Silicon photonics
Which is a correct advantage of a grating coupler for connecting optical fiber to a silicon photonic circuit?
- AIt couples to fiber with lower loss than an edge coupler, over a wide wavelength range and independent of polarization
- BThe chip facet is polished and the fiber is butted against it from the side, making alignment with the fiber easy
- CIt can be placed anywhere on the chip surface, and optical characteristics can be measured on the wafer by bringing a fiber down from above
Show answer and explanation
Answer: C. It can be placed anywhere on the chip surface, and optical characteristics can be measured on the wafer by bringing a fiber down from above
A grating coupler forms periodic grooves (a diffraction grating) in the waveguide that diffract light obliquely upward to couple it to a fiber placed above. Waveguides need not be routed to the chip edge and measurements can be made on the wafer before dicing, so a major advantage is that many chips can easily be tested automatically. On the other hand, the diffraction condition depends on wavelength and polarization, so the coupling bandwidth is narrow, about a few tens of nm, it is hard to support more than one specific polarization, and its loss is generally higher than an edge coupler's. Coupling at the facet is what an edge coupler does.