I. Introduction
Since its first demonstration in 1990 [1], multiphoton microscopy (MPM) has emerged as an indispensible tool for visualizing subcellular structures or dynamics in numerous biological samples, both ex vivo and in vivo. One focused research area in MPM is to visualize multiple structures labeled with different colors, i.e., multi-color MPM, which enables tracking multiple immune cell populations [2] and visualizing different compositions of the sample [3]. On the detection side, fluorescence signals of different colors can be detected simultaneously with several photomultiplier tubes (PMTs) equipped with proper color filters. For example, 3 PMTs can be used to detect 3 fluorescent signals from 3 fluorophores [3]. On the excitation side, it has been demonstrated that multi-color femtosecond pulses, whose center wavelengths match the multiphoton absorption peaks of fluorophores, are suited for efficient multiphoton signal generation from multiple fluorophores [3], [4].