Dissertations / Theses on the topic 'THZ FREQUENCY'
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Parvex, Pichaida Taky. "Astrometric precision spectroscopy: Experimental development of a dual-frequency laser synthesizer based on an optical frequency comb." Tesis, Universidad de Chile, 2018. http://repositorio.uchile.cl/handle/2250/159288.
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Ingeniero Civil EléctricoLa tecnología de terahercios se encuentra en un estado de desarrollo atrasado con respecto a las tecnologías usadas en las bandas adyacentes, como la óptica infrarroja o la electróni- ca de microondas. En particular, no se poseen fuentes compactas de radiación que operen dentro esta banda logrando buenos niveles de potencia y amplios rangos de frecuencia. Las útiles propiedades de la radiación de terahercios como su capacidad de detectar moléculas complejas, buena resolución espacial y ser radiación no ionizante, hacen que el desarrollo de tecnología para esta banda sea un área con creciente interés. En el contexto del desarrollo de una nueva línea de investigación sobre espectroscopía molecular, en el Laboratorio de Terahertz y Astrofotónica de la Universidad de Chile, se realiza este trabajo que consiste en el desarrollo experimental de un sistema láser para la ali- mentación de fotomezcladores. Este sistema tiene como objetivo la generación de dos señales ópticas de alta estabilidad y coherencia, cuya diferencia de frecuencias puede ser ajustada de forma continua dentro del rango de 10 GHz a 300 GHz. Para esto, se utiliza un esquema basado en un peine de frecuencias óptico sobre el cual se enclava por inyección un láser de diodos de frecuencia sintonizable. Esto consigue tener una fuente infrarroja de alta precisión dentro de un gran rango. Además, se genera una segunda señal por medio de modulación en amplitud (AM), la cual es sintonizable dentro de un rango igual al espaciado producido por el peine óptico. En conjunto, estas señales logran abarcar un amplio espectro de frecuencias de forma continua sin perder estabilidad ni calidad de las señales. En este trabajo se logra implementar los subsistemas para la generación de cada una de las señales requeridas y se estudia la capacidad de estos para trabajar dentro del rango deseado. Para la señal generada por enclavamiento por inyección, se logra probar el concepto dentro de un rango reducido, principalmente por falta de un buen sistema de medición de altas frecuencias. Para la señal generada por modulación AM, se logran resultados positivos en todo el rango de diseño. Finalmente, se proponen modificaciones al sistema para mejorar su desempeño.
Este trabajo ha sido parcialmente financiado por Conicyt, a través de su fondo ALMA para el desarrollo de la astronomía, Proyecto 31140025, QUIMAL, Proyecto 1500010, CATA-Basal PFB06 y Fondecyt 1151213
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Dolasinski, Brian David. "Nonlinear systems for frequency conversion from IR to RF." University of Dayton / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1417804168.
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Suizu, Koji, Kodo Kawase, and 晃道 川瀬. "Monochromatic-Tunable Terahertz-Wave Sources Based on Nonlinear Frequency Conversion Using Lithium Niobate Crystal." IEEE, 2008. http://hdl.handle.net/2237/11170.
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Wang, Cheng. "Wideband and fast THz spectrometer using dual-frequency-comb on CMOS." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/118025.
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Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 71-75).
Millimeter-wave/terahertz rotational spectroscopy of polar gaseous molecules provides a powerful tool for complicated gas mixture analysis. Here, a 220-to-320 GHz dual-frequency-comb spectrometer in 65-nm bulk CMOS is presented, along with a systematic analysis on fundamental issues of rotational spectrometer, including the impacts of various noise mechanisms, gas cell, molecular properties, detection sensitivity, etc. The spectrometer utilizes two counter-propagating frequency-comb signals to seamlessly scan the broadband spectrum. The comb signal, with 10 equally-spaced frequency tones, is generated and detected by a chain of inter-locked transceivers on chip. Each transceiver is based on a multi-functional electromagnetic structure, which serves as frequency doubler, sub-harmonic mixer and on-chip radiator simultaneously. In particular, theory and design methodology of a dual transmission line feedback scheme are presented, which maximizes the transistor gain near its cut-off frequency fmax. The dual-frequency-comb scheme does not only improve the scanning speed by 20 x, but also reduces the overall energy consumption to 90 mJ/point with 1 Hz bandwidth (or 0.5 s integration time). With its channelized 100-GHz scanning range and sub-kHz specificity, wide range of molecules can be detected. In the measurements, state-of-the-art total radiated power of 5.2 mW and single sideband noise figure (NF) of 14.6~19.5 dB are achieved, which further boost the scanning speed and sensitivity. Lastly, spectroscopic measurements for carbonyl sulfide (OCS) and acetonitrile (CH3CN) are presented. With a path length of 70 cm and 1 Hz bandwidth, the measured minimum detectable absorption coefficient reaches [alpha] gas,min=7 .2 x 10-7 cm- 1 . For OCS, that enables a minimum detectable concentration of 11 ppm. The predicted sensitivity for some other molecules reaches ppm level (e.g. 3 ppm for hydrogen cyanide (HCN)), or 10 ppt level if gas pre-concentration with a typical gain of 10 5 is used.
by Cheng Wang.
S.M.
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Paquet, Romain. "Nouvelles sources lasers pour génération THz." Thesis, Montpellier, 2016. http://www.theses.fr/2016MONTS017.
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Cette thèse porte sur la conception, la réalisation et l'étude expérimentale d'une source laser bifréquence de haute cohérence émettant à 1 µm en vue d'obtenir par photomélange un émetteur THz. Nous nous intéressons plus particulièrement aux lasers à semiconducteur émettant par la surface en cavité externe verticale (VeCSEL), l'objectif étant d'obtenir un fonctionnement laser bifréquence robuste en régime continu, basé sur la coexistence simultanée de deux modes transverses de Laguerre-Gauss. La sélection de seulement deux modes transverses est réalisée grâce à des masques de pertes insérés intracavité dans le plan transverses. Les caractéristiques du laser bifréquence, telles que l'équilibre entre les puissances des deux modes, le caractère monofréquence de chacun des deux modes, l'accordabilité de l'écart de fréquence, la simultanéité de l'émission et la cohérence du battement THz obtenu, sont étudiées. Enfin, la génération THz par photomélange est effectuée grâce au VeCSEL bifréquence et à une photodiode UTC commercialeThis work focuses on the design, realization and experimental study of highly coherent dual-frequency laser sources emitting at 1 µm for THz radiation generation by photomixing. We are particularly interested in vertical-external-cavity surface-emitting laser (VeCSEL), the aim being to obtain a robust dual-frequency continuous wave operation, based on simultaneous coexistence of two Laguerre-Gaussian transverse modes. We design intracavity transverse selective losses mask to select only the two Laguerre-Gaussian modes. The stable and simultaneous dual-frequency operation, the beat-frequency tunability range and the temporal coherence was specifically studied. We demonstrated THz emission by seeding a uni-travelling-carrier photodiode by an optically-pumped dual-frequency vertical-external-cavity surface-emitting
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Cluff, Julian. "Time domain THz spectroscopy of semiconductors." Thesis, University of Bath, 2000. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.311454.
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Niklas, Andrew John. "Characterization of Structured Nanomaterials using Terahertz Frequency Radiation." Wright State University / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=wright1347461386.
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Thoma, Petra [Verfasser]. "Ultra-fast YBa2Cu3O7-x direct detectors for the THz frequency range / Petra Thoma." Karlsruhe : KIT Scientific Publishing, 2013. http://www.ksp.kit.edu.
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Sung, Chieh. "Interaction of a relativistic electron beam with radiation in the THz frequency range." Diss., Restricted to subscribing institutions, 2008. http://proquest.umi.com/pqdweb?did=1679290761&sid=1&Fmt=2&clientId=1564&RQT=309&VName=PQD.
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Wang, Yuekun. "In0.53Ga0.47As-In0.52Al0.48As multiple quantum well THz photoconductive switches and In0.53Ga0.47As-AlAs asymmetric spacer layer tunnel (ASPAT) diodes for THz electronics." Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/in053ga047asin052al048as-multiple-quantum-well-thz-photoconductive-switches-and-in053ga047asalas-asymmetric-spacer-layer-tunnel-aspat-diodes-for-thz-electronics(5fd73bd5-aef3-476b-be1b-7498da3f9627).html.
