Novel narrow linewidth 785 nm diode laser with enhanced spectral purity facilitates low-frequency Raman spectroscopy

摘要

Raman Spectroscopy enables fast, sensitive and label-free chemical analysis of a large range of materials and has becomea routine analytical tool in a wide range of material science and process-control applications.As the Raman signal is weak it is critical that the illumination laser has a very high level of spectral purity, for efficientdetection of the Raman signal. Most materials can be characterized by studying Raman shifts down to 100 cm-1, but insome cases, for instance for determining the crystallinity of pharmaceutical compounds, it is required to study Ramanshifts in the low-frequency regime; <100 cm~(-1).785 nm is the most common illumination wavelength for Raman spectroscopy as it offers the best compromise betweenRaman signal strength and fluorescence background suppression. In this paper, we present a novel design for a frequencystabilized785 nm diode laser using a highly reflective volume Bragg grating (VBG) element that offers not only a narrowspectral linewidth and low wavelength drift, but also a very high level of spectral purity. Using the VBG reflected light asoutput from the laser suppresses Amplified Spontaneous Emission (ASE) from the diode so that a very high level of sidemodesuppression ratio (SMSR) in the laser output is reached within just a few cm~(-1) away from the main peak without anyexternal spectral filtering. This enhanced spectral purity directly from the laser enables simpler, more compact and morecost-efficient detection of Raman shifts in the very low frequency range.