Improving spatial resolution in non-linear optical diagnostics
Lead Research Organisation:
University of Oxford
Department Name: Engineering Science
Abstract
This project falls within the EPSRC Engineering research area.
The research I am proposing to carry out is concerned with a laser-based temperature diagnostic called LITGS (Laser Induced Thermal Grating Spectroscopy). This technique involves crossing two pulsed laser beams. Within the interference region, the constructive and destructive superposition of light waves from both beams leads to the formation of a fringe pattern. The fringe spacing is determined by the crossing angle and the wavelength of the laser beams used.
Within the high-intensity regions of the fringe pattern (where constructive superposition has taken place), energy from the beams is absorbed and stored in the excited states of the molecules. Subsequent 'quenching' of these molecules results in localised heating and hence a local pressure rise, leading to a density modulation within the interference region called a thermal grating. A third 'probe' laser beam is shot at the thermal grating. A fraction of the intensity of the probe beam is coherently-scattered into a different direction and then measured by a detector. The resulting signal is oscillatory in nature and by measuring this oscillation frequency the average temperature of the gas within the interference region can be determined.
Using optical engines, the LITGS is a non-contact technique can be utilised to measure the evolution of the temperature field within an IC engine during combustion. Since the production of emissions (e.g. NOx gases) is highly temperature dependant, this information will enable the development of low-emission engines. Given the health implications and increasing public concern over these emissions as well as the resultant government legislation regarding them, the use of LITGS has the potential to revolutionise the industry. However, the LITGS technique has one major drawback: poor spatial resolution. The temperature measured is the average within the extended interference volume rather than the temperature of a point. Therefore, for my DPhil, I am hoping to investigate two possible approaches to improve the spatial resolution of this technique, and therefore unlock its potential for use in IC engines.
One approach is to analyse the signals gathered from the scattered probe beam to obtain information about the temperature distribution within the interference region. This will allow more useful data to be obtained without modifying the technique at all.
Another approach is to decrease the size of the interference region set up by the two pulsed laser beams. One way to do this is to increase the crossing angle, but this leads to a decrease in fringe spacing which is undesirable. Instead, it is proposed that a Moiré fringe pattern is used. This is created by using two different fringe patterns, created by using two pairs of pulsed laser beams with different crossing angles. One pattern is then overlaid by the other forming a new Moiré fringe pattern due to the constructive and destructive superposition of the two component fringe patterns. I have already modelled the phenomenon using Matlab during my final year project and found that the Moiré fringe pattern volume (with equivalent fringe spacing) should be significantly smaller than a conventional interference pattern volume. Furthermore I have designed an experiment to create a Moiré fringe pattern and manufactured a holographic sensor to be used to detect the fringe pattern. Therefore, the first part of my DPhil research will be spent making modifications to the holographic sensor and then using it in an experiment to create and map out a Moiré fringe pattern. Following this, I will design, set up and conduct LITGS measurements using a Moiré fringe pattern to demonstrate the power of the technique when resolving temperature within smaller systems.
The research I am proposing to carry out is concerned with a laser-based temperature diagnostic called LITGS (Laser Induced Thermal Grating Spectroscopy). This technique involves crossing two pulsed laser beams. Within the interference region, the constructive and destructive superposition of light waves from both beams leads to the formation of a fringe pattern. The fringe spacing is determined by the crossing angle and the wavelength of the laser beams used.
Within the high-intensity regions of the fringe pattern (where constructive superposition has taken place), energy from the beams is absorbed and stored in the excited states of the molecules. Subsequent 'quenching' of these molecules results in localised heating and hence a local pressure rise, leading to a density modulation within the interference region called a thermal grating. A third 'probe' laser beam is shot at the thermal grating. A fraction of the intensity of the probe beam is coherently-scattered into a different direction and then measured by a detector. The resulting signal is oscillatory in nature and by measuring this oscillation frequency the average temperature of the gas within the interference region can be determined.
Using optical engines, the LITGS is a non-contact technique can be utilised to measure the evolution of the temperature field within an IC engine during combustion. Since the production of emissions (e.g. NOx gases) is highly temperature dependant, this information will enable the development of low-emission engines. Given the health implications and increasing public concern over these emissions as well as the resultant government legislation regarding them, the use of LITGS has the potential to revolutionise the industry. However, the LITGS technique has one major drawback: poor spatial resolution. The temperature measured is the average within the extended interference volume rather than the temperature of a point. Therefore, for my DPhil, I am hoping to investigate two possible approaches to improve the spatial resolution of this technique, and therefore unlock its potential for use in IC engines.
One approach is to analyse the signals gathered from the scattered probe beam to obtain information about the temperature distribution within the interference region. This will allow more useful data to be obtained without modifying the technique at all.
Another approach is to decrease the size of the interference region set up by the two pulsed laser beams. One way to do this is to increase the crossing angle, but this leads to a decrease in fringe spacing which is undesirable. Instead, it is proposed that a Moiré fringe pattern is used. This is created by using two different fringe patterns, created by using two pairs of pulsed laser beams with different crossing angles. One pattern is then overlaid by the other forming a new Moiré fringe pattern due to the constructive and destructive superposition of the two component fringe patterns. I have already modelled the phenomenon using Matlab during my final year project and found that the Moiré fringe pattern volume (with equivalent fringe spacing) should be significantly smaller than a conventional interference pattern volume. Furthermore I have designed an experiment to create a Moiré fringe pattern and manufactured a holographic sensor to be used to detect the fringe pattern. Therefore, the first part of my DPhil research will be spent making modifications to the holographic sensor and then using it in an experiment to create and map out a Moiré fringe pattern. Following this, I will design, set up and conduct LITGS measurements using a Moiré fringe pattern to demonstrate the power of the technique when resolving temperature within smaller systems.
Organisations
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509711/1 | 01/10/2016 | 30/09/2021 | |||
| 1798307 | Studentship | EP/N509711/1 | 01/10/2016 | 01/03/2021 | Priyav Shah |
| Description | As explained in my research proposal, the aim of my research is to increase the resolution of a laser-based temperature diagnostic called LITGS (Laser Induced Thermal Grating Spectroscopy). When two laser beams are crossed together, a fringe pattern is formed within the region where both beams overlap ('interaction region'). In the presence of a suitable gas (based on the wavelength of the laser beams used), a third beam, when be directed at this interaction region, will be scattered off this region to produce a signal beam. This signal is then analysed to determine the temperature of the gas. The spatial resolution of this technique is limited by the size of the interaction region. I had proposed overlapping four laser beams to create a 'Moiré fringe pattern' which would be smaller in size than the standard one created by two beams. During the initial stages of my research, I was able to experimentally prove that this was indeed the case, and preliminary results suggested a decrease in size of about 90%. During my PhD, I have also investigated another approach to increasing the spatial resolution of LITGS. This is by splitting up the signal beam into different parts, and mapping each part to where it originated within the interaction region. By doing this, we can essentially split up the interaction region into smaller sections, and measure the temperature within each section simultaneously. At the International Symposium on Combustion I presented results proving that the signal beam can indeed be split up and mapped back to parts of the interaction region. Since then I have successfully implemented this concept in a LITGS experiment. |
| Exploitation Route | I intend to build upon my findings by independently implementing both methods to common combustion problems in order to characterise each solution. Following this, my findings may be put to use in any application where a spatially resolved, non-intrusive temperature measurement is required - for example, within an optical engine. |
| Sectors | Aerospace, Defence and Marine,Energy,Environment,Pharmaceuticals and Medical Biotechnology,Transport |