  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.
