Research Of All-optical Logic Gates Based On Resonance And Kerr Nonlinear Effect

All-optical network overcomes the problem of electronic bottleneck in information transmission.It reduces the information transmission congestion and has the merit of high quality,a large capacity of information conveying,transparent transmission rate,high flexibility and reliability,etc.All-optical network is the highest stage of the development of optical fiber communication technology.All optical switch as the key component of all optical communication technology,fully optical computing,optical transmission system,etc,can realize all kinds of logical operation on light domain without photoelectric conversion and becomes a research hotspot in recent years.The specific research works and results are as follows:1.This paper introduces the nonlinear equation of optical pulse in optical fiber and deduces the coupled mode equations of optical fiber coupler.The structure of the two cores coupler and the switching characteristics are discussed.By using the coupled-mode theory,one has analyzed the switching characteristics of fiber coupler when the low energy continuous light and high energy continuous light incident into it respectively.The optical fiber coupler works in linear state when the low energy continuous light incidents into it,while it works in nonlinear state,when the high energy continuous light incidents into it.2.This paper theoretically investigates all-optical logical gates based on the resonant nonlinearity in erbium-doped fiber coupler.The resonant nonlinearity in rare-earth-doped fibers,which is not the same as the Kerr nonlinearity induced by third order susceptibility,is mainly yielded by the resonant transition of the rare-earth elements from the ground state to metastable state.This type of resonant nonlinearity is usually many orders of magnitude larger than that provided by Kerr effect.When strong pump light at 980 nm strikes the erbium-doped fiber,erbium ion yields optical transition between the ground states⁴I_?15?/2and the metastable states(⁴I_?13?/2by absorbing the photon of the pump light.Because of the population inversion,the signal nearby 1550 nm will be amplified.The propagation constant is changed because of the gain and the resonant nonlinearity caused by the pump.Both the change of refractive index and gain coefficient in the coupling region are regarded as the real and imaginary part of the variation of the complex propagation constant.This paper introduces three level model diagram of erbium ions and discusses the principle of resonance nonlinear effect,and deduces the coupled-mode equations based on the resonance nonlinear of all optical logic gates.According to the coupled-mode equations one does the work of MATLAB simulation.When the initial phases of two input signals are the same,a signal and a pump light in port A are coupled input into channel Iby a wavelength division multiplexer?WDM?.Because the wavelength of the pump light is different from that of the signal light,the pump light can remain in the channel I,thus one arm of the coupler is pumped to provide gain while the other arm is the absence of the gain.As a result the propagation constant of the channel I is changed because of the gain and the resonant nonlinearity caused by pump.Consequently,the modification of the phase difference,between channel I and channel II,leads to the switching of the output signal in port C and D.One can realize different logic gates by controlling the pump power and the two input signals.One can also get the various logic operations by adjusting the initial phase of two input signals.One also shows the three dimensional plots of the pump light,the phase difference and the extinction ratio.Compared with the other undoped couplers the erbium-doped fiber coupler can drop the switching threshold power,thus it avoids damaging the fiber.3.This paper investigates all-optical logic gates based on wavelength conversion in a nonlinear directional coupler.Firstly one introduces various wavelength conversion technologies,and discussed the principle of them,and compared the advantages and disadvantages of various wavelength conversion techniques.In this paper,one studies the wavelength conversion technology based on the optical fiber cross phase modulation.In my scheme,to avoid damaging the fiber,the coupler is constructed from bismuth oxide-based nonlinear optical fiber?Bi-NLF?.Due to the ultra-high nonlinearity of the fiber,the threshold power of switching can be reduced.Here a weak continuous-wave?cw?light and a pump light with high power which is the input signal amplified are coupled into channel I by WDM.The pump light and the weak light will produce cross phase modulation,and lead to the conversion between the two waveguides.The information on the pump light will transfer to the weak cw light with diverse wavelength because the on-off of the weak cw light is controlled by two pumps which carry information.All-optical logic gates based on wavelength conversion exhibit high switching contrast and have a better switching characteristic.Based on predecessors' research of the switch characteristics in optical fiber couplers,this paper fully studies all-optical logic gates based on resonant nonlinearity and all-optical logic gates based on wavelength conversion.Previous studies of all optical logic gates are generally based on Kerr effect and the switching threshold power is up to tens of kilowatts.All-optical logic gates based on resonant nonlinearity can drop the switching threshold power,which greatly reduce the damage to the fiber and the switching threshold power is in milliwatt magnitude.All-optical logic gates based on wavelength conversion using high nonlinear optical fiber exhibit high switching contrast and have a better switching characteristic,which provide a theoretical basis for the study of low cost,high speed,and practical all-optical switches.