Optical Modulation Characteristics And Devices Research Based On Graphene-coated Microfiber Composite Waveguide

Since single-layer graphene was successfully prepared in 2004,graphene has shown excellent photoelectric properties and has been applied in various fields of scientific research,such as optoelectronic devices,sensors,energy storage,flexible electronics,composite materials and biomedicine.In the field of optoelectronic devices,graphene has excellent physical and chemical properties,including its linear dispersive band structure,which results in a very wide spectral response range.The absorption rate of single layer graphene is only 2.3%,and the absorption rate is superimposed with the increase of layers.Graphene produces a Pauli blockade on the passing light,which results in saturated absorption.Graphene has an ultra-fast carrier relaxation time of picosecond magnitude.In addition,it has strong flexibility and can be easily transferred to optical structure.At present,it has been applied in Q-switched and mode-locking,optical modulation,photoelectric detection,sensing and other fields.With the rise of two-dimensional materials research,microfibers have entered the research field of scholars.Microfibers are fabricated from standard singlemode fiber by fused taper,which have the advantages of perfect compatibility with communication fibers and simple fabrication.When light propagates in micro fibers,part of the light propagates outside the microfibers in the form of evanescent field.Graphene films with atomic layer thickness can be attached to the surface of microfibers,while evanescent fields propagating outside microfibers can easily interact with graphene.In this paper,the optical modulation characteristics of graphene microfiber composite waveguide devices modulated outside the cavity and the application of the devices in intracavity modulation to achieve Q-switched and mode-locked excitation are studied.In the study of optical modulation characteristics of composite waveguide devices,we found SBS phenomenon in the devices.In order to study the non-linear effect,we compared the microfiber graphene composite waveguides with different diameters in experiments,and found that the smaller the diameter,the stronger the non-linear effect and the lower the threshold value in the microfiber graphene composite waveguides.The effects of graphene waveguides and graphene-free waveguides on the generation of non-linear effects are also compared.Under the same diameter,the non-linear effects will occur in the composite waveguides with graphene,but not in the waveguides without graphene.In theory,we also calculated the non-linear threshold of the device and simulated the mode field distribution of microfibers.Through experimental comparison and theoretical analysis,we find that graphene with high third-order nonlinearity coefficient enhances the nonlinearity of devices and reduces the threshold of nonlinearity.This work improves the basic research of composite waveguide,and makes the device take a new step in practical application.In addition,we use graphene microfiber composite waveguide as saturable absorber(SA)in laser to realize active Q-switching.We use passively Q-switched laser pulse to modulate the composite waveguide in the laser cavity,so as to change the Q value in the laser cavity and realize active Q-switching.Finally,we fabricated composite waveguides using graphene oxide carboxylate and microfibers,also applied them in mode-locked lasers to achieve passive mode-locking.The application of composite waveguide in intracavity tuning can effectively improve the damage threshold of materials and enhance the interaction between materials and light.The application of composite waveguide in intracavity tuning can effectively improve the damage threshold of materials and enhance the interaction between materials and light.