Analysis And Optimization Of The Direct UV-written Microring Resonant Wavelength Demulti/multiplexer

With the rapid increase of global message volume, optical communication system's demand for bandwidth and capacity improves constantly. Wavelength division multiplex technology is capable of realizing large capacity and multifunction. It has great potential in network service application and becomes preferred technology in optical fiber communication. Microring resonator (MRR) can be used as a basic element of the optoelectronic devices such as filters, multiplexers, modulators and others. MRR devices have promising applications due to their simple structure, easier fabrication and compactness. Usually we can serially cascade multiple MRRs to flatten the passband and to obtain the box-like spectral response, therefore, MRRs are promising candidates applying to DWDM systems.This paper includes three parts:1. The single-mode optical waveguide is the basic element in integrated optoelectronic circuits and optoelectronic devices including MRR wavelength multiplexers. Based on the electric-magnetic theory, first the Marcatili's method about the straight rectangular waveguide is introduced, and the Melloni's technique about the bent rectangular waveguide is described. Then by using the coupled mode theory (CMT) and the transfer matrix technique (TMT), the bending coupling is analyzed in detail between two bent rectangular waveguides or between a straight waveguide and a bent one. Furthermore, some expressions are derived, such as the amplitude coupling equation, the bending coupling coefficient and the amplitude coupling ratio. As an example, analysis and discussion are performed for the case of the vertical bent coupling, and a conclusion is reached as: the amplitude coupling ratio is a function of the bent radius, the coupling layer thickness, so by selecting their proper values, the appropriate amplitude coupling ratio can be obtained. This provides the theoretical basis and design principle of MRR wavelength multiplexers.2. In order to describe the design principle of some MRR wavelength multiplexers with more complicated structures, first parameters optimization of single MRR wavelength multiplexer is carried out. On the base of introducing the theory of mode describing transmission characteristics of the guided wave, the coupling of bent rectangular dielectric optical waveguides is analyzed in detail in terms of the coupled mode theory in dielectric optical waveguides, and the amplitude coupling equation of bent waveguides is derived using the method of transmission matrix. Those provide the sufficient theoretical basis for describing the design principle of MRR. Then the microring resonant equation is derived from the transfer function of light intensity of MRR with a single ring using the coupled mode equations, which is the direct theoretical basis for the operating principle of MRR. By means of describing the designing principle of MRR, a conclusion is arrived that the microring resonant order m is an key parameter which determines the others such as the microring radius R, the free spectrum range FSR and others. These parameters include the width 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 AR, free spectrum range FSR, and the maximum number Nmax of output channels in an FSR. To be emphasized, optimization design of the MRR wavelength multiplexer plays a determinative role in realizing the demultiplexing function of it. If the relative parameters are selected improperly, it would bring about difficulties in fabrication of the device, and its demultiplexing function desired is unable to arrive.In the simulation of the single MRR wavelength multiplexer, the transmission characteristics are analyzed and discussed, including the bent loss resulting from microrings, leakage loss caused by the substrate with high refractive index, output spectrum, inserted loss and crosstalk. The results show that the bending loss is much smaller than the propagation loss when the radius of microring is larger than 10 (am, the leakage loss is so small that it can be neglected when the thickness of the confined layer between the core and substrate is sufficiently small. Though the single MRR wavelength multiplexer possesses basic demultiplexing function, it still has some deficiencies, for example, the coupling layer between the microring and channel is too thin for the fabrication, the spectral response is convex, the non-resonant light is strong, and hence the crosstalk is large. Finally, It is difficult to be fabricated by UV-written.3. Finally, it is also the core of this thesis. On the basis of the work above, in order to improve the performance of the single MRR wavelength multiplexer, and more conducive to fabricate in the laboratory by UV-written, we propose some novel MRR wavelength multiplexers. And these novel MRR wavelength multiplexers are suited to be fabricated by UV-written. We designed the 1X8 three smooth square (and double smooth octagon) microring resonant wavelength demulti/multiplexer, in which every filter element contains three so-called smooth square (or octagon) rings. The coupling length of the smooth square between double smooth square rings (or the channel and the square ring) becomes large. It will make the device can be fabricated by direct UV-written. In order to improve the Lorentzian spectral response of the single MRR wavelength multiplexer, usually we can serially cascade multiple MRRs to flatten the passband and to obtain the box-like spectral response. By serially cascading double or triple rings, and by properly selecting the amplitude coupling 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 reduced greatly.Currently in the study of MRR devices, a lot of theoretical and experimental researches have been done on the MRR filters, but there are only a few reports on the investigations of the MRR wavelength multiplexers. In this thesis we propose some MRR wavelength multiplexers with novel structures. Some novelties of this thesis are as follows:(1) New ideas first proposed for production of MRRWDM by UV-written. Because it is quite difficult to fabricate traditional MRRDWDM by etching method, so we use the characteristics of strong laser photon energy, the ultraviolet will be introduced directly into the production of MRRWDM to replace the traditional etching technology for photonic processing, and has some innovative.(2) We have designed a MRR wavelength multiplexer, in which every filter element contains a so-called "smooth square ring". By adding the coupling length of the smooth square, the thickness of coupling layer between the ring and channel can be increased efficiently compare with the single MRR structural device for the same amplitude coupling ratio, which is of benefit to the fabrication of the device by UV-written.(3) The single MRR wavelength multiplexer has a convex spectral response and strong non-resonant light, hence possesses large crosstalk. We have designed two kinds of MRR wavelength multiplexers, one is with double series-coupled rings in every filter element, and another is with triple series-coupled rings in every filter element. By serially cascading double or triple rings, the box-like flat spectral response can be formed, and the crosstalk can be reduced greatly.(4) We have presented novel formulas of the transfer functions and the amplitude coupling ratios for characteristic analysis and parameter optimization of the above MRR wavelength multiplexers. Using the presented technique and its relative formulas, we can easily and conveniently perform the formulized analysis of the transmission characteristics. The presented technique, we think, is valuable and helpful for the design and optimization of the MRR wavelength multiplexers as well as some other similar structural devices.