Applied Basic Research On Multi-scenario Short- Reach Optical Interconnects Based On Orbital Angular Momentum Modes

Conventional copper interconnects are limited by the capacitance and resistance characteristics of the metal wires.Bits of information will overlap with each other when the frequency of signal is increased to a certain level.With the rapid development of big data,cloud computing,Internet of things and industrial Internet,copper interconnects have been unable to meet the application demand.Optical interconnects are kind of information transmission scheme with high-frequency optical waves as carriers,which can meet the demand of high speed and large capacity bandwidth.Nowadays,optical interconnects carry over 90% of the world's information transmission,and are regarded as an important cornerstone for the realization of “Cyberpower” strategy and “Broadband China” strategy.However,the well-known physical dimension resources of light waves(such as time,amplitude,frequency/wavelength,polarization,phase,etc.)have been utilized to the extreme.In order to ensure the continuous increase of transmission capacity,it is necessary to explore and develop the “new physical dimension resource” of light waves.Fortunately,as a set of transverse spatial dimension of light waves,orbital angular momentum(OAM)modes,due to their natural orthogonality and in principle infinite values,can provide effective and feasible solutions for the sustainable development of optical interconnects.In this dissertation,the physical characteristics of OAM modes are analyzed and discussed from the basic definition,the generation and detection of OAM modes are also studied both theoretically and experimentally.Based on the application scenario of short-reach optical interconnects,three kinds of basic network schemes(multiplexing,coding and multicasting)are presented and some key issues in multi-scenario applications such as free-space OAM-based optical interconnects,all-fiber OAM-based interconnects and underwater OAM-based optical interconnects are also investigated.The major contents of this dissertation are summarized as follows.1)From the basic definition of OAM,the relationship between other spatial modes and OAM modes is elaborated,as well as its angular momentum value,rotation characteristics,orthogonal characteristics,divergence characteristics and security characteristics are analyzed and discussed.(1)Especially for the analysis of security characteristic,a quantized penalty expression is proposed as the evaluation criterion.Besides,the spectrum diffusion and crosstalk characteristics of OAM modes subjecting to different eavesdropping degrees are also discussed.(2)In addition,from the basic physical characteristics of OAM modes, three basic short-reach optical interconnects schemes(multiplexing,coding and multicasting)are proposed and analyzed,laying a foundation for the application research on multi-scenario short-reach optical interconnects.2)From the implementation principle,the generation and detection of OAM modes can be divided into two categories,i.e.mode conversion and phase control,which are analyzed and elaborated respectively in this dissertation.(1)OAM modes and helical-intensity modes are generated at the wavelength of 2 ?m on the chip using the designed and fabricated reflective metasurface devices which extend the application band range of transverse mode of light waves.Due to the self-interference effect of the device,the generated helical-intensity modes are robust and insensitive to the environmental noise.This special structured light has potential applications in optical imaging and detection.(2)The metasurface-based fiber samples are fabricated through the etch and process on the facet of the fiber which can successfully generated OAM modes in the fiber.The OAM modes are also detected using the Fourier transform method.The results show that due to the broadband characteristics of the metasurface structure,the fabricated metasurface-based fiber samples can work from 1480 to 1640 nm with the high phase purity of more than 93%.3)The application of OAM-based short-reach optical interconnects in free space are studied experimentally.(1)The outdoor 260 m OAM-multiplexing,-multicasting and-coding shortreach optical interconnects links are verified.The link security characteristics are also analyzed and discussed.In detail,the multiplexing link realizes the two OAM channels multiplexing and the aggregate transmission capacity reaches 80 Gbit/s.The multicasting link realizes 1 to 9 channels information distribution and the power distribution of the multicasted channel can be controlled flexibly.The coding link realizes 25 GHz ultra-fast coding for information transmission using the mapping betwwen time domain and spatial domain.(2)Vector mode-based(possessing polarization singularity)coding short-reach optical interconnects link is demonstrated in the experiment,realizing 4 bits/symbol encoding transmission capacity.Besides,bit error rate(BER)is measured and image transmission is investigated.(3)The short-reach coding optical interconnects link based on polarizationarray structured light is proposed.The encoding transmission capacity up to 96 bits/symbol and 1024 bits/symbol are realized respectively by using the vector-like mode array and ultrahigh density polarized mode array.Besides,the transmissions of image and video information are also studied with the influence of atmospheric turbulence.4)The application of OAM multiplexing all-fiber short-reach optical interconnects are studied experimentally.(1)The principle and fabrication process of the all-fiber mode selective coupler are introduced for OAM mode generation/detection and multiplexing/demultiplexing.(2)For the data center application scenario,the commercial small-form factor-pluggable+(SFP+)optical transceiver and fabricated mode selective coupler are utilized to realize 2.6-km unidirectional short-reach optical interconnects in multi-mode fiber and 2-km full-duplex short-reach optical interconnects in few-mode fiber based on OAM multiplexing technique.The power transmission matrix and crosstalk are measured,and the BER performance is also evaluated.5)The application of OAM-based short-reach optical interconnects under the water are studied experimentally.(1)The influence of underwater bubbles and tiny obstacles on the lightwave transmission with different spatial modes(Gaussian,OAM and Bessel)is evaluated.The results show that the Bessel mode with diffraction-free and self-recovery characteristics has the best performance.(2)Underwater OAM multicasting short-reach wireless optical interconnects link is demonstrated in the experiment.1 to 4 channel distribution based on OAM multicasting is achieved,which enriches the application range of underwater short-reach optical interconnects.(3)Considering some huge obstacles which can hardly be removed under the water,the non-line-of-sight short-reach optical interconnects link based on OAM mode is realized by using the internal reflection characteristics of the air-water interface.Besides,an adaptive feedback system is proposed to compensate the beam drift and enhance the stability of the optical interconnects link.(4)On the basis of previous works,a set of underwater OAM-based short-reach optical interconnects prototype is designed,developed and tested.