Silicon Photonics: Fundamentals and Devices

10.9 Tapers

All the methods of coupling, described in Section 10.8, do not work when the guide dimension is reduced. The frequently used method to overcome the large differences in effective index, core size, and symmetry between the fiber and the waveguide is to employ a waveguide taper.

As mentioned in this chapter, some loss is incurred while coupling light from fiber to waveguides, because there is mismatch between the mode field diameters in the two media. The loss is not appreciable for coupling of fibers to silica-based waveguides, since the RIs have very close match. However, when a SOI waveguide is employed, the mode field dimension in high-index Si is considerably smaller than that in the core of the fiber. For example, in a SOI rib guide with a 4 μm thick epilayer, 3 μm rib width, and 2 μm etch depth, the mode field radius is 1.6 μm, whereas the fiber mode field radius is 5.2 μm. The coupling loss in this case is more than 4.7 dB. To reduce this coupling loss, it is necessary to use a mode transformer or a taper. Tapers reduce the coupling loss by an adiabatic modal transformation. Essentially there is a gradual, smooth, and lossless transition from a large cross-sectional waveguide area to a smaller one. The angle of the taper should be very small to achieve the smooth transition, and the surface roughness should also be as small as possible. Tapers may be of different forms. Lateral tapers are easily fabricated, since they involve an etching process from the top of ...