Research Of Optical Logic Gates Based On Sagnac Interferometer

With the development of the dense wavelength division multiplex(DWDM),the communication capacity of single fiber grows rapidly,which puts great pressure on electronic exchange,and drives people to pay considerable attentions to optical exchange to build all-optical network.As the key components of all-optical exchange and all-optical computing,all-optical logic gates naturally attract attention,as far many scholars have proposed a lot of theoretically feasible plans.But all those devices do not have the ability to handle complex logic operation,and their prices are relatively high.What is worse,all those efforts are still in the experimental stage,it needs more efforts to completely replace the electronic control logic gate and eventually achieve commercialization.In this paper,one also does some research on this subject,taking advantage of the Kerr effect and the Pockels effect.One studies optical logic gates based on Sagnac interferometer by inserting electro-optical crystal and erbium-doped fiber amplifier(EDFA)into the Sagnac loop,respectively.The specific research works and results are as follows:Firstly,by using the coupled-mode theory one obtains the coupling matrix of the linear symmetric couplers,and simply analyses the transmission characteristic with the matrix.One also has introduced the fundamental structure of the Sagnac interferometer and its operational principle.Utilizing the coupling matrix of fiber coupler one gets the transmission equation of the symmetric and asymmetric Sagnac interferometer to discuss their switching characteristic and logic function.The result shows that if one can design an asymmetric structure either in the coupler or in the fiber loop,this kind of interferometer structure can act as optical switching and optical logic gate.Secondly,one designs an optical logic gate with the Sagnac interferometer based on the Pockels effect.The circulators can distribute the two opposite propagating beams in the loop to two lithium niobate(LiNbO3)electro-optical waveguides,which have the same parameters and are added to reverse direct-current(DC)modulation voltage,thus the loop becomes asymmetry.By using the coupling matrix one establishes the theoretical model of this scheme to discuss its switching characters and logical function,in the case that the initial phases of the input signals are the same and different.When the initial phases are same,the curves of the transmission and the extinction ration show that changing the DC voltage can make the output route switch periodically between the output ports,and selecting appropriate values for the DC voltage,this structure can realize the switching and different logic gates such as OR and XOR.When the initial phases are not same,one researches the three-dimensional curve of extinction ration as the function of the initialphase difference and the DC voltage.According to the curves one finds when only one signal is input,its phase have no influence on the output,and if there are two signals input,the dephasing can also realize switching and let the extinction ration range more widely,thus one can get better performance logic gates.One can conclude that by altering the modulation voltage or the initial phase of signals,a variety of logic functions can be realized.In additional,the optical beams all perform in the optical domain without any optical-to-electronic conversion,and the DC voltage is only used to change the refractive index of the LiNbO3 waveguides,so this device can also be called all-optical decive.Lastly,the paper mainly studies the all-optical logic gates with EDFA-based Sagnac interferometer.By placing an EDFA in the non-centric position of the loop,the Kerr effect can result in the two opposite propagating beams obtaining different nonlinear phase,namely,realize asymmetry in the loop.Through studying the normalized and relative transmission and the extinction ration,one can obtain the switching characters and logical function.The results show that:the pump added on the EDFA and the initial dephasing of the input signals can both realize switching.When the initial phases are same,the smaller the straight coupling coefficient is,the larger the threshold power is,and the bigger the gain is when the two outputs have the same power.What's more,it doesn't mean that the bigger the gain is,the greater the output power of one port is.The influence of the coupling coefficient on the extinction ration is about its span,slope and extreme value,and the effect is not obvious.When taking the dephasing into consideration,the range of the extinction ration will be wider,and the logic function will have better performace.