Nonlinear Optical Properties Of PbSe Quantum Dots And Application In Mode-locked Fiber Lasers

Ultrafast lasers are widely used in laser medical,laser welding,precision micromachining,coherent optical communication,laser weapons and other fields because of the advantages of good beam quality,high repetition frequency,narrow output pulse width and peak power.Ultrafast lasers are currently produced mainly by Q-modulation or mode-locked technology.Passive mode-locked fiber lasers have the advantages of conventional pulsed solid-state pulsed lasers and are of great interest to researchers because of their simple structure,small size,no collimation,and easy integration with fiber systems.The saturable absorption technique is one of the most important means to achieve mode-locked pulses.Zero-dimensional quantum dots have size-dependent linear and nonlinear optical absorption properties and can be prepared in different sizes by changing the synthesis conditions,making them a promising application as saturable absorbers(SA)in passive mode-locked fiber lasers.In this paper,the preparation,characterization,linear and nonlinear optical absorption properties of PbSe quantum dots were investigated.Firstly,SA based on PS-PbSe quantum dot composite film was prepared under the condition of polystyrene solution as the film-forming agent,and the output pulse characteristics of a modulated Q-locked fiber laser based on PS-PbSe quantum dots were investigated.Secondly,a dual-wavelength passive mode-locked fiber laser was built by directly applying PbSe quantum dot solution droplet to the end face of the fiber jumper as SA to realize the dual-wavelength mode-locked pulse output,and changing the pump power of the laser can realize the conversion between single-wavelength and dual-wavelength operating states.The main research contents are as follows:(1)High-quality PbSe quantum dots solutions with uniform size distribution were prepared by a modified hot-injection method,and the size distribution of PbSe quantum dots ranged from 3.18 nm to 7.89 nm with an average particle size of 5.18 nm±0.5 nm.PS-PbSe quantum dots composite films with uniform thickness were prepared by spin coating using polystyrene solution as the film-forming agent.The nonlinear optical absorption properties of PS-PbSe quantum dots were measured by a two-optical path detection device.The results showed that the modulation depth of PS-PbSe quantum dots was about 32%,and the saturation intensity was 0.01 MW/cm~2.When the pump power is increased to 140 m W,the laser is in mode-locked operation.The maximum output power of the laser was 84.9m W when the pump power was increased to 1000 m W.In addition,the effects of different resonant cavity lengths and output coupling ratios on the output pulse characteristics at the same pump power were investigated.(2)By changing the synthesis conditions of PbSe quantum dots,PbSe quantum dots solutions with a size distribution between 2.87 nm and 6.75 nm were prepared with an average particle size of4.8 nm±0.7 nm.PbSe quantum dots were directly drop-coated onto the end face of the fiber jumper as SA,and the modulation depth of about 8.7%and saturation intensity of 0.106 MW/cm~2 were measured.By adjusting the gain and loss of the resonant cavity,a dual-wavelength conventional soliton output was achieved,and the laser could be switched between single-wavelength and dual-wavelength operation modes.The center wavelengths of the dual-wavelength pulses are 1531 nm and1557 nm,respectively,and the corresponding 3 d B spectral bandwidths are 4.5 nm and 2.1 nm,respectively,as well as the fundamental repetition frequencies of 9.422408 MHz and 9.422976 MHz.