Optical measurements are increasingly used to monitor, for example, human physiology, consumables, drugs, cosmetics, plants, soil, water, air, and industrial processes. The development of new types of light sources plays a key role in this, and during the last few decades, it has especially been lasers that have transformed our understanding of what can be done with light.
However, lasers are monochromatic, which limits their use in spectroscopy. Because there is only one wavelength available, only one molecular species can generally be measured at a time, for example the concentration of carbon dioxide. Supercontinuum lasers are new types of light sources that combine the characteristics of lasers with a broad spectrum.
“You could say that these are pretty much the ultimate light sources for spectroscopy. Due to a wide range of wavelenghts available, we can measure multiple chemical species at once”, Aalto says.
Medicine is one of the potential fields of application for this technology. The exhaled air carries out the metabolites of various cells, whether they belong to healthy tissue, tumors, or microbes. Detecting multiple different molecular species with very high sensitivity could facilitate early detection of various cancers or coronavirus infections, for example.
“We can use two high quality mirrors to bounce laser light back and forth, to increase the interaction length between light and the gas sample to tens of kilometers. This makes it possible to have extremely sensitive measurements”, Aalto explains.
A record sensitivity for one of these types of techniques was achieved in this work. However, the possible uses of supercontinuum sources are not limited to laboratories. As a second application, a laser radar (lidar) for monitoring the internal temperature of a power plant combustion chamber was developed.
“Having a real-time 3D map of temperature inside the combustion chamber would be very useful for process control, especially when using inhomogeneous fuel, such as waste, which requires continuous optimization of the feed”, Aalto specifies.
In this thesis work, a prototype supercontinuum lidar was built and demonstrated for remote thermometry. The final product could be used to optimize combustion processes, allowing more energy to be generated from the same amount of fuel with less harmful emissions. The research on the supercontinuum lidar continues in the Photonics laboratory.
The doctoral dissertation of M.Sc. (Tech) Antti Aalto in the field of physics titled Advanced Optical Techniques for Gas Sensing Using Supercontinuum Sources will be publicly examined in the Faculty of Engineering and Natural Sciences at Tampere University on Friday 6 November 2020 at 12:15 PM in Tietotalo building auditorium TB109, Korkeakoulunkatu 1, Tampere. The Opponent will be Professor Zhipei Sun from Aalto University via remote connection. The Custos will be Professor Juha Toivonen from the Faculty of Engineering and Natural Sciences.
The event can be followed via remote connection.
The dissertation is available online at http://urn.fi/URN:ISBN:978-952-03-1743-0