Gas-phase broadband spectroscopy using active sources: progress, status, and applications

Broadband spectroscopy is an invaluable tool for measuring multiple gas-phase species simultaneously. In this work we review basic techniques, implementations, and current applications for broadband spectroscopy. We discuss components of broadband spectroscopy including light sources, absorption cells, and detection methods and then discuss specific combinations of these components in commonly used techniques. We finish this review by discussing potential future advances in techniques and applications of broadband spectroscopy

First International Conference on Laboratory Research for Planetary Atmospheres

Proceedings of the First International Conference on Laboratory Research for Planetary Atmospheres are presented. The covered areas of research include: photon spectroscopy, chemical kinetics, thermodynamics, and charged particle interactions. This report contains the 12 invited papers, 27 contributed poster papers, and 5 plenary review papers presented at the conference. A list of attendees and a reprint of the Report of the Subgroup on Strategies for Planetary Atmospheres Exploration (SPASE) are provided in two appendices

Transversely Excited Multipass Photoacoustic Cell Using Electromechanical Film as Microphone

A novel multipass photoacoustic cell with five stacked electromechanical films as a microphone has been constructed, tested and characterized. The photoacoustic cell is an open rectangular structure with two steel plates facing each other. The longitudinal acoustic resonances are excited transversely in an optical multipass configuration. A detection limit of 22 ppb (10−9) was achieved for flowing NO2 in N2 at normal pressure by using the maximum of 70 laser beams between the resonator plates. The corresponding minimum detectable absorption and the normalized noise-equivalent absorption coefficients were 2.2 × 10−7 cm−1 and 3.2 × 10−9 cm−1WHz−1/2, respectively

High resolution spectroscopy and applications

xlii, 221 leaves : ill. ; 29 cmWith the ever increasing spectral resolution now achievable with modern spectrometers we do not just observe dark lines anymore but molecular absorption features at very high resolution. These features are governed by the kinematics of the molecules and can be examined carefully by using high resolution spectroscopy. A detailed description of the setup and performance of the tunable diode laser spectrometer (TDL) system used in my research is presented. This tunable laser system has been used to complete several high resolution line shape studies on the 1+ 2+ 4+ 5 and 1+ 3 combination bands of acetylene. The major focus of the presented results has been on the temperature dependence of the retrieved parameters and the identification of the best fitting line shape profile used in the present investigation. Improved spectral resolution also requires better understanding of its effects on the recorded data that could not have been observed on data obtained with low resolution instruments. One of these effects is known as line mixing and it occurs when neighbouring transitions interfere with each other causing a slight asymmetry in the spectral profile. In this Thesis I have examined the line mixing effect in CO2 spectra and the temperature dependence of these line mixing coefficients. The analysis is performed by using two common line mixing models; (1) the energy correct sudden approximation and (2) the exponential power gap law. Finally I will present a project based on the application of high resolution spectroscopy that involves monitoring N2O concentration at remote locations through the use of a long path gas cell and a tunable diode laser spectrometer system. The initial testing and setup of this monitoring system will be presented and discusse

