Study Of Glass-Based Ion-Exchange Multimode Integrated Optical Splitter

Comparing with singlemode fiber, multimode fiber has larger core, larger numerical apertures and stronger ability to assemble power and is easier to align with fiber set. Large core plastic optical fiber is especially cheap, light, easy to handle and so on. The field where effect of multimode loss and chromatic dispersion is unimportant could be composed of multimode fiber because of low-cost advantage. It is necessary to research multimode waveguides and integrated multimode optical waveguide devices which are indispensable in multimode systems. The representative devices are multimode optical splitters.Firstly, we describe the type of multimode splitters and fabrication methods. Then we introduce theory of ion-exchange process and how to design multimode Y branch optical splitters. Lastly we do some experiment of how to fabricate the device with Li⁺-Na⁺ ion-exchange technology.We compare fiber multimode splitter with waveguide multimode splitter, the later is much better. The advantages of ion-exchange technology are low-cost, easily fabricated and so on. So ion-exchange method is the most popular one to fabricate distributed index multimode devices.The shortcoming of ordinary multimode Y branch optical power splitter is that the branch characteristic strongly depends on the excited modal distribution. Different input conditions including different input modes and different input positions will excite different modes and different intensity distribution of these modes in highly multimode waveguides. The interference of these modes will contribute to distribute unstably, i.e. different input conditions contribute to different mode distribution, at last contributes to different branch ratio. The performance of the splitter with different taper structure is analyzed and simulated with two-dimensional beam propagation method (2D-BPM). A novel Y branch splitter is simulated and designed through Beamprop software of Rsoft Company.When try to fabricate multimode waveguide splitter with glass ion-exchange process, there are many factors to be considered, such as exchange equipment,exchange sources, exchange temperature. We can quickly fabricate multimode waveguide devices by Li⁺-Na⁺ ion-exchange method. The disadvantage of this process is that because of large difference ion radius of Li ion and Na ion, greater exchange stress will present after ion-exchange. This will contribute to large loss of waveguide. Appropriate concentration of exchange sources such as LiNO₃ and Li₂SO₄ and high exchange temperature should be considered. High temperature requires of special exchange equipment, and it also needs special mask. In this thesis, experiment and analysis about these problems are presented. We try our best to do some useful work for future multimode optical devices.