Mode-locked And Q-switched Fiber Lasers Based On Gold Nanowires And Cuprous Sulfide Nanomaterials Saturable Absorbers

Pulsed fiber lasers have important applications in the fields of national defense,material processing,optical fiber communication,sensing and detection.At present,there are mainly two ways to realize pulsed laser in optical fiber:active and passive modulation（mode-locking/Q-switching technology）.Compared with active modulated fiber laser,passive mode-locked or Q-switched fiber lasers have the characteristics of simple structure,narrow output pulse width and good stability,and their core device is saturable absorber.The saturable absorber is a device with nonlinear absorption characteristics.The commonly used saturable absorbers in past research were semiconductor saturable absorber mirrors（SESAMs）,but such saturable absorbers usually require an optical lens to couple the light into and out of the fiber,which increased the complexity of the fiber laser cavity structure.In order to realize the all-fiber structure of pulsed lasers,researchers have explored a series of new saturable absorbers,including single-walled carbon nanotubes,graphene,and topological insulators.However,due to the limitation of their zero band gap structure and number of structural layers,their application in fiber lasers is also limited.For the first time,our research team proposed the idea of saturable absorption by utilizing the local surface isoplasmon resonance（LSPR）characteristics of gold nanomaterials,and realized pulse laser output in near infrared band by using gold nanoparticles and gold nanorods as saturable absorbers.However,mode-locked laser output is not realized in the mid-infrared band.During the doctoral study,the author carried out systematic research on how to expand the working wavelength range of gold nanomaterial saturable absorbers and how to realize longer wavelength mode-locked pulsed laser,and developed two kinds of saturable absorbers with gold nanowires and Cu_1.97S nanomaterials.Their nonlinear optical properties and their applications in fiber lasers are systematically studied,and the following research results are obtained:1.Cu_1.97S nanocrystals of different sizes with non-stoichiometric ratios and Cu_1.97S nanorods with different aspect ratios were synthesized.The composition and structure of the synthesized materials were analyzed by X-ray diffraction and transmission electron microscopy.The result of research shows that Cu_1.97S nanocrystalline and Cu_1.97S nanorods have regular morphology and good crystallizability.The absorption spectra were measured separately.The results show that the absorption peak of Cu_1.97S nanocrystals can be covered to about 1200 nm,and the absorption peak of Cu_1.97S nanorods can be covered to about 1800 nm.Such a band indicates that the absorption peak of Cu_1.97S nanorods can be used in the development of near infrared wavelength pulsed fiber lasers.2.Cu_1.97S nanocrystals with different sizes and Cu_1.97S nanorods with different aspect ratios were mixed with sodium carboxymethyl cellulose to prepare the corresponding saturable absorber films.It is found that these Cu_1.97S nanocrystals have good saturable absorption properties at 1.56μm band.When it is placed in an erbium-doped fiber ring laser cavity,the Q-switched laser output at 1.56μm band can be achieved,respectively.In addition,the saturable absorption characteristics of Cu_1.97S nanorods in the bands of 1.56μm and 2μm were also tested,respectively.The results showed that Cu_1.97S nanorods also had good saturable absorption characteristics.The Q-switched pulse laser output in the band of 1.56μm and the Q-switched pulse laser output in the band of 2μm were realized for the first time,respectively.The working wavelength of Cu_1.97S nanomaterials is extended to near infrared band.3.The ultra-fine gold nanowires（GWNs）have been used to achieve saturable absorber covering the working wavelength range of 0.5～25μm.In order to expand the optical response range of gold nanomaterials,ultrafine GWNs materials were prepared.X-ray diffraction and high resolution transmission electron microscopy showed that the synthesized GWNs had good crystallinity and large aspect ratio.These structural and morphological features lay the material foundation for expanding the resonant absorption of GWNs.In addition,it is found that its working wavelength range can cover 0.5～25μm band,such a wide band indicates that it can be used in the development of near infrared and middle infrared pulse fiber lasers.4.Mode-locked pulse laser output of 1.56、2、2.8μm and Q-switched pulse laser output of 2μm were realized by using GWNs saturable absorber.Firstly,the saturable absorption properties and nonlinear absorption property at 1.56、2、2.8μm bands were tested.The results show that GWNs saturable absorbers have a large modulation depth（ΔT=13.8%@2800 nm）,third-order nonlinear absorption coefficient(β=-0.994 cm GW^-1@2800 nm),and a wide range of nonlinear optical response.These results indicate that it can be used in the development of near-infrared and mid-infrared pulsed fiber lasers.A 1.56μm erbium-doped quartz fiber laser,a 2μm thulium-doped quartz fiber laser and a 2.8μm holmium and prasedymium co-doped fluoride fiber laser were constructed.For the first time,the mode locked pulsed laser output based on GWNs saturable absorbers was achieved at 1562.2 nm、1970 nm and 2864 nm.For the first time,the mode-locking wavelength range of gold nanomaterial saturable absorber was extended to 2.8μm.