Coherency-broken Bragg filters: surpassing on-chip rejection limitations

TL;DR

This paper introduces a novel non-coherent cascading approach for on-chip optical filters that overcomes phase error limitations, achieving record high rejection levels exceeding 80 dB in silicon photonics.

Contribution

The paper presents the first experimental demonstration of coherency-broken Bragg filters that enable high rejection without fabrication error sensitivity.

Findings

Achieved over 80 dB rejection in silicon Bragg filters.

Demonstrated non-coherent cascading overcomes phase error limitations.

Validated the approach with experimental on-chip results.

Abstract

Selective on-chip optical filters with high rejection levels are key components for a wide range of advanced photonic circuits. However, maximum achievable rejection in state-of-the-art on-chip devices is seriously limited by phase errors arising from fabrication imperfections. Due to coherent interactions, unwanted phase-shifts result in detrimental destructive interferences that distort the filter response, whatever the chosen strategy (resonators, interferometers, Bragg filters, etc.). Here we propose and experimentally demonstrate a radically different approach to overcome this fundamental limitation, based on coherency-broken Bragg filters. We exploit non-coherent interaction among modal-engineered waveguide Bragg gratings separated by single-mode waveguides to yield effective cascading, even in the presence of fabrication errors. This technologically independent approach allows seamless combination of filter stages with moderate performance, providing a dramatic increase of on-chip rejection. Based on this concept, we experimentally demonstrate on-chip non-coherent cascading of Si Bragg filters with a record light rejection exceeding 80 dB in the C-band.