| Electro-optic modulators are key devices in optical transmission networks and optical information processing systems,which had been widely used in optical fiber communication,optical fiber sensing,microwave photonics,quantum communication and other technical fields.There are many kinds of materials for electro-optic modulators.Among them,lithium niobate material is an important material due to its broadband optical transparent window of 0.35 μm～5 μm,large electro-optic coefficient,and relatively stable physical and chemical properties.It becomes the preferred material for making electro-optic modulators utilized in many fields such as photonic integrated circuits(PIC),quantum photonic integrated circuits,and programmable quantum processors.At present,thin-film lithium niobate electro-optic modulators suffer with the mutual constraints of various performance parameters and indicators such as modulation bandwidth,insertion loss,modulation efficiency,and manufacturing cost,and thus further research and discussion are needed.This paper conducts an in-depth discussion on the working principle and performance parameters of thin-film lithium niobate electro-optic modulators.Many problems,e.g.microwave and light wave speed matching,transmission line system impedance matching,the loss source in such modulator,its thermoelectric effect related with stability issues,high cost issues caused by lithium niobate waveguides etching,and so on,have been researched.Through modeling and analysis due to the finite element analysis and the beam propagation method,a thin-film lithium niobate electro-optic modulator with high bandwidth,low loss,excellent stability and low manufacturing cost is designed.The main contents and innovations of this paper are following:(1)In this paper,a low-loss broadband multimode interference coupler had been proposed,which was composed of shallow etched grooves,wedge-shaped waveguides and SiO2 outer cladding on x-cut thinfilm lithium niobate waveguide.Such structure was designed and optimized by the full vector beam propagation method,and its modal characteristics and energy transmission mode were studied.By optimizing the structural parameters,the size of the coupler is 43 μm × 5μm,in which the length of the multimode region is about 21 μm and the width is 5 μm,and the length of the internal shallow etched structure is 7 μm and the width is 1.25μm.Such coupler had a loss of about 0.01 dB at the wavelength of 1550 nm in TE polarization mode.When satisfying EL<1 dB,its operating bandwidth could reach～1314 nm(～962 nm to～2276 nm).Our proposed coupler has the advantages of low loss,large operation bandwidth,simple manufacturing process,and large manufacturing tolerance,and strong application potential in hybrid quantum photonic integrated circuits and photonic integrated circuits.(2)In this paper,the I-shaped microstructure electrode combined with the silicon nitride on lithium niobate waveguide had been proposed and used in electro-optic modulator,which can be utilized to improve its speed matching between the electrical signal and the optical signal,solve its the impedance matching problem of the transmission line system,and enhance the interaction strength of electro-optic.Due to the LiNbO3 chemical inertness and low toughness characteristics,the difficulty about its microstructure design had been solved.By simulation,the thickness,width,spacing,segmental electrode length of the I-shaped microstructure electrode and the width and thickness of the silicon nitride loaded waveguide in the hybrid waveguide had been optimized.Thus,in our proposed modulator,the optical field confinement factor of the lithium niobate slab waveguide can reach to～76.5%,and the waveguide loss is about 0.3 dB/cm when the electrode spacing is 5.5 μm.The characteristic impedance of our proposed modulator is about 50.1 Ω,and its modulation bandwidth reaches 220 GHz.The modulation efficiency Vπ·L is about 2.08 V-cm.Our proposed design structure could enlarge the modulation bandwidth of the modulator and reduce the manufacturing cost.(3)Aiming at the thermoelectric effect of thin-film lithium niobate,the influence of the thermoelectric effect on the modulator had been analyzed.Then,a feasible solution for eliminating the thermoelectric effect of lithium niobite had been proposed by using graphene film half-wrapped the modulator to eliminate thermal charge generated by the thermoelectric effect of lithium niobate.In this paper,the validity and feasibility of the scheme are verified by building a simulation model,which improves the working stability of the thin-film lithium niobate electro-optic modulator. |
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