Ultra-Compact Thin-Film Lithium Niobate Optical 90° Hybrid with 4 × 4 Mmi Coupler

As the crucial element in the coherent transceivers, traditional optical 90° hybrid suffer from large footprint due to weak optical field confinement. In this paper, an ultra-compact optical 90° hybrid based on x-cut TFLN has been proposed and demonstrated. The optical 90° hybrid is realized by utilizing a 4 × 4 MMI coupler with an ultra-compact footprint of ~ 9 μm × 107 μm. In addition, a novel method has been proposed to evaluate CMRR. The demonstrated optical 90° hybrid shows low excess loss of 0.55~1.7 dB, low phase errors < ± 4.5°, high common mode rejection ratio (CMRR) <−17 dBe and excellent fabrication tolerance over the wavelength range of 1520-1605 nm. It will promote the development of integrated transceivers on TFLN for advanced quadrature phase shift keying (QPSK) data communication, photonic imaging and microwave photonics.