Broadband High-Energy All-Fiber Laser at 1.6 $\mu$ m

A broadband high-energy all-fiber laser at 1.6-μm window with erbium-doped fiber mode locked by nonlinear polarization rotation technology is demonstrated. The direct output optical spectrum centered at 1.6 μm has a bandwidth of 52.4 nm. The fiber laser delivers a pulse energy of 3.9 nJ with 9.3-MHz repetition rate at 220-mW 976-nm pump power. Based on it, a wavelength-tunable optical source with a wavelength tuning range from 1.6 to 1.8 μm is demonstrated through soliton self-frequency shift. To satisfy much longer wavelength demands in some special application scenario, e.g., deep brain imaging with multi-photon microscopy, a chirped pulse amplifier was constructed to boost the pulse energy to 14 nJ, and the wavelength can be tuned from 1.6 to 1.94 μm with those amplified pulses. These fiber sources at the L-band and a longer wavelength can explore a wider scope in deep biotissue imaging area for lower water absorption.