Application of cheap lasers in shifted excitation Raman difference spectroscopy

Raman spectroscopy has increasing importance in a wide field of applications: particularly in real time monitoring of chemical processes, testing of foodstuffs, identification of ingredients in unknown material mixtures etc. Many materials of interest have resonance wavelength close to the excitation wavelength. Resonant Raman spectroscopy can be used to advantage in these cases. The disadvantage of this technology is the presence of a strong fluorescence background in the Raman spectrum. A combination of the mechanism of resonant Raman spectroscopy with shifted excitation Raman difference spectroscopy can be used to suppress the fluorescence background. The applicability of inexpensive green lasers for this purpose and their tunability by temperature and current is investigated in this paper. The setup consists of two pigtailed lasers at a wavelength of 532 nm with a small wavelength difference switched by a fiber switch with a frequency up to 50 Hz. Every switching pulse triggers an optical spectrometer to measure the backscattered light. A resonant Raman spectrum with a minimized fluorescence background is obtained by subtraction of the two different spectra. The specific wavelengths of the two lasers were set by thermal tuning. The Raman spectra of Isopropanol and Carbon Tetrachloride have been measured in order to verify the setup.