Swept-Source NIR-II Raman Excitation Spectroscopy–a Fast Alternative to Conventional Spontaneous Raman Spectroscopy

Various improved Raman techniques such as coherent, resonant, and surface-enhanced Raman scattering, have been reported to boost Raman signals over spontaneous Raman spectroscopy (RS). However, a high-speed spontaneous RS system remains highly desirable for its simple instrumentation, full-spectrum acquisition, and label-free detection. Recently, swept-source lasers were used to generate Raman excitation spectra utilizing single-pixel detectors with high sensitivity for spontaneous Raman measurements with low laser power. However, the longstanding speed issue with conventional commercial RS remains unaddressed. Herein, we develop a swept-source Raman excitation spectroscopy system for high-speed measurements. Furthermore, we investigate the effects of key factors including detector transimpedance, bandpass filters, and modulated detection on system performance and propose guidelines for optimization. The presented system can capture major Raman peaks from caffeine and pork fat at up to 10 and 30 ms/spectrum, respectively. Moreover, it can capture Raman spectral images from a titanium oxide phantom at 1.5 ms/pixel and take five full spectra from a 30- $\mu $ m polystyrene bead moving at 1 mm/s. High-speed acquisitions also show better stability, signal-to-noise ratio, and dynamic range when averaged compared to conventional measurements at longer integration times. This work establishes the foundation for further development of Raman excitation spectroscopy for demanding applications like label-free flow cytometry.