Diode laser spectroscopy in extended wavelength ranges

Abstract: Diode laser spectroscopy performed in the ultraviolet, visible and infrared spectral regions is presented. The accessible wavelength range for visible and near-infrared diode lasers is extended by the use of sum- and difference-frequency generation. Sum-frequency generation to the ultraviolet spectral region was employed using a blue and a red diode laser. Mercury spectroscopy was demonstrated with the generated ultraviolet beam. Difference-frequency generation with two near-infrared diode lasers has been utilised to extend the wavelength range to the mid-infrared spectral region. With each of the three wavelengths resonant with a molecular species, detection of molecular oxygen, water vapour, and methane was achieved simultaneously. By employing frequency modulation techniques, detection limits are improved by orders of magnitude compared to direct absorption measurements. Frequency modulated absorption spectroscopy in the red spectral region was performed for sensitive long-path absorption monitoring of nitrogen dioxide. The possibility of employing frequency modulation techniques for sensitive absorption measurements in the blue and ultraviolet spectral region has also been investigated. Atomic spectroscopy in the blue and red spectral region has been demonstrated, providing a pedagogical demonstration of the theory for Doppler broadening. A new instrument, using a coupled cavity diode laser, has been constructed and employed for aerosol particle sizing and identification. The size of a single aerosol particle is determined by an extinction loss signal and identification is performed using a diffraction image. A compact fluorosensor utilising a violet diode laser as an excitation source combined with an integrated spectrometer and fibre-optic sampling for the monitoring of fluorescence signatures was developed. The potential of this new instrument was demonstrated in measurements on vegetation and human malignant skin lesions.