| With the increasing demand for miniaturization of photonic devices,the optical waveguide nano wires have drawn more attentions.The optical waveguide nano wires can be mainly divided into two parts.The first part is the microfiber which is pulled by a standard single mode fiber using the mature flame-brushing technique.The other part is the planar optical waveguide.The diameter of microfiber is usually less than ten micrometers,which leds to the advantages of large evanescent field,strong light field,high nonlinearity,low loss and relative easiness of coupling to single mode fibers.The material of planar optical waveguide is usually silicon or silicon nitride,whose refractive index is far larger than the substrate material.As the difference of refractive index is large,it results in strong light confinement,which can confine light in a much smaller area.It will obtain greater light power density with the same incident power.At the same time,the processing technology of the waveguide is more mature,and it is convenient to design and adjust the waveguide dispersion.In this thesis,we mainly focus on the nonlinear frequency conversion in the optical waveguide nanowires and study the effects of the external control on the frequency conversion.The outline of our research is as follows:(1)The influence of optical forces on nonlinear frequency conversion of suspended waveguides.We theoretically study the process of frequency conversion on suspended silicon nitride waveguides,which includes the shift of phase matching wavelength and conversion efficiency.In the whole process,the optical forces make the suspended waveguides deform,changing the light field distribution.And the light field distribution changes the optical forces.The optical forces,waveguides deformation and light field distribution interact with others,and finally reach a relatively stable state.When the input power increases,the corresponding phase matching wavelength red shifts.When the input power is 25 mW,the phase matching wavelength shifts 90 nm(3.6 nm/mW).In addition,the optical forces also influence the conversion efficiency.When the input wavelength is fixed,the conversion efficiency increases with the increasing input power.The maximum conversion efficiency appears when the input wavelength shifts 50 nm.This can greatly improve the integration and achieve all-optical control.(2)The influence of refractive index on nonlinear frequency conversion of microfibers.As the existence of large evanescent field,light field energy is not only in the optical fiber,but also in the surrounding environment.Owing to that,the microfiber is sensitive to the surrounding media.We change the media around the microfiber,for example,changing the air into water,and the phase matching wavelength of second harmonic generation shifts from the 774 nm into 1085 nm,to achieve a wide range bandwidth control.At the same time,we can change the kind of media to achieve greater bandwidth control.This not only simplifies the preparation of sample,making it easier,but also obtains a wider bandwidth,to achieve phase matching easily.(3)The influence of temperature on nonlinear frequency conversion of microfibers.We change the temperature around microfibers.The thermal expansion effect makes the microfiber diameter change,and the thermo-optic effect makes the refractive index of microfiber and air change.They finally make the phase matching condition change.Through our theoretical simulation,we found that when the input wavelength is 1550 nm and the temperature changes 75K,the phase matching diameter shifts from 774 nm to 773 nm(shifts 1 nm),which is around 0.12%of original phase matching diameter.It is almost negligible.Therefore,using temperature to control the second harmonic generation is not ideal.In summary,we prove that the optical forces and external refractive index can influence frequency conversion of optical waveguide nanowires by theoretical simulation.These novel control methods provide a new way for all-optical control. |
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