KaSI: a Ka-band and S-band Cross-track Interferometer

A dual-frequency system is needed to better understand natural processes that constitute the environment and seasonal cycles of the Earth. A system working at two different wavelengths acquiring data simultaneously will give a valuable dataset since the conditions on the ground will be exactly the same. Hence, elements such as wind, soil moisture or any other changes on the ground will not interfere in the mea- surements. This thesis explains how an S-band radar was built and tested. Moreover, the experiments done with a Ka-band radar used as a scatterometer are explained as well as the data processing and analysis. Finally, the two systems are used to get dual-frequency measurements from an airborne platform. The dual-frequency data is explored, showing the differences in normalized radar cross-section between frequencies and discussing the interferometric measurements

Internally Sensed Optical Phased Arrays

The performance of existing ground-based space debris laser ranging systems can be improved by directing more light onto space debris by coherently combining multiple lasers using an optical phased array (OPA). If the power delivered to target is sufficiently high then these systems may also provide the capability to remotely manoeuvre space debris via photon radiation pressure and/or ablation. By stabilising the relative output phase of multiple lasers, OPAs form a coherent optical wave-front in the far field. Since the phase of each laser can be controlled independently, they also have the ability to dynamically manipulate the distribution of optical power in the far field, potentially enabling them to compensate for atmospheric turbulence. This beam-forming functionality, combined with their inherent scalability and high power handling capabilities make OPAs a promising technology for future space debris laser ranging and manoeuvring systems. In this thesis, we describe the iterative development of a high-power compatible internally sensed OPA, which---in contrast to externally sensed OPAs that sense the output phase of each laser externally using free-space optics---relies on the small fraction of light that is reflected back into the fibre at the output of the OPA to stabilise its relative output phase. This allows internally sensed OPAs to be implemented entirely within fibre without any dependence on free-space optics at the output, offering potential advantages over externally sensed techniques when operating in the presence of shock and vibration. A proof-of-concept experiment demonstrated the viability of internal sensing, but also highlighted a number of weaknesses that would affect its utility, specifically in supporting high optical powers greater than 100s of mW. An improved high-power compatible internally sensed OPA was designed to overcome these restrictions by isolating sensitive optical components from high optical powers using asymmetric fibre couplers. This concept was initially demonstrated experimentally using slave lasers offset phase-locked to a single master laser, and then again using fibre amplifiers in a master oscillator power amplifier configuration. The experimental demonstration of the fibre amplifier compatible OPA stabilised the relative output phase of three commercial 15 W fibre amplifiers, demonstrating a root-mean-squared output phase stability of $\lambda/194$, and the ability to steer the beam at up to 10 kHz. The internally sensed OPA presented here requires the simultaneous measurement, and control of the phase of each emitter in the OPA. This is accomplished using digitally enhanced heterodyne interferometry and digitally implemented phasemeters, both of which rely heavily on high-speed digital signal processing resources provided by field-programmable gate-arrays