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This thesis is concerned with terahertz (THz) technology from both optical and electronic approaches. On the optical front, the investigation of optimised photoconductive switches included the characterisation, fabrication and testing of devices which can generate and detect THz radiation over the frequency range from DC to ~ 2.5 THz. These devices incorporated semiconductor photoconductors grown under low temperature (LT) Molecular Beam Epitaxy (MBE) conditions and using distributed Bragg reflectors (DBRs). The material properties were studied via numerous characterisation techniques which included Hall Effect and mid infrared reflections. Antenna structures were fabricated on the surface of the active layers and pulsed/continuous wave (CW) signal absorbed by these structures (under bias) generates photocurrent. With the help of the DBRs at certain wavelengths (800 nm and 1550 nm), the absorption coefficient at the corresponding illumination wavelength increased thus leading to significant increase of the THz output power while the materials kept the desirable photoconductive material properties such as high dark resistivity and high electron mobility. The inclusion of DBRs resulted in more than doubling of the THz peak signals across the entire operating frequency range and significant improvements in the relative THz power. For the THz electronic approach, a new type of InP-based Asymmetric Spacer Tunnel Diode (ASPAT), which can be used for high frequency detector, was studied. The asymmetric DC characteristics for this novel tunnel diode showed direct compatibility with high frequency zero-bias detector applications. The devices also showed an extreme thermal stability (less than 7.8% current change from 77 K to 400 K) as the main carrier transport mechanism of the ASPAT was tunnelling. Physical models for this ASPAT diode were developed for both DC (direct current) and AC (alternating current) simulations using the TCAD software tool SILVACO. The simulated DC results showed almost perfect matches with measurements across the entire temperature range from 77 K to 400 K. From RF (radio frequency) measurements, the intrinsic diode parameters were extracted and compared with measured data. The simulated zero biased detector circuits operating at 100 GHz and 240 GHz using the new InGaAs-AlAs ASPAT diode (4*4 micrometer square) showed comparable voltage sensitivities to state of the art Schottky barrier diodes (SBDs) detectors but with the added advantage of excellent thermal stability.
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Suzanovičienė, Rasa. "Investigation of carrier kinetics in semiconductors by terahertz radiation pulses." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2010. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2010~D_20101116_163924-89818.
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Creation of ultrafast semiconductor components is inconceivable without understanding various processes of picoscond duration in semiconductors. These processes, as electron energy relaxation time or nonequiriblium carrier capture are very important for semiconductor photonics and terahertz range devices. Since now, the most popular tool of measuring ultrafast processes in semiconductors was picosecond or femtosecond laser pulses. In spite of excellent time resolution, optical pump – probe methods have a significant imperfection. Interpretation of the results can be very complicate. Also, the measured result can be affected by few variable parameters or interaction of various physical phenomenon. Therefore determinate results can be hardly related with electron time dependent characteristic. The aim of this dissertation was to measure electron energy relaxation times and electron life times by using terahertz pulses in narrow – gap semiconductors used for photoconductive terahertz emitters or detectors. In this dissertation, electron characteristic times witch describe various processes in semiconductor, were studied. These measurements were performed by optical pump – terahertz probe technique and time domain terahertz spectroscopy. The emission of terahertz pulses from the semiconductor surface, illuminated by femtosecond laser pulses, was investigated.Ultrasparčių puslaidininkinių komponentų kūrimas reikalauja gilesnio supratimo apie tai, kaip puslaidininkiuose vyksta fizikiniai procesai, trunkantys kelias pikosekundes ar net mažiau nei vieną pikosekundę. Tokie reiškiniai, kaip elektronų impulso ir energijos relaksacija bei nepusiausvyrųjų krūvininkų pagavimas yra labai svarbūs puslaidininkinių fotonikos ir terahercinio diapazono prietaisų veikimui. Iki pastarojo meto pagrindinis ultrasparčiųjų procesų puslaidininkiuose tyrimo įrankis buvo optiniai metodai, kuriuose elektronų dinamikai stebėti buvo pasitelkiami pikosekundinių ar femtosekundinių lazerių impulsai. Nepaisant išskirtinai didelės šių metodų laikinės skyros, optinio kaupinimo-zondavimo matavimų rezultatus yra palyginti sudėtinga interpretuoti. Šie rezultatai dažniausiai yra įtakojami kelių sistemos parametrų kitimo ir įvairių fizikinių reiškinių tarpusavio sąveikos, todėl sunkiai susiejamas su kuria nors elektronų laikine charakteristika. Disertacijos darbo tikslas – naudojant terahercinės spinduliuotės impulsus išmatuoti elektronų impulso ir energijos relaksacijos trukmes keliuose siauratarpiuose puslaidininkiuose bei jų gyvavimo trukmes medžiagose, skirtose fotolaidžių terahercinės spinduliuotės emiterių ir detektorių gamybai. Šioje disertacijoje yra pateikiami įvairių charakteringų elektroninius procesus puslaidininkiuose apibūdinančių trukmių matavimų naudojant terahercinės spinduliuotės impulsus rezultatai. Tokie tyrimai atlikti ir optinio žadinimo –... [toliau žr. visą tekstą]
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Petersen, Eliot. "Cavity Enhanced THz Generation in Nonlinear Crystals Pumped by Near-IR Fiber Lasers." Diss., The University of Arizona, 2012. http://hdl.handle.net/10150/222833.
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A coherent optical THz (1.5 THz, 200 µm) source was developed based on pulsed, near IR, fiber lasers, and frequency mixing in nonlinear crystals. The generated THz frequency is determined by the difference frequency of two high peak power pulsed fiber lasers at 1550 nm and 1538 nm. When incident to the crystal, the near IR lasers induce a polarization at their beat frequency which generates the THz radiation. The pulsed fiber lasers are single transverse mode, have high pulse energy and peak powers of 0.38 mJ and 128 kW respectively. They are transform limited at a few ns in duration with very good beam quality of M² ≈ 1.2. The pulse seed was created by modulating a constant laser beam with an electro-optic modulator. An arbitrary waveform generator was used to pre-shape these pulses to compensate for pulse distortion caused by pump gain depletion in the subsequent fiber amplifiers. Pre-amplifiers were constructed using commercial erbium doped silica fiber. Special, highly doped, large core, phosphate fiber was developed in-house to further amplify the pulses, while avoiding nonlinear scattering processes such as stimulated Brillouin scattering and stimulated Raman scattering. THz generation was achieved in both ZnGeP₂ and GaP which were chosen based on their low pump and THz absorption, as well as high nonlinear coefficient. Angle tuning was used to phase match all three optical frequencies in ZnGeP₂ thanks to its birefringence. Layers of GaP ~500 µm thick were pressed together alternately rotated 180° around the normal to quasi-phase match the pump and THz frequencies. To increase the efficiency of the THz generation an external optical cavity was used to enhance and recycle the IR pump pulses. The nonlinear crystal was placed inside the cavity and 151 times enhancement of THz power was observed.
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Suzanovičienė, Rasa. "Krūvininkų kinetikos puslaidininkiuose tyrimai naudojant terahercinės spinduliuotės impulsus." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2010. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2010~D_20101116_163938-18022.
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Ultrasparčių puslaidininkinių komponentų kūrimas reikalauja gilesnio supratimo apie tai, kaip puslaidininkiuose vyksta fizikiniai procesai, trunkantys kelias pikosekundes ar net mažiau nei vieną pikosekundę. Tokie reiškiniai, kaip elektronų impulso ir energijos relaksacija bei nepusiausvyrųjų krūvininkų pagavimas yra labai svarbūs puslaidininkinių fotonikos ir terahercinio diapazono prietaisų veikimui.
Iki pastarojo meto pagrindinis ultrasparčiųjų procesų puslaidininkiuose tyrimo įrankis buvo optiniai metodai, kuriuose elektronų dinamikai stebėti buvo pasitelkiami pikosekundinių ar femtosekundinių lazerių impulsai. Nepaisant išskirtinai didelės šių metodų laikinės skyros, optinio kaupinimo-zondavimo matavimų rezultatus yra palyginti sudėtinga interpretuoti. Šie rezultatai dažniausiai yra įtakojami kelių sistemos parametrų kitimo ir įvairių fizikinių reiškinių tarpusavio sąveikos, todėl sunkiai susiejamas su kuria nors elektronų laikine charakteristika.
Disertacijos darbo tikslas – naudojant terahercinės spinduliuotės impulsus išmatuoti elektronų impulso ir energijos relaksacijos trukmes keliuose siauratarpiuose puslaidininkiuose bei jų gyvavimo trukmes medžiagose, skirtose fotolaidžių terahercinės spinduliuotės emiterių ir detektorių gamybai.
