different terahertz devices.

Terahertz wave (T-ray) technologies have become a popular topic in scientific research over the last two decades, and can be utilized in nondestructive evaluation (NDE) techniques. This study suggests an optimal scanning technique method for honeycomb sandwich composite panels, where skins were utilized with two different skins, namely, carbon fiber-reinforced plastic (CFRP) skin and glass fiber-reinforced plastic (GFRP) skin, as layers of the panel surfaces. Foreign objects were artificially inserted between the skins and honeycomb cells in the honeycomb sandwich composite panels. For this experiment, optimal T-ray scanning methods were performed to examine defects based on the angle between the one-ply thin fiber skin axis and the angle of the electric field (E-field) according to the amount of conductivity of the honeycomb sandwich composite panels. In order to confirm the fundamental characteristics of the terahertz waves, the refractive index values of the GFRP composites were experimentally obtained and analyzed, with the data agreeing with known solutions. Terahertz waves (T-rays) were shown to have limited penetration in honeycomb sandwich composite panels when utilized with a skin of carbon fibers. Therefore, T-rays were found to interact with the electrical conductivity and electric field direction of honeycomb sandwich composite panels with glass fiber skins. The T-ray images were obtained regardless of the electric field direction and the fiber direction. In the honeycomb sandwich composite panels with carbon fiber skins, the T-ray images with higher signal-to-noise (S/N) ratios depended on the scanning angle between the angle of the carbon fiber and the angle of the electric field. Thus, the angle of optimum detection measurement was confirmed to be 90° between the E-field and the fiber direction, particularly when using a carbon fiber skin.

We explored the dynamics of frequency-modulated (FM) pulses in a cascaded fiber configuration comprising one active and one passive optical fiber with multiple fiber Bragg gratings (FBGs) of different periods inscribed over the fiber configuration length. We present a theoretical formalism to describe the mechanisms of the FM pulse amplification and pulse compression in such fiber cascades resulting in peak powers up to ~0.7 MW. In combination with the decreasing dispersion fibers, the considered cascade configuration enables pico- and sub-picosecond pulse trains with a sub-terahertz repetition rate and sub-kW peak power generated directly from the continuous optical signal.

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/10251/86148
TESIS

In this dissertation, an all-fiber-based single frequency nanosecond pulsed laser system at ~ 1918.4 nm in master-oscillator-power-amplifier (MOPA) configuration is present. The nanosecond pulse seed is achieved by directly modulating a continuous wave (CW) single frequency fiber laser using a fast electro-optical modulator (EOM) driven by an arbitrary waveform generator (AWG). One piece of single mode, large core, polarization-maintaining (PM) highly thulium-doped (Tm-doped) germanate glass fiber (LC-TGF) is used to boost the pulse power and pulse energy of these modulated pulses in the final power amplifier. This laser system can work in both high power and high energy regime: in high power regime, to the best of our knowledge, the highest average power 16 W and peak power 78.1 kW are achieved for single frequency transform-limited ~2.0 ns pulses at 500 kHz and 100 kHz repetition rate, respectively: In high energy regime, nearly 1 mJ and half mJ pulse energy is obtained for ~15 ns pulses at 1 kHz repetition rate and 5 kHz repetition rate, respectively. Theoretical modeling of the large-core highly Tm-doped germanate glass double-cladding fiber amplifier (LC-TG-DC-FA) is also present for 2&mum nanosecond pulse amplification. A good agreement between the theoretical and experimental results is achieved. The model can simulate the evolution of pump power, signal energy, pulse shape and the amplified stimulated emission (ASE) in the amplifier. It can also be utilized to investigate the dependence of the stored energy in the LC-TGF on the pump power, seed energy and repetition rate, which can be used to design and optimize the LC-TG-DC-FA to achieve higher pulse energy and average power. Two channel of high energy nanosecond pulses (at 1918.4 nm and 1938 nm) are utilized to generate THz wave in a quasi-phase-matched (QPM) gallium arsenide (GaAs) based on difference frequency generation. THz wave with ~ 5.4μW average power and ~18 mW peak power has been achieved. Besides, one model is built to simulate a singly resonated THz parametric oscillator. The threshold, the dependence of output THz energy on pump energy has been investigated through this model. One pump enhanced THz parametric oscillator has been proposed. The enhancement factor of the nanosecond pulses in a bow-tie ring cavity has been calculated for different pulse duration, cavity length and the transmission of the coupler. And the laser resonances in the ring cavity have been observed by using a piezo to periodically adjust the cavity length. We also build an all-fiber thulium-doped wavelength tunable mode-locked laser operating near 2&mum. Reliable self-starting mode locking over a large tuning range (>50 nm) using fiber taper based carbon nanotube (FTCNT) saturable absorber (SA) is observed. Spectral tuning is achieved by stretching another fiber taper. To the best of our knowledge, this is the first demonstration of an all-fiber wavelength tunable mode-locked laser near 2&mum.

