Study On Common Tapered Fiber And Optimization For Filtering Characteristics Of Fiber Loop Mirror Based On The Tapered Fiber

The tapered fiber is an important research field in optical waveguide, by heating part of it into molten state and then pull two ends of it to let it become long and thin,the shape and the optical properties of tapered fiber will get changed. Due to the special structure characteristics of tapered fiber, the mode field radius, the normalized frequency, the distribution of optical power, the mode, the effective cross-sectional area, the density of light intensity, the nonlinear coefficient, the dispersion effects and many other optical properties in optical fiber have been changed and the tapered region also has the properties of focusing light and so on. So tapered fiber is widely applied in optical fiber devices, sensors, microprobe, the generation of super-continuum spectrum and many other fields.Fiber loop mirror filter is one kind of optical filter, it’s main filter principle is to divide a laser beam into two beams through optical fiber coupler, the two breams respectively spread in the opposite direction along the fiber loop mirror then meet again and get interfered in the optical fiber coupler, so from the output side we can get the filtering spectrum. The high-birefringence fiber is added in fiber loop mirror so that we can get better filtering effect, this structure has many advantages, such as simple structure, flexible to design, high stability, a lot kind of filtering effects can be get by adjusting the polarization controller, polarization independent,anti-electromagnetic interference, low cost, etc. With the rapid development of optical fiber communications, the requirements of fiber optic transmission capacity and transmission rate are also raised. Such a lot advantages of the high-birefringence fiber loop mirror filter has attracted many scholars’ attention and it is also widely researched and used in many fields.The main content of the thesis is arranged as follow:1 Theoretically analyze the unique optical properties of tapered fiber and do some researches on its applications in optical fiber devices, sensors, microprobe and super-continuum spectrum such a lot kind of fields, it shows that tapered fiber has unique advantages and plays an important position in the developments of modern science and technology.2 Theoretically deduce the transmission functions of first-order, second-order,two cascaded first-order and two cascaded second-order high-birefringence fiber loop mirrors filter, summarize the filtering characteristics of the output spectrum of different structures and high-order structures about high-birefringence fiber loop mirrors filter, we also do some researches on the filtering characteristics when the length of high-birefringence fiber in high-order-high-birefringence fiber loop mirrors structures get changed.3 Design an optimized structure of fiber loop mirror filter which is based on tapered fiber and our purpose is to get simply made and better filtering performance filter. Studies have shown that after adding tapered fiber into two cascaded second-order-high-order-high-birefringence fiber loop mirrors, the 3d B bandwidth is significantly compressed and the quality factor is further improved, it can reach about5.2 easily, at the same time the tunable range of the notch depth from 10 d B to 35 d B is more flexible to be tuned and the output waveform becomes more stable.In this paper we have done very deep researches both on common tapered fiber and high-birefringence fiber loop mirror, finally we combine the optical properties of tapered fiber and the filtering characteristics of high-birefringence fiber loop mirror filter to get simply made and better filtering effect in order to make contributions for the rapid development of optical communication, also to show the wide application and the important position in the development of modern science and technology of tapered fiber.