High power/energy ultrafast fiber lasers have broadband applications in material processing, medicine, advanced manufacturing and other fields. Compared with solid-state lasers, fiber lasers have the advantages of compact system, flexibility, good heat dissipation and high beam quality. However, due to the serious nonlinear effect inside the fiber, the single-pulse energy and average power of the ultrafast fiber lasers, especially with an all-fiber structure, still lag behind that of the solid-state lasers. In recent years, Mamyshev oscillator has attracted much research attention thanks to its potential in producing high energy ultrafast pulses. Recently, a research group from National University of Defense Technology reported a Mamyshev oscillator based on all-polarization-maintaining fiber with core/cladding diameter of 10/125 μm, and realized a 153 nJ single pulse energy with a compressed pulse width of 73 fs. Furthermore, through adjusting the cavity parameters, a maximum 5th order harmonic mode-locking operation was obtained with an output average power of 3.4 W and a compressed pulse width of 100 fs. Recently their work entitled "All-PM Fiber Mamyshev Oscillator Delivers Hundred-Nanojoule and Multi-Watt Sub-100 fs Pulses" was published on Ultrafast Science.
The pulse energy and average power of ultrafast fiber lasers have been greatly improved in recent years based on the Mamyshev oscillator with cascade spectral broadening and offset spectral filtering effect. The continuous light and weak pulses will be blocked in the Mamyshev oscillator. Due to the sufficient spectral broadening, the strong pulse can be survived in the cavity after two filters with different central wavelengths, and the ultrafast laser with large pulse energy can be obtained. However, most of the previous results adopted spatial structures and introduced a large number of free-space elements for signal collimation and coupling, which render the system cumbersome and susceptible to interference.
"Our aim is to realize high power/energy ultrafast fiber lasers with all-fiber structure," explained A/Prof. Can Li. In the Ultrafast Science paper, a Mamyshev oscillator based on all-polarization-maintaining fiber with core/cladding diameter of 10/125 μm was reported. Compared with the conventional Mamyshev oscillators, which generally consist of two stages of gain fiber amplification, there is only a single amplification arm in the proposed laser. A piece of passive fiber was used to broaden the optical spectrum in the other arm, alleviating the nonlinear phase accumulation inside the cavity and making the system more compact in the meantime.
This research group respectively realized the high performance ultrafast fiber lasers with single pulse energy of 153 nJ and average power of 3.4 W, which represent the highest records in pulse energy and average power obtained from an all-fiber ultrafast laser oscillator with picosecond/femtosecond pulse duration. "By adopting the fiber with larger core diameter and new pulse evolution mechanism that has a higher tolerance of nonlinearity phase accumulation, all-fiber ultrafast lasers with higher power/pulse energy are expectable," says Professor Pu Zhou.
This work is supported by the National Natural Science Foundation of China and the Director Fund of State Key Laboratory of Pulsed Power Laser Technology.
