Research Of Silicon Device Related To Polarization

With the continuous development of silicon-based optoelectronic technology,the level of integration of silicon devices becomes higher and higher with different functions of independent components integrated into a small chip.Optical signals are required to couple into chip through fiber first,to realizes power dividers,phase modulator,and other functions.However,during this coupling procedure,polarization mismatch in between fiber and chip device occurs;meanwhile the silicon device itself is sensitive to polarization.Therefore,sending optical signal directly into silicon devices from fiber affects the performance of devices significantly due to uncertainty of light polarization in fiber.So far,polarization diversity is an effective means to solve this problem in recent years,which processes polarizations of light separately,according to different needs.During all kinds of polarization devices,polarization beam splitters(PBSs)and polarization rotators(PRs)play a crucial role.Although there have been many schemes proposed to realize polarization splitting and polarization rotation,but these devices are still difficult to achieve high extinction ratio and the large bandwidth simultaneously.Therefore,this dissertation focuses on the polarization devices,starts from basic physical principles,explores new schemes of polarization devices to solve the contradiction between high extinction ratio,large bandwidth and small footprint of device,and proposes two new schemes of a polarization beam splitter and a polarization rotator.The main research efforts are summarized as follow.1.A polarization beam splitter based on slot directional coupler is proposed.Its basic principle is the transverse coupling mode theory.Using slot formed in a coupling waveguide,the effective refractive index of TM0 mode of the input waveguide is the same as that of the coupling waveguide,which fully meets the coupled condition,while optical coupling is suppressed for TE0 mode.Firstly,we analyzed the electric field distribution of the waveguide modes and the variations of effective refractive index.Secondly,we compared the extinction ratio and operation bandwidth through the simulation analysis of the two kinds of polarization beam splitters with slot.Next,we designed the structure of a taper introduced into the PBS to improve the performance of the device.Finally,the PBS realizes extinction ratio larger than 24.6 dB from 1.5 ?m to 1.6 ?m for TM and larger than 20 dB from 1.521 ?m to 1.584 ?m for TE.The maximum of ER for TE is as high as 37.4 dB at 1552 nm.At last,we investigated a tolerance analysis of the proposed PBS.The design effectively alleviates the tradeoff between wide bandwidth and high extinction ratio in DC-based PBS has smaller size and larger.tolerance,which facilitates fabrication and on-chip integration.2.A novel polarization rotator is proposed based on TM0->TE1 mode converter.Its operation principle is that,when TM0 mode injects,the mode converter turns TM0 mode into TE]mode,which is then splits into two TE0 beams with phase difference of ?/2 by a Y-type structure.Due to the phase shifter,the two TE0 beams with same phases are injected into a wider waveguide and combined into a TE0 beam,realizing TM0 to TE0 conversion.When TE0 mode injects,it is splits into two TE0 modes with same phase by Y type structure,then with an extra phase difference of ?/2 is introduced into one of the two TE0 modes,the two TE0 modes with phase difference of ? combine via the second Y-type structure to fonn a TE1 mode,at last,the output mode converter turns TE1 mode into a TM0 mode,realizing TE0 to TM0 conversion.We analyze and carefully design the key parts of the proposed PR separately,including:two TM0->TE1 mode converters,two Y-type structures and phase shifter.Secondly,we combined mode converter,Y-type structure and phase shifter.Next,we optimized the length of the mode converter and got all structure parameters of polarization rotator.Finally,we simulated the extinction ratio and bandwidth characteristics of the device.The whole structure is symmetric and the PR realizes mode conversion of TE0->TM0 and TM0->TE0.The ER is larger than 20 dB and insertion loss is less than 0.4 dB in an ultra-wide wavelength range from 1.45 ?m to 1.65 ?m for both TM0 and TE0 modes.At last,we investigated a tolerance analysis of the proposed PR.The design effectively alleviates the tradeoff between wide bandwidth and high extinction.Moreover,the proposed PR realizes TE0->TM0 and TM0->TE0 polarization rotation simultaneously,which makes adjustment of the polarization state of light on chip more flexible.