Cost-Effective Quasi-Parallel Sensing Instrumentation for Industrial Chemical Species Tomography

Chemical Species Tomography (CST) has been widely applied for imaging of critical gas-phase parameters in industrial processes. To acquire high-fidelity images, CST is typically implemented by line-of-sight Wavelength Modulation Spectroscopy (WMS) measurements from multiple laser beams. The modulated transmission signal on each laser beam needs to be a) digitised by a high-speed analogue-to-digital converter (ADC); b) demodulated by a digital lock-in (DLI) module; and c) transferred to high-level processor for image reconstruction. Although a fully parallel data acquisition (DAQ) and signal processing system can achieve these functionalities with maximised temporal response, it leads to a highly complex, expensive and power-consuming instrumentation system with high potential for inconsistency between the sampled beams due to the electronics alone. In addition, the huge amount of spectral data sampled in parallel significantly burdens the communication process in industrial applications where in situ signal digitisation is distanced from the high-level data processing. To address these issues, a quasi-parallel sensing technique and electronic circuits were developed for industrial CST, in which the digitisation and demodulation of the multi-beam transmission signals are multiplexed over the high-frequency modulation within a wavelength scan. Our development not only maintains the temporal response of the fully parallel sensing scheme, but also facilitates the cost-effective implementation of industrial CST with very low complexity and reduced load on data transfer. The proposed technique is analytically proven, numerically examined by noise-contaminated CST simulations, and experimentally validated using a lab-scale CST system with 32 laser beams.Comment: Submitted to IEEE Transactions on Industrial Electronic

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

In Vivo Vascular Imaging with Photoacoustic Microscopy

Photoacoustic (PA) tomography (PAT) has received extensive attention in the last decade for its capability to provide label-free structural and functional imaging in biological tissue with highly scalable spatial resolution and penetration depth. Compared to modern optical modalities, PAT offers speckle-free images and is more sensitive to optical absorption contrast (with 100% relative sensitivity). By implementing different regimes of optical wavelength, PAT can be used to image diverse light-absorbing biomolecules. For example, hemoglobin is of particular interest in the visible wavelength regime owing to its dominant absorption, and lipids and water are more commonly studied in the near-infrared regime. In this dissertation, one challenge was to quantitatively investigate red-blood-cell dynamics in nailfold capillaries with single-cell resolution PA microscopy (PAM). We recruited healthy volunteers and measured multiple hemodynamic parameters based on individual red blood cells (RBCs). Statistical analysis revealed the process of oxygen release and changes in flow speed for RBCs in a capillary. For the first time on record, oxygen release from individual RBCs in human capillaries was imaged with nearly real-time speed, and the work paved the way for our following study of a specific blood disorder. We next conducted a pilot study on sickle cell disease (SCD), measuring and comparing the parameters related to RBC dynamics between healthy subjects and patients with SCD. In the patient group, we found that capillaries tended to be more tortuous, dilated, and had higher number density. In addition, abnormal RBCs tended to have lower oxygenation in the inlet of a capillary, from where they flowed slower and released a larger fraction of oxygen than normal RBCs. As the only imaging modality able to observe the real-time dynamics of the oxygen release of individual RBCs, PAM provides medically valuable information for diagnostic purposes. As the last focus of this dissertation, we tackled the limited view problem in PAM by introducing an off-axis illumination technique for complementing the original detection view. We demonstrated this technique numerically and then experimentally on phantoms and animals. This simple but very effective method revealed abundant vertical vasculature in a mouse brain that had long been missed by conventional top-illumination PAM. This technique greatly advances future studies on neurovascular responses in mouse brains

Laser absorption spectroscopic tomography with a customised spatial resolution for combustion diagnosis

