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Investigations On The Propagation Properties And Nonlinear Spectroscopy Of Ultrafast And Ultrastrong Laser In Combustion Fields

Posted on:2017-01-26Degree:DoctorType:Dissertation
Country:ChinaCandidate:H L LiFull Text:PDF
GTID:1108330482991883Subject:Microelectronics and Solid State Electronics
Abstract/Summary:PDF Full Text Request
Combustion diagnostics is significant for understanding combustion reaction dynamics and designing efficient and low-pollution combustion systems. Optical methods, especially the laser-based spectroscopic techniques that can provide real-time, high-sensitivity and noninvasive detection, have been extensively employed for mapping the spatial distribution of combustion intermediates. However, most of laser spectroscopic techniques can only probe one species at a time owing to the limitation that the laser is required to be resonant with the detected species. In this thesis, a novel analysis technique based on femtosecond laser filaments have been introduced in applications to the combustion field for combustion diagnosis. Femtosecond laser filamentation is regarded as a dynamic balance between the Optical Kerr self-focusing and the defocusing effect induced by the self-generated weak plasma. The laser intensity in the filament core is clamped to 1013~1014 W/cm2, which is high enough to excite the molecules to highly excited states by multiphoton excitation, resulting in the “fingerprint” fluorescence. In principle, multiple molecular species can be simultaneously sensed by only using one laser system. The main contents of the thesis are listed as below:1. We have studied the feasibility of femtosecond laser filament in application to combustion diagnostics and experimentally demonstrated simultaneous sensing of multiple intermediate species by femtosecond filament-induced nonlinear spectroscopy(FINS). The “fingerprint” fluorescence from the intermediates of an ethanol-air flame induced by femtosecond laser filament excitation was measured at a right angle to the laser propagation direction. It was found that multiple intermediates, such as the free radicals C2, CH, OH, NH, CN, N2, and the atomic species C and H, can be simultaneously probed. In addition, the FINS spectra was measured at different positions along the central vertical axis of the flame. Analysis of the spectra revealed the fluorescence intensity of the intermediates increases firstly and then decreases when the distance between the filament and the burner wick is increased, which is consistent with the combustion property of a diffusion flame.2. We have experimentally determined the two fundamental physical parameters of femtosecond laser filamentation in the ethanol-air flame, i.e., critical power and clamping intensity. By measuring the dependence of the focal position of a femtosecond laser in the flame on the input laser power, the critical power for self-focusing of a 800 nm and 35 fs Ti: Sapphire laser in an ethanol-air flame is determined, which is about a quarter of the usually quoted value in air. To measure the clamping intensity, a piece of metal foil was inserted into a flame and the pinhole was drilled on the foil by a single filament formed in the flame. By measuring the size of the pinhole drilled by the filament and the transmitted laser energy through the pinhole, the clamping intensity of the filament inside the flame was determined, which is about half of the value in air.3. We have observed amplified spontaneous emission(ASE) induced by femtosecond laser filament in an ethanol-air flame. By measuring the FINS spectrum in the backward direction of the laser propagation, it is found that the intensity of the spectral band at 388 nm from the 2 2B X? ?? ? ? transition of the intermediate CN increases exponentially with the increase of the laser pulse energy, but that of the spectral band at 474 nm from the Swan band of the C2 radical increases linearly as the laser pulse energy increases. The exponential growth behavior of the emissions from the CN molecule can also be observed as the interaction length of the filament with the flame was changed longer, and the laser pulse energy was fixed.4. We have also studied the mechanism of the fluorescence emissions induced by femtosecond filament in a flame. Both the spectrum of a laminar ethanol-air flame induced by nanosecond laser-induced breakdown and the emission spectrum of the flame itself without the laser excitation were measured. By comparing the difference between these two spectra and the FINS spectrum, it was found that the fluorescence induced by femtosecond laser filament in the flame mainly comes from the multiphoton excitation by the filament, not from the photodissociation of parent molecules.In summary, we have studied the nonlinear propagation properties of femtosecond laser in flames and proposed a novel technique for combustion diagnostics. We found that the spectra of the combustion field induced by the FINS technique consist of rich information of combustion intermediates, indicating the feasibility of the FINS technique in application to combustion diagnostics. The two fundamental physical parameters of femtosecond laser filament in a flame, i.e., critical power and clamping intensity, have been determined, providing insights into understanding the nonlinear interaction between the femtosecond laser filament and the combustion intermediates. When measuring the fluorescence induced by femtosecond laser filament from the backward direction, amplified spontaneous emission from the CN molecule has been observed, which benefits overcoming the quenching effect in practical combustion environment. By comparing the spectra of the flames obtained from nanosecond laser-induced breakdown spectroscopy, emission spectroscopy, and FINS, the fluorescence of the flame induced by the FINS technique was ascribed to the filament excitation. The studies in the thesis provide insights of femtosecond laser filament in application to combustion fields and opens up a new way for combustion diagnostics by using the FINS technique.
Keywords/Search Tags:Femtosecond laser filament, Filament-induced fluorescence, Combustion diagnostics, Critical power, Clamping intensity, Photodissociation, Amplified spontaneous emission
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