Compact and robust dual-color linearly polarized illumination source for three-photon °uorescence imaging

The miniaturized femtosecond laser in near infrared-II region is the core equipment of three-photon microscopy. In this paper, we design a compact and robust illumination source that emits dual-color linearly polarized light for three-photon microscopy. Based on an all-polarization-maintaining passive mode-locked fiber laser, we shift the center wavelength of the pulses to the 1.7 mu m band utilizing cascade Raman effect, thereby generate dual-wavelength pulses. To enhance clarity, the two wavelengths are separated through the graded-index multimode fiber. Then we obtain the dual-pulse sequences with 1639.4 nm and 1683.7 nm wavelengths, 920 fs pulse duration, and 23.75 MHz pulse repetition rate. The average power of the signal is 53.64mW, corresponding to a single pulse energy of 2.25 nJ. This illumination source can be further amplified and compressed for three-photon fluorescence imaging, especially dual-color three-photon fluorescence imaging, making it an ideal option for biomedical applications.