The thesis work concerns development and application of four versatile nonlinear optical techniques, based on exploiting ultrashort laser pulses, for diagnostic purposes in gases and flames. The four techniques, all laser-based, are two-photon laser-induced fluorescence (TPLIF), hybrid femtosecond/nanosecond (fs/ns) rotational coherent anti-Stokes Raman scattering (fs/ns RCARS), fs-laser-induced grating spectroscopy (fs-LIGS), and backward lasing. Special characteristics of fs-laser pulses, such as short pulse duration, high peak power even at low pulse energy, and broad spectral bandwidth, advance the development of these techniques to a new level.
In the TPLIF project, two-photon excited fluorescence of CO in CH_4/air flames was investigated and compared for ns, picosecond (ps), and fs-laser excitation. Moreover, based on fs-laser excitation, simultaneous interference-free two-photon excited fluorescence imaging of H and O atoms in turbulent flames was performed for the first time. In comparison with previous studies, it has been demonstrated that significantly larger measurement areas can be visualized in single-shot acquisitions.
A concept for lasing in the backward direction, facilitated by fs-laser excitation, was developed and demonstrated for range-resolved detection of H atoms in flames. The technique shows great potential for stand-off measurements in devices with only one optical access. On the fundamental level, studies were performed to uncover the physical mechanism responsible for the lasing effect.
With the developed hybrid fs/ns RCARS technique, the RCARS signal can be recorded with a high spectral and temporal resolution, simultaneously, allowing Raman linewidths to be measured on a single-shot basis. This capacity is of great importance for thermometry as it, in principle, eliminates the need for pre-knowledge about the chemical composition and availability of simulated linewidths. With increased fs-laser pulse energy, additional lines in the spectrum were observed due to Stark splitting.
The pioneering work on fs-LIGS was performed in heated flows of N_2 gas with temperatures varying from room temperature to 750 K. The thermal grating was formed by resonant multi-photon absorption, based on 800-nm fs pulses, and the generated LIGS signals were detected time-resolved in single-shot acquisitions. The results show that the method works very well for single-shot thermometry in nitrogen, with a measurement uncertainty of ± 1 K for room temperatures and ± 14 K for 600 K, as an example. Increasing the laser pump energy above a certain threshold causes ionization and generates a plasma density grating.
It is my hope and belief that the developed laser-based diagnostic techniques can contribute important tools for the larger research community in thermal energy conversion, and support the urgent transition to a sustainable energy system.
- Bood, Joakim, Supervisor
- Aldén, Marcus, Assistant supervisor
|Award date||2021 Dec 10|
|Place of Publication||Lund University|
|ISBN (electronic) ||978-91-8039-076-7|
|Publication status||Published - 2021 Nov 16|
Place: Lecture Hall Rydbergsalen, Department of Physics, Professorsgatan 1, Faculty of Engineering LTH, Lund University, Lund. Zoom: https://lu-se.zoom.us/j/61138635115?pwd=cWxrZlg4MHFpQ2Q5a2QzQm1rdHVOZz09 Passcode: 525570
Webinar ID: 611 3863 5115 Passcode: 525570
Name: Ewart, Paul
Affiliation: University of Oxford, United Kingdom.
- Atom and Molecular Physics and Optics
- laser diagnostics
- femtosecond laser
- nonlinear interactions
- ultrafast spectroscopy
- multi-photon excitation
- laser-induced fluorescence
- rotational coherent anti-Stokes Raman spectroscopy
- laser-induced grating spectroscopy
- backward lasing technique