| In 1987, E. Yablonovitch and S. John pointed out that the behavior of photons can be changed when propagating in the material with periodical dielectric constant, and termed such material Photonic Crystal. As known, photonic crystal has characteristics as followed: Photon Band Gap, defect states, Light Localization and so on. These characteristics make it able to control photons, so it may be used to manufacture some high performance devices which have completely new principles or can not be manufactured before. The research on photonic crystals will promote its application and development on integrated photoelectron devices and optical communication.Photonic crystal's characteristics make it very useful in the aspect of all optical devices manufacture, such as Omni directional mirror, optical waveguide, resonance cavity and filter. Especially when defects are brought into photonic crystal structure, there will be a narrow transmitting peak in the wide PBG which has a very high transmitting rate value. This peak is very useful in optical information processing and optical communication. More important, if the defect layer is consisted of nonlinear material, according to change the intensity of input light, this peak will change with the intensity; this change has lots of application on designing of optical devices, especially on optical switches. Traditional optical switches are mainly micro-mechanism switches, which change optical path and optical channel with small action of lens or fibers. But the switches have some shortcomings such as slow switch time and bad reliability, with the improvement of communication speed the switch will can't satisfy the require. The photonic crystal with nonlinear dielectric defect layer can just provide us a optical switch with new principles. This optical switch has advantages to micro-mechanism switches as follow: good stability, high on/off ratio, small size, easily integrated; and photonic crystal has agile structures, so the structure is easily designed; in addition it's all optical device, it is in favor of realization of all optical communication system. Moreover nonlinear photonic crystal can be applied to limiter, which has very good limiting effect.The E and the H part of the electromagnetism are calculated in space and time domains with the FDTD method, in the style of disperse sampled alternately. Thus FDTD methods can analyze the distribution and transmission of electromagnetism in photonic crystals dynamically and accurately, and can provide the characteristics of light pulse's electric field in time domain and frequency domain, which is convenient for the design and analysis of photonic crystals.In this paper, according to programming we get a program which is used to simulate electromagnetism wave transmitting in dielectric with FDTD (Finite-Difference Time-Domain) method, and testify its correctness. Then one dimension photonic crystal structure and that with nonlinear dielectric defect layer are analyzed, their PBG characteristics and rules are as following:1. Make the structure of one-dimension photonic crystal and analyze its PBG characteristics. According to the analysis we know: the PBG( the frequency band in which the transmitting rate is rarely 0) of one dimension photonic crystal withλ0 / 4 structure is symmetrical of central wavelengthλ0 , and the PBG is existed not only in wavelengthλ0 , but alsoλ0 /3,λ0 /5,λ0 / 7……; the width of the PBG will be smaller with the increasing of the dielectric layers number, and the transmitting rate of PBG will be closer to 0, which means the edge of the PBG will be steeper; if the difference of the dielectrics grows up, the PBG will be wider and the transmitting rate in PBG will be lower, but the center of the PBG won't change, and the PBG will always be symmetrical of the center wavelength; and the analysis result tells that it's not only the structure ofλ0 / 4 can give PBG in photonic crystal, some other arbitrary thickness period dielectric layer structures are testified, there are also PBG, but its regulation is not so obvious.2. The PBG characteristics of photonic crystal with defect layer are analyzed. If a defect dielectric layer whose optical distance isλ0 / 2 is introduced into the photonic crystal, there will be a narrow transmitting peak whose transmitting rate is rarely 1 in the primary PBG center, but the width and the location of the PBG don't change in this course, and the location of transmitting rate can be justified to be an arbitrary wavelength in the PBG. To optimize photonic crystal structure, we discuss the effect of the defect dielectric layer location and the number of layers besides the defect layer, and find out that: if the defect layer is away from the center of the photonic crystal, the transmitting rate of the peak will be lower; and when the number of layers besides the defect layer grows up to some amount , the transmitting rate of the transmitting peak will also be lower, just because the dielectric layers besides the defect layer can function as a reflector mirror. So finally we adopt the structure composed with thirteen layers whose defect layer has aλ0 / 2 optical distance and is located in the center.3. The change of the nonlinear dielectric's refractive index caused by the change of the light intensity may effect on the movement of the transmitting peak, the rule of the effect is analyzed. When the incident light is weak which can not cause the change of the refractive index, the transmitting peak locate at 1 which is the center wavelength; the transmitting peak move left to the longer wavelength, and at the same time at the right edge there will be a new transmitting peak, and with the increase of the refractive index, both of the two transmitting peaks will move to the longer wavelength together; when the refractive index is double of the former value, the new transmitting peak also locates at 1, at this time the PBG is very similar to that when the refractive index don't change, the width of the PBG is nearly the same, just the side of the PBG changes obviously. The transmitting peak could point to any wavelength of the PBG according to change the refractive index of the nonlinear dielectric with pump light, and the transmitting characteristics of other wavelength don't change.4. Design the structure of the optical limiter and the optical switch, and optimize the parameters of the optical switches according to the actual wavelength of pump light and the working wavelength of optical communication. The principle of the optical limiter and optical switches is got from the discuss above, then we analyze and confirm the static working point and get the structure parameters of the optical limiter and the optical switch. We get the photonic crystal limiter working at 1.06μm and the photonic crystal optical switch working at 1.55μm, but we notice that there is not a laser which has proper wavelength to be the pump source. Then we adjust the parameters of the optical switch according to the regulations got from the chapter four and chapter five: make the PBG blue shift with the decrease of the common layers light course, but thus the transmitting peak will be away of the wavelength of 1.55μm, and then make the PBG red shift according to increase the light of the defect layers, finally we get the parameters of the optical switch whose pump wavelength is 1.06μm, and working wavelength is 1.55μm after adjusting them time after time, and then we simulate the function course and result of the optical limiter and the optical switch with the program.According to the analysis and research of this paper, we discuss the PBG characteristics of one dimensional PC and one dimensional PC which has nonlinear dielectric defect layer, and discuss the rules of the change of PBG caused by the change of the structure parameters, and design the structure of PC optical limiters and optical switches according to the rules, which provides reference and theory evidence to its realization. |
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