[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Accurate and Traceable Determination of Raman Shifts in Reference Material by Continuous-Wave Stimulated Raman Spectroscopy

摘要： Most Raman spectrometers require frequent calibration in order to provide accurate and repeatable results, and the calibration interval decreases for operation at high resolution. Wavenumber calibration commonly uses emission lines from gas-discharge lamps (e.g. mercury, neon, argon) that provide well-defined frequencies with very low uncertainties. However, it is often impractical to align lamps properly at the sample position, and the lamp emission lines provide absolute frequencies rather than Raman shifts. A more practical alternative is the use of a reference sample with a known Raman spectrum, which allows for the calibration of Raman spectrometer regardless of the excitation wavelength. Polystyrene is commonly used as a reference as it is easily available and practical to handle. We report on the application of continuous-wave stimulated Raman spectroscopy (CW-SRS) for accurate measurement of Raman shifts in polystyrene samples. Due to the narrow linewidth of CW radiation, CW-SRS achieves spectral resolution that is several orders of magnitude below those obtained with common Raman spectrometer or pulsed-based SRS. Furthermore, CW-SRS enables direct determination of the Raman shift by measuring the difference of pump and probe frequencies. For our application, we used an 852 nm diode laser as probe and a widely tunable external cavity diode laser as pump covering the wavelength range from 781 nm to 787 nm. Stimulated Raman gain was measured by modulating the pump amplitude at 10.4 MHz and detecting scattered light on the probe with a balanced photodetector and spectrum analyser. High frequency modulation and balanced detection ensured shot noise limited operation, which was confirmed by the scaling of the noise level and signal with probe power up to 4.5 mW (Fig. 1 (b)). Polystyrene spectra in the range 968 cm-1 to 1059 cm-1 were recorded by scanning the pump wavelength. Both pump and probe wavelengths were measured accurately by a wavelength meter traceable to a frequency standard. Values and uncertainties of the Raman shift of two Raman transitions in polystyrene were calculated according to the Guide to the Expression of Uncertainty in Measurement (GUM) [1]. The method was applied to two different polystyrene samples: a thin plate taken from a tool cabinet and a NIST traceable reference sample for FTIR. For the later, Raman shifts of 1001.15 ± 0.04 cm-1 and 1031.33 ± 0.21 cm-1 were measured. Compared with existing standards [2], we achieve up to one order of magnitude improvement on the uncertainty of the measured Raman shifts, thereby improving the metrology capability for characterization of reference material for Raman spectroscopy. Furthermore, shot noise operation paves the way towards the development of quantum-enhanced SRS with potential application in biological imaging.