Objective Passively mode-locked fiber lasers have been widely studied because of their simple structure, low cost, and high reliability. All-fiber passively mode-locked fiber lasers have outstanding advantages in terms of stability, reliability, and packaging technology because of the absence of spatial optical components. Their output pulse energy and average power are generally low because of fiber nonlinearity and poor power tolerance ability of fiber devices. To improve their pulse energy and average power while ensuring strong anti-interference stability, we developed an all-fiber all-polarization-maintaining dumbbell-shaped, ytterbium (Yb)-doped mode-locked fiber laser based on the nonlinear optical loop mirror (NOLM) dumbbell-shaped structure. By introducing all-polarization-maintaining large-mode-area fiber, high-power fiber components, and optimized cavity, we realized an average power of 5.5 W and pulse energy of 0.68 μJ in rectangular dissipative soliton resonance mode-locking.

Methods Figure 1(a) shows the laser structure, comprising an amplifier and two equivalent cavity mirrors composed of NOLMs. The NOLM1 on the left has a coupling ratio of 10∶90, and a nonreciprocal phase shifter (NRPS) with a phase shift of 3π/4 is inserted into the ring. The NOLM2 on the right has a coupling ratio of 50∶50, and a bandpass filter with a center wavelength of 1064 nm and a 3-dB bandwidth of 13 nm is connected to the ring. All the fibers in the cavity are single-mode, polarization-maintaining large-mode-area double-clad fibers with a core diameter and a numerical aperture of 10 μm and 0.08, respectively, which can reduce the nonlinear effects in the cavity. NOLM1 acts as an equivalent output cavity mirror and an equivalent saturable absorber, whereas the bandpass filter added in the NOLM2 stabilizes the laser center wavelength and dissipates soliton resonance. The insertion of NRPS can change the reflective NOLM's power-dependent reflectivity curve to achieve saturated absorption characteristics. A reasonable selection of NRPS phase shifts can also change the self-starting performance of NOLM-based mode-locked lasers.

Results and Discussions When the pump power of the laser is low, it runs in continuous wave state. When the pump power increases to 2.1 W, the laser turns to a mode-locked state, and when it drops to 1.8 W, it goes to a continuous wave state. Figure 2 shows the output pulse's characteristics at 1.8-W pump power. Moreover, the pulse shape is approximately a flat-topped rectangle with steep edges, with pulse width of 156 ps, repetition frequency of 8.1 MHz, pulse energy of 13 nJ, and peak power of 84 W. When the pump power is increased, the pulse width and the output pulse energy increase linearly, and the pulse peak power and repetition frequency remain unchanged. Figure 3 shows the variations of the pulse waveforms, average power, pulse width, and peak power at different pump powers. When the pump power is increased from 1.8 to 22.7 W, the output pulse width linearly increases from 156 ps to 8.1 ns, the average laser output power from 106 mW to 5.5 W, and the corresponding pulse energy from 13 nJ to 0.68 μJ. Hence, it can be concluded that the laser is operating in the dissipative soliton resonance mode-locked state.

Conclusions On the basis of an all-fiber, all-polarization-maintaining dumbbell-shaped, Yb-doped mode-locked fiber laser, we realized a high-power large-energy rectangular dissipative soliton resonance mode-locking. The pulse width can be adjusted from 156 ps to 8.1 ns. When the pump power reached 22.7 W, the average output power, pulse energy, and peak power increased to 5.5 W, 0.68 μJ, and 84 W, respectively. Benefiting from the all-polarization maintaining structure, we found that the laser exhibited excellent anti-interference stability. Compared with the previously reported maximum output power of all-fiber, Yb-doped mode-locked resonators, we achieved an increase of more than 100% in the average power and a significantly improved pulse energy.