Towards in-cylinder chemical species tomography on large-bore IC engines with pre-chamber

A feasibility study is presented and the achieved key design milestones towards the first application of Chemical Species Tomography by Near-IR Absorption Tomography on a heavy duty, large-bore marine engine to visualise relative mixture strength are described. The engine is equipped with pre-chamber ignition and operates using Liquefied Natural Gas with > 88.9 % methane content. Operation of the engine under maximum-load conditions is a key design requirement, with peak cylinder pressure and mean temperature exceeding 127,510 Torr (170 bar) and 850 K respectively. The near-IR spectroscopic behaviour of methane is examined for suitable absorption and reference regions for the above application. In particular, the spectroscopic absorption around the 2ν3 transition region near 1,666 nm is approximated by simulation using data from the HITRAN database under worst-case conditions. The simulation results are compared with methane spectra experimentally acquired at high temperature and ambient pressure. Interference from other chemical species as well as measurement linearity is also investigated. A 31-laser beam tomographic imaging array is proposed, which has been optimised to achieve higher spatial resolution performance in the vicinity of the pre-chamber’s orifices. To enable optical access, a novel, minimally-intrusive method is presented, utilising standard fibreoptics and collimator

A stability and spatial-resolution enhanced laser absorption spectroscopy tomographic sensor for complex combustion flame diagnosis

A novel stable laser absorption spectroscopy (LAS) tomographic sensor with enhanced stability and spatial resolution is developed and applied to complex combustion flame diagnosis. The sensor reduces the need for laser collimation and alignment even in extremely harsh environments and improves the stability of the received laser signal. Furthermore, a new miniaturized laser emission module was designed to achieve multi-degree of freedom adjustment. The full optical paths can be sampled by 8 receivers, with such arrangement, the equipment cost can be greatly reduced, at the same time, the spatial resolution is improved. In fact, 100 emitted laser paths are realized in a limited space of 200mm×200 mm with the highest spatial resolution of 1.67mm×1.67 mm. The stability and penetrating spatial resolution of the LAS tomographic sensor were validated by both simulation and field experiments on the afterburner flames. Tests under two representative experiment states, i.e., the main combustion and the afterburner operation states, were conducted. Results show that the error under the main combustion state was about 4.32% and, 5.38% at the afterburner operation state. It has been proven that this proposed sensor can provide better tomographic measurements for combustion diagnosis, as an effective tool for improving performances of afterburners

Photoacoustic tomography setup using LED illumination

Photoacoustic tomography (PAT) is a hybrid imaging modality that combines optical contrast with ultrasound resolution. Most of the PAT configurations are based on high-energy solid-state lasers such as Nd:YAG laser. In this work, a PAT system that uses light-emitting diode (LED) as a light source is introduced. The system is designed so that the imaged target can be stationary. The target is illuminated by a LED light source from one side and the pressure wave is measured using an acoustic transducer that is rotated around the target. Image reconstruction is based on Bayesian approach to illposed inverse problems. The system was tested with light absorbing targets also in limited-view and sparse angle measurement situations. The results show that LED-based instrumentation and advanced reconstruction methods can form a potential PAT system that can also be applied in limited-view and sparse angle photoacoustic tomography

Two dimensional angular domain optical imaging in biological tissues

Optical imaging is a modality that can detect optical contrast within a biological sample that is not detectable with other conventional imaging techniques. Optical trans-illumination images of tissue samples are degraded by optical scatter. Angular Domain Imaging (ADI) is an optical imaging technique that filters scattered photons based on the trajectory of the photons. Previous angular filters were limited to one dimensional arrays, greatly limiting the imaging capability of the system. We have developed a 2D Angular Filter Array (AFA) that is capable of acquiring two dimensional projection images of a sample. The AFA was constructed using rapid prototyping techniques. The contrast and the resolution of the AFA was evaluated. The results suggest that a 2D AFA can be used to acquire two dimensional projection images of a sample with a reduced acquisition time compared to a scanning 1D AFA

Tomographic Laser Absorption Imaging of Combustion Gases in the Mid-wave Infrared