Thulium-doped fibre laser in the 2 μm wavelength region for gas sensing

The transition 3F4->3H6 of trivalent Thulium is widely studied for generating lasers at wavelength near 2 μm. For decades, tuneable continuous wave narrow line-width sources in this wavelength region have been proved to be very useful as spectroscopic tools for trace gas detection. Semiconductor lasers are often not readily available at a reasonable cost with the specific wavelengths required to provide a close ‘match’ to the key absorption features of the gases of interest. Well-designed fibre laser-based systems, however, can overcome this limitation by offering potentially much wider wavelength ranges, coupled with their distinctive and valuable features such as stability, narrow linewidth and high tuneability at room temperature. In this work, a compact ‘all-fibre’ laser system has been specifically designed, developed and evaluated, as this type of laser systems is highly desirable for ‘in-the-field’ applications. This takes full advantages of the active fibres based on silica glass host compared to other non-oxide glass hosts in terms of their chemical durability, stability and crucial structural compatibility with readily available telecommunication optical fibres. Ideal host composition for Thulium and efficient pumping scheme posses major challenges restricting the production of commercially deployable efficient ‘all-fibre’ lasers in the 2 μm wavelength region. The aim of the thesis work is to address these challenges. The work presented in this thesis demonstrates a modulated Thulium-doped ‘all-fibre’ tuneable laser in the 2 μm wavelength region suitable for detection of a number of gases of interest. The scope of work includes the fabrication and optimization of the active fibre with the core composition suitable for the creation of an effective Thulium-doped fibre laser. Codoping of Ytterbium is explored to investigate the energy-transfer mechanism from Ytterbium to Thulium and thereby opening up the opportunity of using economic pump laser diodes emitting at around 0.98 μm. In this respect, both Thulium- and Thulium/Ytterbium-doped single-mode single-clad silica optical fibres are designed and fabricated for a systematic analysis before being used as laser gain media. The optical preforms having different host compositions, Thulium-ion concentrations and proportions of Ytterbium to Thulium are fabricated by using the Modified Chemical Vapour Deposition technique coupled with solution doping to enable the incorporation of rareearth ions into the preforms. A thorough investigation of the basic absorption and emission properties of Thulium-doped silica fibres has been performed. The step-wise energy-transfer parameters in Thulium/Ytterbium-doped silica fibre have been determined quantitatively from spectroscopic measurements along with migrationassisted energy-transfer model. A set of tuneable Thulium-doped ‘all-fibre’ lasers, offering a narrow line-width in the 2 μm wavelength region, is created by using fabricated Thulium-and Thulium/Ytterbium-doped fibres as gain media and fibre Bragg grating pairs under in-band pumping at 1.6 μm and/or pumping by an economical laser diode at 0.98 μm, utilizing Ytterbium to Thulium energy- transfer. The host composition and the dopnat concentration in the single-mode single-clad fibre configuration are optimized to achieve maximum lasing efficiency. The tuning of laser wavelength has been achieved by using relaxation/compression mechanism of the fibre Bragg grating pair used to confine the laser cavity. A new set of laser resonators has also been formed by using a combination of a high reflective fibre Bragg grating with a low reflective broadband mirror, fabricated at the end of the fibre through silver film deposition, to enable only one fibre Bragg grating to be tuned. The stability of the laser output power, line-width and shape have been monitored throughout the tuning range. This is followed by the design of a compact, high-Q, narrow line-width and low threshold microsphere laser resonator, operating in the 2 μm wavelength region, by coupling a Thulium-doped silica microsphere to a tapered fibre. In the microsphere, laser emission occurred at wavelengths over the range from 1.9 to 2.0 μm under excitation at a wavelength of around 1.6 μm. The designed modulated tuneable Thulium-doped ‘all-fibre’ laser, operating at a wavelength range centred at a wavelength of 1.995 μm, has been tested for CO2 gas detection. Both the modulation of the fibre laser, through pump source modulation and the ‘locking’ detection mechanism have been utilized to eliminate laser intensity noise and therefore to obtain a compact gas sensor with high sensitivity. The absorption spectrum, the line-strength and the concentration level of CO2, have been monitored using the absorption spectroscopic technique. The measured minimum detectable concentration of CO2 obtained using the system confirms the claim that it is capable of detecting trace gases at the ppm level. The stable laser performance achieved in the sensor system illustrates its potential for the development of practical, compact yet sensitive fibre laser based gas sensor systems

Gas-phase broadband spectroscopy using active sources: progress, status, and applications

Broadband spectroscopy is an invaluable tool for measuring multiple gas-phase species simultaneously. In this work we review basic techniques, implementations, and current applications for broadband spectroscopy. We discuss components of broadband spectroscopy including light sources, absorption cells, and detection methods and then discuss specific combinations of these components in commonly used techniques. We finish this review by discussing potential future advances in techniques and applications of broadband spectroscopy

Diode laser cavity-based techniques for quantification of trace species in laminar sooting flames

