Research On Novel Micro/Nano-Photonic Structures For Polarizatio Control Of Superluminescent Diode

Inertial navigation system is a critical military and civilian strategic resource due to its high precision,strong anti-interference capabilities,and good concealment,which has been widely concerned from governments worldwide.Fiber optic gyroscope(FOG),as a crucial component of inertial navigation system,is advancing towards great precision and miniaturization.Superluminescent diode(SLD)is commonly used as light source in FOG attributed to its broad spectrum.Unfortunately,due to the inherent limitation of epitaxial structure in the active region,the optical gain of the TE0 mode is much greater than that of the TM0 mode,resulting in a high degree of polarization(DOP)of SLD,and thus hampering highprecision detection of FOG.To effectively solve this issue,various solutions have been proposed and implemented,including introducing tensile strain into SLD active region,optimizing ridge waveguide parameters(e.g.reducing waveguide width and increasing waveguide height),and integrating silicon-based polarization controllers,etc.Among them,the silicon-based polarization controllers are particularly attractive owning to their capabilities of significantly reducing DOP,compatibility with CMOS fabrication processes and diversity in structures.Currently,the prevalent polarization control approach involved is removing the TM0 mode and preserving the TE0 mode.However,incomplete removal of the TM0 mode may lead to a risk of deterioration in DOP.Therefore,in this thesis,we adopted new mechanisms to get two silicon-based polarization controllers.One device designed by the inverse scheme works through the polarization loss mechanism,the other device designed by the forward scheme works through the polarization rotation mechanism.In detail,the polarization loss mechanism attenuates the TE0 mode selectively,and the polarization rotation mechanism converts partial TE0 and TM0 modes into orthogonal TM0 and TE0 modes,both of which balance the power between TE0 and TM0 modes so that DOP is reduced significantly.The details and innovations are as follows:(1).A simple and efficient scheme has been proposed by introducing an adaptive projection strategy into the topological inverse design of micro/nano photonic devices with a feature that a two-level hierarchical hyperbolic tangent projection function with both linear increment and differentiation,enabling micro/nano photonic devices hold good performance and full binarization simultaneously.Subsequently,design of waveguide bends,crossings,and power beam splitters demonstrates the advantages of the proposed scheme including good universality,improved optimization efficiency(approximately 30%)and low dependence on initial structures compared with conventional topological inverse design scheme,which shows great prospects in the design of silicon-based photonic devices and chips.(2).A silicon-based "selective loss type" polarization controller has been designed by the proposed scheme.The device with a footprint of 4.0×1.6 μm2 exhibits the average transmittance of 96%for TM0 mode and only 15%for TE0 mode across the entire O band(1260-1360 nm).Thus,the DOP can be reduced from 73-80%to below 7%.(3).A silicon-based "cut-cornered" polarization rotator with both subwavelength grating and tapered structure has been designed by the forward design scheme,realizing the efficient conversion from TM0/TE0 mode to TE0/TM0 mode.The device with a footprint of only 0.34×3.99 μm2 exhibits insertion loss of below 0.2 dB,polarization conversion efficiency approaching 100%,and high polarization extinction ratio of 26-38 dB(30 dB@1310 nm)across the entire O band(1260-1360 nm).After introducing a maximum random fabrication tolerance of±20 nm,the average transmittance of TM0/TE0 to TE0/TM0 conversion decreased 1.1%(1.3%@1310 nm).(4).Based on the above polarization rotator,an ultra-compact siliconbased "polarization rotating type" polarization controller with a footprint of only 0.3×2.2 μm2 has been designed by reducing the period width and number of the subwavelength grating.The device operates in O band(1260-1360 nm)with low insertion loss(0.12-0.15 dB,0.12 dB@1310 nm),satisfactory polarization conversion efficiency(44-50%),and broad bandwidth(60-85 nm),which lowers the DOP of SLD from 60-90%to below 7%and shows great potential values.