This dissertation describes advancements in mid-infrared laser absorption tomography for spatio-temporal measurements of thermochemistry in reacting flows relevant to combustion systems. Tunable laser absorption spectroscopy is combined with tomographic reconstruction techniques to resolve small diameter ( < 1 cm) non-uniform flow fields with steep spatial gradients, leveraging emerging mid-wave infrared photonics. Multiple novel measurement methods, hardware configurations, and image processing techniques were investigated. Initially, a mid-infrared laser absorption tomography sensing method was developed for quantitative measurement of CO and CO2 concentrations and temperature distributions in turbulent premixed jet flames using a translation-stage-mounted optical system. This sensing approach was used to examine effects of varying fuel structure on carbon oxidation over a range of Reynolds number regimes. It was found that spatial and temporal resolution is limited in this method due to the finite laser beam size (~ 1 mm) and the slow mechanical translation of the optical system. To address these limitations, a novel laser absorption imaging (LAI) technique, that expands a single laser beam and replaces the detector with a high-speed infrared camera, was introduced to achieve enhanced spatial and temporal resolution for thermo-chemical imaging. As a demonstration of this new technique, distributions of combustion species were imaged in both axisymmetric and non-axisymmetric flow fields using linear tomography algorithms. For non-axisymetric flows, the limited view tomography problem often results in a blurring effect and artifacts in the reconstructed flow-field. In an effort to address these issues, state-of-the-art deep learning neural networks were developed and applied to solve the limited angle inversion. Initial results suggest that deep neural networks have potential to more accurately predict flame structures with fewer projection angles than linear tomography. This work provides a foundation for a new approach to quantitative time-resolved 3D thermo-chemical imaging in high-temperature reacting flows

Diseño y desarrollo de un sistema optoacústico de múltiples longitudes de onda basado en fuentes de diodos láseres de alta potencia: generación de señales optoacústicas con nanopartículas para aplicaciones biomédicas

Durante los últimos años, el rápido avance de las tecnologías ópticas para la obtención de imágenes biomédicas hace posible revelar importantes informaciones biológicas a partir de la interacción entre la luz y el tejido. El interés emergente en nuevas técnicas de obtención de imágenes biomédicas está motivado por la necesidad de detectar células malignas y otras enfermedades durante las etapas precoces de evolución. La limitada profundidad de penetración de la energía óptica en medios biológicos se debe principalmente al alto nivel de dispersión óptica. Además, la difusión de la luz en los tejidos biológicos limita la resolución espacial de las imágenes adquiridas. La técnica optoacústica sobresale estos problemas combinando el alto contraste de la imagen óptica con la alta resolución espacial de los sistemas de ultrasonido en los tejidos profundos. Asimismo, la baja dispersión de las ondas ultrasónicas producidas en los tejidos biológicos facilita la adquisición de imágenes de alta resolución. Dos importantes aspectos a considerar demás en las aplicaciones optoacústicas para una imagen funcional son el uso de agentes de contraste óptico para mejorar la absorción de energía óptica en aquellas áreas donde la dispersión es dominante y la cantidad de energía óptica suministrada por las fuentes láseres para penetrar en profundidad. La necesidad de fuentes láseres compactas y de bajo coste con las características requeridas por las aplicaciones optoacústicas ha impulsado los estudios presentados en esta tesis, proponiendo el uso de diodos láseres de alta potencia en lugar de los clásicos láseres de estado sólido. Generalmente, los láseres de estado sólido como el Nd:YAG y los osciladores ópticos paramétricos se utilizan para la generación de señales optoacústicas, pero su uso en el ambiente clínico está limitado por sus altos costes, bajas frecuencias de repetición y tamaños voluminosos. Por otro lado, los diodos láseres de alta potencia emergen como una potencial alternativa, debido a sus relativamente bajos costes, altas frecuencias de repetición requeridas para una rápida adquisición de imágenes y tamaños compactos. Sin embargo, la potencia de los diodos láseres de alta potencia es todavía relativamente baja en comparación con los láseres de estado sólido y por esta razón se necesita combinarlos para conseguir la cantidad de potencia óptica requerida para las aplicaciones optoacústicas. Un sistema optoacústico basado en diodos láseres de alta potencia ha sido implementado y mejorado a lo largo de los estudios presentados en esta tesis. Se han realizado experimentos optoacústicos a diferentes longitudes de onda utilizando varios tipos de absorbentes colocados en cubeta de cuarzo u hospedados dentro de un “phantom” que simula la dispersión óptica de un tejido blando. Soluciones de nanotubos de carbono, óxido de grafeno y nanoparticulas de oro se han utilizado como absorbentes a lo largo de los experimentos. Los primeros experimentos realizados en espacio libre para enfocar la luz en los absorbentes se han mejorado mediante el uso de fibras ópticas en una segunda etapa. Por último, se han propuesto barras de diodos láseres comercialmente disponibles para sustituir los diodos láseres de alta potencia con el objetivo de aumentar la potencia óptica para futuras implementaciones en los sistemas optoacústicos. Las simulaciones ópticas han demostrado la posibilidad de enfocar el haz emitido por barras de diodos láseres de diferentes longitudes de onda en fibras ópticas por medio de microlentes cilíndricas. En una segunda etapa, las barras de diodos láseres han sido ensambladas en un único sistema para simular un sistema de múltiples longitudes de onda. Los haces han sido combinados por medio de espejos dicroicos y enfocados en una fibra óptica multimodo. Este trabajo de investigación ha abierto nuevas líneas de investigación en el desarrollo de fuentes láser de alta potencia para la endoscopia optoacústica y la tomografía en aplicaciones biomédicas.Over last few years, the rapid growth of optical technologies for biomedical imaging makes possible to reveal important biological information of tissues from light-tissue interaction. The emerging interest on new biomedical imaging techniques is motivated by the necessity to detect malignant cells and other diseases at early growth stages. The limited penetration depth of optical energy in biological media is primarily due to the high level of optical scattering. In addition, the diffusion of light in biological tissues limits the spatial resolution of the images acquired. The optoacoustic technique overcomes these issues combining the high contrast of optical imaging with the high spatial resolution of ultrasound systems in deep tissues. As well, the low scattering of the ultrasound waves produced in the biological tissues facilitates the acquisition of high-resolution images. Two more important aspects to be considered in optoacoustic applications for a functional imaging are the use of optical contrast agents to increase the absorption of optical energy in those areas where the scattering is dominant, and the amount of optical energy delivered by laser sources to penetrate in depth. The necessity of compact and cost-effective laser sources with the characteristics required by optoacoustic applications has encouraged the studies presented in this thesis, proposing the use of high-power diode lasers instead of the classical solid state lasers. Generally, solid-state lasers like Nd:YAG and optical parametric oscillators are used for the generation of optoacoustic signals, but their use in clinical environment is limited by their high costs, low repetition rates and bulky sizes. On the other hand, high-power diode lasers emerge as a potential alternative, due to their relatively low costs, high repetition rates required for fast image acquisition and compact sizes. However, the power of high-power diode lasers is still relatively low compared to solid-state lasers and for this reason they need to be combined in arrays to reach the amount of the optical power required for optoacoustic applications. An optoacoustic setup based on small arrays of high-power diode lasers has been implemented and improved along the studies presented in this thesis. Optoacoustic experiments have been performed at different wavelengths using several kinds of absorbers hosted in a quartz cuvette or embedded within a phantom that simulates the optical scattering of a soft tissue. Solutions of carbon nanotubes, graphene oxide and gold nanorods have been used as absorbers in the experiments. The first experiments done in free space to focus the light in the absorbers have been improved by using optical fibers in a second stage. Lastly, some commercially available diode laser bars have been proposed to replace the high-power diode lasers with the aim to increase the optical power for future implementations in the optoacoustic systems. Optical simulations have demonstrated the possibility to focus the beam of diode laser bars operating at different wavelengths into optical fibers by means of cylindrical microlenses. In a second step, the diode laser bars have been assembled together to simulate a multi-wavelength system. The beams have been combined by dichroic mirrors and focused in a multi-mode optical fiber. This research work has opened up new lines of investigation in the development of high-power laser sources for optoacoustic endoscopy and tomography in biomedical applications.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y AutomáticaPresidente: Alexander A. Oraevsky.- Secretario: José Antonio García Souto.- Vocal: Ana Pilar González Marco

