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What Is an Arrayed Waveguide Grating?

Paul Reed
Paul Reed

An arrayed waveguide grating is used in fiber optic transmission systems to permit a single optical fiber to carry multiple channels or communication bands. Fiber optic cables use very thin glass fibers to transmit light signals containing voice or data communications. In the late 20th century, fiber optics rapidly expanded the speed and amount of data that could be carried over telephone, television and computer networks, and began replacing wire or co-axial cable networks.

Light passes through air or fiber cables as a series of waves, similarly to waves in water. The principle of light diffraction, where light passing through fibers of slightly different lengths exits at slightly different phases or angles, is the basis for an arrayed waveguide grating. Light exits each of the fibers in the waveguide at a slightly different point in the wave because each fiber has a different length, and the light takes more or less time to travel its length. When these out-of-phase frequencies interact, they create a diffraction pattern, which is a series of evenly spaced light signals, each with its own frequency.

Woman holding a disc
Woman holding a disc

Different frequencies of the light signal are used for different communication bands, and the arrayed waveguide grating is used to combine or multiplex these individual bands into a single fiber cable. This technology is referred to as wavelength division multiplexing (WDM), and permits many conversations or data streams to be combined. The process can be reversed at the other end of a transmission line, with the combined signals separated in a de-multiplexing waveguide.

There are few parts to an arrayed waveguide grating. The incoming fiber cable is connected to a mixing zone, with multiple fiber cables. The arrayed waveguide is lined up in a row at the other end of the zone. At the opposing end is a collection or focusing zone where the different wavelengths or channels are separated by diffraction and enter multiple fiber cables.

One issue with arrayed waveguide grating technology is that it is affected by temperature. As temperatures rise or fall, the optical fibers change length by very small amounts. These sight changes can change the diffraction pattern leaving the grating and cause a loss of signal quality. Early waveguides were heated to maintain an artificial temperature above normal room or outdoor temperatures to prevent signal loss, but caused additional operating costs.

Unheated arrayed waveguide gratings have been developed in the 21st century that use temperature-compensated waveguides. One non-heated system uses copper strips connected to the focusing zone that move slightly with changes in temperature. This can be calibrated to keep the focused light frequencies in the correct position for the exiting fiber optics to collect the signals.

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