Research On High Damage Threshold Carbon-based Mode-locker And Its Applications In Fiber Lasers

Ultrafast fiber lasers have important value in basic research and practical application owing to their extremely short duration and high peak power laser pulses.Their applications in strong field physics,industrial processing,eye surgery,laser communication and ultrafast imaging are worth mentioning.Mode-locking technology is one of the important technologies for generating ultra-short laser pulses.Compared with actively mode-locking,the passively mode-locking technology has the following advantages:simple operation,high cost-effectiveness,and the ability to generate shorter and higher energy pulses.Especially,carbon-based mode-lockers have been widely studied and applied due to their fast recovery time,wide operating band,polarization independence,and low cost.Similar to the commonly used semiconductor saturable absorber mirrors,transmission-type carbon-based mode-lockers made of polymer materials are prone to damage under laser irradiation,which limits their application and practicality.However,there are few studies on laser damage of carbon-based nanomaterial composites,and the understanding of the mechanism,process and phenomenon of laser heating effects and thermal damage is very limited.The research of high damage threshold mode-lockers has fallen into a bottleneck,which seriously hinders the further research and development of carbon-based mode-lockers.Therefore,studying the laser damage mechanism of saturable absorbers and increasing their damage threshold are of great significance for the research of new passively mode-locking technology.In this paper,based on single-walled carbon nanotubes,the laser heating effect and thermal damage mechanism of nanomaterial composite mode-lockers have been studied.We researched and optimized the scheme of using the sol-gel method to increase the damage threshold of the mode-locker,and applied the high damage threshold mode-locker to the soliton mode-locked fiber lasers.1.In this paper,a carbon-based saturable absorber,carbon nanotube-polyvinyl alcohol composite film(CNT-PVA)is prepared.Based on the heat conduction equation,COMSOL software is used to simulate the thermal effect generated in the saturable absorber under laser irradiation.This provides a theoretical basis for the study of laser thermal damage of mode-lockers in subsequent experiments.In the experiment,a method for judging whether the saturable absorber is damaged is proposed.This method is realized by observing the change of the incident laser transmittance with time and the morphology of the saturable absorber after laser irradiation This method does not require a high-power laser source,the experimental device is simple,and the measurement method fits the actual application of the mode-locker,which has a stronger guiding significance for the application of the mode-locker.Based on this method,the damage threshold of the saturable absorber is determined,and the damage process of the saturable absorber is discussed through the experimental results.2.The high-temperature resistant silica is prepared by the sol-gel method,and the composite film is prepared by combining it with carbon nanotubes.However,the obtained film has a small geometric size and is fragile,which is not conducive to being used as a mode-locker in a fiber laser.On this basis,new composite films were prepared using methyltriethoxysilane(MTES)and methyl methacrylate(MMA)modification methods.This is the first time that these two methods have been applied to saturable absorbers.The two methods realized the modification of silica sol by adding organic substances,and the geometrical size and flexibility of obtained films have been improved respectively,which enhances the practicability of the mode-lockers.In addition,we also used a variety of characterization methods to study the morphology,composition,and light absorption characteristics of the films.At the same time,the importance of the heat treatment process for the preparation of stable high damage threshold mode-lockers was explored through experiments.3.The nonlinear absorption and laser thermal damage characteristics of CNT-PVA and CNT-SiO2(carbon nanotube-silica)mode-lockers are studied and compared.It is found through experiments that the pulse repetition frequency of the laser source has a great influence on the saturation absorption characteristics of CNT-PVA,its modulation depth decreases with the increase of the repetition frequency.However,CNT-SiO2 is almost unaffected due to its high thermal damage threshold.The thermal damage measurement results of the two show that the transmittance of CNT-PVA gradually decreases with time under laser irradiation with a power density of 76.4MW/m2,damage caused.However,the transmittance of the CNT-SiO2 film remains constant over time even under the incident light of 127.3MW/m2,and there is no trace of damage on the surface after the laser irradiation,its damage threshold is significantly improved compared to CNT-PVA.4.High damage threshold mode-lockers are applied to ultrafast fiber lasers to realize the single pulse laser output of traditional soliton,dispersion-managed soliton,and dissipative soliton.This is the first time that a CNT-SiO2 saturable absorber has been used to achieve dispersion-managed soliton output in a fiber laser.After realizing mode-locking,the mode-locker can operate continuously and stably in the laser for more than 12 hours under 600m W high-power pump light.The above results prove the practicability of the prepared high damage threshold mode-lockers.