Study Of The Mode-locking Of Er-doped Fiber Laser Based On Novel Two-dimensional Materials

As an important optical modulator for converting continuous wave into pulsed laser output,saturable absorber has always been an important research direction in laboratory and industry.Currently,the commercial saturable absorber is semiconductor saturable absorber mirrors(SESAMs).By adjusting design parameters,it can achieve the output of pulse laser ranged from nanosecond to femtosecond and keep excellent mode-locked stability.However,the preparation of SESAMs is involved in the use of molecular beam epitaxy technology,which is extremely complex and costly.In addition,SESAMs usually has a low damage threshold and a narrow operating wavelength range,which is only decades of nanometer.Therefore,it is necessary to customize corresponding SESAMs for lasers with different wavelengths,which also leads to its poor reusability.Therefore,it is still a long-term goal to explore the saturable absorbers with low cost,high damage threshold,good reusability and good mode-locking stability.At present,studies of saturable absorber based on transition metal sulfides,black phosphorus,topological insulators,transition metal carbides and other two-dimensional materials are all in full swing.However,different materials have their own disadvantages.For example,the very small light absorption coefficient and modulation depth of graphene limit its application in strong optical field.Transition metal sulfides usually exhibit an indirect band gap in the multilayer structure,which is unfavorable for the photon transition in the structure.The stability of black phosphorus has not been effectively solved,which limits its application potential.Therefore,it is a very necessary work to explore new two-dimensional saturable absorber materials.Based on the above objectives,the mode-locking application of three new two-dimensional materials,indium bismuth(InBi),germanium selenide(GeSe)and palladium selenide(PdSe₂)in erbium-doped fiber lasers were studied.Their excellent mode-locking performance will help them become potential candidates as optical modulators.The concrete research contents are as follows:(1)Picosecond erbium-doped mode-locked fiber laser based on InBi saturable absorber was realized.The suspension of InBi nanosheets was prepared by liquid phase exfoliation,and the dispersion sample of InBi nanosheets with an average thickness of 32.8 nm was obtained by centrifugation.Then,the dispersion was mixed with 4%polyvinyl alcohol(PVA),and after drying the InBi saturable absorber(SA)films were successfully prepared.The modulation depth of the InBi SA is about 3.21%,and the saturated light intensity is about 100 MW/cm~2.Further,an erbium-doped mode-locked fiber lasers based on InBi SA was achieved successfully,where can be observed that continuous wave transformed into mode-locking pulse when pump power is equal to 100 m W,and stable mode-locking state was kept until the pump power of300 m W.Signal-to-noise ratio(SNR)is up to 53 d B,fundamental frequency is equal to 9.58MHz,central wavelength is 1562.27 nm,3-d B bandwidth is 2.98 nm,and pulse width is about859.97 fs of this mode-locking signal.(2)Erbium-doped mode-locked fiber laser based on GeSe saturable absorber was realized,and the target of adjusting mode-locked pulse width was achieved by regulating the size of GeSe nanosheets.The GeSe nanosheets dispersions with different sizes(GeSe-5000 rpm and GeSe-3000 rpm)were prepared by liquid phase exfoliation and gradient centrifugation method,and corresponding saturable absorber films were prepared by combining PVA.Furthermore,fiber lasers based on GeSe-5000 rpm and GeSe-3000 rpm SA are built,respectively,and their mode-locking operation was proved at the same time.The fundamental frequency is 9.56 MHz,the central wavelength are 1563.7 nm and 1566.21 nm,the pulse widths are 815 fs and 1.75 ps,SNR are equal to 61.2 d B and 54.56 d B,3-d B bandwidths are 3.15 nm and 1.47 nm,and the boot thresholds were 260 m W and 190 m W,respectively.(3)Erbium-doped mode-locked fiber laser based on PdSe₂ saturable absorber was realized.PdSe₂ nanosheets and PdSe₂ quantum dots solutions were prepared by alcohol and N-methyl-pyrrolidone(NMP)respectively,and their homologous saturable absorber films were prepared by solidifying dispersion sample with PVA.Furthermore,erbium-doped mode-locked fiber lasers based on PdSe₂ nanosheets and PdSe₂ quantum dots are built respectively.Besides,the fundamental frequency mode-locking and the second harmonic mode-locking were realized based on PdSe₂nanosheets at the same time.For the fundamental frequency,repetition frequency is about 9.20 MHz,SNR is about 59.27 d B,the central wavelength is 1562.23 nm,the 3-d B bandwidth is 1.32 nm.For the harmonic mode-locking,SNR is 29.42 d B,the repetition frequency is 18.42 MHz.In addition,the mode-locking phenomenon was also observed in the fiber laser based on PdSe₂ quantum dots.The fundamental frequency is 9.89MHz,the central wavelength is 1565.3 nm,and the 3-d B bandwidth is 1.23 nm.