In recent years, silicon-on-insulator (SOI) technology has focused remarkable attention due to its high index contrast, which enables a high confinement of the propagating waveguide mode and a great integration density. However, the sub-micron waveguide dimensions imply a large difference between the transverse electric (TE) and the transverse magnetic (TM) modes, giving rise to a strong birefringence. The extremely wide range of applicability of this platform increases the interest in the enhancement of the current polarization beam splitters (PBS) performance. Different approaches such as Mach-Zehnder interferometry based PBSs , Bragg grating waveguides , directional couplers , photonic crystals , slotted  and plasmonic  waveguides or multimode interference couplers (MMI)  have been proposed with this purpose. Nevertheless, these schemes present different drawbacks like large footprints, experimental set-up limitations, limited bandwidths, efficiency restrictions, tight fabrication tolerances or complex fabrication techniques.
In this work, the novel PBS proposed is a MMI based on sub-wavelength grating (SWG) technology. SWGs are periodic structures of alternating materials, most commonly silicon and silicon dioxide, with a pitch much smaller than the wavelength of the propagating light, hence suppressing diffractive effects. These widely used structures can be considered as a homogeneous medium with an equivalent refractive index which is the average between the indices of both materials. By adjusting their geometric parameters, particularly the duty cycle, the equivalent index can be engineered opening the way to enhanced ultra-compact devices. SWGs have recently been demonstrated to be especially interesting in MMI couplers providing ultra-broadband bandwidths and notably efficiencies . Therefore, the present design not only benefits from the inherently low losses of MMI devices, but also from the index engineering of subwavelength structures. Furthermore, the high degree of inherent birefringence of these structures provides our MMI with an anisotropic character, which can be advantageously engineered by tilting the SWG structures in the multimode region. The SWG segments in the multimode region are tilted with respect to the optical axis of the device. Progressively-tilted input and output inverse tapers are also implemented, improving coupling efficiency and reducing losses. By selectively tuning the propagation constants of each polarization, large differences in their Talbot self-imaging length can be implemented. As a result, the beat length for the TE and TM polarizations are highly disparate, enabling a compact polarization splitter configuration. With this technique, a more efficient device is obtained with a reduced footprint, low insertion losses and extinction ratios, and broad bandwidth. The polarization splitter implemented on SOI platform allows a one-step and simple fabrication process.
Alaine Herrero, José Manuel Luque González, Aitor V. Velasco, Alejandro Ortega Moñux, Pavel Cheben, and Robert Halir, "High efficiency polarization beam splitter based on anisotropy-engineered MMI (Conference Presentation)," Proc. SPIE 10683, Fiber Lasers and Glass Photonics: Materials through Applications, 1068310 (Presented at SPIE Photonics Europe: April 24, 2018; Published: 23 May 2018); https://doi.org/10.1117/12.2307263.5788853938001.
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