Acoustic Waves

The concept of acoustic wave is a pervasive one, which emerges in any type of medium, from solids to plasmas, at length and time scales ranging from sub-micrometric layers in microdevices to seismic waves in the Sun's interior. This book presents several aspects of the active research ongoing in this field. Theoretical efforts are leading to a deeper understanding of phenomena, also in complicated environments like the solar surface boundary. Acoustic waves are a flexible probe to investigate the properties of very different systems, from thin inorganic layers to ripening cheese to biological systems. Acoustic waves are also a tool to manipulate matter, from the delicate evaporation of biomolecules to be analysed, to the phase transitions induced by intense shock waves. And a whole class of widespread microdevices, including filters and sensors, is based on the behaviour of acoustic waves propagating in thin layers. The search for better performances is driving to new materials for these devices, and to more refined tools for their analysis

Laser Systems for Applications

This book addresses topics related to various laser systems intended for the applications in science and various industries. Some of them are very recent achievements in laser physics (e.g. laser pulse cleaning), while others face their renaissance in industrial applications (e.g. CO2 lasers). This book has been divided into four different sections: (1) Laser and terahertz sources, (2) Laser beam manipulation, (3) Intense pulse propagation phenomena, and (4) Metrology. The book addresses such topics like: Q-switching, mode-locking, various laser systems, terahertz source driven by lasers, micro-lasers, fiber lasers, pulse and beam shaping techniques, pulse contrast metrology, and improvement techniques. This book is a great starting point for newcomers to laser physics

Novel sources of near- and mid-infrared femtosecond pulses for applications in gas sensing, pulse shaping and material processing

In this thesis the design, construction process and the performance of two femtosecond optical parametric oscillators and one second–harmonic generation femtosecond pulse shaper is described. One oscillator was applied to gas sensing while potential applications of other devices are outlined. ATi:sapphire oscillator was used to pump a periodically–poled lithium niobate– based optical parametric oscillator. This signal–resonant device was configured to produce broadband idler pulses tunable in the range of 2.7–3.4 μm. This wavelength coverage was matched to the ν3 optical absorption band of methane, and Fourier–transform spectroscopy of a CH4:N2 mixture was implemented by employing a mid–IR silica photonic bandgap fibre simultaneously as a gas cell and an optical waveguide. Methane sensing below a 1% concentration was demonstrated and the main limiting factors were identified and improvements suggested. Another optical parametric oscillator was demonstrated which was pumped by a commercial Yb:fibre master oscillator/power amplifier system and was based on a periodically–poled lithium niobate crystal. The signal was tunable between 1.42–1.57 μm and was intended as a source for a subsequent project for waveguide writing in silicon. The oscillator was a novel long–cavity device operating at 15 MHz. The 130 nJ pump pulse energies allowed for 21 nJ signal pulses at a pump power of 2 W. The performance of the oscillator was characterised via temporal and spectral measurements and the next steps of its development are outlined. Finally a pulse shaper based on second harmonic generation in a grating– engineered periodically–poled lithium niobate crystal was demonstrated. Pulses from a 1.53 μm femtosecond Er:fibre laser were compressed and then used as the input to the shaper. The performance of the shaper was tested by performing cross–correlation frequency–resolved optical gating measurements on the output second harmonic pulses and this confirmed the successful creation of multiple pulses and other tailored shapes including square and chirped pulses, agreeing well with theoretical calculations

Paraxial application of auxiliary devices with waveguide mediated laser irradiation for applications in medical biophotonics

Laser-based medical applications offer minimally-invasive alternatives to traditional procedures; however, the simplistic method of open-air laser transmission poses ocular hazards and negative side effects, along with a fundamentally limited efficacy for patients of darker complexion due to strong optical absorption in the epidermis. Additionally, the traditional irradiation method also inhibits the incorporation of additional technologies that might otherwise enhance therapeutic effects or provide diagnostic benefits, as doing so would otherwise occlude the laser beam path. The research presented herein addresses each of these considerations individually, first by transmitting laser light into tissue through direct contact with a selective-release waveguide, and thereafter incorporating auxiliary equipment on its rear face. Metal clad planar optical waveguides are demonstrated for the transmission of laser light into samples of porcine skin through direct transmission, governed by scaling evanescent leaking through a designated active area by controlling thin film thickness. In one manifestation, an ultrasonic pulser was incorporated to modulate tissue optical properties and thereby improve transmission of light through epidermal and dermal tissues by increasing forward anisotropy; whereas in another, a high frequency ultrasonic transducer was incorporated to detect photoacoustically generated pressure waves to determine depth profiles of chromophores in skin as a foundation for clinical backward-mode photoacoustic tomography.Includes biblographical reference

