Optimization Design And Analysis Simulation Of Polymer-on-silicon Electro-optic Switches

Optical switches and arrays are key components in optical communication networks, and asthe application of data communication and dense wavelength division multiplexing (DWDM)systems, it is an urgent need of stable and feasible network management for the complicatednetwork topology. The functions, including optical add/drop, optical interconnects, opticaldomain optimization, optical routing, optical protection and optical recovery, all depend onoptical switches. Compared to other type based optical switches, electro optic switches utilizingthe electro optic effect take the advantages of fast switching speed, large transmission capacity,et al, and meanwhile the poled polymers possess the advantages of large electro opticcoefficient, fast response speed, et al, therefore, researches on such switches including bothexperiment and theory have been increasing dramatically. It is an issue to design and fabricatepolymer based electro optic switches with low driving voltage, wide output spectrum, lowinsertion loss and crosstalk, high cutoff switching frequency, et al, for assuring normalperformances of optical communication system.For enhancing the electro optic modulation efficiency, minimizing the device size,decreasing the voltage length product, releasing the fabrication tolerance, enlarging thespectrum, analyzing the time frequency response characteristics and analyzing the skin effectcharacteristics, the main contributions of this paper are as: the improve design and novel designon the structures of several kinds of directional coupler electro optic switches, Y fed couplerelectro optic switches and MZI electro optic swites, and the optimum design and analysis ontime frequency response characteristics and skin effect characteristics of severalimpedance matched and shielded push pull high speed polymer electro optic switches. Theworking principle of each device are thoroughly described, the parameters of the electrode andwaveguide are optimized, the switching performances are simulated, the analytical theory anddesign technique are checked, and the advantages and disadvantages for the designed switchesare discussed. This paper aims to provide related data source and essential theoretical basis forthe fabrication of such switches in laboratory.For the polymer on silicon electro optic switches, the structures of medium absorption ribwaveguide, single metal clad rib waveguide and double metal clad rib waveguide aredemonstrated, and their mode propagation characteristics are analyzed in detail. Because of theexistence of the high refractive index substrate, the propagation mode will be modified toquasi guided mode, and this will lead to leakage loss. Simulation results based on the materialparameters used in the design reveals that a small leakage loss is achieved when the under bufferlayer exceeds certain thickness, and then the under buffer layer can be treated as half infinite soas to ignore the existing of the substrate. Moreover, when the upper buffer layer exceeds certain thickness, it can also be treated as half infinite so as to ignore the effect of the upper confinedlayer on the mode propagation. Therefore, when the upper/under buffer layers both exceedcertain thickness, the influences of the substrate and upper confined layer to the modepropagation characteristics can be both neglected, and the waveguide in the passive region cansimply be regarded to be three layer medium absorption rib waveguide; when the upper/underbuffer layers both exceed certain thickness, the influences of the upper/uner electrodes to themode propagation characteristics can be both neglected, and the waveguide in the active regioncan simply be regarded to be three layer medium absorption rib waveguide. For the proposedthree kinds of rib waveguide, the mode effective refractive index and mode loss coefficient canbe solved by using the effective index method, the pertuebation theory and the differentiationmethod. On these basises, in terms of the equavilent effective method and the Marcatiliapproximation method, the mode distribution of the rib waveguide and the directional couplingbetween the two rib waveguides are analyzed.The electro optic modulation performances of the poled polymer thin film are analyzed interms of the electro optic modulation theory, and the refractive index changes along thex direction and y direction are obtained under the vertical poling and vertical modulating. Sincethe refractive index changes along the two directions are different, the effective index method isstill reasonable, and the Marcatili approximation method should be revised for accuracy. Severalelectrode structures including the coplanar waveguide electrodes, the push pull micro stripelectrodes and the push pull shielded electrodes are presented. The formulation expressions ofthe microwave effective refractive index, the microwave propagation constant and themicrowave characteristic impedance are obtained. When the buffer layer possesses certaindefinite thickness, the conformal transforming method and image method are demonstrated foranalyzing the coplanar waveguide electrodes. When the electrode is infinitely thin and finitelythick, the point matching method and extended point matching method for analyzing thecoplanar waveguide electrodes, micro strip electrodes and shielded electrodes are illustrated,respectively. For the proposed medium parameter model, the electric potentials in differentregions are expanded in terms of the Fourier transformation, and the solutions to the unknowncoefficients are obtained by using the boundary conditions. Under the low and high microwaveswitching frequencies, the equivalent electrode models and the impedance matching principleare described, and the connection wires