Arrayed Waveguide Gratings (AWGs)

Before reading this you may find the following tutorials useful:
Optical Networks, Wavelength Division Multiplexing (WDM)

Arrayed Waveguide gratings (AWGs) look like a bad comb-over sported by a balding, middle-aged man. However, they serve an altogether more useful purpose than lightly covering a scalp. They are formed by a series of curved sections of silica (the material that makes optical fiber) that guide incoming light, hence the name “waveguides.” These waveguides are usually fixed into position upon some kind of base that forms optical circuits (in a similar way to electronic circuit boards).

Like on a running track curve, the inner waveguides are shorter in length than the outer ones. An optical signal of many wavelengths joins the start of the array, on the left, and then a percentage of the signal travels down each waveguide. So every wavelength is traveling down each waveguide, and they are all traveling at the same speed. Now, when they reach the end of the array, on the right, they are all staggered slightly in time — the inner waveguide signal arriving first, and the outer one getting through last.

Arrayed Waveguide Grating These time-delayed signals now combine very cleverly to spit out each individual wavelength in its very own waveguide. I will now go through a few pages of equations to explain this combination process.

Well... maybe not. Ask your father.

So. At the end you have a pure signal of each wavelength in its own waveguide, and a multiplexed (combined) signal has been successfully demultiplexed (separated) into its individual wavelengths. It should be noted that the number of waveguides used does not have to equal the number of wavelengths that are to be demultiplexed. In general, the more waveguides that are used, the better the AWG can completely separate each wavelength.

There are several nice features of AWGs that make them quite a useful item for future optical networks. The fact that they are based in so-called “optical integrated circuits” means that it is easy to integrate them with other functions like amplification, rather than making them a separate box that makes things more complicated. They can also be simply reversed: So in the demultiplexer just described, separate wavelengths could be sent into the system from the right, and they would then exit the left-hand side combined — so the demultiplexer is now a multiplexer. AWGs can also split or combine many wavelengths at a time, compared with fiber Bragg gratings, which require a separate section for each individual wavelength.

Arrayed waveguide gratings are just now beginning to be manufactured, and in the future they may be increasingly used in a wide variety of devices.

Key Points

  • Curved sections of silica acting as waveguides
  • Each waveguide slightly different in length — like a running track bend
  • Incoming signal is split — every wavelength then travels down each waveguide
  • Time-delayed signals recombine to give each wavelength its own waveguide
  • Can be reversed to act as a multiplexer, rather than a demultiplexer
  • Usable in optical integrated circuits, easily combined with other functions

Further Reading

Optical Crossconnects, Fiber Bragg Gratings (FBGs)

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