Design And Fabrication Of Multimode PLC Optical Power Splitter

As a solution to Fiber to the Home(FTTH),Passive Optical Network(PON)requires a large quantity of passive optical devices,with optical power splitters and wavelength division multiplexers as its key elements.Design and fabrication of multimode Planar Lightwave Circuit(PLC)optical power splitter was carried out. This compact device has low insertion loss,good uniformity and large assembly tolerances,and it's also insensitive to wavelength and easy to fabricate.At the stage of design,to solve the problem of modal noise,optical power splitter is divided into two parts by their functions-scrambling section and splitting section. The function of scrambling section is to make optical signals coming from multimode optical fibers reach steady-state mode power distribution;the function of splitting section is to split optical signals and guide them to different output ports. Based on ray tracing method,scrambling section was modeled by non-sequential ray tracing of ZEMAX.1×4 and 1×8 scrambling sections were designed,and lateral, longitudinal and angular assembly tolerances were calculated.By comparison,it was concluded that 1×4 scrambling section should use rectangular structure,and 1×8 scrambling section could use either rectangular or tapered structure.Based on Beam Propagation Method(BPM),splitting section was modeled by BeamPROP.For splitting section composed of straight waveguides,the angleθbetween the waveguide and the axis should not exceed 25°;for splitting section composed of S-bend waveguides,the distance L between the waveguide's output port and the axis should not exceed 2.2mm.At the stage of fabrication,soft lithography was used to fabricate the device.The master was made by Computer Numerical Control(CNC)engraving system.During knife grinding,the method of semi-grinding was effectively improved.Since there was limitation on engraving precision,new structures with waveguide width as large as 200μm were designed and the master was engraved.After the master was polished, stamp was made and micro-structures were formed,and the whole process only took several hours.Soft lithography proves to be convenient,time-saving and low-cost, which are its great advantages.After fabrication,laboratory test of the device was carried out.It proved to have large lateral assembly tolerance,which resulted in relaxed coupling conditions.However, the outcome of the test didn't conform well to the theoretical expectation,which can be explained from two aspects:first,it was hard for the knife used in engraving to have knife-edge diameter smaller than 0.1mm,and the engraved waveguides had tapered walls;second,assembly tolerance in the thickness direction was quite small, and the experimental setup was too simple to avoid the assembly error.If new device with large tolerance in the thickness direction is designed and techniques are effectively improved to deal with these problems,performance of the device can be greatly improved.