The Research On The Characteristics Of Fused Taper Optical Couplers

With the rapid development of optical communication technology, optical devices are used more widely. And the fused tapered couplers has been the most widely used one. The fused taper couplers are one kind of the most representative devices in terms of its various characteristics. In order to meet the evolving needs of fiber optical communications industry, fiber optical couplers are developing towards multi-function, high-performance direction. To get the characteristics of different types of optical fiber couplers based on the study of the performance of all kinds of optical fiber couplers is necessary for the optimization of the device, which lays foundation on design of more kinds of couplers. At first, this article made a comprehensive overview for the structure of fiber, optical transmission in optical fiber, fiber coupler’s own characteristics, the common processing production method, and common theoretical analysis method.In this character, the structure of fiber and optical transmission in fiber are analyzed. In addition, the concept of optical fiber couplers’ split ratio, loss and bandwidth are introduced. Thirdly, three kinds of production methods of optical fiber couplers are described, and the advantages and disadvantages of these three methods are briefly summarized. And the most widely used method of fused tapered fiber is introduced in detail. It’s also depicted several theoretical analysis methods for coupler, include coupled-mode theory, finite element method and beam propagation method. Beam propagation method is used to analyze the feature of couplers in this article.In the second part, based on the gradient characteristic of taper of optical fiber and the change of degree of fusion between two fibers of fused fiber coupler in the drawing process, we construct the changing model of waveguide structure of optical fiber coupler. the relationship between optical power and stretching length is obtained by numerical simulation with beam propagation method, as well as the optical field distribution and loss curve during drawing process. It’s revealed that the coupling effect between two fibers will become more significant with the increasing of the degree of fusion and the length of drawing, which is in accordance with experimental results. It’s also showed that large hydrogen flow and low speed will bring large degree of fusion. According to the distribution of light field, which is obtained by simulation of coupler of different stretching length, the coupling effect between fibers is related to the stretching length and the degree of fusion. Long stretching length and large degree of fusion make coupling effect strong. When the stretched length is short, the coupling of taper region can be ignored. On the one hand, it’s revealed that fibers become thinner and coupling effect becomes stronger in taper region with the increasing of stretched length. On the other hand it is also showed that the energy leaks out of the fiber and it leads to excess loss.In the third part, the model of broad band couplers is established, namely antisymmetric coupler, and the model is simulated by the beam propagation method. The splitting ratio, loss and bandwidth are obtained. Compared with symmetric coupler, the energy between two fibers is transformed incompletely, and coupler with large variation in the degree of fusion comes with more loss. Meanwhile, the ratio of diameter and degree of fusion make a difference to the band of such couplers. It’s found that coupler with broad band and less loss will be got when the stretching length, fire and speed are chosen appropriately. In this article, the experiment is combined with simulation, and find out characteristics of process parameters affect the coupler. These works is beneficial to the actual processing and production of the coupler.