Near infra-red single-photon detection using Ge-on-Si heterostructures

This Thesis investigates the design of Ge-on-Si single-photon avalanche diode (SPAD) detectors combining the many advantages of low-noise Si single-photon avalanche multiplication with the infrared sensing capability of germanium. The devices were simulated by using electric field modelling software to predict key aspects of the device behaviour in terms of the current-voltage characteristic and electric field. The devices were then characterised in terms of their single-photon performance. A 25 m diameter device showed a single-photon detection efficiency of ~ 4 % at a wavelength of 1310 nm and a temperature of 100 K when biased at 10 % above the breakdown voltage. In the same condition, a dark count rate of ~ 6 Mcs-1 was measured. This resulted in the lowest noise equivalent power of ~ 1 × 10-14 WHz-1/2 of Ge-on-Si SPADs reported in the scientific literature. At the longer wavelength of 1550 nm, the single-photon detection efficiency was reduced to ~ 0.1 % at 125 K and 6 % of relative excess bias. Although further investigation needs to be carried out, a potential major advantage of these devices compared to the InGaAs/InP SPADs could be that of reduced afterpulsing since a small increase (a factor of 2) in the normalised dark count rate was measured when the repetition rate was increased from 1 kHz to 1 MHz. Finally, the fill-factor enhancement of 32 × 32 Si CMOS SPAD arrays resulting from the integration of high efficiency diffractive optical microlens arrays was investigated. A full characterisation of SPAD arrays integrating microlens arrays in terms of improvement factor and spatial uniformity of detection is presented for the first time in the scientific literature in a large spectral range (500-900 nm) and different f-numbers (from f/2 to f/22) by using a double telecentric imaging system. The highest improvement factor of ~16 was measured for a SPAD array integrating microlens arrays, combined with a very high spatial efficiency uniformity of between 2–6%

Integrated optics technology study

The status and near term potential of materials and processes available for the fabrication of single mode integrated electro-optical components are discussed. Issues discussed are host material and orientation, waveguide formation, optical loss mechanisms, wavelength selection, polarization effects and control, laser to integrated optics coupling fiber optic waveguides to integrated optics coupling, sources, and detectors. Recommendations of the best materials, technology, and processes for fabrication of integrated optical components for communications and fiber gyro applications are given

Integrated optics technology study

The materials and processes available for the fabrication of single mode integrated electrooptical components are described. Issues included in the study are: (1) host material and orientation, (2) waveguide formation, (3) optical loss mechanisms, (4) wavelength selection, (5) polarization effects and control, (6) laser to integrated optics coupling,(7) fiber optic waveguides to integrated optics coupling, (8) souces, (9) detectors. The best materials, technology and processes for fabrication of integrated optical components for communications and fiber gyro applications are recommended

Multimode optical waveguides and lightguides for backplane interconnection and laser illuminated display systems

The aim of the research in this thesis was to design, model, analyse and experimentally test multimode optical waveguides and lightguides for manipulating infrared light for optical backplane interconnections and visible light for laser illuminated display systems. Optical Input/Output Coupling loss at the entry and exit of polymer waveguides depends on optical scattering due to end facet roughness. The input/output coupling loss was measured for different end facet roughness magnitudes and the waveguide surface profiles due to different cutting methods (dicing saw and three milling routers) were compared. The effect of the number of cutting edges on the router, the rotation rate and translation (cutting) speed of the milling routers on the waveguide end facet roughness was established. A further new method for reducing the end facet roughness and so the coupling loss, by curing a layer of core material at the end of the waveguide to cover the roughness fluctuations, was proposed and successfully demonstrated giving the best results reported to date resulting in an improvement of 2.8 dB, even better than those obtained by use of index matching fluid which is impractical in commercial systems. The insertion loss due to waveguide crossing having various crossing angles was calculated using a beam propagation method and ray tracing simulations and compared to experimental measurements. Differences between the results were resolved leading to an understanding that only low order waveguide modes at no more than 6 degrees to the axis were propagating inside the waveguide. Several different optical designs of multimode waveguide for the light engine of a 3D autostereoscopic laser illuminated display system were proposed. Each design performed the functions of laser beam combining, beam shaping and beam homogenizing and the best method was selected, designed, modelled, tested, and implemented in the system. The waveguide material was inspected using spectroscopy to establish the effect of high power optical density on the material performance showing an increased loss particularly in the shorter wavelengths. The effect of waveguide dimensions on the speckle pattern was investigated experimentally and the speckle contrast was reduced to below the threshold of human perception. Speckle contrast was also recorded for the first time along the axis of the 3D display system and normal to it in the viewing area and the speckle characteristics at each stage were investigated. New algorithms for analysing speckle were used and the perceptual ability of human eyes to detect speckle size and contrast were taken into account to minimise perceived speckle patterns. The effect of the core diameter of optical fibres on the speckle pattern was investigated and it was shown that the speckle spot diameter is reduced by increasing the fibre core diameter. Based on this experiment, it was suggested that speckle reduction is more effective if the optical fibre used in the display system has larger diameter. Therefore, a slab waveguide of 1 mm thickness and 20 m width was used for laser beam combining, homogenising and beam shaping and a uniformity of 84% was achieved with just 75 mm length. The speckle was also completely removed at the output of the waveguid

