PCF-Based Cavity Enhanced Spectroscopic Sensors for Simultaneous Multicomponent Trace Gas Analysis

A multiwavelength, multicomponent CRDS gas sensor operating on the basis of a compact photonic crystal fibre supercontinuum light source has been constructed. It features a simple design encompassing one radiation source, one cavity and one detection unit (a spectrograph with a fitted ICCD camera) that are common for all wavelengths. Multicomponent detection capability of the device is demonstrated by simultaneous measurements of the absorption spectra of molecular oxygen (spin-forbidden b-X branch) and water vapor (polyads 4v, 4v + δ) in ambient atmospheric air. Issues related to multimodal cavity excitation, as well as to obtaining the best signal-to-noise ratio are discussed together with methods for their practical resolution based on operating the cavity in a “quasi continuum” mode and setting long camera gate widths, respectively. A comprehensive review of multiwavelength CRDS techniques is also given

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

Fiber laser system for the detection of trace gas

The research presented in this thesis is on the development of a trace gas detection system based on intracavity absorption spectroscopy (ICAS). The developed system was capable of detecting nitrous oxide (N2O) and acetylene (C2H2) gas at sub-ppmv levels. The P (12) rotational line of N2O at ~1522.20 nm, and the R (5) and R (4) rotational lines of C2H2 at ~1522.22 nm were used for detection. A fiber Bragg grating was incorporated into the ICAS cavity to enhance the system sensitivity by allowing the selection of strong absorption lines for detection. By operating the fiber laser based ICAS system at threshold, multiple circulations of the amplified spontaneous emission inside the cavity enhanced the system detection sensitivity. Further, the laser wavelength contained multi-longitudinal modes, which improved the system sensitivity. The system sensitivity was explored for two intra-cavity gas cells: a multi-pass Herriott cell and a gas cell based on a hollow-core photonic crystal fiber (HC-PCF). The system operated at room temperature and a polarization-maintaining erbium-doped fiber was used as a saturable absorber to help eliminate mode hopping in the laser cavity

Supercontinuum Absorption Spectroscopy for Combustion Diagnostics

During recent years, sensors and diagnostic systems have seen an increase in demand, due to stricter legislative regulations for certification, as well as industry trends, such as Internet of Things and Industry 4.0. In addition, recent scientific discoveries (for example gravitational wave detection) are the result of international collaborations in the field of sensors and diagnostics. The ability to measure process-relevant parameters, preferably in situ and disturbance-free, is essential for improving performance of various systems, from chemical plants to internal combustion engines and energy power plants. Only with precise knowledge of the parameters of these processes, an improvement in efficiency and a reduction of pollutant emissions is achievable. Given the process optimizations in the last decade, the conditions under which a diagnostic system has to obtain valid measurements have significantly harshened. Most of the requirements can only be fulfilled with multi-scalar and multi-species measurements. To obtain such measurements, an optical diagnostic system is often unavoidable, especially for in situ measurements. Given its robustness and versatility, absorption spectroscopy offers great possibilities for such measurements. With the recent arrival of Supercontinuum Laser Light Source (SCLs), which offer broad spectral coverage in pulsed form, the concept of Supercontinuum Laser Broadband Absorption Spectroscopy (SCLAS) was developed, relying on a dispersion in time to record optical spectra. Given the broad spectral coverage, it is possible to derive multiple scalars including species concentrations, pressure and temperature purely optical. Furthermore, such broad coverage is essential for measurements in high-pressure environments (i.e. within the cylinder of an internal combustion engine). Based on an extensive discussion of the underlying effects and processes, necessary spectroscopic models and algorithms were developed to process the obtained measurements. Based on these models, several test cases for SCLAS were investigated, including static tests to quantify accuracy and uncertainty, as well as steady-state laminar flames. Based on the knowledge of these experiments, SCLAS was transferred to transient systems including high-pressure cells and was applied for in-cylinder measurements at a transparent engine test bed. In addition, based on the results of the validation and application tests, new spectroscopic models were developed to fully utilize the potential of SCLs in general and SCLAS in particular. These new models were evaluated against standard practices and found to be an improvement with regards to complexity and speed of data-processing. Furthermore, these models, as opposed to standard gas absorption spectroscopy models, allow for modelling of liquids as well as complex non-discrete absorbing species, such as propane and AdBlue (DEF). Overall, the diagnostic technique SCLAS was proven in comparison to established techniques, while advanced approaches to measure in situ in high-pressure high-temperature processes were developed and tested

