| Summary: | In this paper, we demonstrate silicon-on-insulator (SOI) channel waveguides with propagation loss and <inline-formula><tex-math notation="LaTeX">$r$</tex-math></inline-formula> = 10 <inline-formula> <tex-math notation="LaTeX">$\mu$</tex-math></inline-formula><inline-formula><tex-math notation="LaTeX">$\text{m}$ </tex-math></inline-formula> bends with bending loss as low as 2 dB/cm and 0.02 dB/90<inline-formula> <tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> in the broad wavelength range of 3.68–3.88 <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula><inline-formula> <tex-math notation="LaTeX">$\text{m}$</tex-math></inline-formula>. We also indicate the limitation of SOI waveguide loss in mid-infrared wavelength by analyzing loss mechanisms. Based on this high-performance mid-infrared waveguide, we systematically analyze the coupling efficiency of various directional couplers, which are critical and commonly used building blocks for on-chip light routing and power splitting. By varying coupling length and sweeping wavelength, the performance of directional coupler with various gap separations is meticulously investigated in 3.68–3.88 <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula><inline-formula> <tex-math notation="LaTeX">$\text{m}$</tex-math></inline-formula>. A experimental database for directional coupler with any power splitting ratio is provided. Our results offer a promising broadband platform for dense wavelength division multiplexing (DWDM) and applications that require precise control of coupling such as racetrack resonator in the mid-infrared region.
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