Simulation And Analysis Of Microring Resonant Wavelength Multiplexers

With the development of modern telecommunication and networks, more andmore attentions have been paid to dense-wavelength-division multiplexing(DWDM) systems. Microring resonator (MRR) can be used as a basic element ofthe optoelectronic devices such as filters, multiplexers, modulators and others.MRR devices have promising applications due to their simple structure, easierfabrication and compactness. Usually we can serially cascade multiple MRRs toflatten the passband and to obtain the box-like spectral response, therefore, MRRsare promising candidates applying to DWDM systems.The single-mode optical waveguide is the basic element in integratedoptoelectronic circuits and optoelectronic devices including MRR wavelengthmultiplexers. Based on the electric-magnetic theory, first the Marcatili's methedabout the straight rectangular waveguide is introduced, and the Melloni'stechnique about the bent rectangular waveguide is described. Then by using thecoupled mode theory (CMT) and the transfer matrix technique (TMT), thebending coupling is analyzed in detail between two bent rectangular waveguidesor between a straight waveguide and a bent one. Furthermore, some expressionsare derived, such as the amplitude coupling equation, the bending couplingcoefficient and the amplitude coupling ratio. As an example, analysis anddiscussion are performed for the case of the vertical bent coupling, and aconclusion is reached as: the amplitude coupling ratio is a function of the bentradius, the coupling layer thickness, and the central deviation, so by selecting theirproper values, the appropriate amplitude coupling ratio can be obtained. Thisprovides the theoretical basis and design principle of MRR wavelengthmultiplexers.In order to describe the design principle of some MRR wavelengthmultiplexers with more complicated structures, first parameters optimization ofsingle MRR wavelength muliplexer is carried out. These parameters include thewidth and thickness a and b of the guide core, mode effective refractive index nc,diffraction order m, ring radius R, radius difference of adjacent microring ?R ,free spectrum range FSR, and the maximum number Nmax of output channels in anFSR. To be emphasized, optimization design of the MRR wavelength multiplexerplays a determinative role in realizing the demultiplexing function of it. If therelative parameters are selected improperly, it would bring about difficulties infabrication of the device, and its demultiplexing function desired is unable toarrived.In the simulation of the single MRR wavelength multiplexer, the transferfunction is presented, and the transmission characteristics are analyzed anddiscussed, including the bent loss resulting from microrings, leakage loss causedby the substrate with high refractive index, output spectrum, inserted loss andcrosstalk. The results show that the bending loss is much smaller than thepropagation loss when the radius of microring is larger than 10 μm, the leakageloss is so small that it can be neglected when the thickness of the confined layerbetween the core and substrate is sufficiently small. Though the single MRRwavelength multiplexer possesses basic demultiplexing function, it still has somedeficiencies, for example, the coupling layer between the microring and channel istoo thin for the fabrication, the spectral response is convex, the non-resonant lightis strong, and hence the crosstalk is large.On the basis of the work above, in order to improve the performance of thesingle MRR wavelength multiplexer, we propose some novel MRR wavelengthmultiplexers. For the strong non-resonant light of the single MRR structure, wedesign a cross-grid array of MRR wavelength multiplexer, which consists of eightfilter elements, each of which contains double identical rings. Calculated resultsshow that the 3-dB bandwidth is about 0.2 nm, and the ratio between –1dB and–10 dB bandwidths is about 0.22. In order to easily fabricate this kind of device,we design a 1× 8 MRR wavelength multiplexer, in which every filter elementcontains a so-called "smooth square ring". By adding the coupling length of thesmooth square, the coupling layer thickness d between the rings and channels canbe increased efficiently compare with the single MRR structural device for thesame amplitude coupling ratio. We find that for the amplitude coupling ratio κ =0.2, d is increased to 0.92 μm for the case of L = 10 μm, while d is only 0.52 μmfor the case of L = 0. In order to improve the Lorentzian spectral response of thesingle MRR wavelength multiplexer, usually we can serially cascade multipleMRRs to flatten the passband and to obtain the box-like spectral response. Byserially cascading double or triple rings, and by properly selecting the amplitudecoupling ratio between the ring and channel as well as that between the two rings,the flat box-like spectral response can be formed, and the crosstalk can be reducedgreatly.Currently in the study of MRR devices, a lot of theoretical and experimentalresearches have been done on the MRR filters, but there are only a few reports onthe investigations of the MRR wavelength multiplexers. In this thesis we proposesome MRR wavelength multiplexers with novel structures. Some novelties of thisthesis are as follows:(1) We have designed a polymer cross-grid array of MRR wavelengthmultiplexer, which has weaker output non-resonant light and lower crosstalk, andcan realize better wavelength demultiplexing compared with the device with asingle ring in every filter element.(2) We have designed a polymer MRR wavelength multiplexer, in whichevery filter element contains a so-called "smooth square ring". By adding thecoupling length of the smooth square, the thickness of coupling layer between thering and channel can be increased efficiently compare with the single MRRstructural device for the same amplitude coupling ratio, which is of benefit to thefabrication of the device.(3) The single MRR wavelength multiplexer has a convex spectral responseand strong non-resonant light, hence possesses large crosstalk. We have designedtwo kinds of MRR wavelength multiplexers, one is with double series-coupledrings in every filter element, and another is with triple series-coupled rings inevery filter element. By serially cascading double or triple rings, the box-like flatspectral response can be formed, and the crosstalk can be reduced greatly.(4) We have presented novel formulas of the transfer functions and theamplitude coupling ratios for characteristic analysis and parameter optimization ofthe above MRR wavelength multiplexers. Using the presented technique and itsrelative formulas, we can easily and conveniently perform the formulized analysisof the transmission characteristics. The presented technique, we think, is valuableand helpful for the design and optimization of the MRR wavelength multiplexersas well as some other similar structural devices.