Coherency‐Broken Bragg Filters: Overcoming On‐Chip Rejection Limitations

Abstract

Selective optical filters with high rejection levels are of fundamental importance for a wide range of advanced photonic circuits. However, the implementation of high-rejection on-chip optical filters is seriously hampered 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.). State-of-the-art high-rejection filters partially circumvent the sensitivity to phase errors by means of active tuning, complicating device fabrication and operation. Here, a new approach based on coherency-broken Bragg filters is proposed to overcome this fundamental limitation. Non-coherent interaction among modal-engineered waveguide Bragg gratings separated by single-mode waveguides is exploited to yield effective cascading, even in the presence of phase errors. This technologically independent approach allows seamless combination of filter stages with moderate performance free of active control, providing a dramatic increase of on-chip rejection. Based on this concept, on-chip non-coherent cascading of Si Bragg filters is experimentally demonstrated, achieving a light rejection exceeding 80 dB, the largest value reported for an all-passive silicon filter.

Packaged, Fiber-Coupled Waveguide-Enhanced Raman Spectroscopic Sensor

“…Also, reflections at the interface between the buried waveguide and the exposed sensing waveguide can be reduced by letting the waveguide enter the sensing region at a shallow angle (rather than at a normal angle). Finally, replacing the directional coupler with either a broadband adiabatic coupler or wavelength-selective filters [10,17,18] would improve the flatness of the received Raman signal and could eliminate the 3 dB insertion loss penalty.…”

Silicon subwavelength modal Bragg grating filters with narrow bandwidth and high optical rejection

“…However, in most cases these solutions require an active control of each element, and none of them showed a narrowband notch response. On the other hand, there have been remarkable achievements in the improvement of the performance of Si Bragg gratings in terms of selectivity, showing bandwidth below 1 nm [8,9,[15][16][17] or rejection, with values exceeding 80 dB [18]. Subwavelength engineering of the grating lattice has shown to be a simple and powerful tool to achieve narrowband notch responses [8,15,17].…”

“…Thus, the achievable rejection level in these filters is limited to ∼ 40 dB due to phase errors arising from fabrication imperfections. Concurrently, we have recently shown that noncoherent cascading of multi-modal waveguide Bragg gratings allows ultra-high rejection levels, exceeding 80 dB [18]. The idea lies in implementing several filter sections with Bragg gratings that couple back-reflections into a high-order waveguide mode, and connect them by singlemode waveguides that radiate back-reflections propagating in a high-order mode.…”

Active On-Chip Dispersion Control Using a Tunable Silicon Bragg Grating

“…Integrated Bragg grating filters are an established and widely used technology on the Silicon-on-Insultor (SOI) material platform. They are applied in a large variety of applications, including, optical filtering [1,2], sensing [3], laser cavity feedback [4] and all-optical signal processing [5,6]. A wide range of device geometries have been demonstrated in order to exercise control over the grating optical characteristics, namely the filter bandwidth [7], ripple [8], extinction and dispersion [2,9,10].…”

“…They are applied in a large variety of applications, including, optical filtering [1,2], sensing [3], laser cavity feedback [4] and all-optical signal processing [5,6]. A wide range of device geometries have been demonstrated in order to exercise control over the grating optical characteristics, namely the filter bandwidth [7], ripple [8], extinction and dispersion [2,9,10]. In turn, the optical characteristics of the grating can be designed through the coupling coefficient, κ, and the grating Bragg wavelength, λB, as a function of the propagation length [10,11].…”