| In recent years,silicon-based optical devices have been popular in the communications field,and all-optical switches have played an important role in optical interconnection and optical communication networks.Due to the advantages of light-controlling-light,all-optical switches can avoid the conversion between optical signals and electrical signals.Therefore,they can achieve an efficient signal processing.As we all know,a large number of alloptical devices are built on semiconductor materials(Si,In P,Ga As,etc.).However,there are two photon absorption(TPA)effects and free carrier absorption(FCA)effects in these materials,which causes a large power consumption and a low tuning efficiency.Then,to eliminate this effect has become a hot research direction.In this thesis,we investigate Silicon intride and graphene to solve this issue.Silicon nitride,due to its nearly insulating properties,offers new opportunity for eliminating the effects of two-photon absorption and free carrier absorption.However,compared to silicon,silicon nitride has a lower thermo-optic coefficient and thermal conductivity,which will result in a lower efficiency.Graphene,an excellent two-dimensional material,has many advantages such as high thermal conductivity(5300 W /m · K),good nonlinearity,and adjustable Fermi energy level.Graphene can also directly be covered on the waveguide of the device,which is different from other metal heating materials that require a buffer layer to avoid the optical loss due to the metal absorption.Therefore,when combining graphene and silicon nitride,we can avoid the twophoton absorption effect and the free carrier effect.Then,a higher efficiency can be obtained.Moreoever,the device in this paper is based on the MachZehnder interferometer(MZI)structure,which has the advantages of large working bandwidth,high process tolerance and easy processing.The research work of this article can be devided into threes parts:(1)We firstly researched the background of silicon-based optical devices and focused on all-optical switching devices.Unlike those reported all-optical switches based on semiconductor materials,our proposed all-optical MZI switche is based on silicon nitride and graphene,in the design,in order to show obvious interference fringes,the lengths of two arms are not equal.And the graphene is transferred onto the upper arm of the MZI device.It can absorb the pump light and then realize the function of light-controlling-light by using the thermo-optic effect.(2)We introduced the graphene material and the whole process of wettransferring graphene.Then,we conducted a detailed design study on the device structure.We simulated the lengths of the two directional couplers(DCs)of the MZI device by the finite-difference time-domain method(FDTD).And the lengths of the two directional couplers were set to 20.1 ?m and 16.1 ?m for achieving a split ratio of 68%: 32% and 50%: 50%,respectively.In order to analyze the thermal properties of the proposed graphene-silicon nitride structure,finite element method(FEM)simulation was used to perform 2D thermal simulation on the device.According to the simulation,we found that when graphene absorbs 50 mw of pump power,the temperature of silicon nitride waveguide rises by 99 K.By calculation,if the length of graphene is300 ?m,the switch function can be realized.(3)Finally,according to the results of simulation,we designed the layout of the device,fabricated the devices and then tested the chip.The experimental results show that our all-optical switch has an extinction ratio of 11 d B.And the time constants of the rising edge and the falling edge are measured to be 689 ns and1.028 ?s,respectively.The tuning efficiency of our device is 0.0106 ? / m W,and the tuning efficiency of our device is independent of the wavelength of the pump light.Compared with the previous reported all-optical switches,the tuning efficiency of our device has a great advantage.It should be noted that our switches have an excellent balance in terms of the switching response time and tuning efficiency. |
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