High-performance thin-film lithium niobate optical 90° hybrid with a novel CMRR assessment method

The optical 90° hybrid uses a 4 × 4 MMI coupler with an ultra-compact footprint of ~9 μm × 107 μm, which is 3 times shorter than previous work. A new method to assess the CMRR using a single unequal Mach-Zehnder interferometer (MZI), reducing fabrication complexity and measurement errors. The device features low excess loss (0.55 to 1.4 dB), low phase errors (< ±4.5°), and high CMRR (< −17.8 dBe) over a wide wavelength range (1530 to 1605 nm).