Research Of High Speed Multi-wavelength THz Modulator Based On Multiple Lattices Photonic Crystal

The terahertz radiation is located midway between microwaves and infrared light.Due to its special prosperities such as rich spectral bandwidth,good penetrability and low-energy photons,many application potentials of the THz wave including spectrum analysis,imaging and broadband communications have been explored.With the general trend of a fast increasing demand of mobile traffic,wireless data rates of several 10 s of Gbit/s are required in the future wireless THz communication system.More spectrum-efficient modulation technology will be a promising direction in the development of THz wireless links.Therefore the research of the multi-wavelength THz modulator is of essential importance to WDM THz wireless communication system.Photonic crystal(PC)with periodic refractive index exhibits band gap and photonic localization.It has the ability to provide extra degree of freedom in manipulating the light transmission.So PC plays an important role in the development of THz functional devices.And it will make a major contribution to the miniaturization and integration of THz communication system.With the improvements of communication technologies and further demands of scientific works,the THz modulator with high speed,low insert loss and large extinction ratio has become the mainstream of the THz modulation skills.In this paper,all kinds of THz modulators are based on the photonic crystal.This dissertation focuses on the research of PC-based multi-wavelength THz modulator in THz wireless communication system.The THz modulator based on multiple lattices consists of waveguides and cavities filled with the nonlinear material.A resonant mode cavity varies dynamically with the light or magnetic field.Consequently,the on/off THz modulation can be achieved.The design principle,device structure and performance of PC-based modulator in different applied fields can be investigated respectively.The main content includes:(1)The coupled mode theory(CMT)is improved to deduce the resonant mode spectrum and mode distribution in coupled cavity based on multiple lattices PC,where the resonant mode spectrum is determined by the intrinsic state coupling coefficientKS,and mode distribution is affected by the linear superposition coefficientK nA.The characteristics of PC-based modulator with direct coupled structure(DCS)and opposite-side coupled structure(OSCS)are studied with the time domain coupled mode theory(TCMT),where DCS contains a single multi-mode cavity and OSCS consists of multi single-mode cavities.The mathematical expressions of the transmittance of the modulator are derived by TCMT when the light waves with multi-wavelengths are incident.(2)Optically controlled multi-wavelength THz modulators based on compound lattice PC have been proposed.Quadruple-wavelength THz modulator(QWTM)based on compound lattice utilizing DCS is designed.To optimize the structural parameters,the influence of point defect location on the modulation performance has been investigated.The optimized QWTM has a high extinction ratio of 44.5dB,low insert loss of 0.23 dB,high channel isolation of 16.42 dB and modulation rate of 7.1GHz.The performance comparison of QWTM with DCS and OSCS provides the basis for realizing six-wavelength THz modulation with OSCS.The six-wavelength modulator has an extinction ratio of 11.2B,insert loss of 0.04 dB,and modulation rate of 4.17 GHz.The simulation results prove that the multi-wavelength(>4)THz modulator based on compound lattice with OSCS has a better performance of insert loss and channel isolation;QWTM with DCS has more advantages in extinction ratio.(3)Optically controlled multi-wavelength THz modulators based on a single cavity and the coupled cavities in triple lattice PC have been proposed respectively.The modulator adopts DCS formed by the introduction of waveguides and the coupled cavity.The time domain coupled mode theory(TCMT)and finite difference time domain(FDTD)method are used to analyze the performance of the modulator.The simulation results show that “on” and “off” THz wave modulation with quadruple wavelengths can be acheived based on the coupled cavity composed of the different point defects,it has a high extinction ratio of 34.08 dB,insert loss of 0.21 dB,and modulation rate of 4GHz.Based on the coupled cavity comprised of the same point defects,the THz modulator can realize five,six,eight wavelength modulation respectively.The THz modulators in triple lattice have advantages of higher extinction ratio,less insert loss and channel crosstalk.(4)Magnet controlled magnetized plasma THz modulators composed of a resonant cavity and waveguide based on PC have been proposed.Base on magneto-optical effect,the Drude dielectric tensor model of magnetized plasma material InSb is applied to the PC resonant cavity and the resonant mode spectrum and mode patterns have been analyzed by using the finite element method and FDTD method respectively.The steady-state field intensity distribution and the time domain steady state response of TE wave propagating perpendicularly or parallel to the external magnetic field are simulated by FDTD.The simulation results show that the defect mode transfer modulation(3.2-3.25THz)and defect mode disappearance modulation at the range of 1.95-2.0THz can be achieved with the perpendicular magnetic field of 0T and 1.26 T when the radius of InSb rod is 8.228?m and 13.997?m respectively.The defect mode disappearance modulator has a better performance with a high extinction ratio of 33.61 dB,low insert loss of 0.36 dB and high modulation rate of 4GHz.When the applied magnetic field parallels to the direction of THz wave,the resonant mode can be decomposed into the left and right hand circular polarized light.Therefore the on/off modulation of left and right hand circular polarized light can be realized simultaneously with the extinction ratio of 29.13 dB,low insert loss of 0.08 dB and high modulation rate of 8GHz.In summary,this article not only analyzed the resonant mode spectrum and mode distribution in coupled cavity based on multiple lattice PC,but also simulated the performance index of multi-wavelength modulator by TCMT and FDTD,and studied the primary factor affecting the performance of the modulator.Optically controlled multi-wavelength THz modulators based on compound and triple lattice,as well as magnet controlled magnetized plasma THz modulators have been proposed.These devices have a high modulation rate reaching over 4GHz.Consequently these results have important academic significance and potential application value for THz wireless modulation technology.