Research On Multiple Mode-Locking Phenomena Based On Cr₂Ge₂Te₆ Saturable Absorber

The emergence of ultrashort pulse has greatly promoted the development of many scientific fields,such as medical testing,laser manufacturing and processing,national defense security.About obtaining ultra-short pulse,active mode locking and passive mode locking are the commonly method in getting ultra-short pulse.Acousto-optic or electro-optic modulators are used in active mode-locked lasers,which can output higher energy and more stable pulse.Compared with active mode locking,the passive mode locking does not require additional modulation devices.A saturable absorber is used to achieve the mode locking output,which is more flexible and convenient.In recent years,two-dimensional(2D)materials as saturable absorbers have shown special advantage.2D materials have been widely used in passive mode-locked fiber lasers due to their layered structural and excellent optical properties.However,for the pulse output characteristics and the thermal stability of the saturable absorber,various 2D materials have their different advantages.Therefore,the study of the 2D material with excellent optical properties as a saturable absorber is an indispensable part of current research.In this paper,a Cr₂Ge₂Te₆(CGT)material as saturable absorber was applied in 1.5?m passive mode-locked fiber laser,which successfully obtained pulse outputs with different performances.The main contents of our studies are as follow:1.The CGT-PVA(polyvinyl alcohol)saturable absorber was prepared,and the material and the saturable absorber were characterized.Based on the CGT material,the nanosheets with average thickness of 6 nm were obtained by the method of liquid phase exfoliation,and the nanosheets were mixed with the PVA solution to prepare a“sandwich”saturable absorber.The material and the prepared saturable absorber were characterized by a series of characterizations.The surface morphology of the CGT material,the thickness of the nanosheet and the transmission microscope image were recorded.For the CGT-PVA saturable absorber,the linear transmission and the nonlinear transmission were recorded.2.The CGT-PVA saturable absorber was used to obtain different pulses.By continuously optimizing the length of the laser cavity,a pulse with a pulse width from 1.4 ps to 550 fs was obtained,and the pulse has excellent time stability.Secondly,the pulse output with the maximum output power of 4.207 m W was obtained.By adjusting the pump power,the single pulse energy is 1.29 n J,which shown that the CGT-PVA saturable absorber has excellent thermal stability.In addition,the dropping method was used to prepare saturable absorber.When it was applied in passively mode-locked fiber lasers,Q-switch pulse was obtained,and the minimum pulse width was 3.5?s,the maximum average output power is 641.2?W,and the corresponding single pulse energy is 21.6 n J.the fundamental repetition frequency was from 13.8 k Hz-29.6 k Hz.The experimental results proved that few-layer CGT exhibits excellent saturable absorption properties,which provides a new opportunity for getting ultrafast mode locked pulse based on CGT as saturable absorber.