| Filter is used to transmit spectrum of definitive broad band in the continuous spectrum or to extract certain radiation in the line-spectra. It has an important role in all domains of science and technology. Especially with the development of wavelength division, multiplexing and dense wavelength division multiplexing systems which are to improve the transmission speed and capacity in optical fiber communication, tunable filter as an important device of such systems is caused attention by research department.The selection of the wavelength which is realized by the filter is the key function of most the system of wavelength division multiplexing. If certain wavelength can through the tunable filter and others are cut off, the data can be got correctly. If the tunable receiver is required, the filter must be tunable. So Basing on diversity theory, there are diversified tunable filters.The mechanical tunable grating filter is realized by a grating and a rotary section. The angle between the incident light and grating is the decisive factor of the transmit wavelength. The tunable performance is obtained by rotating the grating. The disadvantages of the mechanical tunable filter are high control voltage, slow response time and big volume.The electro-optiQ tunable filters are obtained by the alteration of the birefringence of the electro-optic material applied voltage. The tunable performance can be realized by rotating the electro-optic material in the electric field. Now with the development of the crystal material science, the e electro-optic tunable filters are used widely. Basing on the theory of Lyot filter and electro-optic effect of liquid crystal, tunable liquid crystal Lyot filter is designed and tested in this paper. It is proved that it has many attractive features:t(1) Its applicable spectral range is broad. Many liquid crystals have very large birefringence and good transparency in both near ultraviolet band, visible band and infrared band.(2) The tunability is' determined by the birefringence of the liquidcrystal, a large birefringence change can be obtained by a small voltage change.(3) The thickness of the liquid crystal and the arrangment of theliquid crystal molecule are flexible, there are no deviation in the outgoing beams.(4) Its used area is wide, its fabrication is simple and its cost is low.Liquid crystal tunable filters as an important device of wavelength division multiplexing and dense wavelength division multiplexing systems is caused attention by research department, especially many foreign research institute are studying it. But in our country its study is lag, so study in such subject is academic and valuable.The theme include four sections. The theory of Lyot filter and FP filter and others are introduced in the first chapter. At the same time parameter and application in optic fiber communication of tunable filter are described in this part. In the second chapter, the basic knowledge and the electro-optic effect of liquid crystal are described in detail. The relation between birefringence of liquid crystal and voltage applied on liquid crystal is yielded by polarized light interferometric test. It is indicated that there will be a large birefringence change if the voltage has a minute variation. Because the liquid crystal molecule has rotary polarization and birefringence property after applying of voltage, the Jones matrix of liquid crystal is introduced so that we can settle problem by method of matrix optics.Tunable liquid crystal Lyot filter is designed and tested in the third chapter. Its feasibility is deduced by method of matrix optics and the relation between transmittance and birefringence. It is tested in the range from 400nm to 1600nm with the experiment setup basing on spectrophotometer. It is realized that tunable liquid crystal Lyot filter has good performance in the range of visible spectrum: narrow band and big fineness and flat curve. But it is not so good in infrared band. The further experiment proved that the tunabl... |
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