2. State of the Art

Waveguide input and output couplers have always been an important issue in optical integrated circuit design. Various coupling schemes have been proposed or demonstrated to overcome the low coupling efficiency between the optical fiber and the waveguides on the chip. Based on the direction at which light is coupled in or out of the waveguides, these schemes fall into one of two categories: vertical coupling (out of plane) and lateral coupling (in plane).

Vertical coupling is typically accomplished with diffractive gratings incorporated into a waveguide layer to provide a conversion between the optical mode in the single-mode fiber (SMF) and the waveguide. The vertical coupling scheme typically requires the fiber to be positioned at some angle to the wafer. Multiple fiber I/O is possible. The drawbacks are that the diffractive nature of this approach relies on interferometric behavior applicable only to a limited wavelength range and therefore may not be suitable for large-spectral bandwidth optical coupling. It also has strong polarization dependence.

In lateral coupling, light is coupled in and out of an exposed cross-section of the waveguide in the lateral direction and this has always been reported in a butt coupling configuration. Typically a lens or a spot-size converter (SSC) is needed for the mode conversion between the fiber and the waveguide. Multichannel coupling of waveguide to fiber arrays has been demonstrated. While the lateral coupling has weak polarization dependence and is insensitive to the input bandwidth, it puts a stringent demand on the alignment of the fiber both vertically and laterally. Also, SSC designs require excellent control of the critical dimensions and lensed fiber or special fiber are often needed for nano-waveguides especially when implemented for short wavelength, which adds to the cost and complicates the fabrication and packaging of the integrated circuits.