Research On Optical Switch And Its Characteristics Based On Graded-Index Multimode Fibers

All photon devices have played a crucial role in the development of all optical networks and laid the foundation for the development of optical computing systems,communication networks,and photonic chips.In all photon devices,optical switch as the basic unit of all optical networks,directly affect the performance of high-density all optical network systems.Therefore,designing and preparing high-performance optical switches has always been a focus of research.The current optical switching devices require continuous application of external energy to maintain their switching state,increasing the static loss of the device itself.One way to solve the above problem is to use non-volatile phase change materials（PCM）to make optical switching devices.At present,research on non-volatile phase change devices is completed on silicon and silicon nitride waveguide platforms,and cannot be compatible with the current carrier fiber of information communication.The thesis proposes a novel fiber optical structure combining phase change materials and designs a non-volatile all optical switch that can be used in all fiber terminal devices.The thesis first investigates the optical properties,coating methods,and excitation conditions of a typical phase change material Ge₂Sb₂Te₅（GST）,and conducts a detailed analysis of the reflectivity of the GST in two different states through experiments and simulations.Subsequently,a detailed simulation introduction was conducted on the refractive index distribution,transmission field form,spot formation at the end of the graded refractive index multimode fiber（GIMMF）,and the thermal field acting on the GST material.Then,based on the simulation and analysis results mentioned above,a reflective fiber structure that can achieve optical switching function is proposed.This structure involves fusing GIMMF onto a single mode fiber（SMF）and depositing the GST thin film on the end face of GIMMF.The implementation principle of this reflective optical switch based on phase change materials and graded-index multimode fiber is to induce phase change of the GST material through pulsed light,resulting in a change in the detection light power after reflection through the GST film,so that a significant difference occurs between the two state of"on"and"off".Realizing true all optical operation using pulse light modulation to transmit light.Finally,a complete set of optical switch implementation and detection device was built,and the results of this experiment verified the effectiveness of GIMMF with a length of 500μm.When the thickness of GST film is 300 nm,the reflective structured optical switch can achieve the switching function with a contrast of 3 dB in hundreds of nanoseconds in the common band of communication（1525 nm to 1610 nm）.In addition,the fiber optical device can also achieve up to 11 non-volatile storage levels.The optical devices based on graded-index multimode fiber and phase change materials proposed in this thesis provide a promising approach for achieving fast all optical switching and storage.This also endows optical fibers with switching and storage functions beyond sensing and communication,taking a step closer to the intelligent optical fibers.