Study Of Ammonia Sensor Based On Graphene-coated Microfiber Modal Interferometer

As one of the most important research topics,optical fiber sensing technology has been widely used in aerospace,information,biomedicine,industry,intelligent structure,and so on.With development of micro/nano technology,microfibers with small size,large evanescent field,high sensitivity and other unique characteristics have attracted attention.One possibility is to combine microfiber structures and nanomaterials,in order to explore the application of optical fibers.On the other hand,graphene,as a two-dimensional honeycomb crystal material formed by sp2 hybridization of carbon atoms,has exhibited excellent physical and chemical properties.Utilization of graphene has opened a variety of new applications for the existing devices including microfibers.It can not only improve the sensing performance for a microfiber-based sensor,but also enable the detection for some new quantities.In our work,graphene has been integrated into a microfiber-based modal interferometer to realize a highly sensitive fiber-type ammonia sensor.It is found that the graphene can enhance light-matter interaction in the microfiber-based modal interferometer so that the sensitivity is improved.When the ammonia molecules are adsorbed by the graphene,the carrier concentration of the graphene is changed,leading to the variation in the effective refractive index of the waveguide modes.As the ammonia concentration changes,the interference fringes shift.We can detect the change in the surrounding ammonia concentration through analyzing the change of the interference spectra.The proposed sensor has advantages of simplification,easy realization,and high sensitivity.In the context of this thesis,the sensing theory of microfiber-based modal interferometer has been analyzed.The interaction between graphene and gas molecules together with the evanescent field effect of microfiber structure has been discussed.The fabrication processes for the graphene(coating)and the microfiber devices have been described.The experiments on ammonia sensing carried out.The influence of structural parameters on the sensing performance has been discussed.