AWGs
Installing fiber infrastructure is expensive, time consuming and disruptive, so it’s important to use the fiber as efficiently as possible. When fiber-optic networks were first installed in the 1990s, very few wavelengths were used, despite the wide transmission window of silica fibers – and the wavelengths needed to be well separated to accommodate the bandpass filters used at the time. As the amount of information transmitted over networks has increased, so has the need for faster and higher-bandwidth networks.
Transmitting many data streams on closely spaced frequencies – for example, 96 channels on a 50GHz spacing – maximizes the spectral efficiency of the silica transmission window. This technology is known as dense wavelength division multiplexing (DWDM). Thin-film solutions work well for small-channel-count DWDM, but the device size and insertion loss increase with channel count and become impractical at channel counts above around 12.
Arrayed waveguide gratings (AWGs) were invented to operate as higher-channel-count DWDM multiplexers and demultiplexers, with the added advantages of small size, uniform insertion loss and wafer-level fabrication. They work by splitting an input light beam or beams into many individual waveguides, each designed to a specific length that provides a particular relative phase profile of the light exiting all the waveguides after propagation. This phase profile is designed to create a diffractive interaction between each wavefront leaving the waveguides, and to impart a propagation angle that depends on wavelength. So a single waveguide input containing many wavelengths can be split into many waveguides containing a single wavelength, and vice versa. This optical operation is completely passive, and the device can be used reversibly to either multiplex or demultiplex channels. Today, AWGs act as the backbone of all modern DWDM networks.