Scaling Optical Phased Arrays

Optical phased array (OPA) technology can improve the useful capabilities of lasers by controlling the relative optical phase of an array of emitting apertures. Combining lasers in this way can produce a beam with increased optical intensity, rapid beam pointing and the potential to perform adaptive optics to correct for atmospheric turbulence. Future applications of optical phased arrays, particularly ground-to-space laser transmission, require both the ability to combine individual high power optical sources and emitter counts greater than in existing implementations. Breakthrough Starshot proposes the ambitious goal of accelerating a light sail to 20% of the speed of light to reach the nearest solar system within a human lifespan, which may require upwards of 100 GW total power from 100 million emitters in a ground-to-space array. At a reduced scale, improved tracking and eventual manoeuvring of orbital space debris could be also improved through the use of a ground-to-space OPA. The research in this thesis presents improvements and techniques for internally-sensed optical phased array designs to allow the scaling to greater numbers of combined optical sources. While primarily motivated by enabling large scale ground-to-space optical phased arrays, consideration is given to more conventional systems and benefits to non-optical phased array applications. A thorough investigation of the limits of digitally enhanced heterodyne interferometry (DEHI), a technique which allows the simultaneous measure of multiple optical phase signals, forms the first major component of this work. In addition to optical phased arrays, this technique has potential applications in acoustic sensing, wavefront sensing, satellite interferometry and fibre frequency references. A combination of analytical, simulation and experimental work was performed to better predict crosstalk between optical phase measurements and establish a set of robust parameters to improve phase measurement performance. Recommendations for requirement dependent parameter choices when using this technique are presented. The optical phased array underpinning this work is internally sensed, implying the ability to measure differential emitter paths without the need for external beam sampling. However, a lingering challenge in previous implementations arose from the double-pass of internal optical pathlengths, resulting in a pi-phase ambiguity in the sensing. The second key challenge addressed in this thesis is a proof-of-concept experimental solution to resolve this ambiguity, demonstrated with a novel waveguide optical head created using three-dimensional laser inscription. In culmination, this research presents a conceptual design for a ground-to-space optical phased array to act as the "photon engine" component of the Breakthrough Starshot program. This design involves a system to interferometrically link multiple sub-arrays in a scalable hierarchy. The active control of differential pathlengths in the hierarchy are enabled using a combination of wavelength division multiplexing and DEHI. Internal array measurements in this design are partnered with measurements of a satellite mounted laser beacon for atmospheric phase sensing and to account for unevenness of the array surface across a kilometre scale. The satellite laser beacon is designed to operate at a different wavelength from the arrays outgoing beam, allowing weak beacon light to be distinguished from high powered emitter scattering. An associated technique established and modelled in this work is how phase measurements can be accurately mapped between wavelengths. Multiple variations of the complete array are modelled to assess fundamental performance limits of the sensing system with realistic system parameters for the combination of 100 million emitters

Optical sampling and metrology using a soliton-effect compression pulse source

A low jitter optical pulse source for applications including optical sampling and optical metrology was modelled and then experimentally implemented using photonic components. Dispersion and non-linear fibre effects were utilised to compress a periodic optical waveform to generate pulses of the order of 10 picoseconds duration, via soliton-effect compression. Attractive features of this pulse source include electronically tuneable repetition rates greater than 1.5 GHz, ultra-short pulse duration (10-15 ps), and low timing jitter as measured by both harmonic analysis and single-sideband (SSB) phase noise measurements. The experimental implementation of the modelled compression scheme is discussed, including the successful removal of stimulated Brillouin scattering (SBS) through linewidth broadening by injection dithering or phase modulation. Timing jitter analysis identifies many unwanted artefacts generated by the SBS suppression methods, hence an experimental arrangement is devised (and was subsequently patented) which ensures that there are no phase modulation spikes present on the SSB phase noise spectrum over the offset range of interest for optical sampling applications, 10Hz-Nyquist. It is believed that this is the first detailed timing jitter study of a soliton-effect compression scheme. The soliton-effect compression pulses are then used to perform what is believed to be the first demonstration of optical sampling using this type of pulse source. The pulse source was also optimised for use in a novel optical metrology (range finding) system, which is being developed and patented under European Space Agency funding as an enabling technology for formation flying satellite missions. This new approach to optical metrology, known as Scanning Interferometric Pulse Overlap Detection (SIPOD), is based on scanning the optical pulse repetition rate to find the specific frequencies which allow the return pulses from the outlying satellite, i.e. the measurement arm, to overlap exactly with a reference pulse set on the hub satellite. By superimposing a low frequency phase modulation onto the optical pulse train, it is possible to detect the pulse overlap condition using conventional heterodyne detection. By rapidly scanning the pulse repetition rate to find two frequencies which provide the overlapping pulse condition, high precision optical pulses can be used to provide high resolution unambiguous range information, using only relatively simple electronic detection circuitry. SIPOD’s maximum longitudinal range measurement is limited only by the coherence length of the laser, which can be many tens of kilometres. Range measurements have been made to better than 10 microns resolution over extended duration trial periods, at measurement update rates of up to 470 Hz. This system is currently scheduled to fly on ESA’s PROBA-3 mission in 2012 to measure the intersatellite spacing for a two satellite coronagraph instrument. In summary, this thesis is believed to present three novel areas of research: the first detailed jitter characterisation of a soliton-effect compression source, the first optical sampling using such a compression source, and a novel optical metrology range finding system, known as SIPOD, which utilises the tuneable repetition rate and highly stable nature of the compression source pulses