Multipoint gas detection using range resolved interferometry

The ability to detect and quantify gas in multiple locations is important in environmental and safety monitoring situations. This thesis describes the first application of Range Resolved Interferometry to the problem of gas sensing at multiple locations. Range resolved interferometry (RRI) is an interferometric signal processing technique that allows the separation of individual interferometric signals from superpositions of multiple interferometers and the rejection of interferometers other than those of interest. This allows the interrogation of the light intensity passing through each interferometer of interest which in turn allows a measure of the absorption of light by gas present within the interferometer arms. The application of the Beer-Lambert Law allows the measurement of a gas concentration from this information. Unlike previous interferometric techniques for multipoint gas measurement, RRI uses injection current modulation of a DFB laser and is therefore, cost effective. The process of applying a ramp modulation to RRI in order to extract spectroscopic information is described along with the post-processing needed to extract gas concentrations from multiple locations simultaneously. Three sensing regions ² < 0.95) and with the ability to measure methane at a concentration of 200ppm with no averaging time. Allen-Werle analysis showed that with sufficient averaging time, a limit of detection as low as 4ppm could be achieved. Cross talk experiments showed that the presence of gas in other sensing regions had no effect on gas concentration measurements. The first use of RRI for spectroscopic measurements required extensive postprocessing to account for the DFB laser’s non-uniform response to sinusoidal modulation as the driving injection current was varied to sweep the laser output wavelength. Application of an envelope function to the sinusoidal modulation provided a stable wavelength response to the sinusoidal modulation and so allowed real-time gas detection with no post processing required. Experiments were performed to establish that the most suitable deployment topology for multipoint sensing is a serial-bus topology and that the amplitude of the sinusoidal modulation must be chosen to provide the chosen balance between the spatial resolution of the system and the signal strength provided by the measurement of light absorption by the gas under test. The ability of RRI to distinguish between interferometers of interest and parasitic interferometers was used to extract the absorption measurements from a gas detection system with optical fringing and was shown to reduce the unwanted signal by a factor of 18

Laser-generated, plane-wave, broadband ultrasound sources for metrology

The accurate quantification of ultrasound fields generated by diagnostic and therapeutic transducers is critical for patient safety. This requires hydrophones calibrated to a traceable national measurement standard over the full range of frequencies used. At present, the upper calibration frequency range available to the user community is limited to a frequency of 60 MHz. However, there is often content at frequencies higher than this, e.g., through nonlinear propagation of high-amplitude pulses or tone-bursts for therapeutic applications, and the increasing use of higher frequencies in diagnostic imaging. To reduce the uncertainties and extend the calibrations to higher frequencies, a source of high-pressure, plane-wave and broadband ultrasound fields is required. This is not possible with current piezoelectric transducer technology, therefore laser-generated ultrasound is investigated as an alternative. This consists of an ultrasound wave generated by the pulsed laser excitation of a thin, planar, layer of light absorbing carbon-polymer nanocomposite materials. The work described in this thesis can be divided into three parts. The first part consisted of the fabrication of various nanocomposites in order to study the effect of different polymer types, composite thickness, laser fluence, and concentration of carbon nanotubes, on the ultrasound generated, as well as their stability. This included an investigation into the nonlinear propagation of MPa range laser-generated ultrasound, and the effect of the bandlimited hydrophone response, using a numerical wave solver (k-Wave). In the second part, the effects on the signal of acoustically reflective and matched backings (the substrates onto which the nanocomposite was coated) were studied. It was found experimentally that the backing material can significantly affect the pressure amplitude when the duration of the laser pulse is longer than the acoustic transit time across the thin nanocomposite layer. An analytical model was developed to describe how the signal generated depends on the backing material, absorbing layer thickness, and laser pulse duration. The model agreed well with measurements performed with a variable pulse duration fibre-laser. Finally, in the third part, a laser-generated, plane-wave, broadband ultrasound source device superficially resembling a standard piezoelectric piston source was designed, fabricated, and tested. The source produced quasi-unipolar pressure-pulse of 9 MPa peak-positive pressure with a bandwidth of 100 MHz, and the ultrasound beam is sufficiently planar to reduce uncertainties due to diffraction to negligible levels for hydrophones up to 0.6 mm in diameter