In this dissertation I report the development of high power pulsed fiber laser systems. These systems utilize phosphate glass fiber for active elements, instead of the industry-standard silica fiber. Because the phosphate glass allows for much higher doping of rare-earth ions than silica fibers, much shorter phosphate fibers can be used to achieve the same gain as longer silica fibers.This single-frequency laser technology was used to develop an all-fiber actively Q-switched fiber lasers. A short cavity is used to create large spacing between longitudinal modes. Using this method, we demonstrated the first all-fiber Q-switched fiber laser in the 1 micron region.In addition to creating high peak powers with Q-switched lasers, created even higher powers using fiber amplifier systems. High power fiber lasers typically produce spectral broadening through the nonlinear effects of stimulated Raman scattering, stimulated Brullion scattering, and self-phase modulation. The thresholds for these nonlinearities scale inversely with intensity and length. Thus, we used a short phosphate fiber gain stage to reduce the length, and a large core fiber final stage to reduce intensity. In this way we were able to generate high peak power pulses while avoiding visible nonlinearities, and keeping a narrow bandwidth.The immediate goal of developing these high power fiber laser systems was to generate narrowband terahertz radiation. Two different wavelengths were combined into the final amplifier stage at orthogonal polarizations. These were collimated and directed into a GaSe crystal, which has a very high figure of merit for THz generation. The two wavelengths combined in the crystal through the process of nonlinear difference frequency generation. This produced a narrowband beam of THz pulses, at higher powers than previous narrowband THz pulses produced by eyesafe fiber lasers.

Les techniques d'imagerie et de spectroscopie térahertz offrent de vastes applications dans le contrôle non destructif ou le contrôle de qualité dans la manufacture industrielle, la pharmaceutique et la biologie, l'archéologie ou encore le monde de l’art. Pour ces applications, la technique de spectroscopie térahertz dans le domaine temporel (THz-TDS) permet une analyse sur une bande passante instantanée très large (0.1-6 THz), mais nécessite généralement de déplacer mécaniquement l’échantillon à imager dans le plan focal du faisceau THz. Le travail de cette thèse porte sur l’adaptation d’un banc THz-TDS pour l’imagerie et la spectroscopie des échantillons fixes, en se basant sur le principe d’un radar à synthèse d’ouverture (SAR), en transmission. En utilisant cette technique, on démontre une reconstruction d'image en 3D avec une résolution inférieure au millimètre de plusieurs échantillons différents. Pour remédier aux temps d'acquisition prolongés, un échantillonnage spatial lacunaire est proposé, réduisant les éléments du réseau synthétique et améliorant la vitesse d'acquisition. De plus, les données reconstruites ne sont pas uniquement utilisées pour l'imagerie mais permettent également la caractérisation des paramètres optiques matériaux (l'indice de réfraction et le coefficient d'absorption) constituant l'objet imagé dans la bande de fréquence de reconstruction. Ainsi, la technique proposée permet la cartographie spectrale 2D de l'indice de réfraction à diverses fréquences térahertz. Enfin, la méthodologie proposée est appliquée à l'imagerie de sortie de guide d'ondes térahertz, illustrant sa grande flexibilité et ses vastes domaine potentielles d’utilisation
Les techniques d'imagerie et de spectroscopie térahertz offrent de vastes applications dans le control non destructif ou le contrôle de qualité dans la manufacture industrielle, la pharmaceutique et la biologie, l'archéologie ou encore le monde de l’art. Pour ces applications, la technique de spectroscopie térahertz dans le domaine temporel (THz-TDS) permet une analyse sur une bande passante instantanée très large (0.1-6 THz), mais nécessite généralement de déplacer mécaniquement l’échantillon à imager dans le plan focal du faisceau THz. Le travail de cette thèse porte sur l’adaptation d’un banc THz-TDS pour l’imagerie et la spectroscopie des échantillons fixes, en se basant sur le principe d’un radar à synthèse d’ouverture (SAR), en transmission. En utilisant cette technique, on démontre une reconstruction d'image en 3D avec une résolution inférieure au millimètre de plusieurs échantillons différents. Pour remédier au temps d'acquisition prolongés, un échantillonnage spatial lacunaire est proposé, réduisant les éléments du réseau synthétique et améliorant la vitesse d'acquisition. De plus, les données reconstruites ne sont pas uniquement utilisées pour l'imagerie mais permettent également la caractérisation des paramètres optiques matériaux (l'indice de réfraction et le coefficient d'absorption) constituant l'objet imagé dans la bande de fréquence de reconstruction. Ainsi, la technique proposée permet la cartographie spectrale 2D de l'indice de réfraction à diverses fréquences térahertz. Enfin, la méthodologie proposée est appliquée à l'imagerie de sortie de guide d'ondes térahertz, illustrant sa grande flexibilité et ses vastes domaine potentielles d’utilisation

