Mode-locked Fiber Laser Based on Lead Sulfide Quantum Dot Saturable Absorber

At present, the most widely used mode-locked pulse fiber lasers mainly use saturable absorber as nonlinear optical element to achieve passive mode-locking. In the past few years, saturable absorber based on zero-dimensional quantum dots have been rapidly developed, which is also related to the excellent nonlinear optical effects of quantum dots themselves. Quantum dots such as PbSe, TiN, CdTe and graphene have been widely used in ultrafast Er-doped fiber lasers. Among various types of quantum dots, PbS quantum dots have the characteristics of small carrier effective mass, large optical permittivity, large exciton Bohr radius and low band gap energy. These characteristics make PbS quantum dots saturable absorber have the advantages of adjustable absorption peak, large third-order nonlinear polarizability and large modulation depth. The optical properties of PbS quantum dots are very sensitive to their size and shape, and the synthesis of high-quality PbS quantum dots with good size and shape distribution is of great significance in the whole experimental research. PbS quantum dots has been studied in the fields of dual wavelength soliton mode-locking, wavelength tunable pulse, ultrafast stretch pulse and high power ultra-shorts pulse. However, in the field of fiber lasers, the prepared quantum dots material is mostly deposited directly onto the jumper head for use, and it is difficult to ensure the repeatability and consistency of the device while the nonsaturable loss is large. To solve this problem, we use the two step spinning coating method to form the quantum dots and use them as saturable absorber to achieve mode-locking in fiber lasers, and achieve good results. We prepare PbS quantum dots by optimized hot-injection method. In the whole process, we use the Schlenk line technology to maintain an anhydrous and anaerobic environment and react under the protection of argon all the time.The prepared quantum dots solution is directly poured into low-temperature methanol in the preparation process to achieve rapid cooling effect, which makes the prepared quantum dots diameter more uniform and stable. After centrifugation and purification, the quantum dots are obtained and then dissolved in toluene and preserved. Transmission electron microscopy is used to characterize PbS quantum dots, and then the diameter distribution of quantum dots is measured by ImageJ software. The average diameter is (6.74 +/- 0.03) nm. Next, we prepare PbS quantum dots film by two-step spiral coating method, cut the film into 3x3 small pieces, remove them with tweezers and place them into the flange to obtain PbS quantum dots saturable absorber. Then the modulation depth, nonsaturable loss and saturated intensity of the fitted films are measured by a twin-detector measurement device, and the repeatability and consistency of the films are tested. These films have modulation depths ranging from 25.5% to 32.6%, nonsaturable loss ranging from 6.42% to 11.58%, and saturated intensity ranging from 4.76 MW/cm(2) to 7.14 MW/cm(2), which has good repeatability and consistency. Finally, we put PbS quantum dots saturable absorber into a self-constructed fiber laser cavity to achieve mode-locking, and find that stable self-starting mode-locking can be achieved starting at a pump power of 47 mW. It is proved objectively that PbS quantum dots saturable absorber has good mod-locking quality during laser evolution by spectrum, pulse trains, frequency spectrum, auto-correlation trace and the relationship of the average output power and single pulse energy with the change of the pump power. In this paper, PbS quantum dots film is prepared by optimized hot-injection method, and then the PbS quantum dots film is prepared by two-step spiral coating method. The PbS quantum dots film is used as saturable absorber in fiber laser to achieve stable mode-locking. The modulation depth and saturated intensity of the prepared films are about 26.46% and 6.75 MW/cm(2), respectively, and the nonsaturable loss is 6.42%. The films have better repeatability, consistency and small nonsaturable loss. The fiber laser use the thin film as a saturable absorber can start self-mode-locking at the output power of 47 mW pump source, and achieve long-term stable mode-locking laser output. The experimental results show that PbS QDs SA has good repeatability and consistency, and achieves mode-locked laser output with center wavelength of 1 564.1 nm, repetition frequency of 19.53 MHz and signal-to-noise ratio of 61 dB in fiber lasers, which lays a foundation for the practical application of PbS QDs SA in fiber lasers in the future. It is expected to be an ideal choice for mode-locked photonic devices.