Wavelength Division Multiplexing with Modal-Engineered Bragg Filters

Introduction

Wavelength division multiplexing (WDM) is a key technology for applications in optical communications and quantum information processing. By allowing multiple wavelengths to be transmitted over a single optical fiber or waveguide, WDM enables more efficient use of available bandwidth. However, implementing WDM on integrated silicon photonics platforms presents challenges, particularly for quantum optics applications that require precise control and low-loss transmission of individual photons.

In a recent work published in a conference paper, researchers from the University of Paris-Saclay and the University of Cote d'Azur have proposed and demonstrated a new approach to WDM based on modal-engineered Bragg filters. Their device combines a tapered multimode Bragg filter with an optimized modal add-drop (MAD) coupler, enabling low-loss selection of a specific wavelength band with high side-lobe suppression.

Proposed Bragg Filter

The key innovation lies in the design of the Bragg filter itself. Traditional Bragg filters consist of a periodic corrugation or modulation of the waveguide geometry, creating a grating that selectively reflects light at the Bragg wavelength determined by the grating period and the effective index of the guided mode. However, these filters often suffer from significant side-lobes in the reflected spectrum, which can limit their usefulness in WDM applications.

The researchers' approach is to engineer the modal properties of the Bragg filter by gradually transitioning from a completely symmetric grating geometry to a completely asymmetric one, as shown in Figure 1.

Fig. 1. Schematic of the geometry of the device.

In the symmetric case, the fundamental mode of the waveguide is phase-matched to the Bragg condition, while in the asymmetric case, the conversion from the fundamental mode to the first excited mode becomes dominant. By apodizing the grating – gradually shifting the symmetry of the corrugation from symmetric to asymmetric along the length of the filter – the researchers can control the modal interaction and suppress the side-lobes of the reflected signal.

Figure 2 shows the calculated reflection spectra for filters with different levels of sidewall asymmetry, illustrating the side-lobe suppression achieved by the apodization approach.

Fig. 2. 2.5D FDTD simulations of the reflection of a non-apodized filter and an apodized filter.

To couple light from the tapered Bragg filter, the researchers employed a modal add-drop (MAD) device, which exploits the multimodal behavior of the filter to efficiently extract the reflected signal. The MAD coupler is designed to be broadband and optimized for the specific modal properties of the Bragg filter.

The device was fabricated on a silicon-on-insulator (SOI) platform using electron-beam lithography and characterized in the laboratory. Figure 3 shows the measured transmission spectra for a non-tapered filter (a) and a filter with a 90 μm taper (b).

Fig. 3. Measured transmitted spectra of a) non-tapered filter and a b) 90 µm tapered filter.

The non-tapered filter exhibits significant side-lobes in the reflected signal, even with asymmetric sidewalls. In contrast, the tapered filter with the modal-engineered apodization approach achieves more than 20 dB of side-lobe suppression while maintaining low insertion loss.

The researchers highlight that the nature of the device allows for cascading multiple stages, potentially enabling even further side-lobe suppression. This opens up new possibilities for highly precise and low-loss WDM in integrated photonics, particularly for quantum optics applications where maintaining the integrity of quantum states is crucial.

Conclusion

This work presents a novel and promising approach to WDM on silicon photonics platforms. By combining modal-engineered Bragg filters with optimized MAD couplers, the researchers have demonstrated a device capable of selecting specific wavelength bands with low loss and high side-lobe suppression. This technology could pave the way for a new generation of high-performance, multichannel WDM devices for applications in communications, quantum information processing, and beyond.

Reference

[2] D. E. Medina et al., "Wavelength Division Multiplexer Based on Modal-Engineered Bragg Filters in Silicon," Centre de Nanosciences et de Nanotechnologies, Université Paris-Saclay, CNRS, Palaiseau, France; Université Côte d’Azur, CNRS, Institut de Physique de Nice (INPHYNI), Nice, France, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.