  A femtosecond frequency-doubled Erbium-doped fiber laser with adjustable
pulse repetition rate is developed and applied in two-photon excited
fluorescence microscopy. The all-fiber laser system provides the fundamental
pulse at 1560 nm wavelength with 22 fs duration for the second harmonic
generation, resulting in 1.35 nJ, 60 fs pulses at 780 nm. The repetition rate
is adjusted by a pulse picker unit built-in within the amplifier chain,
directly providing transform-limited pulses for any chosen repetition rate
between 1 and 12 MHz. We employed the laser source to drive a scanning
two-photon excited fluorescence microscope for ex vivo rat skin and other
samples imaging at various pulse repetition rates. Due to compactness, ease of
operation, and suitable pulse characteristics the laser source can be
considered as an attractive alternative for Ti:sapphire laser in biomedical
imaging.
