A non-uniform segmented grating coupler with a metal reflective layer is designed by finite difference time domain simulation. The grating coupler is on a 400 nm thick z-cut lithium niobate thin film waveguide. A metal reflective layer is added to reduce the leakage to the substrate, and a non-uniform grating structure is constructed to improve the mode mismatch. The non-uniform grating coupler consists of three segmented grating regions, with a gap between two adjacent grating regions. The grating periods of these three grating regions are not completely the same, namely Λ1, Λ2, and Λ3, Λ1 is shorter than Λ2 and Λ3. According to grating coupling principle, the key structural parameters of the grating coupler such as the gap between the grating regions, the grating period, the duty cycle are discussed with respect to the grating coupling efficiency. Through FDTD simulation calculation, the coupling efficiency under theoretical simulation (TM mode, at 1550 nm) can reach nearly 76%. The proposed non-uniform segmented grating coupler is expected to be fabricated and applied to the input-output coupling of waveguide devices.
A spot-size converter is proposed based on lithium niobate on insulator (LNOI) waveguide. The spot-size converter is composed of the adiabatic taper and the low refractive index cladding. The evanescent wave coupling method is used to complete the conversion between fiber mode field and waveguide mode field, and the high-efficiency edge coupling between the fiber and LNOI waveguide can be realized. The parameters of the spot-size converter are designed and optimized, and the performance of the whole device is simulated. The total length of the spot-size converter is only 500 μm. An ultra-high numerical aperture (UHNA) fiber with mode field diameter (MFD) of 3.2 μm is applied to couple with LNOI waveguide. The coupling loss is 0.4 dB for TE mode and 0.47 dB for TM mode at the wavelength of 1550 nm. It can cover the whole C-band well. The proposed spot-size converter is expected to be used in high-density monolithic integrated optical system and gives a favorable reference for on-chip light source coupling.
Lithium niobate on insulator (LNOI) thin film maintains the attractive material properties of lithium niobate, while also offering a stronger optical confinement and a high optical element integration density. The structure of integrated chips needs to be redesigned because the LNOI chip is quite different from the single crystal LN one. Beam splitter has been widely used in integrated optical devices, such as MZM modulator, AM modulator and light switch. In this paper, two kinds of beam splitters, including directional coupler (DC) and Multi-Mode interference (MMI) structure were designed by using finite difference time domain. The gap and coupling length of directional coupler were analyzed. An MMI beam splitter structure was designed and simulated. The key parameters of beam splitter such as free spectral range, splitting ratio and propagation loss were compared. The analysis of the two beam splitter structures can supply guidance for the design of the LNOI devices.
Integrated array optical switch puts forward higher requirements for switch time and switch voltage. In order to achieve lower switch voltage and shorter switch time, the theoretical model of coplanar waveguides (CPW) electrode is established for Lithium niobate (LiNbO3) optical switch, and a novel structure with thickening buffer layer between electrodes is proposed in this paper. Then the modulation bandwidth and electro-optic overlap integral are qualitatively analyzed and optimized by finite element method (FEM). The simulation results show that the electro-optic overlap integral increases gradually with the raising of buffer layer thickness between electrodes. The switch voltage of the optical switch is about 5.7V, which is lower than the traditional electrode structure. The switch time is about 0.48ns. This new structure contributes to reducing the half-wave voltage of the modulator and can be potentially used in the field of electro-optic modulation.
A 1×4 LiNbO3 electro-optic switch is proposed based on Mach-Zehnder interference structure. The optical switch is composed of three 1×2 MZI switch units, each of which includes Y branch, interference arm and directional coupler. The phase difference of the light propagating on two interference arms is generated through the loaded bias voltage, and the switching between the two output ports of the MZI switch unit can be realized. The parameters of Y branch and directional coupler of optical switch are designed and optimized, and the performance of the whole device is simulated. The total length of the device is 5.8cm and the insertion loss is 0.54dB. The extinction ratio bandwidth larger than 20dB reaches 100nm, and the maximum extinction ratio at the wavelength of 1550nm is 33dB. The function of optical switch with four-channel gating is realized. The proposed optical switch is expected to be used in the fields of optical interconnection and optical signal monitoring on high-speed integrated chips.
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