NASA Tech Briefs, April 2000

Topics covered include: Imaging/Video/Display Technology; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Programs; Mechanics; Bio-Medical; Test and Measurement; Mathematics and Information Sciences; Books and Reports

Geosynchronous synthetic aperture radar for Earth continuous observation missions

This thesis belongs to the field of remote sensing, particularly Synthetic Aperture Radar (SAR) systems from the space. These systems acquire the signals along the orbital track of one or more satellites where the transmitter and receiver are mounted, and coherently process the echoes in order to form the synthetic aperture. So, high resolution images can be obtained without using large arrays of antennas. The study presented in this thesis is centred in a novel concept in SAR, which is known as Geosynchronous SAR or GEOSAR, where the transmitter and/or receiver are placed in a platform in a geostationary orbit. In this case, the small relative motions between the satellite and the Earth surface are taken to get the necessary motion to form the synthetic aperture and focus the image. The main advantage of these systems with respect to the current technology (where LEO satellites with lower height are considered) is the possibility of permanently acquire images from the same region thanks to the small motion of the platform. Therefore, the different possibilities in the orbital design that offer this novel technology as well as the geometric resolutions obtained in the final image have been firstly studied. However, the use of geosynchronous satellites as illuminators results in slant ranges between 35.000-38.000 Km, which are much higher than the typical values obtained in LEOSAR, under 1.000 Km. Fortunately, the slow motion of the satellite makes possible large integration of pulses during minutes or even hours, reaching Signal-to-Noise Ratio (SNR) levels in the order of LEO acquisitions without using high transmitted power or large antennas. Moreover, such large integration times, increases the length of the synthetic aperture to get the desired geometric resolutions of the image (in the order of a few meters or kilometres depending on the application). On the other hand, the use of long integration time presents some drawbacks such as the scene targets decorrelation, atmospheric artefacts due to the refraction index variations in the tropospheric layer, transmitter and receiver clock jitter, clutter decorrelation or orbital positioning errors; which will affect the correct focusing of the image. For this reason, a detailed theoretical study is presented in the thesis in order to characterize and model these artefacts. Several simulations have been performed in order to see their effects on the final images. Some techniques and algorithms to track and remove these errors from the focused image are presented and the improvement of the final focused image is analysed. Additionally, the real data from a GB-SAR (Ground-Based SAR) have been reused to simulate a long integration time acquisition and see the effects in the image focusing as well as to check the performance of compensation algorithms in the final image. Finally, a ground receiver to reuse signals of opportunity from a broadcasting satellite have been designed and manufactured. This hardware is expected to be an important tool for experimental testing in future GEOSAR analysis.Aquesta tesi s'emmarca dins de l'àmbit de la teledetecció, en particular, en els sistemes coneguts com a radar d'obertura sintètica (SAR en anglès) des de l'espai. Aquests sistemes adquireixen senyal al llarg de l'òrbita d'un o més satèl·lits on estan situats el transmissor i el receptor, i processa els ecos de forma coherent per a formar l'obertura sintètica. D'aquesta manera es poden aconseguir imatge d'alta resolució sense la necessitat d'emprar un array d'antenes molt gran. El treball realitzat en aquest estudi es centra en un nou concepte dins del món SAR que consisteix en l'ús de satèl·lits en òrbita geostacionària per a l'adquisició d'imatges, sistemes coneguts com a Geosynchronous SAR o GEOSAR. En aquest cas, els petits moviments relatius dels satèl·lits respecte de la superfície terrestre s'empren per a aconseguir el desplaçament necessari per a formar l'obertura sintètica i així obtenir la imatge. El principal avantatge d'aquests sistemes respecte a la tecnologia actual (on s'utilitzen satèl·lits en orbites més baixes LEO) és la possibilitat d'adquirir imatges d'una mateixa zona de forma permanent gràcies als petits desplaçaments del satèl·lit. Així doncs, en aquesta tesi s'estudien les diferents possibilitats en el disseny orbital que ofereixen aquests sistemes així com les resolucions d'imatge que s'obtindrien. Tot i així, l'ús de satèl·lits en òrbita geoestacionària, resulta en una distància entre el transmissor/receptor i l'escena entre 35000-38000 Km, molt més gran que les distàncies típiques en els sistemes LEO per sota dels 1000 Km. Tot i així, el moviment lent de les plataformes geostacionàries fa possible la integració de polsos durant minuts o hores, arribant a nivells acceptables de relació senyal a soroll (SNR) sense necessitat d'utilitzar potències transmeses i antenes massa grans. A més a més, aquesta llarga integració també permet assolir unes longituds d'obertura sintètica adients per a arribar a resolucions d'imatge desitjades (de l'ordre de pocs metres o kilòmetres segons l'aplicació). Malgrat això, l'ús de temps d'integració llargs té una sèrie d'inconvenients com poden ser la decorrelació dels blancs de l'escena, l'aparició d'artefactes atmosfèrics deguts als canvis d'índex de refracció en la troposfera, derives dels rellotges del transmissor i receptor, decorrelació del clutter o errors en el posicionament orbital, que poden afectar la correcta focalització de la imatge. Així doncs, en la tesi s'ha fet un detallat estudi teòric d'aquests problemes per tal de modelitzar-los i posteriorment s'han realitzat diverses simulacions per veure els seus efectes en una imatge. Diverses tècniques per a compensar aquests errors i millorar la qualitat de la imatge també s'han estudiat al llarg de la tesi. Per altra banda, dades reals d'un GB-SAR (SAR en una base terrestre) s'han reutilitzat per adaptar-les a una possible adquisició de llarga durada i veure així de forma experimental com afecta la llarga integració en les imatges i com millora l'enfocament després d'aplicar els algoritmes de compensació. Per últim, en la tesi es presenta un sistema receptor terrestre per a poder realitzar un anàlisi experimental del cas GEOSAR utilitzant un il·luminador d'oportunitat. Els primers passos en el disseny i la fabricació del hardware també es presenten en aquesta tes

