Two-photon bioimaging with a multi-kilowatt peak-power optical pulse source using a 1-μm wavelength mode-locked semiconductor laser

We have developed a multi-kilowatt peak power 1-μm optical pulse source for two-photon microscopy. Utilizing an external-cavity hybrid mode-locked semiconductor laser, we were able to generate picosecond optical pulses at a 500-MHz repetition rate. With a semiconductor optical amplifier driven by synchronized electronic gating pulses, the optical pulse repetition rate was sub-harmonically extracted at 1-100 MHz. At a 10-MHz repetition rate, optical pulses were then amplified to a peak power of greater than 2 kW with a two-stage Yb-doped fiber amplifier. Using this light source, we successfully obtained clear two-photon images of mouse brain neurons expressing green fluorescent proteins.