Objective Various pulse shaping processes, including convention soliton, stretched pulse, similarity, and dissipative soliton, are formed in present passively mode-locked fiber lasers based on the diverse distribution positions of dispersion in the cavity. The development of soliton pulses has raised the single pulse energy to a new level, making fiber lasers cater to the needs of fields, such as optical metrology, biomedicine, and laser micromachining. Dissipative solitons are usually generated from lasers with a large net normal dispersion owing to the effects of dispersion, nonlinearity, gain, and loss. The spectral amplitude modulation introduced by the spectral filter plays a key role in forming the dissipative soliton. Therefore, various filters are used in the lasers. The birefringent filter has been widely used owing to its flexible filtering bandwidth and good fiber compatibility. In addition, noise-like pulses have also been extensively studied in normal dispersion lasers.

Both dissipative solitons and noise-like pulses can be generated in Ytterbium (Yb)-doped fiber lasers by reasonably adjusting the cavity parameters such as the pump power. Although pulse state switching has been verified in many experiments, few reports on the multiple switching of dissipative soliton and noise-like pulses in Yb-doped fiber lasers are available. In this study, we design a Lyot filter with a stable and powerful comb filtering using a pair of polarization-maintaining 45° tilted fiber gratings as polarizers and section of polarization-maintaining fiber as the birefringent medium. Therefore, an all-normal-dispersion Yb-doped fiber laser can achieve stable dissipative soliton mode-locking. By increasing the pump power unidirectionally in the dissipative soliton mode-locking state, the laser realizes multiple switching of dissipative soliton and noise-like pulse.

Methods Two polarization-maintaining 45° tilted fiber gratings are separated by a length of polarization-maintaining fiber with a particular splicing angle in the Lyot filter used in the experiment. It can be used as a comb filter in the laser cavity and a fiber-type polarizer because of its unique structure. To generate linear polarization light, the first grating couples the TE polarization component out of the fiber core and causes the TM polarization component to propagate in the fiber core. Linear polarization light accumulates linear phase shift in the polarization-maintaining fiber owing to the particular splicing angle between the grating and the polarization-maintaining fiber. The linear phase shift is transferred to the amplitude modulation in the second grating, resulting in comb filtering. The specific splicing angle is designed to be 45° for the filter to have the maximum-filtering modulation depth.

Results and Discussions The laser realizes stable dissipative soliton mode-locking with a pump power of 177 mW by finely adjusting the polarization controller, and its spectrum is shown in Fig. 4(a). The switching of the mode-locking pulse state can be observed while keeping the polarization controller and only increasing the pump power. When the pump power is increased to 323 mW, the spectrum gradually broadens under the influence of enhanced self-phase modulation. At this time, the pulse generated by the laser is still a dissipative soliton. Then, the pump power is continuously increased up to 455 mW in Fig. 4(e).

The sharp edges of the spectrum gradually disappear and become smooth. The autocorrelation trace in Fig. 4(f) has a wide base with a narrow peak, typical of noise-like pulses, indicating that the laser is working in a noise-like pulse regime. When the increasing pump power reaches 691 mW, the laser generates a dissipative soliton pulse again. The pulsed state switching of the laser can be attributed to the switching of the feedback mechanism of nonlinear polarization rotation. Fig. 5 shows the relationship between the instantaneous power and nonlinear polarization rotation transmittance. The sinusoidal transmission spectrum indicates that the nonlinear polarization rotation can occur in positive and negative states. The critical power of positive and negative feedback is named critical saturation power. After achieving stable mode-locking, the intracavity instantaneous power increases owing to the continuously increasing pump power. When the instantaneous power in the cavity exceeds the critical saturation power, the feedback mechanism of nonlinear polarization rotation switches from positive to negative feedback state, which causes the pulse state of the laser to switch from dissipative soliton to noise-like pulse. When the instantaneous power reaches the critical saturation power again, the feedback mechanism switches from the negative feedback state to the positive feedback state, so that the mode-locked pulse state of the laser also switches back to the dissipative soliton.

Conclusions We integrated a compact Lyot filter with a pair of polarization-maintaining 45° inclined fiber gratings in an all-normal-dispersion Yb-doped fiber laser. The laser realizes stable dissipative soliton mode-locking at a pump power of 177 mW. Furthermore, the pulse state of the laser recognizes switching from dissipative soliton to noise-like pulse and then to dissipative soliton by only continuously increasing the pump power from 177 mW to 691 mW. As adjusting the state of the polarization controller during switching is not necessary, it has higher controllability and accuracy, and the laser can be designed as a compact multifunctional light source.