Watt-level widely tunable near-infrared all-fiber femtosecond laser

Abstract

Due to the low scattering and absorption in tissues, the femtosecond laser at 1600–1880 nm (NIR-IIb-c) has aroused great interest for biomedical multiphoton microscopy. Here, we present a compact all-fiber femtosecond laser system based on the integration of coherent Raman amplification and soliton self-frequency shift in large-mode-area (LMA) fiber amplifiers. The output wavelength can be continuously tuned from 1580 to 1860 nm, covering both the NIR-IIb and NIR-IIc biological imaging windows. The output power increases with frequency shifting, exceeding the watt level at ∼1700 nm. The system delivers up to ∼1.6 W average power with pulse energy of ∼29.57 nJ and pulse duration of ∼235 fs at 1860 nm. Numerical simulations combined with experimental results reveal the critical role of LMA fiber composite configuration in enhancing Raman soliton generation, frequency shift. By integrating a second-harmonic generation module, the tunable spectrum can cover the 805–930 nm range (NIR-I window), with a maximum output of 362 mW and pulse duration of ∼298 fs. The proposed system provides a robust, commercially accessible all-fiber solution for high-power, wavelength-tunable femtosecond sources, offering significant potential for advanced multiphoton microscopy and other biomedical imaging applications.