Near-Infrared Coherent Raman Spectroscopy and Microscopy

Abstract

Coherent Raman Scattering (CRS) spectroscopy and microscopy is a widely used technique in biology, chemistry, and physics to determine the chemical structure as well as provide a label-free image of the sample. The system uses two coherent laser beams one of which is constantly tuned in wavelength. Thus, a tunable laser source or optical parametric oscillator (OPO) is commonly used to achieve this requirement. However, the aforementioned devices are extremely expensive and work only for a specific wavelength range. In this study, we replace an OPO system with a photonic crystal fiber (PCF) in order to significantly reduce the cost and increase the flexibility of our microscopy system. Here, by exploiting the nonlinear phenomenon in the fiber called the soliton self-frequency shift (SSFS), we are able to shift the pulse central frequency while preserving its shape. Also, by switching to a near-infrared (NIR) source, the undesired fluorescence is reduced while the penetration depth increases. Moreover, the NIR laser source is more biologically friendly as each photon carries less energy than the visible laser counterpart. This reduces the probability of the photodamage effect. Based on this system, we designed and implemented CRS microscopy and spectroscopy, using Coherent anti-Stokes Raman Scattering (CARS) and Stimulated Raman Scattering (SRS) spectroscopy techniques.