Cavity-enhanced absorption spectroscopy (CEAS) has been applied for the first time to in situ measurements of acetylene in sooting flames using a near-infrared diode laser. This is motivated by the role of acetylene as a major precursor in soot formation and the need for accurate measurements of acetylene to understand soot formation (with the eventual goal of reduced emissions). Vertical profiles of acetylene have been investigated in two flat flame burners with some consideration of the effect of radial profiles on the results. This thesis builds on the novel application of continuous-wave cavity ring-down spectroscopy (cw-CRDS) to in situ flame measurements of acetylene: in this work, refinements are made to the technique, which is developed to the point of generating reliable data in standard flames of interest in studies of soot formation. A key advantage of the cw-CRDS approach is that it is an absolute method, not requiring calibration. Nevertheless, the slow data acquisition led to the consideration of alternative approaches. This resulted in research on CEAS, which forms the bulk of this thesis. The advantages of the CEAS technique over cw-CRDS are highlighted, whilst noting the agreement between the two techniques. The CEAS approach allows spectra to be acquired much more rapidly and with better spectral resolution, as well as having a somewhat simpler experimental set-up. This has enabled the acquisition of a large dataset of broad scans over the full scanning range of the diode lasers employed. This thesis also goes on to explore the application of the CEAS technique to flame measurements of OH radical. The recovered profiles of acetylene concentration show good agreement for both techniques and follow the expected trend of higher acetylene concentration with increasing equivalence ratio. They also show a trend of decreasing acetylene concentration with increasing height above the burner surface. This would be consistent with the consumption of acetylene, including in reactions forming polycyclic aromatic hydrocarbons and ultimately soot. However, this is contrary to model predictions also shown in the thesis.Cavity-enhanced absorption spectroscopy (CEAS) has been applied for the first time to in situ measurements of acetylene in sooting flames using a near-infrared diode laser. This is motivated by the role of acetylene as a major precursor in soot formation and the need for accurate measurements of acetylene to understand soot formation (with the eventual goal of reduced emissions). Vertical profiles of acetylene have been investigated in two flat flame burners with some consideration of the effect of radial profiles on the results. This thesis builds on the novel application of continuous-wave cavity ring-down spectroscopy (cw-CRDS) to in situ flame measurements of acetylene: in this work, refinements are made to the technique, which is developed to the point of generating reliable data in standard flames of interest in studies of soot formation. A key advantage of the cw-CRDS approach is that it is an absolute method, not requiring calibration. Nevertheless, the slow data acquisition led to the consideration of alternative approaches. This resulted in research on CEAS, which forms the bulk of this thesis. The advantages of the CEAS technique over cw-CRDS are highlighted, whilst noting the agreement between the two techniques. The CEAS approach allows spectra to be acquired much more rapidly and with better spectral resolution, as well as having a somewhat simpler experimental set-up. This has enabled the acquisition of a large dataset of broad scans over the full scanning range of the diode lasers employed. This thesis also goes on to explore the application of the CEAS technique to flame measurements of OH radical. The recovered profiles of acetylene concentration show good agreement for both techniques and follow the expected trend of higher acetylene concentration with increasing equivalence ratio. They also show a trend of decreasing acetylene concentration with increasing height above the burner surface. This would be consistent with the consumption of acetylene, including in reactions forming polycyclic aromatic hydrocarbons and ultimately soot. However, this is contrary to model predictions also shown in the thesis

Segmented hollow core photonic crystal fiber as a gas cell

Developing a unique gas cell based on hollow core photonic crystal fiber (HC- PCF) improving its ability to be used in measurements which require higher temporal resolution. Traditional HC-PCF is an excellent candidate for a gas cell due to its extremely small filling volume combined with its small footprint, however a detriment it suffers from is an extraordinarily long fill and evacuation time, a matter of hours for cells surpassing 10 m. The HC-PCF in this work was segmented into smaller lengths, with each segment allowing gas to enter and exit the fiber. By stringing multiple segments of HC-PCF together one can increase to overall length of the gas cell, thus lowering the detection limit of the system, without increasing the fill and evacuation time required to saturate the HC-PCF with gas. By segmenting HC-PCF the fill time can be reduced to a matter of minutes, which makes it an attractive candidate for use in a remote monitoring setting due to the fact that it can operate in the telecommunications band, reducing the cost of implementing such a system, as well as operating at room temperature, removing the need to power an active cooling system