Ultrawide-Space and Controllable Soliton Molecules in a Narrow-Linewidth Mode-Locked Fiber Laser

We propose and demonstrate ultrawide-space and quantized-controllable soliton molecules in a narrow-linewidth passively mode-locked Er-doped fiber ring laser. Stable soliton molecules with a central wavelength of 1565.7 nm and a narrow linewidth of 0.30 nm are observed with a pulse repetition rate of 1.665 MHz. Each soliton in molecules has a pulse width of 12.0 ps, and adjacent solitons have an ultrawide space as large as 12.8 ns which is ~1067 times of the pulse width. Interestingly, the soliton number in a molecule can be continuously tuned from 2 to 7 by changing the pump power. Furthermore, the features of the quantized-controllable soliton molecules in optical and radio-frequency spectral domains are comparatively studied.