with the requirement of the impedance match for eachelectrode structure are discussed.As the core content of this thesis, based on the proposed theory and design techniques onthe waveguide and electrode, for the directional couler electro optic switches, Y fed couplerelectro optic switches and the MZI electro optic switches, the material parameters including therefractive index, the loss coefficient and the extinction coefficient and the dimension parametersincluding the thickness, width and length of each layer are defined as variables. Structural modelwith parameters, thorough analysis and derivative process are presented. Under the centrloperation wavelength of 1550 nm, to obtain low switching voltage, low insertion loss andcrosstalk, wide spectrum, small device size and relaxed fabrication tolarence, the structuralparameters are optimized, and the switching features are simulated and discussed. The detailsare as the following three aspects. First, for the dorectional coupler electro optic switch, a novel crosscronect push pullmicro strip four section electrode structure is proposed, and the electro optic modulationefficiency along the vertical direction is greatly larger than that of the coplanar waveguideelectrodes. The designed device shows a low switching voltage of 1.645 V, which is 1/5 times ofthat of the switch with coplanar waveguide electrodes. Besides, the insertion loss is less than1.98 dB, and the crosstalk is less than–30 dB for this device. To release the fabrication tolerance,two kind of electro optic switches with two section and three section reversed electrodes arethen designed and optimized, respectively. Simulation results show that, for the two sectionreversed electrodes based device, the electro optic coupling region length is 4751 m, the crossand bar state voltages are 1.22 V and 2.65 V, and the insertion loss and crosstalk are less than2.21 and–30 dB, respectively, and by slightly adjusting the state voltages, the crosstalk of thetwo ports can be both dropped to less than–60 dB, within the fabrication error range of theelectro optic coupling region length from–100 to 100 m. whereas for the three sectionreversed electrodes based device, the bar and cross state voltages are 1.409 V and 2.889 V, theinsertion loss and crosstalk are less than 3.71 and–30 dB, respectively, and by slightly adjustingthe state voltages, the crosstalk of the two ports can be dropped to less than–50 and–60 dB,respectively, within the fabrication error range of the electro optic coupling region waveguidelength from–500 to 500 m.Second, to minimize the device size and meanwhile to decrease the switching voltage, twokinds of integrated Y fed coupler electro optic switches with single section and double sectionreversed electrodes are designed, respectively. Under the same parameter condition, calculationresults prove that the electro optic coupling length is 1 2 times of that of the directionalcoupler electro optic switch, while the switching voltages of the two are almost identical. TheY fed coupler electro optic switch with double section reversed electrodes has two differentswitching conditions as: for the first condition, the electro optic coupling region length is 1.8478times of that of the directional coupler electro optic switch, and the switching voltage is 0.4783times of the latter switch; and for the second condition, the electro optic coupling region lengthis 0.7654 times of that of the directional coupler electro optic switch, and the switching voltageis 2.7877 times of the latter switch.Third, the MZI electro optic switch with two critical 3 dB directional couplers areoptimized and designed. The results shows that the output spetrum is relatively narrow, which isless than about 60 nm, due to the phase shift error resulting from the wavelength variation. Toeliminate the phase shift arising from the wavelength variation in the electro optic region of thetraditional MZI switch, the auto phase compensating asymmetric first order PGC MZI switchand the symmetric N th order PGC MZI switch are designed. By optimizing the PGC structure,the phase shift error can be effectively eliminated, so a stable phase shift is obtained. Simulationresults show that for making the crosstalk to drop to less than–30 dB, the output spectrum of theasymmetric first order PGC MZI switch can be increased from 60 nm to 110 nm. Furthermore,the spectrums can be expanded to as large as 320, 390 and more than 435 nm, respectively, forthe symmetric first order, second order and third order PGC MZI electro optic switches.However, as the increase of the order number, the device fabrication becomes complicated andthe insertion loss increases. So the order number should be selected properly. As another core content of this thesis, firstly, to increase the switching speed and enablenormal high frequency switching function, design and optimization of the impedance matcheddirectional coupler electro optic switch and multimode interference (MMI) MZI electro opticswitch are carried out, and their time and frequency domain characteristics are analyzed,respectively. The cutoff switching frequency can be up to 114.7 and 263 GHz, respectivelyunder the step change voltage. However, we find that it is hard to realize simultaneously thehigh electro optic modulation efficiency, well impedance match and less refractive indexmismatch between lightwave and microwave, for these two electro optic switches. Therefore,secondly, the push pull shielded electrodes are introduced, and the shielded directional couplerelectro optic switch and the shielded Y fed coupler electro optic switch are optimized anddesigned, which makes the increase of the electro optic modulation efficiency and the match ofthe impedance can be realized by optimizing different electrode parameters. The analyticaltheory on skin effect