polarization-and wavelength- agnostic nanophotonic beam splitter

International audienceHigh-performance optical beam splitters are of fundamental importance for the development of advanced silicon photonics integrated circuits. However, due to the high refractive index contrast of silicon-on-insulator platforms, state-of-the-art nanophotonic splitters are hampered by trade-offs in bandwidth, polarization dependence and sensitivity to fabrication errors. Here, we present a new strategy that exploits modal engineering in slotted waveguides to overcome these limitations, enabling ultra-broadband polarization-insensitive optical power splitters with relaxed fabrication tolerances. The proposed splitter design relies on a single-mode slot waveguide that is gradually transformed into two strip waveguides by a symmetric taper, yielding equal power splitting. Based on this concept, we experimentally demonstrate −3 ± 0.5 dB polarization-independent transmission for an unprecedented 390 nm bandwidth (1260-1650 nm), even in the presence of waveguide width deviations as large as ±25 nm. Silicon-on-insulator (SOI) platforms are becoming established as an enabling technology for next-generation photonic circuits for a wide range of applications, including telecom and datacom applications 1-3 , radio-over-fibre systems 4,5 , bio-sensing 6,7 , LIDAR 8 and absorption spectroscopy 9,10 , to name a few. Such applications would benefit from the low-cost and large-volume fabrication offered by existing CMOS facilities as well as from the high density of integration enabled by the high refractive index contrast of SOI platforms. Furthermore, the high modal confinement of Si wire waveguides provides strong light-matter interactions with great potential for exploitation in opto-electronics, sensing and non-linear optical devices 11,12. However, the index contrast and modal confinement of SOI platforms also pose important challenges for the realization of high-performance SOI circuits, including a strong sensitivity to small geometric deviations, strong modal dispersion, and large birefringence between the transverse electric (TE) and transverse magnetic (TM) modes. Hence, SOI circuits typically operate in a single polarization state, within a limited bandwidth and with tight fabrication tolerances. Nevertheless, polarization-independent devices have emerged showing very similar performance for both TE and TM polarizations 13. High-performance SOI fibre-chip interfaces have been demonstrated to yield broadband and high-efficiency coupling with negligible polarization dependence and relaxed fabrication tolerances 14-16 , but such performance enhancements are still sought after in other essential SOI components. Specifically, beam splitters are particularly sensitive to the effects related to high index contrast and tight mode confinement in Si wires and would greatly benefit from achieving ultra-broadband dual-polarization operation. Over the past few years, several beam splitters with different advantages and limitations have been proposed. Directional couplers (DCs) are based on two parallel waveguides separated by a gap, enabling straightforward tuning of the power-splitting ratio by adequately selecting the coupling length. Due to their mode-beating-based operational principle, DCs are intrinsically narrowband in nature and suffer from a low tolerance to fabrication errors and a strong polarization dependence 17-19. The constraints on fabrication tolerances have been alleviated by engineering the excitation of odd and even modes in shallow-etched 20 and fully etched DCs 21-24 , but only single-polarization operation and dual-polarization operation over a limited bandwidth have been demonstrated