Nanosecond square-wave pulses of flexibly tunable domain walls in a nonlinear polarization rotation fiber laser

Nanosecond square-wave pulses of color domain wall (CDW) and polarization color domain wall (PCDW) are realized in a nonlinear polarization rotation (NPR) fiber laser. CDW and PCDW can be flexibly switched by purely changing the polarization state in the cavity. The pulse width of PCDW can be tuned from 57 ns to 1013 ns smoothly. The duty cycle can be tuned from 0 to 1, accompanied by the energy transfer from long to short wavelengths. The dual-wavelength operation collapses into single-wavelength operation when the duty cycle reaches 0 or 1. A wavelength tuning range of ∼12 nm is also achieved for PCDW. The signal to noise ratios of CDW and PCDW are 58 dB and 66 dB respectively, and the envelope modulations of the radio-frequency (RF) spectrums are periodical. All those results can be reproduced by properly setting the pump power and polarization state. This work provides a platform with good flexibility, stability, and repeatability for investigating domain walls, and offers strong support for the further applications of square-wave pulse fiber lasers.