Combustion is a widely used energy conversion technology. However, post-combustion gas emissions have adverse effects on climate change. To address the urgent need for carbon neutrality, efforts are being made to develop cleaner fuels and improve combustion efficiency. Accurate in situ measurements of temperature and species concentration are crucial for analysing and diagnosing the combustion process. In industrial applications, probed-based measurement methods are commonly used to detect temperature and species concentration in the combustion, favoured by their simplicity. However, the probe-based techniques are limited in their spatial resolution, as only point-wise measurements can be provided by them. Additionally, their principle often restricts their temporal resolution, which limits their ability to capture the dynamics of the combustion process. To overcome these limitations, researchers are actively working on developing rapid and multi-dimensional in situ techniques for temperature and species concentration monitoring. Laser Absorption Spectroscopy (LAS) has gained significant attention for its non-intrusive nature and fast response in combustion diagnostics. LAS techniques use an emitter-receiver configuration to measure the line-of-sight light intensity absorbed by species in the gaseous medium. By collecting multiple line-of-sight measurements from different angles, LAS enables tomographic measurement of the combustion process. However, implementations of LAS tomography face challenges due to the physical dimensions of the emitter and receiver and the optical access to industrial combustors. These limitations lead to incomplete measurements, which are key factors of ill-posed problems and artefacts in the reconstructed images. The artefacts lead to inaccuracy and unreliability of the diagnostic results. Increasing physical sampling density is one of the most straightforward ways to alleviate the ill-posed problem caused by inadequate line-of-sight measurements. Improvements in sensors have been demonstrated in previous research, such as optimising laser beam arrangement and reducing the spacing of neighbouring laser beams. In this work, a novel design of a miniature and modular sensor is firstly introduced. It reduces the beam spacing between adjacent laser beams, allowing for a more precise and detailed reconstruction of temperature and species concentration distributions. Meanwhile, modular design allows for customisation and adaptation to various measurement requirements. This flexibility in deployment reduces the cost of the LAS technique. The application of small beam spacing in characterising the non-uniformity of the combustion process has also been demonstrated in this thesis. A multi-channel LAS sensor is developed and applied to exhaust measurements of a commercial auxiliary power unit. The results show that the small beam spacing allows a detailed understanding of the exhaust plume at the mixing zone between the exhaust gas and surrounding air. This spatial information can be used to improve the accuracy of temperature and species concentration measurements. On the other hand, prior knowledge, such as smoothness and sparsity of the measurement target and beam arrangement of the LAS tomographic sensor is used to provide extra physical information to the ill-posed inverse problem. To incorporate the beam arrangement information into the reconstruction process, a new meshing scheme is proposed in this thesis. The scheme dynamically allocates smaller meshes in the beam-dense regions and coarser meshes in the beam-loose regions. This adaptive meshing scheme ensures a finer resolution in detailing the combustion zone where the beams are closely spaced while maintaining the integrity of the physical model by using less resolved reconstruction in the bypass flows or regions where the beams are further apart. As a result, the proposed meshing scheme improves the reconstruction accuracy of the combustion zone. Overall, this PhD project designed and developed LAS tomographic sensors and methods that enable accurate and fast measurement of gas temperature and species concentration in combustion processes with a customised spatial resolution. The main contributions of this thesis include the design and prototyping of a miniature and modular optical sensor for flexible LAS tomography; the development of a multi-channel LAS sensor for simultaneously monitoring exhaust gas temperature and water vapour concentration in gas turbine engines; and the development of a size-adaptive hybrid meshing scheme to improve the reconstruction of target flow fields

Developing Wavefront Shaping Techniques for Focusing through Highly Dynamic Scattering Media

One of the prime limiting factors of optical imaging in biological applications is the diffusion of light by tissue, which prevents focusing at depths greater than the optical diffusion limit of ~1 mm in soft tissue. This greatly restricts the utility of optical diagnostic and therapeutic techniques, such as optogenetics, microsurgery, optical tweezing, and phototherapy of deep tissue, which require focused light in order to function. Wavefront shaping extends the depth at which optical focusing may be achieved by compensating for phase distortions induced by scattering, allowing for focusing through constructive interference. However, due to physiological motion, scattering of light in tissue is deterministic only within a brief speckle correlation time. In in vivo soft tissue, this speckle correlation is on the order of milliseconds. Because wavefront shaping relies on deterministic scattering in order to compensate for the resulting phase distortion, the wavefront must be optimized within this brief period. This presents a challenge as the speed of digital wavefront shaping has typically been limited by the relatively long time required to measure and display the optimal phase pattern due to the low speed of cameras, data transfer and processing, and spatial light modulators. In order to overcome these restrictions, wavefront shaping techniques which minimize the time required in measurement and display are therefore vital. In this dissertation, I will describe our efforts to improve the speed of wavefront shaping without sacrificing the performance of the systems. To this end, we have successfully developed several systems which are capable of full-phase wavefront shaping with latencies of 9 ms or less. In addition, we report an all-digital alignment compensation protocol, which may be used to obtain optimal alignment in digital optical phase conjugation systems, a key component when acquiring the best possible focusing performance

