High Performance Silicon Photonics Filters for Quantum Applications

Introduction

Silicon photonics has emerged as a promising platform for integrated quantum optics due to its high index contrast, compact footprint, and compatibility with advanced semiconductor manufacturing processes. One of the key building blocks in silicon photonic quantum circuits are optical filters, which play a crucial role in spectral filtering, pump rejection, and signal conditioning. In this tutorial, we will explore two recent advancements in high-performance integrated optical filters based on Bragg gratings: a polarization-independent contra-directional coupler and a thermally tunable notch filter with controllable sidelobes.

Cladding Modulated Bragg Gratings

At the core of these advanced filtering devices lies the cladding modulated Bragg grating structure. Bragg gratings are waveguides with a periodic perturbation along the propagation direction, characterized by two key parameters: the spatial period of the perturbation (Λ), which sets the central wavelength of the filter, and the grating strength (κ), which is related to the reflectance and bandwidth of the filter.

Cladding modulated Bragg gratings, as shown in Figure 1, provide control over the grating strength by tuning the separation (s) between the waveguide core and the loading segments. This approach enables exquisite adjustment of the grating strength without requiring small feature sizes, greatly facilitating fabrication.

Figure 1: Schematic (top view) of a cladding modulated Bragg grating waveguide consisting of a homogeneous waveguide and a set of periodic loading segments with a periodicity, Λ, separated from a distance s from the core.

Polarization Independent Contra-Directional Couplers

In conventional Bragg filters, the reflected signal is sent back to the input port, requiring an external optical circulator or a beam splitter/combiner for its extraction, which introduces additional losses. Contra-directional couplers (CDCs) provide an elegant solution to this problem by coupling the reflected signal to a parallel waveguide instead of the input waveguide.

Figure 2a illustrates the schematic of a cladding modulated CDC, consisting of two asymmetric waveguides with a periodic perturbation. The resonance condition for the light to be coupled to the parallel waveguide is given by the equation:

λᴄᴅᴄ = (n₁ + n₂)Λ,

where n1 and n2 are the effective indices of the supermodes of the structure.

Figure 2: a) Schematic (top view) of a cladding modulated CDC. b) Measured response at the through port of the design for both polarizations.

In this work, a CDC device was designed for a 400 nm thick silicon nitride platform, targeting pump rejection around 1300 nm. By carefully engineering the geometrical parameters, polarization independence was achieved, satisfying the condition:

(n₁ᵀᴱ + n₂ᵀᴱ) = (n₁ᵀᴹ + n₂ᵀᴹ)

The fabricated device demonstrated an impressive 40 dB rejection of the pump wavelength for both polarizations, as shown in Figure 2b, while exhibiting low losses (<0.5 dB) for the single-photon source wavelength, excited at ~10 nm from the pump.

Tunable Notch Filters

Quantum optical systems often rely on non-linear phenomena that generate desired quantum light sources. However, these non-linear effects can also generate spurious intermodulation products on a periodic grid. To address this challenge, a tunable notch filter with controllable sidelobes was developed using an array of heating elements on top of the device, as depicted in Figure 3a.

Figure 3. a) Schematic (top view) of a single band cladding modulated Bragg filter with a periodic array of heater on top of the filter and c) its simulated and measured spectrum observing the formation of multiple bands when the heaters are feed with a 30 mA current.

By applying heat to the array, a set of periodically spaced sideband reflections is generated, with a separation given by:

FSR = λ²/(n𝓰Λʜ')

where ng is the group index of the grating mode, and ΛH is the heater period.

This technique was experimentally demonstrated in a 220 nm thick silicon-on-insulator (SOI) platform. Figure 3b shows the transformation of a single notch spectrum into a multiple sideband notch filter with 3 nm spacing when the heaters are turned on, effectively filtering out the undesired intermodulation products.

Conclusion

The presented work showcases the potential of silicon photonics platforms for applications demanding high-performance filtering, such as quantum optics. The cladding modulated Bragg filter architecture enables the design of polarization-independent contra-directional couplers and tunable notch filters with controllable sidelobes. These advancements pave the way for more efficient and versatile integrated quantum optical systems, further solidifying the role of silicon photonics in this emerging field.

Reference

[1] J. M. Luque-González et al., "High Performance Silicon Photonics Filters for Quantum Applications," Telecommunication Research Institute (TELMA), Universidad de Málaga, CEI Andalucía TECH, Louis Pasteur 35, 29010 Málaga, Spain; National Research Council Canada, 1200 Montreal Road, Bldg. M50, Ottawa K1A 0R5, Canada; Department of Electronics, Carleton University, Ottawa, ON K1S 5B6, Canada, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.