Advances in Optofluidics

Optofluidics a niche research field that integrates optics with microfluidics. It started with elegant demonstrations of the passive interaction of light and liquid media such as liquid waveguides and liquid tunable lenses. Recently, the optofluidics continues the advance in liquid-based optical devices/systems. In addition, it has expanded rapidly into many other fields that involve lightwave (or photon) and liquid media. This Special Issue invites review articles (only review articles) that update the latest progress of the optofluidics in various aspects, such as new functional devices, new integrated systems, new fabrication techniques, new applications, etc. It covers, but is not limited to, topics such as micro-optics in liquid media, optofluidic sensors, integrated micro-optical systems, displays, optofluidics-on-fibers, optofluidic manipulation, energy and environmental applciations, and so on

On-line analysis of bacterial metabolism by modern spectroscopic laser techniques

In order to best determine how bioprocesses develop and how to run bioreactors most efficiently, innovative new analytical techniques are required to supplement, or even supersede, conventional methods, many of which are invasive, require sampling and/or do not provide on-line data analysis. Spectroscopic laser techniques are powerful analytical tools that are capable of real-time, non-invasive monitoring of multiple variables simultaneously. Furthermore, spectroscopy offers high selectivity and sensitivity, including the ability to distinguish different isotopomers and isotopologues, which enables isotopic labelling studies to provide greater mechanistic insights into metabolic pathways. This thesis describes the development of several spectroscopic techniques and their applications in studying different metabolic modes of Escherichia coli batch cultures. On-line analysis is achieved in the gas-phase using cavity-enhanced Raman spectroscopy (CERS), White cell FTIR spectroscopy and photoacoustic detection in a differential Helmholtz resonator (DHR) as well as in the liquid-phase using Raman spectroscopy. The spectral analysis and quantitation of over twenty parameters is discussed, including growth substrates such as glucose and ammonia, metabolites such as acetate, ethanol and formate, headspace gases such as H2, O2 and CO2, and other process variables measured in situ such as the pH and optical density (OD). The first bacterial study conducted is a revisitation of the classical E. coli experiment of glucose-lactose diauxie. A new approach for studying mixed sugar metabolism is presented using both the CERS and DHR techniques to distinguish 13CO2 produced from 13C-glucose metabolism from the subsequent production of 12CO2 from unlabelled lactose. Next, gas-phase FTIR and liquid-phase Raman are developed for batch culture analysis and applied to monitoring mixed-acid fermentation. Finally, two further isotopic labelling studies are conducted by using CERS alongside FTIR and liquid-phase Raman analysis. Nitrate and nitrite reduction by E. coli to the major and minor end-products of ammonium and nitrous oxide is studied, respectively. 15N-labelling is used to give mechanistic insights through interpretation of the different 14N/15N-isotopomer products. The final study focuses on the fermentative pathways of E. coli in the absence and presence of O2. Using 13C- and D-labelled formate, evidence is found that the formate hydrogenlyase (FHL) complex can be assembled and functional under micro-aerobic conditions, which could remove some barriers to biotechnological applications such as biohydrogen generation