Šioje disertacijoje yra pateikiami įvairių charakteringų elektroninius procesus puslaidininkiuose apibūdinančių trukmių matavimų naudojant terahercinės spinduliuotės impulsus rezultatai. Tokie tyrimai atlikti ir optinio žadinimo –... [toliau žr. visą tekstą]Creation of ultrafast semiconductor components is inconceivable without understanding various processes of picoscond duration in semiconductors. These processes, as electron energy relaxation time or nonequiriblium carrier capture are very important for semiconductor photonics and terahertz range devices. Since now, the most popular tool of measuring ultrafast processes in semiconductors was picosecond or femtosecond laser pulses. In spite of excellent time resolution, optical pump – probe methods have a significant imperfection. Interpretation of the results can be very complicate. Also, the measured result can be affected by few variable parameters or interaction of various physical phenomenon. Therefore determinate results can be hardly related with electron time dependent characteristic. The aim of this dissertation was to measure electron energy relaxation times and electron life times by using terahertz pulses in narrow – gap semiconductors used for photoconductive terahertz emitters or detectors. In this dissertation, electron characteristic times witch describe various processes in semiconductor, were studied. These measurements were performed by optical pump – terahertz probe technique and time domain terahertz spectroscopy. The emission of terahertz pulses from the semiconductor surface, illuminated by femtosecond laser pulses, was investigated.
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Smith, Ryan Michael. "Terahertz frequency analysis of gaseous and solid samples using terahertz time-domain spectroscopy." Diss., University of Iowa, 2012. https://ir.uiowa.edu/etd/3386.
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Developments in semiconductor and laser technologies have facilitated development of terahertz (THz)-frequency (˜2-200 cm-1) technologies. Results published in the literature as far back as the early 20th century demonstrate the utility of this frequency range for myriad applications, but the improved performance of modern THz technologies has renewed interest in THz-frequency analysis. Material presented in this dissertation focuses on three applications of terahertz time-domain spectroscopy (THz-TDS): quantitation of gas-phase molecular species, analysis of polymeric materials, and investigation of dental tissue/composite structures.
Gas phase species were quantified individually at concentrations ranging from several parts per million to several parts per thousand using various chemometric methods. Quantitative model robustness was evaluated by comparison of model precision, and partial least-squares (PLS) regressions provided the greatest precision. Species were quantified in mixtures using PLS with errors of prediction below the permissible exposure limits (PELs) set by the Occupational Safety and Health Administration. The effect of spectral broadening as a result of overall sample pressure was investigated, and species were analyzed in mixtures at various overall pressures. Errors of prediction were again near or below the PELs, demonstrating the utility of this method for atmospheric analysis. Chemical selectivity available in THz spectral features was evaluated and compared to selectivity available in infrared frequencies.
Spectral parameters measured in the THz frequency range also provide insight into structural properties of polymeric materials. In some cases, spectral peaks may be used to identify the temperature at which phase changes occur within these materials. THz refractive index spectra were found to be a sensitive and non-destructive tool for identification of phase transition temperatures.
The time-resolved measurement of THz-TDS makes it particularly useful for rapid, non-destructive analysis of layered structures. Ordinarily, the strength of bonds between dental tissues and composite materials are evaluated in the laboratory using destructive failure analyses. Transparency of dental tissues and composite materials used for restorative procedures to THz pulses allows investigation of interfaces between these materials. Refractive index spectra indicate locations in which delamination has occurred between bonded layers. These results provide an overview of unique capabilities of the THz-TDS method in real-life spectral analyses.
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Kantemur, A., Q. Tang, and H. Xin. "Design of volumetric sub-THz negative refractive index metamaterial with gain." IEEE, 2016. http://hdl.handle.net/10150/622669.
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Conventional passive metamaterials always suffer from the limitation of loss and dispersion due to fundamental causality issue. Especially it becomes severe due to material loss at terahertz frequency. Our work resolves the loss problem by introducing gain device into the metamaterial structure. A passive volumetric metamaterial is firstly designed on the quartz substrate. A negative resistance is inserted into the wire of the structure to provide the gain. We have identified resonant tunneling diodes that work up into THz frequency and shown in simulation that simultaneous negative index and gain can be obtained.
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Majeed, Farhat. "Screen Printed and Graphene and Carbon Nanotube Antennas at UHF and EHF - Design, Modelling and Measurement." Thesis, Griffith University, 2018. http://hdl.handle.net/10072/376514.
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Researchers seek to design small meander line antennas for radio frequency
identification (RFID) applications, while taking into consideration the
achievement of best performance parameters of the antenna: impedance, return
loss, voltage standing wave ratio (VSWR), radiation efficiency, gain, directivity
and etc. Various planar meander line antennas were proposed which have the
lowest resonant frequency and maximum radiation efficiencies for a fixed area,
using multi-parameter optimization routines. The design in planar form with
maximum radiation efficiency was optimized with the inclusion of matching
structure for complex impedance matching with RF transceiver chip CC1101 for
433 MHz RFID applications. The design was also optimized for impedance match
with commonly used transmission lines (50 ohms characteristic impedance) for
2.45 GHz applications. The antennas were fabricated using circuit in plastic (CiP)
technique with screen printed silver conductor on 3mm FR4 and plastic substrates
respectively. The technique allows the creation of a waterproof antenna for
underground (mining geophysics, environmental monitoring, etc.) and underwater
(swimmer communications, tidal monitoring, etc.) applications, because the
antenna unit is completely sealed in a thin plastic coating. The impedance, band
width, return loss and radiation efficiency of these antennas are the main
parameters reported.
Since the discovery of graphene, its electrical, mechanical and electronic
properties have been explored. The high conductivity, low fabrication cost,
flexibility, controllable isotropic and anisotropic behavior are some of the main
properties of graphene which make the technology fascinating for electrical
engineers and antenna designers. It can be used for antenna applications in the
Terahertz frequency band either as a two dimensional, infinitely thin, planar sheet
or in the form of one dimensional rolled up graphene known as carbon nanotubes.
A pole-zero analysis on the input impedance of carbon nanotube (CN) dipole
antennas with different lengths was used to explain the damped plasmonic
resonances of these dipoles with increasing length. We used model based
parameter estimation to approximate the input impedance of the antennas with a
rational function in the complex frequency domain. Despite the dispersive nature of CNs, the imaginary part of the poles and zeros are respectively the integer
multiples and odd multiples of the imaginary part of the first pole and zero.
However, the real parts of impedance at the poles are almost constant, while the
pattern was not observed for the real part of zeros. Carbon nanotube dipoles
operating between 43-53GHz are well matched if the source impedance is much
higher than 50 ohms, and even higher than 12.9kΏ. The fundamental resonances
(f0) of carbon nanotube dipoles plotted versus their inverse-half-length (1/L) are
linearly related, but the intercept of the fitted straight line is non-zero unlike that
for perfect electric conductor (PEC) dipoles. This leads to a non-linear variation
in wavelength scaling of CN dipoles. The resonant CN antennas are relatively
much shorter than PEC dipoles of equivalent size.
The fundamental properties of carbon nanotube straight wire dipole and Yagi-
Uda antennas were determined using theoretical and numerical modelling. The
insertion loss, radiation efficiency and Q-factor have been reported using
classical Hallén’s-type integral equation, based on quantum mechanical
conductivity. Contrary to the properties of metal antennas with dimensions less
than 0.01 m, the addition of parasitic elements in a Yagi-Uda antenna with carbon
nanotubes does not lead to significant changes in the radiation characteristics as
the induced current in the parasitic elements is very small. The radiation
efficiency of the Yagi-Uda antenna shows little change. The radiation efficiency
of these antennas when compared with the Pfeiffer’s maximum efficiency bound
lies well below the maximum efficiency limit. The -factor of carbon nanotube
antennas limited by their radiation efficiency was compared to the fundamental
limits proposed by Chu and Thal and although the -factor of the nanotube
antennas is very small, it is still larger than the fundamental limits. This analysis
was also extended to carbon nanotube loop antennas operating in transmitting
mode. The extinction properties of carbon nanotube loop antennas were also
investigated and compared with ordinary metallic loop antennas.
Since graphene is an ideal candidate for THz antenna applications due to its
excellent electrical and mechanical properties, the conductivity and scattering
properties of monolayer graphene were explored over a wide frequency range.
Two monolayer graphene samples having different dimensions printed on a
plastic (PET) substrate were measured using horn waveguides at microwave frequencies. Cascade matrix theory was applied to calculate scattering parameters
of monolayer graphene patch on PET substrate assuming a frequency
independent surface conductivity (manufacture’s specification) and to deembed
the surface conductivity of graphene from measured transmission
parameter s21. De-embedding from measured reflection parameter s11 was
performed using transmission line theory. The theoretically modelled Sparameters
were in good agreement with measured S-parameters of graphene.
