High-Integration multimode waveguide grating based CWDM4 MUX/DEMUX with Flat Wide Passband and Ultra-Low Crosstalk for 2xFR4 Module Applications

TL;DR

This paper introduces a compact CWDM4 MUX/DEMUX device with low crosstalk, flat wide passband, and high integration using cascaded multimode waveguide grating filters, suitable for 2xFR4 module applications.

Contribution

It presents a novel cascaded multimode waveguide grating design achieving ultra-low crosstalk and flat bandwidth in a compact footprint for CWDM4 systems.

Findings

Crosstalk below -25 dB achieved

Insertion loss under 1 dB

Flat-top bandwidth of approximately 18 nm

Abstract

This work presents a compact, low-crosstalk CWDM4 MUX/DEMUX utilizing cascaded multimode waveguide grating filters. The individual filters are designed with finite Gaussian apodization and positive dispersion, enabling strong unilateral sidelobe suppression while maintaining a minimum feature size compatible with UV lithography. By cascading these filters, we demonstrate a DEMUX that achieves channel crosstalk below -25 dB, insertion loss under 1 dB, and a flat-top bandwidth of approximately 18 nm. The entire device occupies a compact footprint of only 1.6 mm x 40 um, with a channel spacing compatible with commercial TIA and driver chips. Furthermore, a series-parallel hybrid cascade configuration can further suppress the crosstalk to -40 dB.