Study On Mode Excitation Of Step-index Multimode Fiber Based On Gaussian Beam

In the field of local area network, optical information processing and optical fiber sensor, the mode excitation of the optical fiber is involved. Especially with the rapid development of optical information processing technology, the optical information processing technology based on optical fiber mode dispersion has drawn people’s attention, but how to accurately excite the mode to meet the conditions has limited its application. Power coupling coefficient is an important parameter for the study on mode excitation. The study on power coupling coefficient of the graded multimode fiber is more mature and there is already analytical solution for it, but for the step-index multimode fiber, there is no perfect analytical solution. However, the optical information processing based on the mode dispersion needs to introduce large mode dispersion by the step-index fiber, so it is important to study the mode excitation of the step-index fiber. In the paper, the main contents are:(1) Firstly, on the basis of the power coupling coefficient, we theoretically analyze the mode filed of multimode fibers and Gaussian beams, and use matlab to simulate the mode filed and the transmission constant of the multimode fiber with particular parameters, which lay theoretical foundations for the behind research.(2) Secondly, we use matlab to numerically simulate the power coupling coefficient of the step index fiber’s mode excitation, and discuss how different excitation conditions (such as offsetting the beam, tilting the beam, changing the beam waist radius and changing the wave-front curvature)influence the excitation of each mode in the fiber. Then we quantitatively compare with the mode excitation of the graded index optical fiber, and the results show that the numerical simulation method is correct.(3) Lastly, we discuss an important application in the optical information processing based on mode excitation--the all optical integrator based on the mode dispersion and study the all optical integrator based on modal dispersion through numerical simulations and experiments. We propose a method using one-dimensional broadening inclined incident mean to develop an all optical integrator, and select a high-speed photo detector with the larger area to receive the output beam of the multimode fiber. At last, we get a more efficient optical integrator based on mode dispersion, and use 50m multimode fiber to achieve the time window with 1.5ns. At the same time, we do integral experiments for the light signal with the integrator, showing that the method has good flexibility, and is easy to debug and control. Compared with the integrator based on single-mode fiber’s group velocity dispersion, the integrator based on mode dispersion uses shorter fiber, and the integration time window has little correlation with wavelength and bandwidth of the light source.