The measured conductivity of graphene using proposed formulations was in good
agreement with the theory. The results were compared with previously published
conductivity measurements of mono and multi layered graphene.
This work on graphene and carbon nanotube antenna properties is a contribution
towards THz antenna applications and we intend to extend this work to explore
the properties of graphene meander line antennas which have not been reported to date.Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Eng & Built Env
Science, Environment, Engineering and Technology
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Mu, Tangjie. "Heteroepitaxy of OP-GaP on GaAs for frequency conversion in mid-IR and THz regions." Thesis, KTH, Tillämpad fysik, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-265627.
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Motzkau, Holger. "High-frequency phenomena in small Bi2Sr2CaCu2O8+x intrinsic Josephson junctions." Doctoral thesis, Stockholms universitet, Fysikum, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-115582.
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In this thesis, the tunneling between individual atomic layers in structures of Bi2Sr2CaCu2O8+x based high-temperature superconductors are experimentally studied employing the intrinsic Josephson effect. A special attention is paid to the fabrication of small mesa structures using micro and nanofabrication techniques. In the first part of the thesis, the periodic Fraunhofer-like modulation of the critical current of the junctions as a function of in-plane magnetic field is investigated. A transition from a modulation with a half flux quantum to a flux quantum periodicity is demonstrated with increasing field and decreasing junction length. It is interpreted in terms of the transformation of the static fluxon lattice of stacked, strongly coupled intrinsic Josephson junctions and compared with theoretical predictions. A fluxon phase diagram is constructed.Numerical simulations have been carried out to complement the experimental data. In the second part of the thesis, different resonant phenomena are studied in the dynamic flux-flow state at high magnetic fields, including Eck-resonances and Fiske steps. Different resonant modes and their velocities, including superluminal modes, are identified. In the third part, different experiments attempting to detect radiation from small mesa structures using different setups based on hot-electron bolometer mixers and calorimeters are described. No distinct radiation with emission powers higher than about 500pW could be detected. Furthermore, the interaction with external GHz-radiation is studied. Resonances attributed to an induced flux-flow are observed, and the reflectivity of the sample can be tuned by switching mesas between the superconducting and quasiparticle state. In the last part, the resistive switching of mesas at high bias is studied. It is attributed to a persistent electrical doping of the crystal. Superconducting properties such as the critical current and temperature and the tunneling spectra are analyzed at different doping states of the same sample. The dynamics of the doping is studied, and attributed to two mechanisms; a charge-transfer effect and oxygen reordering
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Sinha, Raju. "Tunable, Room Temperature THz Emitters Based on Nonlinear Photonics." FIU Digital Commons, 2017. http://digitalcommons.fiu.edu/etd/3172.
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The Terahertz (1012 Hz) region of the electromagnetic spectrum covers the frequency range from roughly 300 GHz to 10 THz, which is in between the microwave and infrared regimes. The increasing interest in the development of ultra-compact, tunable room temperature Terahertz (THz) emitters with wide-range tunability has stimulated in-depth studies of different mechanisms of THz generation in the past decade due to its various potential applications such as biomedical diagnosis, security screening, chemical identification, life sciences and very high speed wireless communication. Despite the tremendous research and development efforts, all the available state-of-the-art THz emitters suffer from either being large, complex and costly, or operating at low temperatures, lacking tunability, having a very short spectral range and a low output power. Hence, the major objective of this research was to develop simple, inexpensive, compact, room temperature THz sources with wide-range tunability.
We investigated THz radiation in a hybrid optical and THz micro-ring resonators system. For the first time, we were able to satisfy the DFG phase matching condition for the above-mentioned THz range in one single device geometry by employing a modal phase matching technique and using two separately designed resonators capable of oscillating at input optical waves and generated THz waves. In chapter 6, we proposed a novel plasmonic antenna geometry – the dimer rod-tapered antenna (DRTA), where we created a hot-spot in the nanogap between the dimer arms with a very large intensity enhancement of 4.1×105 at optical resonant wavelength. Then, we investigated DFG operation in the antenna geometry by incorporating a nonlinear nanodot in the hot-spot of the antenna and achieved continuously tunable enhanced THz radiation across 0.5-10 THz range. In chapter 8, we designed a multi-metallic resonators providing an ultrasharp toroidal response at THz frequency, then fabricated and experimentally demonstrated an efficient polarization dependent plasmonic toroid switch operating at THz frequency.
In summary, we have successfully designed, analytically and numerically investigated novel THz emitters with the advantages of wide range tunability, compactness, room temperature operation, fast modulation and the possibility for monolithic integration, which are the most sought after properties in the new generation THz sources.
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Kaltenecker, Korbinian J. [Verfasser], Hanspeter [Akademischer Betreuer] Helm, Markus [Akademischer Betreuer] Walther, and Bernd-Michael [Akademischer Betreuer] Fischer. "Breaking the diffraction limit : experiment and theory on sub-diffraction limited focusing and imaging realised in the THz frequency regime." Freiburg : Universität, 2016. http://d-nb.info/1119899370/34.
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Arenas, Daniel J. "Devices and materials for THz spectrosopy [sic] GHz CMOS circuits, periodic hole-arrays and high-frequency dielectric materials /." [Gainesville, Fla.] : University of Florida, 2009. http://purl.fcla.edu/fcla/etd/UFE0024735.
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Mei, Jialin, Kai Zhong, Maorong Wang, Pengxiang Liu, Degang Xu, Yuye Wang, Wei Shi, Jianquan Yao, Robert A. Norwood, and Nasser Peyghambarian. "Compact high-repetition-rate terahertz source based on difference frequency generation from an efficient 2-μm dual-wavelength KTP OPO." SPIE-INT SOC OPTICAL ENGINEERING, 2016. http://hdl.handle.net/10150/622890.
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A compact optical terahertz (THz) source was demonstrated based on an efficient high-repetition-rate doubly resonant optical parametric oscillator (OPO) around 2 mu m with two type-II phase-matched KTP crystals in the walk-off compensated configuration. The KTP OPO was intracavity pumped by an acousto-optical (AO) Q-switched Nd:YVO4 laser and emitted two tunable wavelengths near degeneracy. The tuning range extended continuously from 2.068 mu m to 2.191 mu m with a maximum output power of 3.29 W at 24 kHz, corresponding to an optical-optical conversion efficiency (from 808 nm to 2 mu m) of 20.69%. The stable pulsed dual-wavelength operation provided an ideal pump source for generating terahertz wave of micro-watt level by the difference frequency generation (DFG) method. A 7.84-mm-long periodically inverted quasi-phase-matched (QPM) GaAs crystal with 6 periods was used to generate a terahertz wave, the maximum voltage of 180 mV at 1.244 THz was acquired by a 4.2-K Si bolometer, corresponding to average output power of 0.6 mu W and DFG conversion efficiency of 4.32x10(-7). The acceptance bandwidth was found to be larger than 0.35 THz (FWHM). As to the 15-mm-long GaSe crystal used in the type-II collinear DFG, a tunable THz source ranging from 0.503 THz to 3.63 THz with the maximum output voltage of 268 mV at 1.65 THz had been achieved, and the corresponding average output power and DFG conversion efficiency were 0.9 mu W and 5.86x10(-7) respectively. This provides a potential practical palm-top tunable THz sources for portable applications.
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Beneš, Adam. "Plazmonické antény pro vysoké vlnové délky." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2021. http://www.nusl.cz/ntk/nusl-443226.
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Tato diplomová práce se zabývá vlastnostmi plazmonických antén v oblasti vysokých vlnových délek. Důraz je kladen na popis rezonančních vlastností jednotlivých antén i antén uspořádaných do periodických polí. Těžiště práce spočívá v počítačovém modelování navýšení magnetického pole v blízkosti antén, které lze využít ve vysokofrekvenční elektronové paramagnetické rezonanci (HFEPR) k zesílení měřeného signálu. Autor se zabývá kvantifikací zesílení v anténách s odlišnou geometrií a navrhuje i geometrii vlastní. Značná část práce se také věnuje snaze rozlišit příspěvky k navýšení magnetického pole od různých zdrojů při měření HFEPR v uspořádání s dvojitou transmisí záření.
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Fang, Qiang. "2 μm Pulsed Fiber Laser Sources and Their Application in Terahertz Generation." Diss., The University of Arizona, 2012. http://hdl.handle.net/10150/232475.
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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.
