Polarization-transparent microphotonic devices in the strong confinement limit

Microphotonic structures that strongly confine light, such as photonic crystals and micron-sized resonators, have unique characteristics that could radically advance technology. However, such devices cannot be used in most applications because of their inherent polarization sensitivity; they respond differently to light polarized along different axes7–9. To take advantage of the distinctive properties of these structures, a general, integrated, broadband solution to their polarization sensitivity is needed. Here, we show the first demonstration of such a solution. It enables arbitrary, polarization-sensitive, strong-confinement (SC) microphotonic devices to be rendered insensitive (transparent) to the input polarization at all wavelengths of operation. To test our approach, we create the first polarization-transparent add–drop filter from polarizationsensitive microring resonators. It shows almost complete elimination of polarization sensitivity over the 60-nm bandwidth measured, while maintaining outstanding filter performance. This development is a milestone for SC microphotonics, allowing the applications of photonic-crystal and microring devices to several areas, including communications, spectroscopy and remote sensing. Polarization sensitivity is a major problem in microphotonics, because the polarization state changes randomly in optical fibres. This makes SC microphotonic devices incompatible with the optical fibres necessary to connect them to the outside world. SC microphotonics should not be confused with microphotonics using weak confinement of light, such as those based on dopedsilica waveguides and ridge waveguides, as these may be fabricated to be individually polarization transparent. This is impossible for SC microphotonic devices as it would require fabrication with subatomic absolute dimensional accuracy (see Supplementary Information). Ridge waveguides, even those made in silicon, are not considered part of SC microphotonics as they offer a weak lateral field confinement that results in substantial bending loss and, consequently, in large resonator size. However, a ridge waveguide etched so deep that it resembles a rectangular waveguide and shares a similar level of confinement will also exhibit similar challenges in polarization transparency. Our approach to polarization transparency is based on polarization diversity with a unique implementation that exploits symmetry to overcome sensitivities to fabrication imperfections that would otherwise prohibit practical applications. Polarization-transparent devices are constructed from polarization-sensitive components using the integrated scheme shown in Fig. 1. The arbitrary polarization state emanating from a fibre is split into orthogonally polarized components travelling in separate arms. By rotating the polarization state in one of these arms, a single polarization is realized on-chip. The two arms then pass through identical sets of polarization-sensitive photonic structures. At the output of the chip, another polarization rotation is performed and the two arms are recombined. The second polarization rotation is carried out to avoid wave interference between the two arms, and the Rotator Rotator Combiner