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Absorption spectrsocopy and hyperspectral tomography based on coherent supercontinuum source

Final Report Summary - ASHTCSCS (Absorption spectrsocopy and hyperspectral tomography based on coherent supercontinuum source)

1. Objectives
This project aims to develop a versatile chemical sensing system based on a novel coherent supercontinuum (SC) laser source and absorption spectroscopy. The system is designed to possess two operational modes, which are the line-of-sight-averaged and tomographic modes to satisfy various application needs. The line-of-sight-averaged mode is more suitable for situations where the temporal resolution is the first priority, for example, a rapid changing turbulent flame with insignificant non-uniformities in temperature and concentration distributions of multiple chemical species. On the other hand, the tomographic mode is targeted to maximize the spatial resolution and is a better option for scenarios where non-uniformities cannot be neglected such as steady laminar flames with large gradients in temperature and chemical species distributions.
2. Work performed so far
Since the beginning of the project, both computational and experimental works have been done. The major works done can be summarized as follows
a) A new tomographic method was proposed with supercontinuum laser source and can simultaneously image the temperature, species concentration, and pressure for the first time using absorption spectroscopy.
b) A new tomographic algorithm was proposed with multiple tunable semiconductor lasers using frequency domain multiplexing and calibration-free wavelength modulation spectroscopy.
c) A new imaging theory, the so-called nonlinear tomography was proposed which extends the definitions of traditional tomography and can provide new mathematical frameworks for the development of new tomographic modalities using the recent advances in laser absorption spectroscopy.
d) An experimental demonstration of tomographic absorption spectroscopy was realized using a supercontinuum radiation source.
e) A new tomographic method was proposed using just one single tunable diode laser with multiple orders of harmonic detections using wavelength modulation spectroscopy.
f) A numerical investigation of high-resolution multispectral absorption tomography was performed for flow thermometry.
3. Main results achieved so far
Since the beginning of the project, fruitful results have been delivered. The major results are summarized as follows
a) A new tomographic method was numerically demonstrated that can simultaneously recover the spatially-resolved information of temperature, species concentration, and pressure for the first time using absorption spectroscopy based on dispersed coherent supercontinuum radiation. The temporal resolution can be as high as MHz, which is critical to resolve the flow fields of supersonic/hypersonic flows. The work was performed in the University of Cambridge. A relevant journal paper was published in ‘Applied Physics Letters’. This work was also presented in two conferences, which are ‘OSA Conference of Optics and Photonics for Energy and Environment (E2) 2013’ and ‘OSA Conference of Laser Applications to Chemical, Security and Environmental Analysis (LACSEA) 2014’, respectively.
b) A new tomographic algorithm was numerically demonstrated that can simultaneously recover the 2D profiles of temperature and species concentration using calibration-free wavelength modulation spectroscopy with 1f-normalized, 2f signal detection. Compared with the previous work, this technique is more suitable for harsher experimental conditions such as high pressure or sooting flames. Also, it is more cost-effective since only tunable semiconductor lasers are required. The work was performed in the University of Cambridge. A relevant journal paper was published in ‘Applied Physics Letters’. This work was also presented in ‘OSA Conference of Laser Applications to Chemical, Security and Environmental Analysis (LACSEA) 2014’.
c) The theory of nonlinear tomography was established and was used as the framework for the development of new modalities of tomography. This theory essentially extends the definition of tomography and makes the combination of the recently developed spectroscopic techniques with tomography possible. The work was performed in the University of Cambridge. The theory was broadcast in a few invited talks such as in the ‘OSA Conference of Laser Applications to Chemical, Security and Environmental Analysis (LACSEA) 2014’ and also the award ceremony of the ‘Masao Horiba Award’ in the Kyoto University.
d) A laser spectroscopic system was built with a supercontinuum radiation source to perform both line-of-sight and tomographic measurements of a flame. The measurements were taken from a Hencken burner. The work was performed in the University of Nuremburg-Erlangen, Germany.
e) Some preliminary numerical results were obtained for a new tomographic method, the so-called multi-harmonic absorption tomography, using just one single tunable diode laser with multiple orders of harmonic detections using wavelength modulation spectroscopy. The technique is the most cost-effective one since only one single inexpensive tunable diode laser is required for tomographic measurements. The work was performed in the University of Cambridge.
f) A systematic numerical investigation of multispectral absorption tomography with a high grid resolution was performed. The relationship between the reconstruction fidelity and the grid resolution was obtained; and a record high grid resolution was demonstrated. A relevant journal paper was published in ‘Applied Physics B: Lasers and Optics’. This work was also presented in ‘Field Laser Applications in Industry and Research (FLARE) 2014’. The work was performed in the University of Cambridge.
The experimental data taken from the University of Nuremburg-Erlangen, Germany will be processed and is expected to be submitted to ‘Optics Express’; and the numerical results of the multi-harmonic absorption tomography is expected to be submitted to ‘Applied Physics Letters’; and An invited review article is in progress and is expected to be submitted to ‘Progress in Energy and Combustion Science’, which is the best journal for combustion research.
4. Conclusion and impact
To summarize, this project had led to fruitful results as can be demonstrated by the long publication list. In addition, it has to be noted that the laser analytical methods that has been developed are of practical use and not only limited to applications to engine measurement, but rather it can be commercialized into portable products which can be taken advantage of to perform in situ online monitoring of industrial flames and provide feedback information for optimized process controls. To cite a positive example, Dr. Xin Zhou and Dr. Jian Wang who graduated from Dr. Ronald Hanson’s group (Stanford University) started their company in China which produce gas sensors and industrial control systems based on Tunable Diode Laser Absorption Spectorscopy (TDLAS). Currently, their company have grown into a large listed company and the growing tendency is expected to continue. The fruits of the current project can also be packaged into promising product of industrial interests.