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Meier, Steffen Marius [Verfasser], Uwe [Gutachter] Czarnetzki, and Horst [Gutachter] Fichtner. "Plasmadichtebestimmung mittels Dual-Frequency-Multichannel-Boxcar-THz-Time-Domain-Spektroskopie / Steffen Marius Meier ; Gutachter: Uwe Czarnetzki, Horst Fichtner ; Fakultät für Physik und Astronomie." Bochum : Ruhr-Universität Bochum, 2018. http://d-nb.info/1161942327/34.
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Seif, Marcelino. "Caractérisation et modélisation des sources de bruit BF dans les transistors bipolaires développés en technologie BiCMOS (sub 0,13µm) pour applications RF et THz." Thesis, Montpellier, 2015. http://www.theses.fr/2015MONTS127/document.
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Les travaux de thèse, présentés dans ce manuscrit, portent sur la caractérisation et la modélisation des sources de bruit basse fréquence dans les transistors bipolaires à hétérojonction Si/SiGe:C issus des filières BiCMOS 130 et 55 nm utilisées pour la réalisation de circuits intégrés dédiés aux futures applications dans le domaine du THz. A partir des mesures réalisées en fonction de la polarisation, de paramètres géométriques (surface et périmètre d'émetteur principalement) et de la température, la composante de bruit en 1/f, associée aux fluctuations du courant de base, a été entièrement caractérisée et les sources de bruit associées localisées. Les paramètres du modèle compact SPICE ont été extraits et comparés avec ceux de la littérature. Pour la technologie BiCMOS 130 nm, la valeur obtenue pour la figure de mérite KB égale 6,8 10-11 µm² ce qui représente le meilleur résultat publié à ce jour, toutes filières de transistors bipolaires confondues. Réalisée sur une plaque entière, l'étude statistique de la dispersion du niveau de bruit en 1/f a permis d'étendre la modélisation compacte de type SPICE. Mesuré sur une large gamme de température, le niveau de bruit en 1/f n'a pas présenté de variation significative. Pour la première fois, une étude complète de la composante de bruit en 1/f associée aux fluctuations du courant de collecteur est présentée et les paramètres du modèle SPICE extraits. Concernant la caractérisation des composantes de génération-recombinaison (présence non systématique), une étude statistique a montré que les transistors de plus petites dimensions étaient les plus impactés. La comparaison entre les différentes technologies montre que ces composantes sont beaucoup plus présentes dans les technologies les moins matures. Quand ces composantes ont été associées à du bruit RTS, une méthode de caractérisation temporelle et fréquentielle a été mise en œuvre. Enfin, dans certains cas, une étude en basses températures a permis d'extraire les énergies d'activation des pièges responsables de ces composantes de génération-recombinaisonThe presented thesis work, in this manuscript, focuses on the characterization and modeling of the low frequency noise sources in heterojunction bipolar transistors Si/SiGe :C derived from 130 to 55 nm BiCMOS technology used in the production of integrated circuits dedicated for THz domain applications. From measurements versus bias, geometrical parameters (emitter area and perimeter) and temperature, the 1/f noise component, associated to the base current fluctuations, has been fully characterized and the associated sources have been localized. The SPICE compact model parameters have been extracted and compared with those of the literature. For the BiCMOS 130 nm technology, the obtained figure of merit value of 6,8 10-11 µm2 represents the best published result so far in all bipolar transistors. The dispersion study of the 1/f noise component, performed over a complete wafer, allowed us to extend the SPICE type compact modeling. Measured over a large temperature range, the 1/f noise did not show any variations. For the first time, a complete characterization of the 1/f component at the output of the transistors is presented as well as the extraction of SPICE parameters. Regarding the characterization of generation-recombination components (unsystematic presence), a statistical study has showed that transistors with small emitter areas (Ae < 1 µm2) are affected more than the transistors with large emitter areas by the presence of g-r components. Comparison between different technologies shows that these components are much more present in the less mature technologies. When these components have been associated to RTS, time and frequency domain method is implemented. Finally, in some cases, a study at low temperatures was used to extract the activation energy of the traps responsible for the generation-recombination components
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Danion, Gwennaël. "Synthétiseur micro-onde à térahertz ultra-stable." Thesis, Rennes 1, 2015. http://www.theses.fr/2015REN1S014/document.
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Le but de cette thèse est la synthèse optique d'ondes millimétriques et submillimétriques avec un très bas bruit de phase. La première partie concerne la réalisation d'un laser biaxe bifréquence dont chacune des deux fréquences est accordable indépendamment et continûment sur 1 THz. Ce laser est caractérisé en bruit d'amplitude et de phase. Nous avons mis en évidence un facteur de couplage entre les fluctuations de puissance de la diode de pompe et le bruit de phase du laser. La deuxième partie concerne le développement d'un système amplificateur qui se compose d'un amplificateur EDFA et d'un SOA par polarisation. Ce système amplificateur permet d'obtenir une puissance de l'ordre de 17 dBm, tout en réduisant le bruit relatif d'intensité (RIN) d'une vingtaine de dB sur 1 GHz. Cet amplificateur est également un actionneur pour la stabilisation de puissance permettant un RIN de l'ordre de -150 dB/Hz de 3 Hz à 5 kHz. La dernière partie concerne la mise en place du banc cavité et de l'asservissement des fréquences du laser sur une cavité ultra-stable. Nous obtenons un bruit de phase, à 10 kHz pour une porteuse à 10 GHz, meilleur que le plancher de bruit d'un analyseur de bruit de phase hautes performances de l'ordre de -115 dBc/ Hz. Le bruit de phase du système est indépendant de la fréquence de battementThe aim of this thesis is the optical synthesis of millimeter and submillimeter waves with a very low phase noise. The first part concerns the development of a dual-axis dual frequency laser, whose the two frequencies are tuneable independently and continuously on 1 THz. This laser is characterized in amplitude noise and phase noise. We have identified a coupling factor between the diode pump and the power fluctuations of the laser phase noise. In the second part, we report the development of an amplifier system which consists of an EDFA and a SOA per polarisation axis. This amplifier system delivers 17 dBm of power and reduces the relative intensity noise (RIN) by 20 dB on a 1 GHz bandwidth. This amplifier is also an actuator for the power stabilization to a RIN of the order of -150 dB/Hz from 3 Hz to 5 kHz. The last part concerns the setup of the cavity bench and the stabilization of the laser frequency on a ultrastable cavity. We obtain a phase noise at 10 kHz of frequency offset on a 10 GHz carrier better than the noise floor of a phase noise analyser with high performance of the order of -115 dBc/Hz. The system phase noise is independent of the beatnote frequency
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Khairuzzaman, Md. "Digitally selected electronically switchable terahertz-over-fibre." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/digitally-selected-electronically-switchable-terahertzoverfibre(7003c9cb-fa46-4933-9348-b1977a446b7f).html.
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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.
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Martínek, Tomáš. "Návrh nerezonančního držáku vzorku pro obecné použití v terahertzové elektronové spinové resonanční spektroskopii." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2018. http://www.nusl.cz/ntk/nusl-382623.
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Cílem diplomové práce je navrhnout konstrukční řešení držáků vzorků pro vysokofrekvenční elektron paramagnetickou resonanci. Předmětem návrhu je vytvořit jednoduchý zamykací systém pro spojování mikrovlnného vlnovodu a držáku vzorku. Dále navrhnout systém s řešením držáku pro více vzorků. Toto unikátní provedení držáku povede k několikanásobné úspoře celkového času měření vzorků. Poslední návrh spočívá v optimalizaci držáku vzorku s možností naklápění osy, kterou lze díky přímému napojení na piezoelektrický rotátor pootáčet s přesností na miliradiány. Oba typy držáku vzorku jsou navrženy s ohledem na automatizaci měření.
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Collombon, Mathieu. "Résonance noire à trois photons sur un nuage d'ions calcium confinés." Thesis, Aix-Marseille, 2019. http://www.theses.fr/2019AIXM0017.
