Design And Fabrication Of Graphene-based Optoelectronic Devices And Their Applications In Optical Communications

With the advent of big data,cloud era and smart city,the high-speed optical communication network develops very fast.The optical communication networks have three kinds of trend.First,the transmission speed and capacity of optical communication backbone network should be improved to satisfy the need of information transmission.Second,high speed and large capacity,tunable,reconstructable,and flexible function for signal processing at the nodes of the network is required to match the high speed and large capacity of information transmission.Third,the modularity and integration of optical communication networks is the mainstream technology of optical communication in the future.In order to improve the capacity of information transmission,the technique of advance modulation format and data multiplex and de-multiplex is used.All-optical signal processing provides a solution to solve the mismatch problem between the information processing of optical communication network node and transmission of information.Continuously improve the optoelectronic integrations and performance is the key development of optical communication devices.Graphene,a two-dimensional atom-layer material,has excellent unique optoelectronic properties.Combination with existing optical communication system and optoelectronic devices,graphene has been paid more and more attention in the field of all-optical signal processing and integrated optoelectronic devices.In this paper,we mainly research on graphene-based optical signal processing and optoelectronic devices.The details are as follows:(1)We theoretically studied the basic optical properties of graphene and explored the preparation process of graphene.(1)The linear optical properties of graphene,such as graphene conductivity and dielectric constant,were studied.(2)The nonlinear optical properties of graphene were studied,mainly the third-order nonlinear effect of graphene.(2)The graphene-based nonlinear optical device was fabricated and applied to all-optical signal processing.(1)We fabricate a nonlinear optical device based on a fiber pigtail cross-section coated with a single-layer graphene grown by chemical vapor deposition(CVD)method.Using such graphene-assisted nonlinear optical device,we experimentally demonstrate tunable wavelength conversion of a 10 Gbaud quadrature phase-shift keying(QPSK)signal by exploiting degenerate four-wave mixing(FWM)progress in graphene.We study the conversion efficiency as functions of the pump power and pump wavelength and evaluate the bit-error rate(BER)performance.The observed optical signal-to-noise ratio(OSNR)penalties for tunable QPSK wavelength conversion are less than 2.2 dB at a BER of 1×10-3.(2)Using the fabricated graphene-assisted nonlinear optical device and employing Nyquist 16-ary quadrature amplitude modulation(16-QAM)signal,we experimentally demonstrate phase conjugated wavelength conversion by degenerate fourwave mixing(FWM)and transparent wavelength conversion by non-degenerate FWM in graphene.We study the conversion efficiency as functions of the pump power and pump wavelength and evaluate the bit-error rate(BER)performance.We also compare the time-varying symbol sequence for graphene-assisted phase conjugated and transparent wavelength conversions of Nyquist 16-QAM signal.(3)By exploiting the degenerate four-wave mixing progress in graphene and adopting(differential)quadrature phase-shift keying signals,we experimentally demonstrate 10 Gbaud two-input(A,B)hybrid quaternary arithmetic functions of doubling and subtraction(2A-B,2B-A)in the optical domain.The measured optical signal-to-noise ratio penalties at a bit-error rate of 2×10-3 are about 7.4 dB for 2A-B and 7 dB for 2B-A.(4)An approach to implementing modulo 4 operations of three-input hybrid addition and subtraction of quaternary base numbers in the optical domain using multiple non-degenerate four-wave mixing(FWM)processes in graphene coated optical fiber device and(differential)quadrature phase-shift keying((D)QPSK)signals is presented.We demonstrate 10-Gbaud modulo 4 operations of three-input quaternary hybrid addition and subtraction(A+B-C,A+C-B,B+C-A)in the experiment.The measured optical signal-to-noise ratio(OSNR)penalties for modulo 4 operations of three-input quaternary hybrid addition and subtraction(A+B-C,A+C-B,B+C-A)are measured to be less than 7 dB at a bit-error rate(BER)of 2×10-3.The BER performance as a function of the relative time offset between three signals(signal offset)is also evaluated showing favorable performance.(3)We fabricate a nonlinear optical device based on graphene-silicon microring resonator(GSMR).Using such graphene-assisted nonlinear optical device,we experimentally demonstrate up and down wavelength conversion of a 10-Gbaud quadrature phase-shift keying(QPSK)signal by exploiting degenerate four-wave mixing(FWM)progress in the fabricated GSMR.We study the conversion efficiency as a function of the pump power.In addition,the resonant wavelength of GSMR is tuned by changing the temperature from 20°C to 40°C.We evaluate the bit-error rate(BER)performance for up and down wavelength conversion.The observed optical signal-to-noise ratio(OSNR)penalties for QPSK up and down wavelength conversion are less than 1.4 dB at a BER of 1×10-3.(4)Graphene possesses unique optical properties.Combination with silicon photonics is the development trend of integrated optoelectronic devices.We systematically and theoretically study and design other graphene optoelectronic devices.(1)Graphene is treated as an anisotropic material.We studied the gate-variable dielectric constant of graphene.Based on the polarization dependent absorption,we design compact broadband transverse-magnetic(TM)-pass polarizer by exploiting graphene-silicon horizontal slot waveguide structure on silicon-on-insulator(SOI)platforms.(2)By exploiting the electro-refraction effect of graphene,we present a graphene-based compact phase shifter.The waveguide structure consists of a silica substrate,high index silicon(Si)vertical slot waveguide,Si3N4 dielectric spacer,two graphene layers,and two metal electrodes.The phase shifter performance is comprehensively studied in terms of working range,insertion loss,bandwidth,V2? for transverse magnetic(TM)and transverse electric(TE)modes.(3)By exploiting the electro-absorption effect of graphene,we present a graphene-based long-range hybrid plasmonic slot(LRHPS)waveguide modulator.The graphene-based long-range hybrid plasmonic slot(LRHPS)waveguide modulator is comprehensively studied.It possesses low insertion loss,high modulation depth and figure of merit(FoM).(4)By exploiting the electro-absorption effect of graphene,we present a graphene-based polarization-insensitive optical modulator.The modulator performance is comprehensively studied in terms of attenuation,insertion loss,modulation depth,and bandwidth.We achieve broadband >16 dB attenuation graphene-based optical modulator over a 35 nm wavelength range(covering C band)with an imbalance of no greater than 1 dB and with insertion losses of less than 2 dB for transverse magnetic(TM)and transverse electric(TE)polarized modes.(5)We exploit split-ring graphene to realize coherent perfect absorption(CPA)in the terahertz(THz)regime.By controlling the relative phase of two counter-propagating coherent beams,the coherent absorption at resonant frequency of 2.91 THz can be tuned continuously from 99.7% to less than 2.1×10-4%,which gives a modulation contrast of 56.7 dB.Moreover,the coherent absorption also can be tuned by varying the gate-controlled Fermi energy based on the electro-absorption effect of graphene,giving a modulation contrast of 19 dB.Center frequency tunable CPA is also achieved using gate-tunable split-ring graphene.