Research On Multi-mode Fiber Variable Optical Attenuator

With the development of multi-mode fiber(MMF)communication technology,MMF variable optical attenuators(VOA)are widely used in MMF communication systems to achieve power equalization and devices protection.At present,the mainstream solution of MMF VOA is designed to insert a continuously variable neutral density filter(NDF)into light path.The NDF is driven by a linear stepper motor to obtain continuous variable attenuation levels.However,it results in slow response.This thesis proposes a MMF VOA with rapid response.Its main contents are as follows:The thesis has systematically studied different solutions of VOAs and compared the differences between single-mode fiber(SMF)VOA and MMF VOA.Considering the mode stability and the response speed of MMF VOA,a new MMF VOA is proposed based on a continuously variable NDF and a rotating stepper motor.The optical beam is reflected by a rotating mirror to scan on the fixed NDF.Thus,continuously variable optical attenuation can be obtained with rapid response.The optical system is simulated and analyzed via Zemax software.According to the simulation results,the coupling efficiency is 81.678%,which verifies the feasibility of the solution.Considering the refletion loss of the optical elements' surface,theoretically,the insertion loss(IL)is 1.5d B,which is expected to be 0.9d B after anti-reflection(AR)coating on the NDF.According the simulation and optimization,the assembly components for experiment are designed and fabricated.The assembling of the MMF VOA and the measurement of it's parameters are completed.The IL is measured to be 2.07 d B,which is expected to be around 1.5d B after AR coating on the NDF.The attenuation range is 2d B ? 25 d B.The slowest response speed is 7ms.The repeatability of attenuation is within 0.1d B.The experiment shows that the IL of cylindrical lens is 0.86 d B.Therefore,an improved solution without cylindrical lens is proposed.A retroreflector is driven by a rotating stepper motor.And the optical beam reflected by the retroreflector scans on the fixed NDF.The simulation result shows that the coupling efficiency is 100%.Based on the simulation,the assembly components for the MMF VOA are designed.The IL is excepted to be 0.5d B.The attenuation range is 0.5d B ? 30 d B.The slowest response speed is excepted to be 14 ms.