Towards a miniaturized application-specific Raman spectrometer

Raman spectroscopy is commonly used in analytical chemistry for identification of molecules based on the analysis of spectral lines in the Raman spectrum corresponding to molecular vibrations. For solids and gases, usually high spectral resolution is required. For many other applications, however, a high spectral resolution is less critical, while compactness and reduced system cost are increasingly important. Several studies have already shown the applicability of low resolution Raman spectroscopy (LRRS) systems to a wide variety of fields, such as in situ monitoring of process control, on-site detection of illicit drugs and explosives, on-site detection of water contamination, and safety analysis of edible oil. In this work, we illustrate how imec's CMOS based hyperspectral (HS) filter technology can be used to build a very compact, low cost, mass-manufacturable Raman spectrometer where spectral resolution can be traded for increased sensitivity, hence better SNR and/or shorter acquisition time. Furthermore, unlike other types of Raman spectrometers, the proposed system can be tailored to the application by only targeting specific Raman bands, e.g., 400-1500cm(-1) and 3000-4000cm(-1), by selecting the right set of HS filters for a given wavelength. This enables further improvements in system performance, or even use for Raman imaging. The HS filter technology is currently already used in various commercially available HS cameras for imaging applications in the VIS-NIR (470-900nm), NIR (600-970nm) and SWIR range (1100-1700nm) to study the reflectance or transmission spectra of the imaged targets. Proof-of-concept Raman measurements have been successfully performed in a laboratory setup where the impact of using different HS filter selections or layouts and different cooled and uncooled cameras can be tested. Furthermore, the measurements closely match the predicted spectra obtained with our in-house developed HS-based Raman spectrometer simulation software, which can be used for design space exploration to optimize each of the system components (optical module, rejection filters, HS filters and image sensor) for the target application.