Cavity-enhanced absorption spectroscopy with a deep UV-LED

The motivation for this project work was to build a portable analyser based on a UV LED to carry out in situ measurements of trace gas species under ambient conditions applied to atmospheric monitoring and breath analysis. Acetone has gained vital importance as a volatile organic compound (VOC) which is found as a pollutant in the atmosphere. Acetone also serves as a biomarker for metabolic processes particularly in people on a ketogenic diet or patients suffering from diabetic ketoacidosis (DKA), a life-threatening condition that can be experienced by Type 1 Diabetic patients. The project work mainly consisted of two parts: design of a portable set-up and as a proof of principle that a UV-LED based acetone detector is feasible instead of the most commonly used laser-based detector. Acetone absorption measurements were carried out in nitrogen background using a 300 nm LED employing the technique of Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy (IBBCEAS). The design of the portable set-up is based on a cage-system to achieve portability, robustness, mechanical stability, insensitivity to temperature and pressure variations, and cost effectiveness. The designed portable set-up has potential applications as a detector providing in situ real-time detection of trace gas species in atmospheric chemistry and in medical diagnostics as a breath acetone analyser. An incoherent light source such as an LED requires a different approach to light collimation and guiding compared to a laser. In the UV range this poses additional challenges due to a lack of off-the-shelf optical components for this purpose. In this project, a stable optical cavity was successfully set-up to carry out acetone absorption measurements in nitrogen background using CEAS. A noise-equivalent absorption coefficient of 4.1 ± 0.3 x 10−6 cm-1Hz-1/2 (corresponding to ~2 ppmv of acetone) was achieved with the CEAS set-up.James Watt Scholarshi

Supercontinuum radiation for ultra-high sensitivity liquid-phase sensing

The real-time detection of trace species is key to a wide range of applications such as on-line chemical process analysis, medical diagnostics, identification of environmentally toxic species and atmospheric pollutant sensing. There is a growing demand for suitable techniques that are not only sensitive, but also simple to operate, fast and versatile. Most currently available techniques, such as spectrophotometry, are neither sensitive enough nor fast enough for kinetic studies, whilst other techniques are too complex to be operated by the non-specialist. This thesis presents two techniques that have been developed for and applied to liquid-phase analysis, with supercontinuum (SC) radiation used for liquid-phase absorption for the first time. Firstly, supercontinuum cavity enhanced absorption spectroscopy (SC-CEAS) was used for the kinetic measurement of chemical species in the liquid phase using a linear optical cavity. This technique is simple to implement, robust and achieves a sensitivity of 9.1 × 10−7 cm−1 Hz−1/2 at a wavelength of 550nm for dye species dissolved in water. SC-CEAS is not calibration-free and for this purpose a second technique, a time-resolved variant called broadband cavity ring-down spectroscopy (BB-CRDS), was successfully developed. Use of a novel single-photon avalanche diode (SPAD) array enabled the simultaneous detection of ring-down events at multiple spectral positions for BB-CRDS measurements. The performance of both techniques is demonstrated through a number of applications that included the monitoring of an oscillating (Belousov-Zhabotinsky) reaction, detection of commercially important photoluminescent metal complexes (europium(III)) at trace level concentration, and the analysis of biomedical species (whole and lysed blood) and proteins (amyloids). Absorption spectra covering the entire visible wavelength range can be acquired in fractions of a second using sample volumes measuring only 1.0mL. Most alternative devices capable of achieving similar sensitivity have, up until now, been restricted to single wavelength measurements. This has limited speed and number of species that can be measured at once. The work presented here exemplifies the potential of these techniques as analytical tools for research scientists, healthcare practitioners and process engineers alike

Enhanced raman spectrometry for environmental gas sensing and human breath analysis

Gas sensing techniques allow for groundbreaking studies in the field of plant-physiological processes, soil-bacteria interactions, as well as early stage monitoring of disease states via human breath analysis. Easy-to-operate, miniaturized, on-site, and cost-efficient gas sensors have attracted great interest in the scientific community in the last years. In this work an innovative fiber-enhanced Raman multi-gas sensor was designed, developed, and tested for manifold applications in the field of clinical diagnosis and environmental science. By combining the versatile Raman spectroscopic technique with state-of-the-art low loss microstructured optical fibers (e.g. HC-PCF), which show very low sample demand, a tremendous signal enhancement was achieved for potential monitoring of a complex volatile anesthetics matrix, or for the diagnosis of metabolic diseases including lactose intolerance, fructose malabsorption, or SIBO. The versatility of the new sensor allows simultaneous identification and quantitative monitoring of various climate-relevant gases and volatiles, especially of stable isotope tracers (e.g. 13C, 15N) and homonuclear molecules (e.g. H2, O2, N2) in a high dynamic concentration range and high chemical selectivity, without cross-sensitivity and the need for sample preparation. Furthermore, the application of a miniaturized, cavity-based Raman multi-gas sensor was applied for profound insights into plant functioning such as the link of more drought-tolerant pine to its greater flexibility in substrate switch for plant respiration under drought and shading. Future investigations on device miniaturization, cost reduction, low maintenance costs, easy operability and calibration, together with low power consumption will enable these Raman instruments further to be used for the elucidation of complex environmental processes and easy-to-apply, point-of-care diagnosis of metabolic disorders and diseases. Thus, it can fill the gap of already well-established analytical techniques

