A portable shifted excitation Raman difference spectroscopy system: device and field demonstration

In this paper, we present a portable shifted excitation Raman difference spectroscopy (SERDS) system applied in outdoor experiments. A dual-wavelength diode laser emitting at 785nm is used as excitation light source. The diode laser provides two individually controllable excitation lines at 785nm with a spectral distance of about 10cm(-1) for SERDS. This monolithic light source is implemented into a compact handheld Raman probe. Both components were developed and fabricated in-house. SERDS measurements are performed in an apple orchard, and apples and green apple leafs are used as test samples. For each excitation wavelength, a single Raman spectrum is measured with 50mW at the sample. Strong background interference from ambient daylight and laser-induced fluorescence obscure the Raman signals. SERDS efficiently separates the wanted Raman signals from the disturbing background signals. For the Raman spectroscopic investigations of green leafs, one accumulation with an exposure time of 0.2s was used for each excitation wavelength to avoid detector saturation. An 11-fold improvement of the signal-to-background noise is achieved using SERDS. The results demonstrate the suitability of the portable SERDS system for rapid outdoor Raman investigations. Copyright (c) 2016 John Wiley & Sons, Ltd.