Ultrafast Light Source at 1.8 μm Based on Thulium-Doped Fibers for Three-Photon Microscopy

Ultrafast laser systems operating around 1.7 μm to 1.8 μm are powerful tools to extend the penetration depth of mulitphoton microscopes [1,2]. However, radiation at this wavelength region is not readily available and thus frequency shifting was necessary to obtain the desired wavelength. Here, we demonstrate an ultrafast thulium-doped fiber laser system generating the spectrum covering the 1.8 μm region, whose output pulses have sufficient intensity for three-photon excitation necessary for multiphoton microscopes.