1. Introduction . Mode-locked fiber lasers offer desirable features of compact size, long-time reliability, freeness of alignment, and flexible customization, which have thus been developed as an attractive light source in numerous fields including time-resolved spectroscopy, laser material processing, and nonlinear optical microscopy [ 1 ]. Especially, ultrafast fiber lasers with spectro-temporal engineering would facilitate broadened advanced functionalities requiring precise timing of pu...