Ironless Inductive Position Sensor for Harsh Magnetic Environments

Linear Variable Differential Transformers (LVDTs) are widely used for high-precision and high-accuracy linear position sensing in harsh environments, such as the LHC collimators at CERN. These sensors guarantee theoretically infinite resolution and long lifetimes thanks to contactless sensing. Furthermore, they offer very good robustness and ruggedness, as well as micrometer uncertainty over a range of centimeters when proper conditioning techniques are used (such as the three-parameter Sine-Fit algorithm). They can also be suitable for radioactive environments. Nevertheless, an external DC/slowly-varying magnetic field can seriously affect the LVDT reading, leading to position drifts of hundreds of micrometers, often unacceptable in high-accuracy applications. The effect is due to the presence of non-linear ferromagnetic materials in the sensor’s structure. A detailed Finite Element model of an LVDT is first proposed in order to study and characterize the phenomenon. The model itself becomes a powerful design tool for possible countermeasures to the interference effect. In particular, a combination of magnetic shielding and DC polarization is proposed to reduce the drift due to the external field. Nevertheless, such solutions cannot lead to complete immunity, given the unavoidable presence of magnetic materials in the sensor. Taking the CERN application as a starting point, this thesis aims at conceiving, modelling and characterizing a valid alternative to LVDTs for harsh magnetic environments, which would guarantee magnetic-field-immune position sensing while keeping all the advantageous properties of LVDTs. The Ironless Inductive Position Sensor (I2PS) is an air-cored structure made of 5 coaxial coils. The position sensing is achieved by spatially-variable magnetic fluxes, which give rise to position-dependent coil voltages, just as for LVDTs. The complete electromagnetic model of the sensor is proposed, showing the working principle and demonstrating the magnetic-field immunity from a theoretical viewpoint. In addition, a high-frequency electromagnetic analysis is performed, in order to model the skin and proximity effects in the conductors and foresee their impact on the sensor’s functioning. The models are validated with FEM simulations and experimental measurements. The thermal behaviour of the sensor is also investigated and an effective compensation algorithm is proposed to cancel the temperature-dependence of the position reading. In addition, a smart real-time reading algorithm is proposed in order to significantly reduce the estimation error of standard three-parameter Sine-Fit algorithms when an additional sinusoidal signal is present on the main waveform. Finally, a generic optimization procedure is proposed in order to maximize the performances of the sensor in terms of sensitivity. Taking this procedure as a guideline, an actual I2PS optimized prototype is designed and manufactured, having the specifications of the LHC collimators application as a reference. The optimized prototype shows immunity to external ramped and sinusoidal fields, as expected. In addition, it is used for the experimental validation of the models and the reading techniques, which demonstrate their effectiveness

