Sub-100-fs deep-red mode-locked fiber laser for multicolor two-photon microscopy

Visible femtosecond lasers, especially within the deep-red wavelengths, are critical for applications such as bioimaging, biomedicine, and material processing. Traditional visible ultrafast lasers, such as Ti:sapphire lasers or nonlinear frequency–converted lasers, face limitations in size, cost, stability, and efficiency. Here, we demonstrate a Pr3+-doped mode-locked fiber laser (MLFL), directly generating sub-100-fs pulses at deep-red wavelength. This approach involves theoretical optimization and experimental validation using a ring cavity with Pr3+:ZBLAN fiber and nonlinear polarization rotation. The resulting ultrafast laser operates at 716.6 nm, delivering pulses with an ∼13-nm bandwidth and 83-fs duration at an ∼73.7-MHz repetition rate. To showcase the feasibility of this 717-nm MLFL for practical applications, two-photon microscopy is demonstrated with outstanding and unique performance regarding the simultaneous multicolor excitation of blue, green, and red dyes, enhanced resolution by 33%, and approximately four times higher excitation efficiency, compared with the conventional excitation wavelength at ∼1064 nm. These advantages confirm its versatility and reliability in biophotonic imaging. Our findings pave the way for compact, efficient sub-100-fs visible fiber lasers for multicolor bioimaging applications.