An overview of lidar imaging systems for autonomous vehicles

Lidar imaging systems are one of the hottest topics in the optronics industry. The need to sense the surroundings of every autonomous vehicle has pushed forward a race dedicated to deciding the final solution to be implemented. However, the diversity of state-of-the-art approaches to the solution brings a large uncertainty on the decision of the dominant final solution. Furthermore, the performance data of each approach often arise from different manufacturers and developers, which usually have some interest in the dispute. Within this paper, we intend to overcome the situation by providing an introductory, neutral overview of the technology linked to lidar imaging systems for autonomous vehicles, and its current state of development. We start with the main single-point measurement principles utilized, which then are combined with different imaging strategies, also described in the paper. An overview of the features of the light sources and photodetectors specific to lidar imaging systems most frequently used in practice is also presented. Finally, a brief section on pending issues for lidar development in autonomous vehicles has been included, in order to present some of the problems which still need to be solved before implementation may be considered as final. The reader is provided with a detailed bibliography containing both relevant books and state-of-the-art papers for further progress in the subject.Peer ReviewedPostprint (published version

Analysis and design of antennas and radiometers for radio astronomy applications in microwave, Mm-wave, and THz Bands

Mención Internacional en el título de doctorWe are living in interesting times for astronomy science, since the birth of the radio astronomy field in the 20th century by Karl Jansky, the availability of new and better radio astronomy receivers is in increasing demand to push the human understanding of the universe. In this thesis, various components (antennas, baluns, antenna-arrays, and radiometers) are proposed for radio astronomy receivers. The proposed designs are belonging to three receiver topologies (direct detection, down-conversion, and up-conversion) that operate at different frequency bands from MHz up to a few of THz. Also, to demonstrate that the same proposed design is capable of working efficiently at different operating frequencies, multiple adjusted designs are presented for several practical radio astronomy and space applications. Firstly, a receiver based on the direct detection of the Electromagnetic (EM) radiation through a radio telescope working on cryogenic cooling conditions. In this part, the focus is on designing conical log-spiral antennas and baluns (balanced to unbalanced transformers) to be used as feeds for VLBI Global Observing System (VGOS) ground-based radio telescopes. The feeds cover the Ultrawideband (UWB) from 2 GHz to 14 GHz with Circular Polarization (CP) radiation and stable radiation patterns. After integration of the feeds to the radio telescope, the whole system operates with high aperture efficiency and high System Equivalent Flux Density (SEFD) over the whole required wide range. The fabrication, assembly, and measurements for single-element and four-elements array are provided for achieving the requirements for single CP and dual CP operation. Also, in the same first part, the proposed single-element feed (antenna + balun) is readjusted for being used for CryoRad spaceborne Earth observations. This feed has a single CP over low-frequency UWB from 400MHz to 2 GHz with low weight and physical size compared to standard horn feeds. The second part of the thesis is dedicated to a THz source to be used as a local oscillator for heterodyne radio astronomy THz receivers in which the down-conversion of the THz radiation to a lower frequency occurs. The source is based on an array of self-complementary bow-tie antennas and photomixers that lies on a dielectric lens. The source can be scaled easily to cover different UWB ranges, three ranges are analyzed from 200 GHz to 2 THz, 100 GHz to 1 THz, and 50 GHz to 0.5 THz. Additionally, in this part, a complete study for the effects of metal losses on such THz planar antennas is performed which are not well-investigated in literature yet, the physical explanations behind such effects are also provided. Although these proposed THz sources themselves can work at room temperature, the receiver probably still needs the cooling for the other receiver components (such as the mixer) to work efficiently at such high frequencies. This is the motivation for the third part of this thesis which presents a different type of radio astronomy receiver that is completely able to work without cooling. The third receiver is based on the nonlinear up-converting of the microwave radiation into the optical domain using Whispering Gallery Mode (WGM) resonators which can work at room temperature efficiently. For such advantage and since this concept is naturally narrow-band, it can be a proper candidate for Cosmic Microwave