Ultra-broadband, Low-loss Wavelength Combiners and Filters: Novel Designs and Experiments in Thin-film Lithium Niobate

TL;DR

This paper introduces novel ultra-broadband, low-loss wavelength combiners and filters on thin-film lithium niobate, with experimental validation demonstrating minimal loss and high extinction ratios across relevant optical wavelengths.

Contribution

The authors develop and experimentally demonstrate compact, fast-quasi-adiabatic wavelength combiners and filters with ultra-low loss and high extinction ratios on a 300-nm TFLN platform.

Findings

Loss below 0.06 dB across 90 nm bandwidth at 1550 nm

Loss below 0.12 dB over 45 nm bandwidth at 775 nm

Extinction ratios exceeding 25 dB at fundamental harmonic

Abstract

Thin-film lithium niobate (TFLN) has emerged as a leading platform for large-scale programmable photonic circuits for quantum and classical applications. As circuits scale in complexity, low-loss routing of broadband pump and signal fields becomes essential. Here, we present closed-form analytical models and experimentally demonstrate compact, fast-quasi-adiabatic driving-optimized wavelength combiners and filters operating at the fundamental harmonic (FH, 1550 nm) and second-harmonic (SH, 775 nm) wavelengths. Our designs achieve ultra-low loss below 0.06 dB across a 90 nm bandwidth at FH, while maintaining extinction ratios exceeding 25 dB. At SH, the loss remains below 0.12 dB over a 45 nm bandwidth with extinction ratios greater than 19 dB. Devices fabricated on a 300-nm TFLN platform exhibit added loss below 0.1 dB across 1550 - 1600 nm, with minimum values of 0.04 dB around 1580 nm and 0.021 dB at 775 nm. Combined with recent advances in on-chip quantum state generation, low-loss interferometers, and detection, these results enable high-fidelity quantum photonic circuits on the TFLN platform.