Quantitative laser-induced fluorescence: Some recent developments in combustion diagnostics

Generation of high-energy, kilohertz-rate narrowband tunable ultraviolet pulses using a burst-mode dye laser system

Typical commercial pulsed dye laser systems used in the generation of narrowband, tunable ultraviolet radiation for planar laser-induced fluorescence (PLIF) imaging are optimized for either high (∼5-10  kHz) repetition rates at comparatively low ultraviolet pulse energies (hundreds of microjoules) or high-output pulse energies (>10  mJ) at comparatively low repetition rates (∼10  Hz). In this work we use a frequency-doubled Nd:YAG burst-mode laser to pump a custom dye laser system for high pulse energies and repetition rates of 7.5, 10, and 20 kHz at 566 nm. The frequency-doubled output of over 2.2 mJ/pulse at 283 nm, which can be used for PLIF imaging of combustion radicals, is an order of magnitude higher per pulse energy as compared with continuously pulsed dye laser systems and is ∼3× higher in overall efficiency than a burst-mode optical parametric oscillator at similar wavelengths. The influence of repetition rate, pump energy, and dye concentration on the output conversion efficiency and pulse-to-pulse stability of the current system is discussed.

A LabVIEW-Based Virtual Instrument System for Laser-Induced Fluorescence Spectroscopy

We report the design and operation of a Virtual Instrument (VI) system based on LabVIEW 2009 for laser-induced fluorescence experiments. This system achieves synchronous control of equipment and acquisition of real-time fluorescence data communicating with a single computer via GPIB, USB, RS232, and parallel ports. The reported VI system can also accomplish data display, saving, and analysis, and printing the results. The VI system performs sequences of operations automatically, and this system has been successfully applied to obtain the excitation and dispersion spectra of α-methylnaphthalene. The reported VI system opens up new possibilities for researchers and increases the efficiency and precision of experiments. The design and operation of the VI system are described in detail in this paper, and the advantages that this system can provide are highlighted.

Degenerate four-wave mixing from laser-populated excited states

Degenerate four-wave mixing (DFWM) from laser-populated excited states, i.e., two-step DFWM (2S-DFWM), has been performed to investigate the possibility of increasing signal quality (i.e., strength or signal-to-noise ratio) when species with low transition probabilities or far-UV transitions are to be detected or when large beam areas are used. Gold atoms, aspirated into an air-acetylene flame, were chosen as a suitable species for this investigation. The 2S-DFWM signal strength was found to be comparable to the ordinary (one-step) DFWM signals for moderately high UV-light intensities but substantially better for low UV-light intensities. This finding implies that DFWM detection of species with lower transition probabilities in the first step as compared with gold (<10(-8) s(-1)) can benefit from the 2S-DFWM technique when moderate or low UV-light intensities are available. Additional possible advantages of using 2S-DFWM are also discussed.