Multi-color laser source for STED microscopy

摘要

We report on a pulsed laser source whose wavelength can be switched between 585 nm, 600 nm, and 616 nm. The pulses are approximately 1 nsec long, the repetition rate is 20 MHz, and the pulse energies are 25 to 50 nJ. The laser source uses a laser diode seed and a series of Yb-doped fiber amplifiers to generate pulsed light at 1060 nm. The 1060 nm light is Raman-shifted in ordinary undoped fiber, then converted to the visible using MgO-doped periodically poled lithium niobate (PPLN). In general, the spectrum of such Raman-shifted light is too broad to be efficiently frequency-converted by PPLN. To overcome this problem, we have used narrow band fiber Bragg gratings to create a dual-wavelength fiber Raman laser. The 1060 nm light is first launched into a length of passive fiber, where the first Raman wavelength is generated. This (broad spectrum) light then synchronously pumps the fiber Raman laser, which supports the second and third Raman wavelengths simultaneously. Either of these wavelengths can be frequency doubled, or the two can be frequency summed, to create any of three visible colors. The PPLN crystal accordingly has three poling regions, and the color produced can be selected by indexing the crystal. The final output is suitable for high speed STED microscopy.