Due to the many transmission modes of the step-type plastic optical fiber, the transmission paths of the various modes are different, and the time for the light to reach the end point after transmission is also different, thereby generating a delay difference and widening the optical pulse. Therefore, the optical fiber has high inter-mode dispersion, a wide transmission band, and a transmission rate that cannot be too high. It is not ideal for communication, and is no longer the focus of research.
The gradient refractive index distribution plastic optical fiber mainly adopts the interface gel method. The process flow is as follows: firstly, the high refractive index dopant is placed in the core monomer to prepare a core mixed solution, and secondly, the initiator which controls the polymerization rate and the molecular weight of the polymer is used. The chain transfer agent is placed in the core mixed solution, and the solution is placed in a hollow tube selected as a cladding material. Finally, the PMMA tube containing the core mixed solution is placed in an oven and polymerized under a certain temperature and conditions. During the polymerization, the PMMA tube is gradually swollen by the mixed solution to form a gel phase on the inner wall of the PMMA tube. In the gel phase, the molecular motion speed is slowed down, the polymerization reaction is accelerated by the "gel action", the thickness of the polymer is gradually thickened, and the polymerization is terminated at the center of the PMMA tube, thereby obtaining an optical fiber having a gradient of the refractive index in the radial direction. The preform is finally put into the heating furnace and heated to form a gradient refractive index distribution plastic optical fiber.
Process optimization of gradient-index plastic optical fiber is mainly considered from two aspects: one is to reduce transmission loss, and the other is to expand transmission bandwidth. Reducing Transmission Loss The transmission loss of plastic optical fibers is mainly the absorption loss and dispersion loss of the core plastic material.
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