Silicon (Silicon Photonics and Planar Lightwave Circuits): Silicon is very inefficient in generating or detecting light in the telecom wavelength window as it is an indirect bandgap semiconductor material. Consequently, waveguides of silicon or doped silicon dioxide (silica) exhibit very low optical loss and are ideal for switching, filtering, or interferometric applications. Silicon is used to form Photonic Integrated Circuits (PICs) in two separate ways:
Planar Lightwave Circuits: First, dopped silica glasses are formed with controllable indices of refraction and are used to make waveguide circuits called Planar Lightwave Circuits or PLCs. For example, a layer of silicon dioxide is formed by oxidizing a standard silicon wafer at high temperature. Then a layer of silica glass is deposited using standard semiconductor tools such as Plasma Enhanced Chemical Vapor Deposition (PECVD) or custom equipment like Flame Hydrolysis. The index of the layer is controlled by adding dopants to the glass. Then the waveguide circuit is formed using standard photolithographic and etching techniques and then covered with a layer of lower index glass using the same process tools described above. PLC devices were the first PICs to enter widespread deployment and have been shipping for almost 20 years. Due to the very low loss, complex optical circuits are possible using silica waveguides. Furthermore, because the waveguides are made of glass, the indices and mode sizes are very close to those of optical fiber, so there is very low coupling loss between the waveguides and fibers, making PLCs ideal for passive optical components. Typical devices are Passive Splitters, Arrayed Waveguide Gratings for DWDM MUX/DMUX, Variable Optical Attenuators, and Optical Switches. For example, an 8×16 Multi-Cast Switch combines eight 1×8 splitters and sixteen 1×8 switches plus numerous taps, crossings, and other element on a single chip. PLC wafers are manufactured in specialized clean rooms and are typically 6-8 inches in diameter.
Silicon Photonics: Secondly, Silicon itself can also be used to make waveguides and other photonic elements. Since a waveguide must be surrounded with lower index material, Silicon waveguides cannot be made directly in the silicon wafer itself, but instead are made in silicon redeposited on a lower index layer in a process called Silicon on Insulator. Silicon waveguides have much higher effective indices of refraction than silica glass (PLC), and therefore can make much smaller devices, but since the waveguide modes do not match optical fiber modes, the coupling losses are much higher. Of particular interest is that Silicon can be engineered to enable high-speed modulators which are now being used in 100G transmission. Silicon Photonic devices can be made in standard CMOS fabs, leading to potentially very low costs when the volumes approach electronics scale. For now, however, most Silicon Photonics devices are made in semi-specialized fabs on wafers with diameters of 8-12 inches. Silicon modulators typically have a high fiber-to-fiber insertion loss and require a high driving voltage. However, it does not require a TEC and hermetic sealing in packaging, which can save significant packaging cost.