Experimental Study On Light-Controllable In-Fiber Mach-Zehnder Interference Filter

Tunable optical fiber Mach-Zehnder interferometric filter plays an important role in optical fiber communication,optical fiber sensing and wavelength tuning of the laser systems due to its characteristics of inherent compatibility with optical fiber,low insertion loss and return loss.All optically tunable fiber Mach-Zehnder interferometric filter usually composing of a pair of couplers and a photosensitive material coated pathway shows the advantages of easy implementation,high tuning precision and low cost over the electrically or acoustically tunable interferometric filter.Tapering a piece of standard single mode fiber down to 10?m is required to enhance the interaction between the evanescent field and functional material.In addition,external disturbance will result in different effect onto the two pathways of the fiber Mach-Zehnder interferometer due to their spatially separated configuration,severely deteriorating the optical tuning performance.To address the issues,a graphene quantum dots assisted optically tunable in-fiber Mach-Zehnder interferometric filter is proposed in this thesis,which possesses high tuning efficiency and integration by combining the high refractive index sensitivity of the Mach-Zehnder interferometer and quantum confinement effect of the graphene quantum dots.The main research contents are as follows.(1)An in-fiber Mach-Zehnder interferometer with high refractive index sensitivity is designed.The effects of the structural parameters on the interferometric spectrum and the response to external refractive index change are studied by the method of finite difference in time domain,providing theoretical guidance for fabricating the interferometer.Meanwhile the optical tuning property of the quantum dots is analyzed by the Lorentz model and the wave function,which is the basis of optical tuning with high efficiency.(2)An in-fiber Mach-Zehnder interferometer is fabricated by femtosecond laser ablation and its response to external refractive index change is also experimentally studied.During the experiment,a piece of standard fiber is firstly polished into a D-fiber and then transfer to the femtosecond laser micromachining system,where a groove symmetric about the fiber axis is fabricated,resulting in an in-fiber Mach-Zehnder interferometer.Finally,the response of the Mach-Zehnder interferometer to external refractive index change is studied by dipping sucrose solutions with different concentration into the rectangular groove.Experimental results show that the refractive index sensitivity of the Mach-Zehnder interferometer with length of 50?m is 1.5866×10~4nm/RIU.(3)An all-optically tunable in-fiber Mach-Zehnder interferometer is fabricated by dipping water soluble nitrogen-doped graphene quantum dots into the rectangular groove.The optical tuning characteristic of the Mach-Zehnder interferometer is studied by employing a UV light with central wavelength of 365nm as the pump.Experimental results show that red shift up to 90nm happens when the pump power density varies from0mW/cm~2 to 41.1mW/cm~2,corresponding to a tuning efficiency of 1.9817nm/(mW/cm~2).