X-Band LLRF Developments for High Power CLIC Test Stands and Waveguide Interferometry for Phase Stabilisation

This thesis describes the upgrade of the first high power X-band RF test for high gradient accelerating structures at CERN, as required for the e+ e- collider research program; Compact Linear Collider, CLIC. Significant improvements to the control system and operation of the first test stand, Xbox-1, are implemented. The design and commissioning of the new Low Level Radio Frequency, LLRF, system is described in detail. The upgrade also encompasses software, interlock systems, timing, safety and control. The new LLRF requires an up-convertor to convert an input signal at 187.4 MHz to 11.806 GHz. The most common method is a phase locked loop, PLL, an alternative method was envisioned which uses single side-band up-convertor. This necessitated the design and manufacture of a custom cavity filter. The up-convertor and PLL are compared and both are implemented in the new LLRF. The new LLRF system is implemented at Xbox1 and used to RF condition a 50 MW CPI klystron, the final output power was 45 MW for a 50 ns RF pulse length. The phase and amplitude of the LLRF, TWT and klystron are characterised with both the PLL and up-convertor. The klystron phase stability was studied using a sensitivity analysis. The waveguide network between the klystron and the accelerating structures is approximately 30 m. This network is subject to environmental phase changes which affect the phase stability of the RF arriving at the structures. A single path inteferometer was designed which will allow a phase measurement pulse at a secondary frequency to be injected into the waveguide network interleaved with klystron pulses. The interferometer is commissioned in the lab and low power measurements validate its operation. The system is then integrated into the high power network at Xbox1 and used to measure phase shifts in the waveguide network which are correlated with temperature

EuPRAXIA@SPARC_LAB Conceptual Design Report

It is widely accepted by the international scientific community that a fundamental milestone towards the realization of a plasma driven future Linear Collider (LC) will be the integration of high gradient accelerating plasma modules in a short wavelength Free Electron Laser (FEL) user facility. To this end, in October 2019 the Horizon2020 Design Study EuPRAXIA (European Plasma Research Accelerator with eXcellence In Applications) will propose the first European Research Infrastructure that is dedicated to demonstrate usability of plasma accelerators delivering high brightness beams up to 1-5 GeV for users. In this report we discuss the EuPRAXIA@ SPARC_LAB project, intended to put forward LNF as host of the EuPRAXIA European Facility. The EuPRAXIA@SPARC_- LAB facility will equip LNF with a unique combination of a high brightness GeV-range electron beam generated in a state-of-the-art X-band RF linac, a 0.5 PW-class laser system and the first 5th generation light source driven by a plasma accelerator. These unique features will enable at LNF new promising synergies between fundamental physics oriented research and high social impact applications, especially in the domain of Key Enabling Technologies (KET) and Smart Specialisation Strategies (S3)

MEMS Technology for Biomedical Imaging Applications

Biomedical imaging is the key technique and process to create informative images of the human body or other organic structures for clinical purposes or medical science. Micro-electro-mechanical systems (MEMS) technology has demonstrated enormous potential in biomedical imaging applications due to its outstanding advantages of, for instance, miniaturization, high speed, higher resolution, and convenience of batch fabrication. There are many advancements and breakthroughs developing in the academic community, and there are a few challenges raised accordingly upon the designs, structures, fabrication, integration, and applications of MEMS for all kinds of biomedical imaging. This Special Issue aims to collate and showcase research papers, short commutations, perspectives, and insightful review articles from esteemed colleagues that demonstrate: (1) original works on the topic of MEMS components or devices based on various kinds of mechanisms for biomedical imaging; and (2) new developments and potentials of applying MEMS technology of any kind in biomedical imaging. The objective of this special session is to provide insightful information regarding the technological advancements for the researchers in the community