  Broad and safe access to ultrafast laser technology has been hindered by the
absence of optical fiber-delivered pulses with tunable central wavelength,
pulse repetition rate, and pulse width in the picosecond-femtosecond regime. To
address this long-standing obstacle, we developed a reliable accessory for
femtosecond ytterbium fiber chirped pulse amplifiers, termed as fiber-optic
nonlinear wavelength converter (FNWC), as an adaptive optical source for the
emergent field of femtosecond biophotonics. This accessory embowers the
fixed-wavelength laser to produce fiber delivered ~20 nJ pulses with central
wavelength across 950-1150 nm, repetition rate across 1-10 MHz, and pulse width
across 40-400 fs, with a long-term stability of >2000 hrs. As a prototypical
label-free application in biology and medicine, we demonstrate the utility of
FNWC in real-time intravital imaging synergistically integrated with modern
machine learning and large-scale fluorescence lifetime imaging microscopy.
