ﻃﺮﺍﺣﻲ ﻭ ﺷﺒﻴﻪﺳﺎﺯﻱ ﻣﺪﻭﻟﻪﺳﺎﺯﻫﺎﻱ ﻧﻮﺭﻱ ﻓﺮﺍﭘﻬﻦﺑﺎﻧﺪ ﻣﺒﺘﻨﻲ ﺑﺮ ﻟﻴﺘﻴﻮﻡ ﻧﺎﻳﻮﺑﻴﺖ ﻻﻳﻪ ﻧﺎﺯﻙ ﺑﺎ ﻛﺎﺭﺍﻳﻲ ﺑﻬﺒﻮﺩ ﻳﺎﻓﺘﻪ

Microwave loss is an important limiting factor for the electro-optic (EO) ban‎dwidth of thin-film lithium niobate (TFLN) modulators. In conventional coplanar waveguide (CPW) structures, reducing RF loss often comes at the expense of an increased half-wave voltage–length product (Vπ L). Here, we employed coaxial transmission lines (TLs) supporting a radial electric field to reduce RF loss by enabling larger electrode gaps, while maintaining a low Vπ L by positioning the optical waveguide closer to the inner conductor. In addition, coaxial TLs support a pure TEM mode with nearly frequency-independent impedance an‎d RF effective index, enabling ultra-wide EO ban‎dwidths. Using a segmented coaxial TL, we demonstrated terahertz ban‎dwidth operation together with high voltage–ban‎dwidth limit in a compact cross-sectional geometry. Unlike CPWs, coaxial TLs exhibit uniform current distribution within each electrode; however, the current density is higher in the inner conductor than in the outer one. To further reduce RF loss, it is therefore necessary to achieve uniform current distribution between both electrodes. To address this limitation, we introduced a parallel-plate TL structure, which supports a TEM mode while providing equal current distribution between the signal an‎d ground electrodes. For the same electrode gap an‎d characteristic impedance, the RF loss of a parallel-plate TL is lower than that of a coaxial TL. Partial electrode segmentation was also employed to further suppress RF loss while using small electrode gaps to reduce Vπ L. This design achieves simultaneously low RF loss an‎d ultra-low half-wave voltage–length product. A further reduction in Vπ L is achieved by introducing a high-permittivity material into the electric-field path. By avoiding a full series junction, near-unity (rather than unity) filling factors are employed, preventing a significant increase in the RF effective index an‎d thereby preserving velocity matching. This structure demonstrates exceptional voltage–ban‎dwidth limit among TFLN Mach–Zehnder modulators (MZMs). To improve manufacturability, the same high-permittivity, near-unity filling factor concept was applied to CPW-based MZMs, resulting in high EO ban‎dwidth an‎d low voltage efficiency. Additional designs presented in this thesis include non-segmented square an‎d circular coaxial TL modulators, which increase the voltage–ban‎dwidth limit by reducing RF loss through larger electrode gaps, as well as segmented coaxial TLs incorporating high-permittivity materials to enable further ban‎dwidth enhancement. In summary, this dissertation employs TEM-based transmission lines to realize ultra-wideban‎d TFLN electro-optic modulators operating in the terahertz frequency regime. It also presents sub-volt half-wave voltage TFLN modulators with EO ban‎dwidths approaching the hundreds of gigahertz, enabling reduced power consumption for high-speed optical applications.

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پريسا كريمي

ابوالقاسم زيدابادي نژاد , محسن مداح علي , مسعود موحدي