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-lint width 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 similar to 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 ill optical and radio-frequency spectral domains are comparatively studied.