A filling material based on a similar refractive index with SiN is designed as the mode converter for thin film lithium niobate (TFLN). Such a design can realize an output mode field compatible with different sizes ranging from 3.5 um-9.2 um. The double-layer mode converter core with SiN has a similar height as the ridge waveguide of TFLN, which is helpful to increase the conversion efficiency. An overall coupling loss of less than 0.6 dB was achieved theoretically at 1310 nm for both modes. The proposed scheme avoids the disadvantage of high reflection when the inclined TFLN section result from dry-etching is directly used as the coupling end face and can improve the performance of integrated TFLN electro-optic modulation on the chip level. Three-dimensional simulation results show that the designed structure is insensitive to fabrication tolerance, which provides a feasible solution for reducing the volume of integrated devices, increasing overall performance and high-density integration.
Periodically poled lithium niobate (PPLN) is a promising platform for realizing high-speed active polarization mode conversion. Especially, the development of thin-film PPLN techniques drives related devices to lower power consumption, higher performance and more integration. However, the wavelength shifting with the temperature variation is still a problem that brings instability and impedes modulation efficiency. In this paper, we first analyzed the temperature characteristics of a well-designed z-cut polarization mode converter based on thin-film PPLN. The simulated modulation voltage is smaller than 5V. Then a temperature-insensitive device was proposed with different coating materials of negative thermo-optic coefficients. Compared to the structure without coating, the wavelength shifting decreases from 0.25nm/°C to 0.07nm/°C, in the meantime, the modulation voltage can still be kept smaller than 5V or even be reduced slightly.
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