Ultra‐Low‐Loss and Athermalized Lithium‐Niobate‐on‐Insulator Photonic Chip for Next‐Generation PONs

Abstract A high‐capacity lithium‐niobate‐on‐insulator (LNOI) optical transmitter for next‐generation passive optical‐networks (PONs) is proposed and demonstrated with ultra‐low temperature‐dependence for the first time by simultaneously introducing Z‐ and Y‐propagation LNOI photonic waveguides. There are five wavelength‐division‐multiplexed channels involved with the center wavelengths of 1270/1300/1342/1537/1577 nm, and the photonic filters are realized by integrating amplitude‐apodized multimode waveguide gratings (MWGs) and mode (de)multiplexers designed with Z‐propagation LNOI photonic waveguides, which are found to be with a low thermo‐optic coefficient and thus enable the athermalization. Meanwhile, high‐speed optical modulators connected at the add ports of the photonic filters for the 1342/1577 nm channels are developed with Y‐propagation LNOI photonic waveguides with the maximal electro‐optic effect. For the fabricated on‐chip optical transmitter, box‐like spectral responses are achieved with ultra‐low excess losses of <0.5 dB and low inter‐channel crosstalk of < −26 dB for all the channels whose bandwidths are respectively ∼16/20/4/15/5 nm as expected. The present LNOI photonic chip is nearly athermal, exhibiting very low temperature‐dependence less than 8.7 pm/°C in the full wavelength range of >300 nm. Finally, high‐capacity data transmissions of 50/100 Gbps on‐off‐keying signals and 100/200 Gbps four‐level‐pulse‐amplitude‐modulated signals are demonstrated, providing a promising solution for next‐generation PONs.