Researches on two key frontier issues in picosecond pulses generated by mode-locked fiber lasers

Narrow band dissipative soliton mode-locked fiber lasers can generate picosecond pulses close to the transformation limit, but due to the limitation of nonlinear phase shift, the output pulse repetition rate cannot be reduced by increasing the cavity length, and the pulse energy is only below 0.1 nJ, which seriously restricts the practical application of such picosecond pulses. In this paper, we propose a method to reduce the pulse repetition rate of narrowband dissipative soliton picosecond fiber laser by using a coupler to extract pulse energy in the cavity, restrain the accumulation of nonlinear phase shift in the cavity, and then allow to increase the cavity length. Using this method, the repetition rate of the laser is successfully reduced from 35.2 MHz to 1.77 MHz, and the time-frequency characteristics of the pulse remain unchanged. We also propose a method based on interstage FBG notch filtering to suppress spectral broadening in picosecond pulse fiber amplification. By simply using the interstage notch filter, the output pulse spectrum width after the first stage fiber amplifier can be narrowed, allowing the use of the second stage fiber amplifier to further increase the pulse energy. In addition, the pulse can also be reshaped into a near Gaussian shape, allowing the second stage fiber amplifier to increase the pulse energy higher by taking advantage of the small slope of the Gaussian pulse spectrum broadening. Using our method, the energy of 10 ps pulse passing through standard single-mode fiber amplifier can be increased from 0.2 nJ to more than 10 nJ under the premise that RMS spectral width is kept within 0.4 nm.