6.1. OPTICAL MATERIAL SYSTEMS
The key material systems used in optical communication componentry include silica fibers, silica on silicon (SOS), silicon on insulator (SOI), silicon oxynitride, sol-gels, polymers, thin-film dielectrics, lithium niobate, indium phosphide, gallium arsenide, magnetooptic materials, and birefringent crystals. The silica (SiO2) fiber technology is the most established optical guided-wave technology and is particularly attractive because it forms in-line passive optical components that can be fused to transmission fibers using standard fusion splicers. It includes fused fiber, doped fiber, patterned fiber, and moving fiber technologies, all described later in the chapter. Silica fibers have been used to produce lasers, amplifiers, polarization controllers, couplers, filters, switches, attenuators, CD compensators, and PMD compensators .
The SOS technology is the most widely used planar technology. It involves growing silica layers on silicon substrates by chemical vapor deposition (CVD) or flame hydrolysis. Both growth processes are lengthy (a few to several days for several to a few tens of microns), and are performed at high temperatures .
The deposited layers typically have a high level of stress. This stress can result in wafer bending, a problem that translates into misalignment between the waveguides on a chip and the fibers in a fiber array unit used for pigtailing. Nevertheless, the wafer-bending problem can be substantially reduced by ...