Free-Form Optics Enhanced Confocal Raman Spectroscopy for Optofluidic Lab-on-Chips

We present an optofluidic lab-on-chip for confocal Raman spectroscopy, which can be used for the analysis of substances. The device strongly suppresses unwanted background signals because it enables confocal detection of Raman scattering thanks to a free-form reflector embedded in the optofluidic chip. We design the system using non-sequential ray-tracing combined with a mathematical code to simulate the Raman scattering behavior of the substance under test. We prototype the device in polymethyl methacrylate by means of ultraprecision diamond tooling. In a proof-of-concept demonstration, we first show the confocal behavior of our Raman lab-on-chip system by measuring the Raman spectrum of ethanol. In a next step, we compare Raman spectra measured in our lab-on-chip with spectra measured with a commercial Raman spectrometer. Finally, to calibrate the system we perform Raman measurements on urea solutions with different concentrations with our proposed experimental proof-of-concept setup. We achieved a detection limit that corresponds to the noise equivalent concentration of 20 mM.