Optical CCD lock-in device for differential Raman spectroscopy

In Raman spectroscopy variations in temperature or a frequency shift of the excitation laser may cause an unwanted shift of the measured spectra. Measurement errors can be avoided if the spectra of two or more different probes are recorded on the same CCD chip in the same Raman experiment. For this purpose we developed an optical lock-in based on a common liquid nitrogen-cooled high-performance spectroscopic CCD camera from Princeton Instruments/Acton. Our recording scheme requires the periodic shifting of the collected charges on the CCD and of the different probes in a perfectly synchronized manner with respect to time and space. The proof of concept is done with a FPGA-based electronic board. It performs the timing of the CCD, laser control, and a synchronized rotation of the cell. The rotating cell carries two probes to perform true on-the-fly differential Raman spectroscopy. The hardware design concept is discussed and experimental results of alcohol-water spectral test measurements are presented. Due to the specific properties of the setup it is possible to determine the peak position of two distinct samples with sub-pixel precision (better than 5%, 90% confidence interval).