Fiber-based light sources for biomedical applications of coherent anti-Stokes Raman scattering microscopy

During the past decade coherent anti-Stokes Raman scattering (CARS) microscopy has evolved to one of the most powerful imaging techniques in the biomedical sciences, enabling the label-free visualization of the chemical composition of tissue in vivo in real time. While the acquisition of high-contrast images of single cells up to large tissue sections enables a wide range of medical applications from routine diagnostics to surgical guidance, to date CARS imaging is employed in fundamental research only, essentially because the synchronized multiple wavelength pulsed laser sources required for CARS microscopy are large, expensive and require regular maintenance. Laser sources based on optical fibers can overcome these limitations combining highest efficiency and peak powers with an excellent spatial beam profile and thermal stability. In this review we summarize the different fiber-based approaches for laser sources dedicated to coherent Raman imaging, in particular active fiber technology and passive fiber-based frequency conversion processes, i.e. supercontinuum generation, soliton self-frequency shift and four-wave mixing. We re-evaluate the ideal laser parameters for CARS imaging and discuss the suitability of different laser concepts for turn-key operation required for routine application in clinics.