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Modal division multiplexing (MDM) can increase the information capacity of a single optical fiber through the separation of modal groups with indential wavelength. How to design mode demultiplexer is its key. G.A.Gasparian,C.S.Ronald and et al present Fresnel lens assembly and graded index rod lens for both accomplishing mode demultiplexing and output coupling. But the fabrication of them is complicated and higher cost. A composite holographic lens (CHL) with three focal points is introduced in this paper.This CHL can separate the mode groups, and converge them upon detectors or intasub-channer. It has also advantages of higher efficieny, lower cost, easier fabrication and so on. Thus the CHL can be used a mode demultiplexer in MDM system. Holographic lens (HL) is one of holographic optical elements (HOE). Its action bases on interference and diffraction of optical wave. When a diverging spherical wave interferes with a in-line converging spherical wave, we record the formed fringes using thick recording material. The recorded fringes are a set of hyperboloid situated in the volume of the material. A volume HL is obtained through processing of developing, fixing . Analyzing diffraction law of volume hologram we can know that the Bragg'ef-fect occurs and there is only one-order diffraction light but two-order or higher-order light. The diffraction efficieny of its one-order light is 100 % theoretically. When the volume HL is illuminated by original diverging spherical wave, a converging spherical wave is only diffracted from the hyperboloidal fringes, and focused at a point. The kind of volume HL is also called in-line HL with two focuses. According to the mode coupling equation of multimode fiber the far field at output end of the fiber consists of a number of con-centric annular rings. When two modes are excited and transtmited in the fiber, they emanate from the output end of the fiber, and are limited respectively to a cone cup angular and a cone angular which are concentric. Using the two emanating waves we design two off-axis sub-HLs correspondingly. Then we correct the two sub-HLs' aberration caused by wavelength shift. We eliminate Frensel reflection effect for diffraction efficieny. The dichrometed gelatin (DCG) having high diffraction effect is used as recording material. The two sub-HL is made at indentical plate and is called CHL. We make a simulate experiment and measure the parameters of the CHL. Higher coupling efficiency and higher signal-to-noise are obtained. Measuring results demostrate that the CHL used as a mode demultiplexer is feasible. Finally some problems about fabricating practical CHL are also discussed in the paper.
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