High-conversion-efficiency mode converters based on cubic spline interpolation

Mode converters play a critical role in mode-division multiplexing (MDM) systems. This paper proposed and demonstrated a design method of high conversion efficiency (CE) and broadband mode converters based on cubic spline interpolation curves. The cubic spline interpolation method provides great flexibility in defining the boundaries of irregularly shaped waveguide curves. This design effectively improves the mode conversion efficiency and can be applied to different mode orders and multimode waveguide widths. For the TE0-TE1 converters, we designed different multimode waveguide widths (1 µm and 1.1 µm) to demonstrate the flexibility of the design approach. At 1550 nm, the conversion efficiencies of the TE0-TE1 converters and TE0-TE2 converters are -0.026<i>d</i><i>B</i> (99.4%) and -0.039<i>d</i><i>B</i> (99.1%), respectively, according to the theoretical results. The mode converters were manufactured using a standard process provided by the foundry, and the measured conversion efficiency for all devices was above -0.419<i>d</i><i>B</i> (90.8%) in the wavelength range of 1530-1565 nm.