Using a Single-Arm Mamyshev Oscillator as a Simple Soliton-Self-Frequency-Shift Platform for NIR-IIb High Energy Femtosecond Laser

Due to a large penetration depth in tissue, a high energy femtosecond (fs) laser in the wavelength window of 1600 nm– 1800 nm (NIR-IIb) is attractive in biomedical applications of multiphoton microscopies. In comparison with bulky all-solid-state optical parameter oscillators (OPOs) in this paper, we propose a new all-fiber scheme to generate a high energy femtosecond fiber laser operating in 1600 nm–1800 nm wavelength. By optimizing the cavity parameters, we theoretically and experimentally demonstrate a single gain-arm erbium Mamyshev oscillator, which can directly deliver low repetition rate (<5 MHz) and nJ-level (∼2.37 nJ) fs (∼124 fs) pulses. Typical SSFS systems require amplifiers between the oscillator and the nonlinear fiber (NLF). As a contrast, in our work, the designed Mamyshev oscillator with a nJ-level fs output pulses directly pumps the NLF, which is a simpler platform to obtain a low repetition rate tunable SSFS laser source in 1600 nm–1800 nm. Then, we fix the SSFS laser wavelength at ∼1750 nm and construct a CPA system to amplify the low repetition rate ∼1750 nm laser. Ultimately, after the amplification, the 1750 nm laser power can reach ∼329 mW, corresponding to a ∼71 nJ pulse energy. The narrowest pulse duration after compression is ∼188 fs. Since the CPA system adopts thulium-doped fibers (TDFS) as the gain fiber, the CPA system is capable of amplifying the SSFS laser wavelength from 1700 nm–1800 nm. This presented all-fiber laser system has simple structure and tunability of laser wavelength, which would be attractive for biomedical applications.