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Polarization Control in Silicon Nitride Photonics for Near-Infrared Applications

Writer's picture: Latitude Design SystemsLatitude Design Systems
Introduction

The silicon nitride (SiN) platform has emerged as a promising candidate for integrated photonics due to its low propagation and nonlinear losses, reduced birefringence, and wide transparency range from visible to mid-infrared wavelengths. However, to fully exploit these advantages, it is crucial to control the polarization state in SiN photonic devices, especially for wavelengths shorter than the telecommunication range (1300-1550 nm). These shorter wavelengths, particularly around 1 μm, are of particular interest in quantum applications, where the low-loss characteristics of SiN provide a significant advantage, and efficient single-photon sources based on InAsP are available.

Principle of Operation

The polarization splitter-rotator (PSR) device presented in this work is designed to operate in the 910 nm – 980 nm wavelength range, covering the emission wavelength of InAsP quantum dots. The device is divided into two main parts: rotation and splitting.

SiN polarization splitter-rotator
Figure 1: (a) Schematic of the SiN polarization splitter-rotator. (b) Calculated effective modal indices as a function of the waveguide width. The hybridization zone is indicated with a dashed circle. (c) Cross-sectional view of the directional coupler at the position (i) marked by the yellow dashed line in panel (a).

  1. Rotation:

    The rotation part of the PSR consists of an adiabatic taper that gradually converts the fundamental transverse magnetic mode (TM0) of the input waveguide into the first higher-order transverse electric mode (TE1). This mode conversion is possible due to the hybridization between the TM0 and TE1 modes, which occurs at a specific waveguide width (1.49 μm) when the cross-sectional symmetry is broken by removing the upper cladding (Fig. 1(b) and (c)).

  2. Splitting:

    The splitting part involves coupling between the TE1 mode of the input waveguide (WG1) and the TE0 mode of the adjacent waveguide (WG2). This coupling is achieved by satisfying the phase-matching condition, where the effective indices of the TE1 mode in WG1 and the TE0 mode in WG2 are equal. The widths of WG1 and WG2 are carefully chosen to meet this phase-matching condition. The input TE0 mode in WG1, not being phase-matched, continues its path to the output through port.

Design and Simulation

The device was designed using finite difference eigenmode (FDE) and eigenmode expansion (EME) solvers. The optimal dimensions, such as waveguide widths, taper length, coupling length, and gap width, were determined to maximize the desired mode conversions and minimize losses.

Experimental Results
performance of the PSR
Figure 2: Simulated (dashed lines) and measured transmissions (solid lines) of the PSR at the through and cross ports for (a) TE mode input and (b) TM mode input.

The performance of the PSR was characterized by two key parameters: Polarization Extinction Ratio (PER) and Insertion Loss (IL). Figure 2 shows both the simulated and experimentally measured transmissions for TE and TM mode inputs over the wavelength range of 910 to 980 nm.

For TE mode input (Fig. 2(a)), the measured PER reaches a maximum of 43 dB at 970 nm and remains above 30 dB across the entire range, with an insertion loss below 1 dB.

For TM mode input (Fig. 2(b)), the PER reaches a maximum of 31 dB at 980 nm and remains above 24 dB, with insertion losses of around 1 dB over the same range.

Conclusion

The experimental demonstration of a highly efficient polarization splitter-rotator on the SiN platform for the 950 nm wavelength band has been achieved. The device, which combines an adiabatic taper and a directional coupler, exhibits low insertion losses (less than 1 dB) and achieves very high PER values, exceeding 30 dB and 24 dB across the entire range of 910-980 nm, with peak values exceeding 40 dB and 30 dB for TE and TM polarizations, respectively.

This work paves the way for efficient polarization control in SiN photonic devices operating in the near-infrared range, enabling the exploitation of the low-loss characteristics of the SiN platform in quantum and other applications that require precise control of the polarization state.

Reference

[2] Z. Mokeddem et al., "Broadband and Efficient Polarization Splitter-Rotator in the Silicon Nitride Platform for the 0.95 µm Wavelength Band," Centre de Nanosciences et de Nanotechnologies, CNRS, Université Paris-Saclay, Palaiseau, France; Advanced Photonics and Electronics Research Centre, National Research Council Canada, Ottawa, Canada; Digital Technologies Research Centre, National Research Council Canada, Ottawa, Canada, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.

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