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Diode laser based light sources for shifted excitation resonance Raman difference spectroscopy in the spectral range between 450 nm and 532 nm

Exciting the Raman effect at a wavelength in resonance with the absorption spectrum of the sample, typically in the visible spectral range, can increase the strength of Raman lines by orders of magnitude. Particularly in this case, the lines can be obscured by fluorescence but also by background light. Shifted excitation Raman difference spectroscopy (SERDS) can recover Raman signatures. This method uses an excitation light source with alternating operation of two neighboring, spectrally stabilized, narrow emission wavelengths. Only the Raman lines follow that change in the excitation wavelength and can be separated from the background. Up to now, internally wavelength stabilized dual wavelength diode lasers for the blue and green spectral range are unavailable. Other concepts, as presented in this work, had to be evaluated. First, the combination of two external cavity stabilized GaN diode lasers will be presented. Low reflection coated laser diodes are externally wavelength stabilized using VBGs and their beams are superimposed. Output powers in the 10- mW range for emission wavelengths of 454 nm and 456 nm will be presented. Second, devices based on frequency doubled GaAs diode lasers will be tested. The wavelength shift is realized by thermal tuning of the heat sink or by applying a current to internal heater elements in the GaAs-DBR-RW-lasers. In this case output powers, up to 50 mW at 488 nm or 515 nm were achieved. Third, dual wavelength Y-branch diode lasers at 1064 nm were frequency converted towards 532 nm with output powers in the 10-mW range using customized SHG waveguide crystals.