Low-loss coupling technique between SOI waveguides and standard single-mode fibers

In this paper, an inverted taper-based technique for highly efficient coupling between SOI waveguides and standard single-mode fibers is reported. The coupling structure is polarization insensitive and it is based on the inverted taper coupled to a fiber-adapted waveguide. In this case, the fiberadapted waveguide is made by using the SiOB2B layer under the Si waveguiding layer of the SOI wafer thus avoiding the use of extra materials such as polymers. The proposed coupling structure is aimed for being integrated with V-groove auto-alignment techniques. Coupling losses of 4.21dB and 4.5dB to 10μm mode field diameter standard single-mode fibers have been estimated for TE and TM polarizations respectively and 1550nm performance. Introduction Efficient coupling between standard single-mode fibers and single-mode waveguides is a key point in silicon photonics. An optical integrated circuit is useless without an interface to the outside world. The small size of single-mode silicon-on-insulator (SOI) waveguides (typically 500nm width and around 200nm thickness) compared with the high diameter of a single-mode fiber (between 8-10μm) makes coupling inefficient. A direct end-fire coupling between a SOI single-mode waveguide and a standard singlemode optical fiber means around 20dB of coupling losses for TE polarization and 1550nm input signal wavelength. Three-dimensional (3D) tapers have been reported to achieve 3D spot-size conversion between the spot-sizes of the waveguide and the fiber [1]. More complex structures such as two different tapers formed at different levels have also been proposed [2]. However, the complexity of the fabrication significantly increases in 3D approaches and the coupling length is typically higher than 500μm. A more elegant and compact solution that is compatible with planar processing techniques is the use of twodimensional (2D) inverted tapers to achieve 3D spotsize conversion [3-9]. In this case, the width of the taper is gradually decreased thus delocalizing the mode profile out of the waveguide core. The tip of the inverted taper can be directly attached to the optical fiber [4,5]. However, in this case, very precise control on the chip facets position is required. Therefore, the mode out of the inverted taper is usually coupled into another fiber-adapted waveguide. This fiber-adapted waveguide is made of low index contrast polymer materials and it is placed on top of the inverted taper [6-9]. However, the inverted taper and the low-index contrast waveguide are optimized for low coupling losses into small core optical fibers with typically 3-4 μm mode field diameter (MFD). In this paper, a new coupling structure based on the inverted taper is proposed for low coupling losses between SOI waveguides and standard single-mode fibers with 10μm MFD. The proposed approach is polarization insensitive and it takes advantage of the SiOB2B layer under the Si waveguiding layer of the SOI wafer to obtain the fiber-adapted waveguide. Furthermore, the structure is designed with the aim of future integration with V-groove structures thus allowing passive alignment and easier packaging [10].