Objective With excellent photoelectric properties, low-dimensional nanomaterials show great promise for applications, such as optical switch, optical communication and industrial materials processing, particularly in combination with ultrashort pulse fiber laser. Passively mode-locking fiber laser is one of the important ways to generate ultrashort pulses. And carbon nanotube, graphene, black phosphorus and so on, have been used in lasing ultrashort pulses. However, low damage threshold is the fatal drawback of low dimensional nanomaterials, resulting in the inability to be applied to high-energy fiber lasers. Functional modified or customized nanomaterials have been studied in generating high-energy pulses. But these fiber lasers have mono-functional and no tunable features. Therefore, it is necessary to establish high-energy and flexible tunable fiber laser to meet the needs of different usage. For this purpose, a PbS quantum dots based passively mode-locking fiber is designed in this paper, where a piece of graded index multimode fiber is added to cavity, acting as a tunable filter and dispersion compensator.

Methods A tunable high-energy passively mode-locking fiber laser is built in this paper. PbS quantum dots is chosen as the high-damage threshold saturable absorber. A 24 cm-long graded index with slightly stretching is performed to implement a tunable filter (Fig.1-2). Because of the principal modes in multi-mode fiber, the group delay resulting from them is in linear relationship with the length of multi-mode fiber. And they are polarization dependent. Therefore, the total cavity dispersion can be controlled by just adjusting the polarization states of fiber lasers, leading to the operation of cavity switchable.

Results and Discussions By properly adjusting the polarization controller, a stretched pulsed fiber laser is established under the pump current of 245 mA (Fig.4). The center wavelength is located at 1 568.6 nm, with 3 dB bandwidth of 11.4 nm, and the full width at half maximum (FWHM) is about 361 fs with a fundamental repetition frequency of 9.55 MHz. Rising the pump to 296 mA and adjusting the polarization state, the operation of fiber laser switches to high-energy region (Fig.5). The center wavelength red-shifts to 1 569.5 nm, with a 3 dB bandwidth of 9.3 nm. The FWHM of pulse is about 20 ns with a near-rectangle waveform, where the leading and trailing edges of pulse are asymmetry resulting from three-order dispersion. When the polarization state is adjusted, the group delay resulting from principal mode accumulates, and the bandwidth of filter narrows down (Fig.2). At the same time, the total cavity dispersion increases to negative dispersion zone. With the pump current increasing, the high-energy dissipative soliton resonance pulse occurs. The FWHM can be in the range from 7.7 ns to 23 ns, and the maximum energy in cavity reaches to 34.8 nJ, indicating the damage threshold is more than60 mJ/cm². It's worth noting that the output spectrum shows asymmetrical sideband in two side of spectrum, which includes dip-type sideband and Kelly sideband. The appearance of dip-type sideband shows local asynchronous dispersive wave and soliton interactions in the cavity.

Conclusions The proposed fiber laser can be flexible transited between stretched pulse and high-energy dissipative soliton resonance pulses using a bandwidth-tunable filter based on multi-mode fiber. The micro-stretched multi-mode fiber not only has the function of filter, but also plays a vital role in compensating dispersion in ultrashort fiber laser. The versatile fiber laser provides diverse solutions for intelligent lasers.