The Quantum Cascade Laser (QCL)-based terahertz-over-fibre (ToF) concept combines the strength of QCLs as ultra-wide bandwidth, high speed data sources, with the mature optical fibre technology. In this thesis, for the first time, by fusing multiple technologies, digitally selected, electronically-switchable ToF concept is experimentally demonstrated. Furthermore, the digital mode selection principle and electronic tuning mechanism provided by novel aperiodic distributed feedback (ADFB) multi-band filters are presented. For the development of electronically tunable ADFB lasers, a range of bound-to-continuum and chirped superlattice terahertz (THz) QCLs are measured across the frequency range 2.9 – 4.5 THz. The availability of these active materials allowed rapid assessments of the optimum design parameters for subsequent measurements. First, a range of photonic lattice-engineered lasers operating at 4.4 THz are characterized and key design parameters identified. Following this initial development, full electrical and spectral characterization of ADFB lasers operating at 2.9 THz are presented. The novelty of this work lies in the first-ever successful demonstration of discretely tunable QCLs, operating at six distinct THz frequencies. The ADFB technology was experimentally applied using various device geometries and gain dynamics. Toward this aim, results are presented for a Y coupled QCL architecture, showing that complex on-chip signal manipulation can be extended into the THz regime. In addition, it is demonstrated that ADFB technology provides broadband multi-channel optical filtering for the entire gain bandwidth. It is shown that discrete, purely electronic, tuning of simultaneous dual colour output can be achieved. Multi band optical filter functions derived from ADFB gratings possess highly nonlinear dispersion across the filter bandwidth and are found to modify the gain-induced, driving current-dependent continuous mode tuning. This thesis, therefore, presents a systematic experimental analysis of the dispersion engineered continuous fine-tuning in THz QCLs. In the final two chapters, the thesis presents, for the first time, transmission of tunable THz signals over standard single-mode optical fibre by up converting 2.9 THz QCL radiation via intra-cavity nonlinear mixing with an optical fibre-injected near-infrared (NIR) carrier in the 1.3 µm band. Discrete and continuous tuning technologies, as developed in chapters 3 – 5, are now successfully transferred to THz sidebands on the NIR carrier, extracted via a butt coupled single mode fibre and recorded using an optical spectrum analyzer. The major novel outcome of this thesis is the first demonstration of electronically tunable phase-matched points in a THz plasmon waveguide. The key breakthrough is the experimental confirmation of the photonic band-gap engineering of group velocity of THz signals – as both ‘fast’ and ‘slow’ switchable side bands are observed. Such novel nonlinear up-conversion of spectrally flexible THz signals may open up new possibilities for ultrafast THz telecom frameworks.

Ces travaux portent sur la génération d'un peigne de longueurs d'ondes à haut-débit pour les télécommunications. Ce manuscrit s'articule autour de trois grandes parties. Dans un premier temps, nous nous sommes intéressés aux cavités laser fonctionnant en blocage de modes passif. Les expériences ont notamment permis de générer à chaque aller et retour dans la cavité des paquets d'impulsions liées et séparées d'une période correspondant à une cadence de 100-160 GHz. Dans la seconde partie, nous montrons la possibilité de générer des horloges à haute cadence (100 GHz - 1 THz) en exploitant les bonnes propriétés des lasers à îlots quantiques à blocage de modes, par l'ajout de filtres. La sélection des fréquences a été réalisée, au début de la thèse, par l'application de points de température induisant des sautsde phase sur un réseau de Bragg à pas variable, puis par l'utilisation d'un filtrage spatial utilisant un appareil commercial. La stabilité des horloges optiques, de fréquences variables pouvant atteindre 1,5 THz, a été validée par des mesures de taux d'erreurs. Enfin, nous montrons qu'il est possible réaliser des convertisseurs de fréquence permettant de récupérer une horloge à la sous harmonique f à partir d'un signal binaire cadencée à N × f. En conclusion, nous dressons les perspectives liées à ces résultats.

The quantum noise based terahertz signal encryption scheme is proposed, a 16 Gbits-1 secure terahertz communication system at 300 GHz with the optical communication realms is demonstrated, taking a significant step toward high-security wireless communications.

The sub-THz inter-chip interconnections are first demonstrated with terahertz photomixers based on standard-process fabricated germanium-silicon photodetectors and bow-tie antennas, featuring a frequency range over 200 GHz.

Prospects and challenges for mobile 6G communications using terahertz frequencies are discussed. For the first time, THz wireless communications with 15 Gbps to multiple mobile users employing a photonic-assisted beam steering antennas is experimentally demonstrated.

We propose and experimentally demonstrate a Nyquist pulse that can improve the error rate of a 311 GHz photonic-terahertz communications system by more than an order of magnitude at a normalized timing-jitter of 22.5% and 1.44 Gbit/s bit rate.