Design and fabrication of optical fibre long period gratings for CO₂ sensing

This thesis investigated the repeatability of the overwrite long period grating (LPG) fabrication method and highlighted the advantage it offers in its ability to tune spectral features thus allowing the manufacture of bespoke sensors. Moreover, LPGs with periods ranging from 100 - 200 μm were written and a novel technique for mapping the transmission data was presented. This method gave a unique overview into the period mediated evolution of attenuation features, which, when designing LPGs that operate at the sensitive phase matching turning point, is invaluable. Further exploration into the overwrite method revealed that the UV irradiation duty cycle used in the fabrication of LPGs was found to influence the presence of harmonics, where a duty cycle of 25% maximised coupling to 2nd order transmission features. LPGs which possessed these additional spectral features within a small wavelength range (600 - 1000 nm) were assessed for their suitability in performing multi-parameter sensing. Ionic liquids were explored as an LPG COThis thesis investigated the repeatability of the overwrite long period grating (LPG) fabrication method and highlighted the advantage it offers in its ability to tune spectral features thus allowing the manufacture of bespoke sensors. Moreover, LPGs with periods ranging from 100 - 200 μm were written and a novel technique for mapping the transmission data was presented. This method gave a unique overview into the period mediated evolution of attenuation features, which, when designing LPGs that operate at the sensitive phase matching turning point, is invaluable. Further exploration into the overwrite method revealed that the UV irradiation duty cycle used in the fabrication of LPGs was found to influence the presence of harmonics, where a duty cycle of 25% maximised coupling to 2nd order transmission features. LPGs which possessed these additional spectral features within a small wavelength range (600 - 1000 nm) were assessed for their suitability in performing multi-parameter sensing. Ionic liquids were explored as an LPG COThis thesis investigated the repeatability of the overwrite long period grating (LPG) fabrication method and highlighted the advantage it offers in its ability to tune spectral features thus allowing the manufacture of bespoke sensors. Moreover, LPGs with periods ranging from 100 - 200 μm were written and a novel technique for mapping the transmission data was presented. This method gave a unique overview into the period mediated evolution of attenuation features, which, when designing LPGs that operate at the sensitive phase matching turning point, is invaluable. Further exploration into the overwrite method revealed that the UV irradiation duty cycle used in the fabrication of LPGs was found to influence the presence of harmonics, where a duty cycle of 25% maximised coupling to 2nd order transmission features. LPGs which possessed these additional spectral features within a small wavelength range (600 - 1000 nm) were assessed for their suitability in performing multi-parameter sensing. Ionic liquids were explored as an LPG CO₂ sensitive coating. It was shown that these materials demonstrate a refractive index change upon exposure to CO₂ which was maintained following mechanical stabilisation using a gelling agent. A coating system for applying the gelled ionic liquid to the surface of an optical fibre was developed and techniques to improve the coating deposition were explored. The sensor demonstrated an 8 nm wavelength shift in response to 20% CO₂, which was reversible by reducing the partial pressure of CO₂ for 25 min.sensitive coating. It was shown that these materials demonstrate a refractive index change upon exposure to CO₂ which was maintained following mechanical stabilisation using a gelling agent. A coating system for applying the gelled ionic liquid to the surface of an optical fibre was developed and techniques to improve the coating deposition were explored. The sensor demonstrated an 8 nm wavelength shift in response to 20% CO₂, which was reversible by reducing the partial pressure of CO₂ for 25 min. sensitive coating. It was shown that these materials demonstrate a refractive index change upon exposure to CO₂ which was maintained following mechanical stabilisation using a gelling agent. A coating system for applying the gelled ionic liquid to the surface of an optical fibre was developed and techniques to improve the coating deposition were explored. The sensor demonstrated an 8 nm wavelength shift in response to 20% CO₂, which was reversible by reducing the partial pressure of CO₂ for 25 min

Recent Progress in Optical Fiber Research

This book presents a comprehensive account of the recent progress in optical fiber research. It consists of four sections with 20 chapters covering the topics of nonlinear and polarisation effects in optical fibers, photonic crystal fibers and new applications for optical fibers. Section 1 reviews nonlinear effects in optical fibers in terms of theoretical analysis, experiments and applications. Section 2 presents polarization mode dispersion, chromatic dispersion and polarization dependent losses in optical fibers, fiber birefringence effects and spun fibers. Section 3 and 4 cover the topics of photonic crystal fibers and a new trend of optical fiber applications. Edited by three scientists with wide knowledge and experience in the field of fiber optics and photonics, the book brings together leading academics and practitioners in a comprehensive and incisive treatment of the subject. This is an essential point of reference for researchers working and teaching in optical fiber technologies, and for industrial users who need to be aware of current developments in optical fiber research areas

Fantasies using optical fibers

The activity carried out during the PhD course has concerned special optical fibers with particular refractive index profiles, from Photonic Crystal Fibers (PCFs) to Plastic Optical Fibers (POFs), which is a research topic in continuous evolution and characterized by a great scientific excitement. The aim of the research of the three year PhD course has been to accurately study, and thus to deeply understand the light guiding mechanisms exploited in these kinds of optical fibers. The unusual guiding properties of PCFs with different air-hole arrangements in the fiber cross-section have been investigated both numerically, through a full-vector modal solver based on the Finite Element Method, and experimentally, by considering samples of large mode area PCFs, as well as of nonlinear fibers. Moreover, the properties of Erbium-Doped Fibers (EDFs) with a particular refractive index profile, that is with a depressed-cladding, have been experimentally characterized. By exploiting the bending loss of these active fibers, amplifiers with different configurations have been realized, which cover larger bandwidths with respect to conventional ones, as well as tunable lasers in S, C and L band. Then, in order to design and realize the pre-amplifier stage for a pulsed high power laser useful for industrial applications, single-mode ytterbium-doped fibers with different doping concentrations, with either a single or a double-cladding, have been considered with the aim to optimize the gain performances. Finally, low cost sensors based on the inexpensive plastic fibers have been proposed as an effective solution to the problem of the liquid level measurement. Sensors for both point and continuous measurements of the liquid level, which can be also exploited to distinguish fluids according to their refractive index