for these two shielded switches are proposed, the formulas of the thresholdfrequency and effective electrode thickness are deduced, and the influences of the skin effect onthe microwave characteristic parameters, response characteristics, insertion loss, crosstalk,cutoff switching frequency and 3 dB modulating bandwidth are thoroughly discussed.Calculation results show that considering the skin effect, the cutoff switching frequency and the3 dB modulating bandwidth of the two switches are up to 172 and 94 GHz, respectively.Simulations in terms of the beam propagation method (BPM) on the designed switches areperformed, and calculation results of our technique are in good agreement with thos of the BPM.This reveals that the proposed theory and design technique possess satisfactory accuracy andfeasiablity on the engineering design.In conclusion, the proposed structures including the crossconnect push pull four sectionmicro strip electrodes, shielded puch pull electrodes, Y fed coupler, wide spectrum asymmetricfirst order PGC MZI and wide spectrum symmetric N th order PGC MZI, the novel analyticaltheory and related formulas on time frequency characteristic and skin effect characteristic andthe presented design technique and optimization process, to our knowledge, have not beenreported thoroughly, which are of certain innovation in this field. These techniques are alsovaluable and helpful for the design and fabrication of such devices in laboratory. Speciallyspeaking, the novelties of this thesis are concluded as follows.(1) We have proposed the novel crossconnect push pull micro strip four section electrodestructure for the directional coupler electro optic switches. The electro optic modulationefficiency is greatly improved due to the special cross connection of the driving voltage. Underthe same parameter condition, the switching voltage of the device with this electrode structure isabout 1/5 times of that of the device with the coplanar waveguide electrode structure, and stillthe complicated dual power source used to drive the devce is avoided.(2) We have designed the Y fed coupler electro optic switches with single section anddouble section reversed electrodes. The electro optic coupling length of the single section baseddevice is 1 2 times of that of the directional coupler electro optic switch, while the switchingvoltages of the two are almost identical. The switch with double section reversed electrodes hastwo different switching conditions as: for the first condition, the electro optic coupling regionlength is 1.8478 times of that of the directional coupler electro optic switch, and the switching voltage is 0.4783 times of the latter switch; and for the second condition, the electro opticcoupling region length is 0.7654 times of that of the directional coupler electro optic switch, andthe switching voltage is 2.7877 times of the latter switch.(3) We have introduced the shielded push pull electrodes in the design of high speedelectro optic switches, which makes the realization of high electro optic modulation efficiency,well impedance match and less refractive index match can be achieved by optimizing differentelectrode parameters. This makes the device design, in a degree, be more convenient.(4) The spectrum of the traditional MZI electro optic switch is only about 60 nm, whichconfines the application of such switches in WDM systems. To overcome this problem, we havedesigned the asymmetric first order PGC MZI switch and the symmetric N th order PGC MZIelectro optic switches. By optimizing the PGC structure, under the same crosstalk level, thespectrum can be improved to 110, 320, 390 and 435 nm, for the asymmetric first orderPGC MZI switch, symmetric first order PGC MZI swich, symmetric second order PGC MZIswitch and symmetric third order PGC MZI electro optic switches, respectively, and withinthese spectrum ranges, the crosstalk is less than–30 dB.(5) We have proposed the novel analytical method and related formulas for analyzing thetime and frequency domain response characteristics for the two designed impedance matcheddirectional coupler electro optic switch and the MMI MZI electro optic switch, under the lowswitching frequency and high switching frequency. Relations between the output powers and theresponse time under the step change voltage are deduced, and the expressions of the delay time,rise time, fall time, switching time and cutoff switching frequency are obtained.(6) We have proposed the novel skin effect analytical method for the shielded push pulldirectional coupler electro optic switch and the shielded push pull Y fed coupler electro opticswitch. The expressions of the threshold frequency and effective electrode thickness are deduced,and the influences of the skin effect on the microwave characteristics parameters, responsecharacteristics, insertion loss, crosstalk, cutoff switching frequency and 3 dB modulatingbandwidth are thoroughly discussed.(7) We have presented the formulas for analyzing the medium absorption rib waveguide,single metal clad rib waveguide and double metal clad rib waveguide, the analytical theory anddesign technique on the electrode, and the analytical techniques on time and frequency domaincharacteristics and those on skin effect characteristics. Comparared to other numericalsimulation methods, the proposed formulation method can provide more detailed optimumdesign and simulation, and they are of simplicity, normalization from and accuracy. With thepresented techniques, we can easily and conveniently perform the structural design, theoreticalanalysis, parameter optimization and characteristic simulation on such kind of electro opticswitches and similar devices. These techniques are also valuable and helpful for the fabricationof the electro optic switches as well as other similar structural devices in laboratory.