Thin film lithium niobate (TFLN) electro-optic modulation devices feature large bandwidth, low loss, and small half-wave-voltage length. However, lithium niobate materials cannot be directly applied to light source fabrication, and current integrated electro-optic modulation systems still require an external III-V laser chip. Three main methods for integrating light sources on the wafer level include flip-chip, micro-transfer printing, and heterogeneous integration. The micro-transfer printing and heterogeneous integration both involve fabricating structures of the laser devices, and the on-chip output power in most reports is less than 10 mW, which is difficult to meet practical applications. In contrast, flip-chip integration based on butt coupling with a high-power laser chip that has passed a stability test can not only achieve high-level integration on the TFLN chip but also has been verified to obtain an on-chip output power larger than 60 mW. However, most TFLN devices are based on the ridge waveguide due to the difficult fabrication, and dry etching of TFLN always results in an end facet with an angle between 40° and 80°, which drastically decreases the coupling efficiency of integrating photonic dies on the chip. On the other hand, low-loss coupling among devices with different mode sizes is still a problem for TFLN. To this end, we propose a spot size converter (SSC) design and prove its effectiveness.

As shown in Fig. 1, the proposed SSC is divided into two parts. Part 1 is the SiN ridge waveguide that is directly connected to the inclined section of the TFLN waveguide, and part 2 is designed to convert the mode distribution similar to the target device through a SiN core waveguide and two thin layers of SiON, with the entire mode converter cladding of SiO₂. This structure reduces the crucial requirements for dry etching of lithium niobate materials as SiN fabrication is more mature. Based on the structure in Fig. 1, we first improve the conversion efficiency between the SSC and the TFLN (part 1) by optimizing the thickness of the un-etched slab layer (H₁), the etched thickness (H₂), and the width of the ridge waveguide (W₁) near the ridge parameters of TFLN [with the refractive index of SiN (n=2.0) similar to that of TFLN]. By employing the structural parameters of part 1, we then optimize the conversion efficiency of part 2. Since most target components have symmetry mode distribution, it is convenient to optimize the efficiency with relatively few parameters. For example, the top and bottom thin layers of the SiN are set to symmetrical distribution, which means we only need to optimize the SiN width at the end face (W₂), the thickness and width of the SiN thin layer (W₃ and H₃), and the distance between them (G). Three-dimensional simulation is applied to analyze the conversion efficiency of different parts.

The overall efficiency of the SSC is defined by η=η₁×η₂×η₃,where η₁ and η₂ are the conversion efficiency of part 1 and part 2 respectively, and η₃ is the mode overlap between the target DFB and the SSC at the left facet. After optimization, η₁ is larger than -0.11 dB as discussed in Fig. 2, and the efficiency is insensitive to the inclined angle between TFLN and SiN ridge waveguide. Even if the inclination angle of TFLN is 20°, the efficiency is still larger than -0.23 dB [Fig. 8(a)], which significantly reduces the impact on dry etching of the TFLN. η₂ is larger than -0.17 dB after optimizing W₂, W₃, and G as shown in Figs. 3 and 4, and η₃ is -0.24 dB for the target DFB [Fig. 3(d)]. These results prove that the overall efficiency η is -0.52 dB. Furthermore, the SSC conversion efficiency for different wavelengths is shown in Fig. 5, and more designs suitable for single-mode fibers with a mode diameter of 10 μm and small mode field spot with a diameter of 3 μm are presented in Fig. 6. Further, the design is insensitive to fabrication tolerances as shown in Fig. 8. This provides a feasible solution for reducing the size of integrated devices and improving the overall performance. The proposed SSC is of significance for on-chip coupling with various passive (active) waveguides when TFLN facet treatment is difficult, especially for high-power hybrid integrated systems on TFLN with a DFB laser by flip-chip, with the typical processing illustrated in Fig. 7.

A filling material based on a similar refractive index with SiN is designed as the core part of an SSC that is compatible with different mode sizes for hybrid integration of DFB laser on TFLN by flip-chip. The SSC conversion efficiency can be greater than -0.28 dB (including part 1 and part 2). The proposed scheme avoids the disadvantage of reflection when the high inclination section after TFLN dry etching is directly adopted as the coupling end face and can improve the performance of integrated TFLN electro-optic modulation on the chip. Three-dimensional simulation results show that the designed structure is insensitive to fabrication tolerances, which provides a feasible solution for reducing the size of integrated devices, decreasing costs, and meeting high-density integration requirements.