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Les travaux présentés dans cette thèse décrivent l’ensemble de la démarche expérimentale qui a été mise en place afin d’observer un piégeage cohérent de population à trois photons sur un nuage d’ions calcium confinés en piège radio-fréquence. La réalisation d’un tel processus cohérent repose sur des contraintes expérimentales fortes concernant la relation de phase entre les trois lasers impliqués dans le processus d’interrogation ainsi que sur leurs finesses spectrales. Notre approche est basée sur l’optimisation des performances d’un laser stabilisé à 411 THz (729 nm) pour atteindre une stabilité de fréquence relative inférieure à 10⁻¹⁴ à 1 s. Ces performances ont ensuite été transférées vers un peigne de fréquence optique. Le comportement de cet instrument ainsi que les performances du transfert sont présentés dans ces travaux. Le peigne est alors utilisé comme oscillateur de transfert pour asservir les deux autres lasers impliqués dans l’interrogation des ions (866 nm et 794 nm). Grâce à ce travail sur la mise en relation des trois sources laser, ainsi que l’optimisation de leur qualités spectrales nous avons pu observer les premières résonances noires à trois photons dans le spectre de fluorescence des ions 40 Ca⁺. Ces résultats originaux ainsi que les premières études systématiques du comportement de la résonance en fonction de différents paramètres expérimentaux sont présentés. L’analyse de ces résultats a permis de définir clairement les conditions expérimentales à respecter pour une potentielle utilisation métrologique de cette résonance qui émane d’une superposition cohérente d’état stable ou métastableThe work presented in this thesis describes the complete experimental set-up and approach to realize three-photon coherent population trapping in a cloud of radiofrequency confined calcium ions. The realization of such a population trapping relies on a stringent conditions of the phase relationship between the three lasers involved in the interrogation process, and also their spectral linewidths. Our experimental approach is based on the optimization of the performance of a frequency-stabilized titanium-sapphirelaser at 411 THz (729nm) at the 10⁻¹⁴ level, in term of relative frequency uncertainties. This laser’s fre-quency stability is subsequently transfered to a optical frequency comb by means of a phase-locked loop. Then the two other lasers involved (866 nm and 794 nm) are phase-locked to the optical frequency combwith the same technique. This work describes the transfer method along with its measured performances. With all the three lasers sharing the same ultra-stable frequency reference we have been able to experi-mentally observe for the first time a 3-photon dark resonance in the fluorescence spectra of the 40 Ca⁺, signature of a coherent population trapping in a 3-photon scheme. The dependence of this resonance at experimental parameters, such as laser powers, laser detunings and local magnetic field, have been studied and are presented in this work. The preliminary results have allowed to explain the behaviour of the dark resonance and explored conditions for the use of the 3-photon dark line as a THz frequency standard
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Ayoub, Anas. "Sources laser ultrarapides performantes dans le moyen IR et le Tz." Thesis, Normandie, 2020. http://www.theses.fr/2020NORMR044.
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La sonde atomique tomographique est un instrument d’analyse de la matière à trois dimensions avec une résolution atomique. Cet instrument s’appuie sur l’effet de champ électrique généré à l’extrémité d’un échantillon taillé sous la forme d’une aiguille nanométrique pour faire évaporer les atomes de surface qui sont collectés par un détecteur à deux dimensions. La mesure du temps de vol des ions dont l’évaporation est déclenchée par une impulsion électrique ou optique permettent de remonter à la composition chimique en plus de la localisation 3D des atomes. Dans les sondes atomiques actuelles, l’évaporation atomique est déclenchée par un laser ultrarapide émettant dans l’UV. Cependant, l’interaction de la lumière UV avec la matière induit un échauffement thermique qui limite la résolution en masse de l’instrument et empêche son exploitation pour l’analyse de matériaux fragiles comme les composants biocompatibles. Ces travaux de thèse visent à étudier des solutions pour favoriser l’évaporation rapide tout en inhibant les effets thermiques indésirables dans le cadre d’une sonde atomique laser. Notre approche consiste à exploiter des impulsions ultracourtes dans le domaine du moyen infrarouge ou du THz en raison de leur grande énergie pondéromotrice associée à une faible énergie de photon. Ce manuscrit rapporte sur le développementd’un banc de génération et caractérisation d’impulsions THz intenses. Le couplage de ces rayonnements avec une nano-pointe métallique polarisée négativement a permis de caractériser le champ proche induit à la surface de la nano-pointe qui est fortement modifié par l’effet d’antenne de cette dernière. La deuxième partie rapporte sur le développement d’une source laser ultrarapide de haute cadence accordable dans le moyen infrarouge autour de 3 mm en exploitant des fibres en verre fluoréThe atome probe tomography is an instrument for analyzing matter in three dimensions with atomic resolution. This instrument relies on the effect of an electric field generated at the end of a sample cut into the shape of a nanoscale needle to evaporate the surface atoms which are collected by a two-dimensional detector. The measurement of the time of flight of the ions whose evaporation is triggered by an electrical or optical pulse makes it possible to measure the chemical composition in addition to the 3D localization of the atoms. In current atome probes, atomic evaporation is triggered by a high-speed laser emitting in the UV. However, the interaction of UV light with matter induces thermal heating which limits the mass resolution of the instrument and prevents its use for the analysis of fragile materials such as biocompatible components. This thesis work aims to study solutions to promote rapid evaporation while inhibiting unwanted thermal effects of the laser in atome probe. Our approach consists in exploiting ultrashort pulses in the mid-infrared or THz domain due to their high ponderomotive energy associated with low photon energy. This manuscript reports on the development of a bench for the generation and characterization of intense THz pulses. Coupling these radiations with a negatively polarized metallic nanotip has made it possible to characterize the near field induced at the surface of the nanotip, which is strongly modified by the antenna effect. The second part reports on the development of an ultra-fast laser source tunable in the mid-infrared around 3 mm using fluoride glass fibers
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Saarinen, Linn. "The Frequency of the Frequency : On Hydropower and Grid Frequency Control." Doctoral thesis, Uppsala universitet, Elektricitetslära, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-308441.
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Variations in the electricity consumption and production connected to the power system have to be balanced by active control. Hydropower is the most important balancing resource in the Nordic system, and will become even more important as the share of variable renewable energy sources increases. This thesis concerns balancing of active power, especially the real-time balancing called frequency control. The thesis starts in a description of the situation today, setting up models for the behaviour of hydropower units and the power system relevant to frequency control, and comparing the models with experiments on several hydropower units and on the response of the Nordic grid. It is found that backlash in the regulating mechanisms in hydropower units have a strong impact on the quality of the delivered frequency control. Then, an analysis of what can be done right now to improve frequency control and decrease its costs is made, discussing governor tuning, filters and strategies for allocation of frequency control reserves. The results show that grid frequency quality could be improved considerably by retuning of hydropower governors. However, clear technical requirements and incentives for good frequency control performance are needed. The last part of the thesis concerns the impact from increased electricity production from variable renewable energy sources. The induced balancing need in terms of energy storage volume and balancing power is quantified, and it is found that with large shares of wind power in the system, the energy storage need over the intra-week time horizon is drastically increased. Reduced system inertia due to higher shares of inverter connected production is identified as a problem for the frequency control of the system. A new, linear synthetic inertia concept is suggested to replace the lost inertia and damping. It is shown that continuously active, linear synthetic inertia can improve the frequency quality in normal operation and decrease wear and tear of hydropower units delivering frequency control.
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Walker, Scott A. Luna Lauro. "Frequency mapping for the operational frequency manager." Monterey, Calif. : Naval Postgraduate School, 2008. http://bosun.nps.edu/uhtbin/hyperion-image.exe/08Mar%5FWalker%5FScott.pdf.
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Thesis (M.S. in Information Technology Management)--Naval Postgraduate School, March 2008.Thesis Advisor(s): Bordetsky, Alex ; Bourakov, Eugene. "March 2008." Description based on title screen as viewed on May 1, 2008. Includes bibliographical references (p. 119). Also available in print.
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Luna, Lauro. "Frequency mapping for the operational frequency manager." Thesis, Monterey, California. Naval Postgraduate School, 2008. http://hdl.handle.net/10945/4231.
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The Department of Defense (DOD) has placed great emphasis on the networking and connectivity of forces over the last several years. Programs include the Global Information Grid (GIG), Force Net, and Net Centric Warfare to name a few. These programs emphasize and stress the warfighter's need to stay connected to their appropriate operational command and control structure during operations. The value of this connectivity is crucial to both the individual warfighter and the command structure as a force multiplier in modern warfare. One solution to this problem of connectivity is giving our operating forces the tools and knowledge of existing network infrastructure that details the information regarding the location, frequency and power out of existing nodes and spectrum analysis. The knowledge and ability of a trained Frequency Manager will allow our forces to use the full electromagnetic spectrum to maintain connectivity with their command structure. The goal of this research is to provide a systematic approach to detecting existing network and telecommunication frequencies and mapping their positions. This information can then used by a Frequency Manager for planning operational test exercises and for operational forces that may operate in an area that is frequency saturated. In these situations and with the knowledge of existing frequencies these forces will be better able to manage, configure, and exploit existing network communications. The experimental study will encompass the collection, data processing, modeling and mapping of existing networks and their electromagnetic effects in both a rural and urban environment using the TNT 802.16 OFDM test bed in the San Francisco Bay area and Camp Roberts.