Dual-frequency-comb two-photon spectroscopy

This thesis reports on experimental demonstrations of a novel direct frequency-comb spectroscopic technique for the measurement of one- and two-photon excitation spectra. An optical-frequency-comb generator emits a multitude of highly coherent laser modes whose oscillation frequencies are evenly spaced and uniquely determined by only two measurable and adjustable radio-frequency parameters and the integer-valued mode number. Direct frequency-comb spectroscopy can traditionally be performed by scanning the comb lines of the frequency comb across the transitions of interest and measuring a signal that is proportional to the excitation by all comb lines in concert. Since the modes that contribute to the excitation cannot be singled out, transition frequencies can only be measured modulo the comb-line spacing with this scheme. The so arising limitations are overcome by the technique presented here, where the first frequency comb is spatially overlapped with a second frequency comb. Both combs of this so-called dual-comb setup are ideally identical except for having different carrier-envelope frequencies and slightly different repetition rates. The interference between the two combs leads to beat notes between adjacent comb lines, forming pairs (with one line from each comb) with an effectively modulated excitation amplitudes. Consequently the probability of excitation by any given comb-line pair is also modulated at the respective beat-note frequency. These beat-note frequencies are spaced by the repetition-rate difference and uniquely encode for individual comb-line pairs, thus enabling the identification of the comb lines causing an observed excitation. In a first demonstration, Doppler-limited one-photon excitation spectra of the transitions 5S_{1/2}-5P_{3/2} (at 384 Thz/780 nm), 5P_{3/2}-5D_{3/2}, and 5P_{3/2}-5D_{5/2} (both at 386 Thz/776 nm), and two-photon spectra of the 5S_{1/2}-5D_{5/2} (at 2x385 Thz/2x778 nm) transition, agreeing well with simulated spectra, are simultaneously measured for both stable Rb isotopes. Within an 18-s measurement time, a spectral range of more than 10 THz (20 nm) is covered at a signal-to-noise ratio (SNR) of up to 550. To my knowledge, this is the first demonstration of both dual-comb-based two-photon spectroscopy and fluorescence-based dual-comb spectroscopy. In a follow-up experiment probing the same sample and two-photon transitions, the Doppler-resolution limit is overcome by implementation of an anti-resonant ring configuration. Cancellation of the first-order Doppler effect makes it possible to resolve 33 hyperfine two-photon transitions. The highly resolved (1 MHz point spacing), narrow transition-linewidth (5 MHz), accurate (systematic uncertainty of ~340 kHz), high-SNR (10^4) spectra are shown to be consistent with basic simulation-based predictions. As the spectral span is, in principle, only limited by the bandwidths of the excitation sources, the acquisition of Doppler-free two-photon spectra spanning 10s of THz appears to be in reach. To my knowledge, this is the first demonstration of Doppler-free Fourier-transform spectroscopy. Lastly, the possibility of extending the technique's scope to applications in the field of biochemistry, such as two-photon microscopy, are explored. To that end, first high-speed, low-resolution (>>1 GHz) experiments are carried out identifying comb-stabilization requirements and measurement constraints due to the limited dynamic range of the presented highly multiplexed spectroscopic technique

Optical MEMS

Optical microelectromechanical systems (MEMS), microoptoelectromechanical systems (MOEMS), or optical microsystems are devices or systems that interact with light through actuation or sensing at a micro- or millimeter scale. Optical MEMS have had enormous commercial success in projectors, displays, and fiberoptic communications. The best-known example is Texas Instruments’ digital micromirror devices (DMDs). The development of optical MEMS was impeded seriously by the Telecom Bubble in 2000. Fortunately, DMDs grew their market size even in that economy downturn. Meanwhile, in the last one and half decade, the optical MEMS market has been slowly but steadily recovering. During this time, the major technological change was the shift of thin-film polysilicon microstructures to single-crystal–silicon microsructures. Especially in the last few years, cloud data centers are demanding large-port optical cross connects (OXCs) and autonomous driving looks for miniature LiDAR, and virtual reality/augmented reality (VR/AR) demands tiny optical scanners. This is a new wave of opportunities for optical MEMS. Furthermore, several research institutes around the world have been developing MOEMS devices for extreme applications (very fine tailoring of light beam in terms of phase, intensity, or wavelength) and/or extreme environments (vacuum, cryogenic temperatures) for many years. Accordingly, this Special Issue seeks to showcase research papers, short communications, and review articles that focus on (1) novel design, fabrication, control, and modeling of optical MEMS devices based on all kinds of actuation/sensing mechanisms; and (2) new developments of applying optical MEMS devices of any kind in consumer electronics, optical communications, industry, biology, medicine, agriculture, physics, astronomy, space, or defense