The Research Of Optical Time Filter Based On Multimode Fiber

Optical filters are usually filtered according to their frequency domain characteristics,which is different from electrical filters.This phenomenon is caused by the coherence of light.In general,optical signals are delayed and then superimposed to produce light interference.The reason for the existence of different modes in multimode fibers is also the result of light interference.Moreover,the structures using splitting and optical delay lines generally have the problems of large loss and large variation length of the delay line.The mode dispersion effect of multimode fiber is utilized to delay the optical signal.We propose an optical integrator based on the modal dispersion effect of multimode fiber.This is an optical integrator that is insensitive to the wavelength and bandwidth of the light source signal.The integrator is mainly composed of multimode optical fiber and lens,and the mode dispersion is used to make the input beam delay in the system to form a time window.During the transmission of light in the system,the signal will be widened due to the correlation between the group velocity and the different modes,and the integration window is superimposed in the time domain.We have found in our research that integrating in the time domain can also be understood as moving average processing in the time domain.The response function in the time domain can be approximated as a square wave,and the response function in the frequency domain can be obtained accordingly.On this basis,we propose an optical filter that is operated based on time-domain filtering.The system proposed in this paper can change the phase modulation of the signal to the amplitude modulation of the signal without using a local oscillator laser,which eliminates the limitations caused by the parameters such as the bandwidth of the laser in the coherent optical communication system and greatly saves the system cost.In the paper,the main content of this thesis is as follows:(1)First,these situations are introduced,that is the research background and significance of optical time domain filters based on multimode fiber and the development of optical filters.The specific principle of the optical time domain filter is analyzed.Based on the integration interval of the square wave type,we study the filtering characteristics of the frequency domain according to the time domain characteristics of the filter system.The carrier recovery method of the optical time domain filter based on the multimode fiber is proposed and verified.(2)We propose an optical time-domain filter system based on fiber dispersion theory.First,we study the effect of fiber dispersion on the input signal pulse.Second,we analyze the optical time-domain filter based on multimode fiber mode dispersion theory.And the filter system is simulated and tested.While proving the effectiveness of the system's time-domain filtering,we simulated it to prove that the system is insensitive to the wavelength and bandwidth of the optical signal.(3)In order for the time domain filter system to be able to excite more fiber modes and the system can select specific modes.The spatial resolution of the system needs to be further improved.We propose an optical filter based on a single mode-multimode-single mode fiber structure.We have studied the basic characteristics and time domain characteristics of the structure.Based on the S-M-S fiber structure,in order to improve the conversion efficiency between the fiber and the optical signal,separate more discrete mode excitation groups to improve the resolution of the system,we have proposed an elliptical core multimode fiber system.It is found that the system based on the elliptical core multimode fiber is more conducive to the separation of the incident beam mode,which is helpful for the application of the system in time domain separation and all-optical space-time mapping.