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Di, Malta Gina. "The development and validation of the Relational Depth Frequency Scale." Thesis, University of Roehampton, 2016. https://pure.roehampton.ac.uk/portal/en/studentthesis/the-development-and-validation-of-the-relational-depth-frequency-scale(b248083e-6a9a-4f62-b92f-cc1b6987b18c).html.
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Background: Relational depth is defined as ‘a state of profound contact and engagement between two people, in which each person is fully real with the Other, and able to understand and value the other’s experiences at a high level’ (Mearns and Cooper, 2005, p. xii). The concept emerged in humanistic therapies and became an area of interest in research on the therapeutic relationship. Evidence suggests relational depth may be associated to psychological growth and therapy outcome. The Relational Depth Inventory (Wiggins, 2007) provided a first instrument to measure presence of relational depth in a significant event. To this day there is no validated instrument to measure the frequency of relational depth in therapy. Aims: The principal aim of this project is to develop and validate a scale that can reliably measure the frequency of relational depth in therapy. Other aims include explorations of the demographic moderators of the frequency of relational depth. Methods: Standard procedures included the creation of an item pool, expert rating of items, and Three-Step Test interviews. A psychometric exploration was used to assess internal consistency in a sample of 556 clients and therapists, convergent validity with the RDI and WAI-SR, divergent validity with a measure of self-compassion, a principal component analysis, and associations in demographic variables. Findings and further research: The 20-item Relational Depth Frequency Scale(RDFS) has excellent reliability in this sample and good initial construct validity. We uncovered two dimensions of relational depth: enduring relational depth and intense moments of relational depth. Therapists had higher relational depth frequency (RDF) than clients. Qualified practitioners had higher RDF than trainees. Individuals who self identified as spiritual had higher RDF than those who self-identified as atheists. Therapists had higher RDF the longer the therapy was. Clients showed lower RDF between the sixth and twenty-fourth session. The RDFS can be used for further research particularly in assessing the predictive validity of relational depth on outcome.
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Geladakis, Dimitrios N. "Comparison of the step frequency radar with the conventional constant frequency radars." Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1996. http://handle.dtic.mil/100.2/ADA328272.
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Thesis (M.S. in Electrical Engineering) Naval Postgraduate School, December 1996."December 1996." Thesis advisor(s): Gurnam S. Gill. Includes bibliographical references (p. 45). Also available online.
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Bhattacharya, Gautam. "Sparse denoising of audio by greedy time-frequency shrinkage." Thesis, McGill University, 2014. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=123263.
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Matching Pursuit (MP) is a greedy algorithm that iteratively builds a sparse signal representation. This work presents an analysis of MP in the context of audio denoising. By interpreting the algorithm as a simple shrinkage approach, we identify the factors critical to its success, and propose several approaches to improve its performance and robustness. We also develop several model enhancements and introduce an audio denoising approach called Greedy Time-Frequency Shrinkage (GTFS). Numerical experiments are performed on a wide range of audio signals, and we demonstrate that GTFS denoising is able to yield results that are competitive with state-of-the-art audio denoising approaches. Notably, GTFS retains a small percentage of a signal's transform coefficients for building a denoised representation, i.e., it produces very sparse denoised results.L'algorithme de Matching Pursuit (MP) construit par itérations une représentation parcimonieuse du signal, au prix d'un coût de calcul élevé. Ce mémoire présente une analyse de l'algorithme de MP dans le contexte du débruitage audio. En interprétant l'algorithme MP comme une méthode de contraction simple (simple shrinkage), nous chercherons à identifier les facteurs essentiels à son succès, puis proposerons plusieurs approches afin d'en améliorer les performances et la robustesse. Plusieurs améliorations du modèle seront ainsi développées, et une approche du débruitage audio dénommée Greedy Time-Frequency Shrinkage (GTFS) sera présentée en détails. Des expérimentations numériques appliquées à un large éventail de signaux sonores démontrent que les résultats obtenus par débruitage GTFS s'avèrent compétitifs face aux méthodes de débruitage audio qui constituent l'état de l'art. En particulier, le GTFS ne retient qu'un faible pourcentage des coefficients de la transformée du signal pour en construire une représentation débruitée, et produit ainsi des résultats débruités très compacts.
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Koller, Angela Erika. "The frequency assignment problem." Thesis, Brunel University, 2004. http://bura.brunel.ac.uk/handle/2438/4967.
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This thesis examines a wide collection of frequency assignment problems. One of the largest topics in this thesis is that of L(2,1)-labellings of outerplanar graphs. The main result in this topic is the fact that there exists a polynomial time algorithm to determine the minimum L(2,1)-span for an outerplanar graph. This result generalises the analogous result for trees, solves a stated open problem and complements the fact that the problem is NP-complete for planar graphs. We furthermore give best possible bounds on the minimum L(2,1)-span and the cyclic-L(2,1)-span in outerplanar graphs, when the maximum degree is at least eight. We also give polynomial time algorithms for solving the standard constraint matrix problem for several classes of graphs, such as chains of triangles, the wheel and a larger class of graphs containing the wheel. We furthermore introduce the concept of one-close-neighbour problems, which have some practical applications. We prove optimal results for bipartite graphs, odd cycles and complete multipartite graphs. Finally we evaluate different algorithms for the frequency assignment problem, using domination analysis. We compute bounds for the domination number of some heuristics for both the fixed spectrum version of the frequency assignment problem and the minimum span frequency assignment problem. Our results show that the standard greedy algorithm does not perform well, compared to some slightly more advanced algorithms, which is what we would expect. In this thesis we furthermore give some background and motivation for the topics being investigated, as well as mentioning several open problems.
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Jones, Graeme Stephen. "Instantaneous frequency, time-frequency distributions and the analysis of multicomponent signals." Thesis, Queensland University of Technology, 1992.
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Li, Shaomeng. "Do more mergers and acquisitions create value for the firm?" Thesis, Brunel University, 2015. http://bura.brunel.ac.uk/handle/2438/10536.
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This thesis is aimed to empirically investigate the performance impact of frequent acquisitions as an aggressive merger and acquisition (M&A) strategy for an acquiring firm. In literature related to the study of M&A, a common question is whether acquisitions improve the performance of acquirers. Neither theoretical nor empirical studies have a clear view on the performance effect of M&A. Some argue positively and some are opposite. Although existing research are mixed for their arguments, a takeover is commonly perceived as a shock to the firm with a constant effect on changing business performance. This static perception of M&A creates a difficulty in explaining why firms acquire others when the performance effect is negative. To address the issue, this thesis examines the M&A effect dynamically with taking into account the role of merger frequency in affecting performance. On the basis of a large sample that consists of about 14,000 acquisitions from more than 100 countries over last 12 years, the thesis finds that the investors perceive a lower value if the acquiring firm is involved in frequent mergers. This is because more mergers are expected to attract considerable amount of management attention away from profitable activities in order to digest the challenges of new business integration at least in the short run. This “digesting constraint” argument is evident by our estimations. Firm becomes less profitable in the short run after a merger shock, and this adverse effect can be more severe if the firm is involved in more frequent mergers. Evidence of the thesis further show that, the effect of merger shocks is not static and persistent, and it changes with time. The shock affects adversely profitability in the short run, usually lasting a couple of years, and then the negative effect on performance could be turned either oppositely if the firm digests the shock successfully, or otherwise, continuously but diminishing over time if the digestion takes longer such as for frequent acquisition. This finding implies that the pace of firm resilience to a merger shock can be affected by its merger strategies. The pace can be slow if the firm pursues frequent mergers aggressively. The performance effect of a merger shock is dynamic and changes with time. The dynamic view for merger shocks from this study opens a new vision for literature in merger studies. Overall the market expectation to a merger effect on changing firm performance is quite consistently related to what has actually happened to the firm after the merger shock.
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Gordon, Jocelynne E. "Nighttime fears in children : origins, frequency, content and severity." Monash University, Faculty of Education, 2004. http://arrow.monash.edu.au/hdl/1959.1/5321.
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Xu, Yiwen. "Frequency analysis of low-frequency field fluctuations detected by the Cluster satellites." Thesis, KTH, Rymd- och plasmafysik, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-91547.
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Ledingham, Jamie Andrew. "The estimation of flood frequency curves by mapping from rainfall frequency curves." Thesis, University of Newcastle Upon Tyne, 2011. http://hdl.handle.net/10443/1320.
