Study On Integrated Light Receiving Devices For Telecommunication

For optical communication systems,the optical receivers are the important components.Key optical devices of optical receiver front ends for different modulation formats and multiplexing technologies include the grating couplers for optical coupling,wavelength-division-multiplexing filters based on arrayed-waveguide gratings(AWGs)and microring resonators(MRRs),the polarization-division-multiplexing devices like polarizers(PCs),the polarization beam splitters(PBSs)and polarization rotators(PRs)as the representative on-chip polarization-handling devices,the optical hybrid such as multimode-interference(MMI)couplers and photodetectors.These optical devices and optical waveguides for optical transmission constitute the basis of the integrated optical path in the optical receivers.Among them,the photodetectors are the core device of the optical receivers.Achieving the large-scale optoelectronic integrated circuit(OEIC)would be the future development direction of optical communication systems.At present,it is still difficult to integrate a large number of optoelectronic devices together.Due to many obstacles in materials and manufacturing,it is necessary to overcome many scientific and technical problems in order to achieve the large-scale OEIC.So a realistic solution is to achieve small-scale integration of some functions first.Therefore,the work of this thesis is based on the study of integrated photodetectors,and exploring the application of photon integration in the optical receivers.We mainly study the integration of optical devices at the front ends of optical receivers,including the integration of the waveguide with photodetectors,the PBS with photodetectors,and the optical hybrid with photodetectors.By introducing micro-nano structures,we have studied the integrated optical devices and new integrated solutions for optical receiver front ends.The main work and innovations of the thesis are as follows:(1)The InP-based waveguide photodiodes have been studied.Based on the model of asymmetric double waveguide structure and the photodiode with partially doped absorption region,a high-speed waveguide photodiode with simple structure and easy fabrication process(only by one epitaxial growth)is designed.The waveguide structure of each part of the waveguide photodiode is designed and simulated by using the finite-difference time-domain(FDTD)method.The mode characteristics in the waveguide and the light field distribution of each waveguide layer when the taper waveguides with different lengths used as matching layers are simulated.The simulation results show that in the case of using a linear taper waveguide with the lenghth of 160 ?m,when the active region length of the photodiode is more than 30 ?m,the light absorption efficiency of the photodiodes can reach 94%,accordingly the theoretical value of the responsivity is about 0.8 A/W.And the electrical characteristics of the active region were studied by using the simulation software SILVACO TCAD.The simulation results show that the theoretical bandwidth of the photodiode is ~26 GHz.Then,based on the comprehensive consideration of fabrication realization,the fabrication process of the waveguide photodiode were determined and a batch of samples were fabricated.Finally,the characteristic parameters of the samples,such as dark current,photocurrent and bandwidth were tested and analyzed.(2)In order to improve the integration of the polarization diversity detection receiver,we propose a novel scheme to monolithically integrate photodetectors with a PBS based on the dielectric transmission gratings.We demonstrate the InP-based two-quadrant photodetector covered over with silicon nitride grating,which serves as both an efficient vertical coupler and a polarization splitter were integrated in a single device.Polarization-diversity detection can be achieved by the grating-based PBS to couple the TE and TM polarized lights into two sets of two-quadrant photodetector.The PBS was designed using the modal method and a numerical simulation method.The simulation results show that the efficiencies of 61% for TE polarized light and 85% for TM polarized light can be coupled into the two sets absorption layers of the two-quadrant photodetector and the polarization crosstalk is-21 dB and-23 dB,respectively.Since the length of the miniaturized device is only 35 ?m,combined with a suitable detector structure,it can be suitably employed in the compact and high-speed polarization diversity detection modules.Compared to conventional polarization detection modules,our approach offers greater advantages in coupling efficiency,size,structural simplicity,and manufacturing.(3)A novel scheme to integrate an optical hybrid based on standing wave with graphene photodetector(GPD)for coherent detection is proposed.There are two main innovations: On the one hand,different from optical hybrids based on MMI couplers,the proposed optical hybrid can be miniaturized by constructing a standing wave field in a silicon strip waveguide.On the other hand,the waveguide-integrated GPD with symmetric electrodes is adopted as an ultrafast self-differential photodetector(SDPD),instead of conventional balanced-photodetectors(BPDs)based on PIN photodiodes in a coherent receiver.By placing two symmetric interdigitated electrodes of the SDPD at different phase points of the standing wave field,it is convenient to realize the integration of the proposed optical hybrid with the graphene-based SDPD,which can effectively reduce the complexity of a coherent receiver and meet the future demand for wide bandwidth.In theory,our scheme would provide a feasible way to realize a high-speed compact coherent receiver.