Multimodal photoacoustic remote sensing (PARS) microscopy combined with swept-source optical coherence tomography (SS-OCT) for in-vivo, non-contact, functional and structural ophthalmic imaging applications

Ophthalmic imaging has long played an important role in the understanding, diagnosis, and treatment of a wide variety of ocular disorders. Currently, available clinical ophthalmic imaging instruments are primarily optical-based, including slit-lamp microscopy, fundus photography, confocal microscopy, scanning laser ophthalmoscopy, and optical coherence tomography (OCT). The development of these imaging instruments has greatly extended our ability to evaluate the ocular environment. Studies have shown that at least 40% of blinding disorders in the United States are either preventable or treatable with timely diagnosis and intervention. OCT is a state-of-the-art imaging technique extensively used in preclinical and clinical applications for imaging both anterior and posterior parts of the eye. OCT has become a standard of care for the assessment and treatment of most ocular conditions. The technology enables non-contact, high-speed, cross-sectional imaging over a large field of view with submicron resolutions. In eye imaging applications, functional extensions of OCT such as spectroscopic OCT and Doppler OCT have been applied to provide a better understanding of tissue activity. Spectroscopic OCT is usually achieved through OCT systems in the visible spectral range, and it enables the amount of light absorption inside the ocular environment to be measured. This indirect optical absorption measurement is used to estimate the amount of ocular oxygen saturation (SO2) which is a well-known biomarker in prevalent eye diseases including diabetic retinopathy, glaucoma, and retinal vein occlusions. Despite all the advancements in functional spectroscopic OCT methods, they still rely primarily on measuring the backscattered photons to quantify the absorption of chromophores inside the tissue. Therefore, they are sensitive to local geometrical parameters, such as retinal thickness, vessel diameters, and retinal pigmentation, and may result in biased estimations. Of the various optical imaging modalities, photoacoustic imaging (PAI) offers unique imaging contrast of optical absorption because PAI can image any target that absorbs light energy. This unique imaging ability makes PAI a favorable candidate for various functional and molecular imaging applications as well as for measuring chromophore concentration. Over the past decade, photoacoustic ophthalmoscopy has been applied for visualizing hemoglobin and melanin content in ocular tissue, quantifying ocular SO2, and measuring the metabolic rate of oxygen consumption (MRO2). Despite all these advantages offered by PAI devices, a major limitation arises from their need to be in contact with the ocular tissues. This physical contact may increase the risk of infection and cause patient discomfort. Furthermore, this contact-based imaging approach applies pressure to the eye and introduces barriers to oxygen diffusion. Thus, it has a crucial influence on the physiological and pathophysiological balance of ocular vasculature function, and it is not capable of studying dynamic processes under normal conditions. To overcome these limitations and to benefit from the numerous advantages offered by photoacoustic ophthalmoscopy, non-contact detection of photoacoustic signals has been a long-lasting goal in the field of ocular imaging. In 2017 Haji Reza et al. developed photoacoustic remote sensing (PARS) for non-contact, non-interferometric detection of photoacoustic signals. PARS is the non-contact, all-optical version of optical-resolution photoacoustic microscopy (OR-PAM), where the acoustically coupled ultrasound transducer is replaced with a co-focused probe beam. This all-optical detection scheme allows the system to measure the photoacoustic pressure waves at the subsurface origin where the pressure is at a maximum. In a very short time, PARS technology has proven its potential for various biomedical applications, including label-free histological imaging, SO2 mapping, and angiogenesis imaging. PARS is an ideal companion for OCT in ophthalmic applications, where the depth-resolved, detailed scattering information of OCT is well complemented by rich absorption information of PARS. This combined multimodal imaging technology has the potential to provide chromophore selective absorption contrast in concert with depth-resolved scattering contrast in the ocular environment. The main goals of this PhD project are to: • Develop a photoacoustic remote sensing microscopy system for in-vivo, non-contact ophthalmic imaging. This is the first time a non-contact photoacoustic imaging has been used for in-vivo imaging of the eye. • Develop a robust and temporally stable multiwavelength light source for functional photoacoustic imaging applications. • Develop a multimodal PARS-OCT imaging system that can image in-vivo and record, simultaneously, functional, and structural information in the anterior segment of a rodent eye. This is the first time a multiwavelength non-contact photoacoustic system is used for in-vivo measurement of oxygen saturation in the ocular environment. • Develop and modify the multimodal PARS-OCT imaging system for non-contact, in-vivo, functional, and structural imaging of the posterior part of the rodent eye