Ultra-Low Noise, Fully Automated Laser System for Coherent Raman Scattering Microscopy

Coherent Raman scattering (CRS) imaging is based on a multiphoton scattering process that employs two near-infrared laser pulses to excite Raman modes in the midinfrared spectral range. Its label-free chemical selectivity has earned CRS a wide scope of applications in biomedical microscopy, including live cell, tissue, or DNA imaging. The most prominent representatives of CRS are coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS). In both cases, two beams—the so-called pump and Stokes beams—interact, giving rise to the generation of a new frequency (CARS) or to an energy exchange between the two beams (SRS).

A team of scientists from University of Stuttgart and University of Glasgow has developed an ultra-low noise, fully automated laser system for coherent Raman scattering microscopy, as reported recently in Advanced Photonics. Based on solid-state femtosecond technology, combined with optical parametric frequency conversion, their multicolor system reaches the shot noise limit at modulation frequencies of 1 MHz and above. Delivering tunable radiation in the 750-nm to 950-nm and 1.4-μm to 2.0-μm ranges, together with a spectrally fixed beam at 1043 nm, it is perfectly suited for coherent Raman microscopy in the range of 1015 to 3695 cm^-1, as well as for multiphoton excitation microscopy. With customizable pulse durations from hundreds of femtoseconds to picoseconds, efficient excitation and spectral resolution down to 13 cm-1 are possible. All three output beams are inherently synchronized and, in addition, the Stokes and pump beams are spatiotemporally overlapped with a precisely controllable temporal delay. The unique robust frequency conversion design requires no active stabilization electronics, which usually negatively affect the system stability, noise, and handling. The system is fully automated and allows wavelength tuning with subnanometer precision via its hardware control panel or by remote access.

In order to evaluate the system performance, including the tuning width and resolution, the team recorded the spectrally very broad SRS response from a D₂O and H₂O mixture, as well as the narrow response of acetone. To demonstrate the capability of the novel light source with respect to imaging applications, they investigated a mixture of micrometer-sized polystyrene and PMMA in a basic confocal scanning microscope setup. With this proof-of-concept, the team shows that the proposed system allows chemical-selective imaging with video frame rates.

Read the original research article in the peer-reviewed, open-access journal Advanced Photonics: H. Linnenbank et al., "Robust and rapidly tunable light source for SRS/CARS microscopy with low-intensity noise," Adv. Photonics 1(5) 055001 (2019).