Sub-kHz-linewidth external-cavity laser (ECL) with Si3_3N4_4 resonator used as a tunable pump for a Kerr frequency comb

Combining optical gain in direct-bandgap III-V materials with tunable optical feedback offered by advanced photonic integrated circuits is key to chip-scale external-cavity lasers (ECL), offering wideband tunability along with low optical linewidths. External feedback circuits can be efficiently implemented using low-loss silicon nitride (Si$_3$ N$_4$) waveguides, which do not suffer from two-photon absorption and can thus handle much higher power levels than conventional silicon photonics. However, co-integrating III-V-based gain elements with tunable external feedback circuits in chip-scale modules still represents a challenge, requiring either technologically demanding heterogeneous integration techniques or costly high-precision multi-chip assembly, often based on active alignment. In this work, we demonstrate Si$_3$N$_4$-based hybrid integrated ECL that exploit 3D-printed structures such as intra-cavity photonic wire bonds and facet-attached microlenses for low-loss optical coupling with relaxed alignment tolerances, thereby overcoming the need for active alignment while maintaining the full flexibility of multi-chip integration techniques. In a proof-of-concept experiment, we demonstrate an ECL offering a 90 nm tuning range (1480 nm–1570 nm) with on-chip output powers above 12 dBm and side-mode suppression ratios of up to 59 dB in the center of the tuning range. We achieve an intrinsic linewidth of 979 Hz, which is among the lowest values reported for comparable feedback architectures. The optical loss of the intra-cavity photonic wire bond between the III-V gain element and the Si$_3$N$_4$-based tunable feedback circuit amounts to approximately (1.6 ± 0.2) dB. We use the ECL as a tunable pump laser to generate a dissipative Kerr soliton frequency comb. To the best of our knowledge, our experiments represent the first demonstration of a single-soliton Kerr comb generated with a pump that is derived from a hybrid ECL