Research On Key Devices And Communication Chips For Visible Light Communication

Visible light communication?VLC?which uses visible light signals as transmission media is a new communication technology.Different from the situation that the spectrum resources of wireless communication tend to be exhausted,the spectrum resources of visible light are abundant,including hundreds of terahertz unused blank spectrum.VLC has the advantages of high bandwidth,strong anti-interference,good confidentiality and no electromagnetic radiation.It is a future-oriented green communication technology which has attracted extensive attention of communication researchers all over the world,and is an important research direction in the field of communication.The rapid development and popularization of light emitting diodes?LEDs?provide strong support for the development of VLC.However,the current commercial LEDs are mainly designed for illumination,their modulation bandwidth is limited,so they are not suitable for high-speed and ultra-high-speed VLC systems.In addition,the electronic integrated chip has approached the limit of classical scale,and it becomes extremely difficult to further reduce the feature size and improve the chip integration.Photonic integrated chip has become a new development trend in the post-Moore era with its unique advantages of high speed and low power consumption.Based on Ga N-on-silicon wafers,seven kinds of LED devices and VLC chips with different structures and functions are explored in this paper,focusing on the two aspects of free space VLC and photonic integration for on-chip communication.The LED devices for free space VLC are explored,which mainly include three kinds of devices.The first is a suspended membrane dual-emission LED.The device consists of two light emitters.The silicon substrate at the bottom of the device is removed by deep silicon etching,which eliminates the absorption of downward-emitting light by the silicon substrate.The suspended membrane structure with controllable thickness is obtained by back epitaxial wafer thinning,and the defective buffer layer is removed,which effectively improves the photoelectric performance of the device.Experiment results show that the device can emit 9 MHz and 25 MHz modulated light signals at the same time,which lays a foundation for further development of light signal transmitting module in VLC multi-input multi-output?MIMO?system.The second is a resonant-cavity light emitting diode?RCLED?with a gold reflector and a dielectric distributed Bragg reflector?DBR?.The device uses the p-electrode located on the upper surface of the p-Ga N layer as the top mirror,and deposits a high reflective DBR on the lower surface of the suspended Ga N membrane as the bottom mirror.The DBR consists of 6 pairs of Si O₂/Ta₂O₅ and the reflectivity at 450 nm is about 95.1%.The experiment and simulation results show that the vertical resonant-cavity composed of metal reflector,Ga N mambrane and DBR can effectively control the bottom emission of the device,which verifies the feasibility of depositing DBR on the bottom of the suspended Ga N membrane of the LED to obtain the vertical resonant-cavity device.The third is a RCLED with two DBR mirrors.Based on the research of vertical resonant-cavity device mentioned above,the structure of the resonant-cavity is optimized.DBRs composed of 8.5 pairs and 7.5 pairs of Ti O₂/Si O₂ are deposited on the upper surface of p-Ga N layer and on the lower surface of suspended Ga N membrane as top and bottom mirrors,respectively,which avoides the absorption of the emitted light by the metal mirror.The peak reflectivity of the top mirror and the bottom mirror are 99.4%and 92.7%,respectively.Experiments show that,compared with the non-resonant LED,The full wave at half maximum?FWHM?of the EL spectrum of the resonant-cavity device is narrower and the spectral intensity is higher.The free space VLC system using the device as the transmitter achieves data transmission up to 200 Mbps,which shows that the resonant-cavity LED has excellent communication performance.Then the in-plane VLC of chips is explored,which mainly includes four kinds of communication chips.The first is a VLC chip integrated with Ga N straight waveguides.The chip consists of three membrane-type multiple-quantum-well diodes?MQW-diodes?with the same structure.Suspended straight waveguides are used to realize light signal coupling between the MQW-diodes.One-to-one and one-to-many on-chip VLC can be realized based on the physical mechanism that MQW-diodes have the dual functions of light emission and light detection.Experiment results show that the eye diagram can be clearly observed when the on-chip data communication is carried out at 1 Mbps.One-to-two high-quality real-time audio data transmission can also be realized based on the chip.The second is a VLC chip integrated with Ga N bent waveguide.On the basis of the straight waveguides VLC chip,device structures are optimized.The chip consists of two membrane-type MQW-diodes,which are used to transmit and receive visible light signals.The transmitter is connected with the input waveguide,and the receiver is connected with the detection waveguide.Experiment and simulation results show that the suspended Ga N bent waveguide can effectively transmit visible light signals.Compared with the straight waveguides VLC chip,the effective size of the active region of the transmitter and receiver in the chip is greatly reduced,the capacitance of the devices and the RC constant of the on-chip communication system are reduced,and the response speed of the chip is greatly improved.Based on this chip,the on-chip visible light data transmission with the rate of 70 Mbps is realized.The third is a full-duplex VLC chip.The waveguide layer and cladding layer can effectively confine the photons generated by the active layer electroluminescence to the device,so the silicon substrate is no need to be removed and the fabrication process is simplified.The physical phenomena of partial overlap between the EL spectra and the photoresponse spectra of MQW-diodes and the physical mechanism of the coexistence of light emission and light detection of devices provide a physical basis for the development of full duplex VLC chips.The chip integrates two waveguide-type MQW-diodes which can transmits visible light signals via free space.Experiment results show that the chip can achieve spatial co-time co-frequency full-duplex?CCFD?light communication.The self-interference cancellation scheme is used to extract the superimposed signals,and a full-duplex audio transmission experiment is performed,opening a promising route toward on-chip parallel information processing.The fourth is a photonic computing prototype chip.The chip integrates two common-base phototransistors,light signals coupling between the emitter and the base is achieved through a suspended Ga N waveguide.Experiment results show that different light signals emitted by two emitters can form a superimposed signal at the collector.The superimposed signal combination representing 00,01,10 and 11 is verified,which provides a research basis for the further development of complex photonic computing chips.