Background (CMB) spectroscopy and space applications. The system design and its performance are analyzed for Ku band at 12 GHz with proposing a novel microwave coupling scheme for enhancing the up-conversion photonic efficiency which is the main limitation for such upconversion systems. Likewise, several high gain 3D-printed Dielectric Resonator Antenna (DRA)s are proposed in both isolated and array configurations to have a direct coupling of the microwave radiation to the proposed scheme. Another practical application for such receiver is presented for CubeSat missions at the mm-wave band (183 GHz) for climate change forecasting. It is clear here that removing the cryogenic cooling conditions decreases satellite weight and cost, which in turn significantly increases its lifetime. Also, it is worth noting that besides the radio astronomy applications, the proposed receivers (and/or their antenna/components) can be used for many other applications. For example, the UWB antennas in the first part can be used as wideband scalable probes for EM compatibility testing or other wireless systems that require single or dual CP such as radar and military applications. This is because the solutions provide constant beam characteristics with good CP polarization purity and stable performance over the operating UWB. In the same way, the proposed THz source in the second part can be used in several THz applications such as very high-speed wireless communications, highresolution imaging for medical and security purposes. This is because of its key benefits as decade bandwidth, compact size, low noise, low power demand, high tunability, and the ability to work at room temperature. For the up-conversion scheme proposed in the third part, due to its high photonic efficiency, low noise level which enables it to work at room temperature, and its scalability from a few GHz up to several THz, it is suitable for low-cost and high sensitivity applications. Specifically, the ones that need to get rid of the hard cryogenic cooling conditions, or at least, relax them and allow the system to work efficiently at higher temperatures. For instance, portable mm-wave and THz systems for quality control, security, and biochemistry. Finally, in this part, the proposed DRA elements and arrays, due to their low cost, high gain, and low losses, can be used for sensing applications and 5G base station antennas.Programa de Doctorado en Multimedia y Comunicaciones por la Universidad Carlos III de Madrid y la Universidad Rey Juan CarlosPresidente: Raed Shubair.- Secretario: Adrián Amor Martín.- Vocal: José Manuel Fernández Gonzále

Dynamically reconfigurable metamaterials using pneumatics, flexibility and structural nonlinearity

Metamaterials are composites consisting of sub‐wavelength resonant elements aiming to manipulate the material's electromagnetic properties. One of the advantages of artificially created materials over natural materials is the possibility to custom design and tune their properties as one desires. Metamaterials continue to draw the attention of the research community as new and significantly enhanced phenomena associated with them are discovered. Significant effort has also been devoted to integrating them with existing structures for potential applications in sensing, defence and next generation devices. Due to the resonant nature of the metamaterial elements, the desired properties are achieved only within a narrow frequency band. For various applications, it is desirable to be able to tune their frequency response. Although the connection between the modification of geometry of the resonators and resultant variations in their response individually and as an effective media has been extensively studied, the area of dynamic tuning could benefit from further investigation. The major contribution made by this work includes investigation of real time tuning possibilities and developing new approaches for altering the shapes and orientations of metamaterial resonators, post fabrication, as means of widening flexibility in the design and improving variety of responses. A novel pneumatic switching approach is demonstrated for alteration of the shape of the resonators via addition or retraction of pneumatic elements, as well as application of this method to the realisation of a switchable graded index lens. Further, suspended resonators with mechanical degrees of freedom have been realised which allow shifts in their position and orientation leading to nonlinear effects. A new microfabricated mesh substrate with significantly reduced mass was developed. Embedding resonators into elastic substrates has also been explored for stretching and conformal adhesion purposes. Most of the work is for metamaterials operating in the microwave frequency range (GHz), except elastic metamaterial intended for far infrared (THz) frequencies. In summary, metamaterial tuning approaches have been extended to dynamic manipulation of both shape and orientation of resonators providing greater flexibility and control over effective material parameters