Fibre-Based Fast T-ray Tomography

Lead Research Organisation: University of Manchester
Department Name: Electrical and Electronic Engineering

Abstract

The aim of this project is to demonstrate THz tomography, operating at millisecond image frame rates. We propose to design, build and characterise a fast multi-channel THz tomography system without moving parts, utilising a new THz source based on short-pulse fibre lasers driven by high-power laser-diodes. The output of the fibre lasers will be combined to produce THz emission by difference-frequency generation. The character of the THz emission from the proposed source allows inexpensive pyroelectric detector arrays to be used. A large number of THz measurement channels will be realised as a parallel and re-configurable data acquisition and processing electronic system. The breakthrough in speed and facility of THz tomography will be achieved via novel solutions in: fibre lasers; THz optical train (difference-frequency generation, beam conditioning, detector geometry); signal acquisition and processing; methods for image reconstruction. The proposed technology will also open the way to fast benchtop THz tomographs with applications in a wide area of science and engineering.We propose a demonstrator experiment which will employ this novel THz tomography system to map major species' concentrations and temperature field distributions in high-pressure flames. These flames are opaque to the usual IR analysis techniques because of scattering from the soot particles, but have been shown by the applicants to be transparent to THz radiation. In addition, many of the species in these flames exhibit absorption resonances in the THz region. THz tomography therefore promises to be the only way to study high pressure flames. This has important implications in our understanding of the combustion processes in, for example, aero engines with the possibility of improving efficiency and reducing emissions of greenhouse gases. We'll demonstrate at least the following performance in the case of high-pressure flames in the presence of soot: image frame acquisition period <1ms; maps of the cross-sectional distribution of major species with spatial resolution (in area) of D2/64; maps of distribution of temperature (T~1000K) of water molecules. This project is a direct consequence of the THz Basic Technology work funded by RCUK and combines established expertise in tomography (Manchester), THz technology (Leeds) and novel laser sources (Southampton). The synergy between the currently running Basic Technology THz work and this project will allow the identification of the spectral behaviour of the chemical species in the flame under realistic conditions and indicate the best choice of spectral lines for temperature mapping. The project is a crucial step towards THz tomography for imaging of gas mixture systems and a variety of other objects, with expected strong impact on the future utility and affordability of the THz technology.

Publications

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Efthymiou S (2013) Pulse Detection by Gated Synchronous Demodulation in IEEE Sensors Journal

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Efthymiou S (2009) Pulsed performance of pyroelectric detectors in Journal of Physics: Conference Series

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Malinowski A (2012) Fiber MOPA based tunable source for terahertz spectroscopy in Laser Physics Letters

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Ozanyan K (2011) Hard-Field THz Tomography in IEEE Sensors Journal

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York T (2011) Agile Sensing Systems for Tomography in IEEE Sensors Journal

 
Description The system is based around a new high-power laser-diode driven fibre-laser THz source, using difference-frequency generation, and pyroelectric detector arrays. The THz measurement channels were realised as a data acquisition and electronic processing system, capable of multi-channel (32channels) THz tomography, to map major species concentrations and temperature field distributions in high-pressure flames operating under sooting conditions. These flames are opaque to the usual IR analysis techniques because of scattering from the soot particles, but we had shown, before the start of the project, that they were transparent to THz radiation. In addition, many of the species in these flames exhibit absorption resonances in the THz region.

The following objectives were achieved in full:

i) A novel THz source based on Fibre Lasers and was designed, built and tested. The THz emission is generated as the frequency difference between two seeding infrared lasers.

ii) THz spectral lines, suitable for water absorption THz temperature tomography were identified and operation of the THz source at these wavelengths was demonstrated. This allows the exact wavelength positioning on the chosen absorption line.

iii) A single line-of-sight (LoS)THz transmission measurement channel utilising the fibre-laser source and room-temperature pyroelectric detectors was designed, built and tested.

iv) The LoS channel was operated with fast switching between two seeds for THz mixing, demonstrating the potential for time-domain multiplexed multi-line THz measurements.

v) A high-pressure burner was designed, built, tested, certified and operated for the purpose of THz spectroscopy and tomography.

vi) In-flame THz time-domain spectroscopy (TDS) of water absorption lines under sooty conditions was demonstrated in the high-pressure burner and the influence of various co-flow conditions was investigated.
Exploitation Route THz tomography in flames has important implications in our understanding of the combustion processes in, for example, aero engines offering the possibility of improving efficiency and reducing the emission of greenhouse gases. The developed technology adds substantially to the development of fast benchtop THz spectroscopy and tomography systems with applications across a wide area of science and engineering. Possible exploitation routes would be through KTA funding or specific government and industrial calls for knowledge transfer
Sectors Chemicals,Electronics,Environment,Healthcare,Pharmaceuticals and Medical Biotechnology

 
Description A proposal was submitted to Home Office for the detection of hazardous substances. The Hard-Filed THz Tomography paper opened a way towards THz time-domain and THz amplitude Tomography. Further projects employed this approach and resulted in definitions of HF THz amplitude tomography (paper accepted at IEEE SENSORS 2014 Conf. and a common project with GSK on de-mineralisation of teeth is currently in place, funded by a CASE studentship)
First Year Of Impact 2012
Sector Healthcare,Pharmaceuticals and Medical Biotechnology,Security and Diplomacy,Transport
Impact Types Economic

 
Title GQSD (Gated Quadrature Synchronous Demodulation) 
Description Gated Quadrature Synchronous Detection (GQSD) is a new signal processing method for pulse signal detection, which combines the principles of "gating" and "synchronous demodulation" that outperforms the existing ones. For low duty cycle pulse trains, the sensitivity and efficiency of CQSD is greatly reduced because of the increased energy spread across the spectrum, leading to deterioration in the SNR. SMGI is the method of choice for pulsed signals, but does not offer immunity against 1/f noise and, when the latter is overwhelming, CQSD becomes the better choice. GQSD is efficient against 1/f noise and hence improves the output SNR. It incorporates a gating function to achieve signal conditioning suitable for synchronous detection, while the QSD function ensures maximum suppression of flicker noise. An advantage of applying GQSD with low bandwidth devices, such as a PED, will be that it allows a nominally slow detector to be exploited at higher than usual frequencies. This is possible, since gained reserve in SNR can be traded against sensitivity, e.g. by reducing the electrical time constant. 
IP Reference  
Protection Copyrighted (e.g. software)
Year Protection Granted
Licensed No
Impact Interest has been stated by Ametek UK. The invention is published in http://www.click2go.umip.com/i/software/gqsd.html for wider dissemination