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Recent large flooding events have reinforced the need for prudent flood risk management. The July 2007 floods in Yorkshire and the Midlands and the November 2009 floods in the Lake District have highlighted the current vulnerability of key infrastructure and the built environment in the UK to flooding. This existing flood risk is coupled with concerns over the potential impacts of future climate change on flood regimes. Therefore, there is a need to develop tools and methodologies to assess the potential impact of likely climate change on flood risk. The link between large rainfall and flow events is first examined, as well as an assessment of the seasonality of these events. This reveals a distinct east-west split in the seasonal concentration of flooding. This work provides a basis for the development of a statistical modelling technique which estimates a catchment flood record on an event basis. The model uses estimates of the flood generating storm and the antecedent conditions to estimate a flow magnitude. The modelled flood record is then transformed into a flood frequency curve using an appropriate statistical method. Extensive testing of the model has assessed its robustness to the length of flood record used in fitting and its sensitivity to the input climate data. Several case studies using the UKCP weather generator show how the method works as well as providing an indication of how future climate changes may affect the flood frequency curve. The frequency curve mapping method developed here performs best on catchments whose flood regime is driven by rainfall. The use of a simple antecedent rainfall accounting method has been shown to perform as well as a quasi-physical soil moisture estimation method. The research undertaken offers several possibilities to develop understanding of flood frequency curves in catchments with short gauged records. This new methodology has the potential for further development and can be used to explore a wide range of future scenarios.
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Williams, N. G. "Frequency domain parameter identification and the statistical properties of frequency response estimates." Thesis, Loughborough University, 1993. https://dspace.lboro.ac.uk/2134/25141.
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Frequency domain techniques in systems theory have their origins in Heavyside's operational calculus (Heavyside, 1889). Such work was later developed by Foster and Campbell (1931), Brune (1931), Nyquist (1932), Black (1934), Darlington (1939) and subsequently Bode (1948). This interest in the frequency domain was due to its appeal to the intuition of the engineer. The dominance of frequency domain techniques was subsequently eroded from the late 1950s through the 1960s by the influence of the space programmes. The space systems being analysed were based on strong theoretical foundations with well-defined sets of differential equations. The analysis led to the development of the state-space methods which were able to cope with the multivariable problems and were amenable to numerical solution. As a result of these developments, control engineering was largely dominated by the state-space approach and the associated areas of LQG optimal control, Kaiman-Bucy filters, observability and controllability. Two factors led to a resurgence of interest amongst academics in the development of frequency domain techniques in the 1970s and 1980s. The first was the development of the Fast Fourier Transform (FFT) (Cooley & Tookey, 1965). This provided an efficient method of analysing the Fourier transforms of signals and allowed the development of spectral methods of obtaining frequency response estimates. The collection of data was greatly speeded up and this enabled frequency domain methods to be increasingly applied to on-line control problems. The second factor was that the developments in the time domain were never fully embraced by practicing engineers in traditional control environments.
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Labib, Amr. "Crack detection in frames using natural frequency degradations." Thesis, Cardiff University, 2016. http://orca.cf.ac.uk/88543/.
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Crack detection at an early stage can prevent catastrophic structural failures. In this thesis, the inverse problem of crack detection in frames is studied. The direct problem of calculating the natural frequencies of beams and frames with multiple cracks is first tackled. A new method for natural frequency calculation is devised. The cracks are modelled as rotational springs. 4 × 4 dynamic stiffness matrices for beams are evaluated in a recursive manner, according to the number of cracks, by applying partial Gaussian eliminations. The resulting transcendental eigenvalue problem is solved using the Wittrick–Williams algorithm to extract the natural frequencies. Additional sign counts resulting from the partial Gaussian eliminations must be accounted for when applying the algorithm. The dynamic stiffness matrix of a frame with multiply cracked members is then assembled. The natural frequency calculation method forms a basis for detecting a single crack in a frame using only natural frequency measurements. Each frame member is discretised into a number of points. Selected natural frequencies are calculated accurately in the uncracked case and when the crack is placed individually at each discretisation point. The variation between the uncracked and cracked frequencies is normalised giving a number of curves corresponding to the selected frequencies. The normalisation is then applied on the measured frequencies. For noise free measurements, point crack locations are obtained. Applying the principles of interval arithmetic, noisy measurements give crack location ranges. Empirical probability distributions are used to graphically represent these ranges and their relative probabilities. Crack severity ranges are then obtained. The detection method is validated experimentally on a frame with scaled down dimensions. The fast Fourier transform is used to convert the time domain vibration signal into the frequency domain. Using higher order natural frequencies, two enhancement procedures for the detection method are devised and applied theoretically.
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Omara, Hisham. "A methodology for determining the load frequency sensitivity." Thesis, University of Manchester, 2012. https://www.research.manchester.ac.uk/portal/en/theses/a-methodology-for-determining-the-load-frequency-sensitivity(fcad5d22-66b2-4964-a32a-461fec5ffcb2).html.
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The last three decades have experienced an impressive growth in loads interfacing with the grid through power electronic devices. These include personal computers and most office equipment as well as industrial induction motors driven by variable frequency drives. The increasing importance of these nonconventional loads has an impact which is yet to be quantified and modelled systematically. This is particularly the case with the natural load-side frequency response. This lack of knowledge compounds with the current uncertainty in the characterization of the load’s natural behaviour under frequency disturbances. The load in power systems is known to have an inherent frequency sensitivity. Because of the complexity and the constant evolution of the aggregate system load, the frequency sensitivity is difficult to estimate. In fact, in practice, utilities use some ill-defined, static and generic values for this sensitivity as part of their dynamic performance assessments and load-frequency control designs. To address this shortcoming, this thesis proposes a methodology based on a bottom-up approach based on aggregate load composition information and load behaviour modelling. The method allows for the development of a deeper understanding of the currently unknown impact of the load mix and other external factors on the natural load-side frequency sensitivity. The thesis then goes on to demonstrate that the load-frequency sensitivity is indeed a time-varying value, changing throughout the course of the day and the year, unlike the industry-wide static value assumption. The methodology allows for the identification of the explanatory factors behind the sensitivity variations. The evidence shows that those explanatory factors are the outside ambient temperature, the time of day and the date. The methodology is also able to model frequency sensitivity within a 10% error margin (using only publicly-available data). A comprehensive sensitivity analysis is undertaken to prove the robustness of the Load Frequency Sensitivity Bottom-Up Methodology (LFS-BUM). Probabilistic characterizations that represent the uncertainty embedded in the load-frequency sensitivity (LFS) at various times reveal that generally the prediction error probability distributions are skewed and sometimes even bimodal. For this reason, Edgeworth series expansions are used to provide analytical formulae for the approximate the probability distribution of the LFS values. The statistical relationship between LFS and the outside ambient temperature is used to generate prediction models for LFS. Error analyses of the models demonstrate an ability to predict LFS within a 7.66% error margin. The methodologies and the results should be of interest most notably to transmission system operators as they attempt to quantify the resources necessary to conduct primary frequency control tasks reliably and at the best possible cost. Furthermore, the bottom-up methodology is used to provide a detailed understanding of the evolution of LFS and thus is of potential use in informing grid operators and planners on the potential impacts of an evolving load mix which is increasingly insensitive/“stiffer” to frequency deviations, especially for frequency control. In addition, it will assist as a primary tool in redesigning frequency regulation function to accommodate for load participation.
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Chen, Shuming. "Filtering in the frequency domain." Thesis, University of Sheffield, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.434612.
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Cassady, Charles Richard. "The frequency distribution of availability." Thesis, This resource online, 1993. http://scholar.lib.vt.edu/theses/available/etd-09052009-040624/.
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Xu, Dongming. "On the testing of the RFID security /." View abstract or full-text, 2009. http://library.ust.hk/cgi/db/thesis.pl?CSED%202009%20XU.
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Paje, Vladimir I. "THE EFFECTS OF CARRIER FREQUENCY AND ANTENNA DIAMETER ON FREQUENCY SELECTIVE MULTIPATH FADING." International Foundation for Telemetering, 2000. http://hdl.handle.net/10150/606502.
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International Telemetering Conference Proceedings / October 23-26, 2000 / Town & Country Hotel and Conference Center, San Diego, CaliforniaThis paper uses ARTM channel sounding data collected at Edwards Air Force Base to investigate the dependency between the characteristics of frequency selective multipath fading and carrier frequency and ground based received antenna diameter. Channel model parameters are obtained from data sets measured at different frequencies and various ground based antenna diameters. The analysis shows that the multipath characteristics do not statistically change between the L-band and S-band carrier frequencies, but do change with antenna diameter. Furthermore, Flight 18 data shows that antenna diversity may be a useful technique to reduce data loss from multipath fading.