Low-Phase-Error Thin-film Lithium Niobate Optical 90° Hybrid with 4 X 4 MMI Coupler

We propose and demonstrate a thin-film lithium niobate 90° optical hybrid based on $4\times 4$ MMI coupler, which features low average phase error $\lt \pm 3^{\circ}$ over the wavelength range of 1535-1595 nm.