Brief Introduction of Arrayed Waveguide Gratings

Wavelength Division Multiplexing (WDM) technology provides an effective approach to the rapid increase of bandwidth and capacity requirement in communication systems and networks. In WDM systems. all kinds of photonics devices based on arrayed waveguide gratings (AWG) have been the key components used as wavelength division multiplexing/demultiplexing.

What’s Arrayed Waveguide Grating (AWG)?
Arrayed waveguide grating (AWG), also known as the optical phased array (PHASAR), phased-array waveguide grating (PAWG), or waveguide grating router (WGR), is a device built with silicon planar lightwave circuits (PLC), that allows multiple wavelengths to be combined and separated in a dense wavelength-division multiplexing (DWDM) system. It has become increasingly popular as a wavelength multiplexer and demultiplexer (MUX/DeMUX) for dense wavelength division multiplexing (DWDM) and very high density wavelength division multiplexing (VHDWDM) applications.

AWG Structure & Working Principle
Based on the substrate, an AWG consist of an array of waveguides (also called phased array) and two couplers (also called the free propagation region – FPR). One of the input waveguides carries an optical signal consisting of multiple wavelengths λ1 – λn into the first (input) coupler, which then distributes the light amongst an array of waveguides, as the following picture.

The light subsequently propagates through the waveguides to the second (output) coupler. The length of these waveguides is chosen so that the optical path length difference between adjacent waveguides, dL equals an integer multiple of the central wavelength λc of the demultiplexer. For this wavelength the fields in the individual arrayed waveguides will arrive at the input of the output coupler with equal phase, and the field distribution at the output of the input coupler will be reproduced at the input of the output coupler. Linearly increasing length of the array waveguides will cause interference and diffraction when light mixes in the output coupler. As a result, each wavelength is focused into only one of the N output waveguides (also called output channels).

Or we can simply understand it by take the following picture for example: The incoming light (1) traverses a free space (2) and enters a bundle of optical fibers or channel waveguides (3). The fibers have different length and thus apply a different phase shift at the exit of the fibers. The light then traverses another free space (4) and interferes at the entries of the output waveguides (5) in such a way that each output channel receives only light of a certain wavelength. The orange lines only illustrate the light path. The light path from (1) to (5) is a demultiplexer, from (5) to (1) a multiplexer.

Types of AWG
Various AWGs are available on the market. In general, they can be divided into two main groups according to the material used: low-index and high index AWGs. Low-index AWGs with a typical refractive index contrast of 0.75% have the advantage of their compatibility with optical fibers, and hence very low coupling losses between output waveguides and optical fibers. The disadvantage of such AWGs is their size, which corresponds with the waveguide curvature that may not lie below a critical value. As a result, increasing the channel counts and narrowing the channel spacing leads to a rapid increase in the AWG size and this, in turn; causes the deterioration in optical performance like higher insertion loss and, in particular, higher channel crosstalk. In contrast to this, high-index AWGs feature a much smaller size but also much higher coupling losses.

As the number of waveguides used to carry the information in DWDM systems is generally a power of 2, the AWGs are designed to separate two different wavelengths, or 4, 16, 32, 64 etc. In addition to this, 40- and 80-channel AWGs are also available. Systems being deployed at present usually have no more that 40 wavelengths, but technological advancements will continue to make higher numbers of wavelengths possible.

The wavelengths being used to transmit the information are usually around the 1550 nm region, the wavelength region in which optical fiber performs best (it has very low loss and low attenuation). Each wavelength is separated from the previous one by a multiple of 0.8 nm (also referred to as 100 GHz spacing, which is the frequency separation). However, they can be also separated by 1.6 nm (i.e. 200 GHz) or another spacing as long as it is a multiple of 0.8 nm. These channel spacings refer to WDM systems. On the other hand, increasing capacity demands mean the present aim is to squeeze even more wavelengths into an even tighter space, which may result in as little as half the regular spacing, i.e. 0.4 nm (50 GHz) or even a quarter, 0.2 nm (25 GHz). Such narrow channel spacings are being used in DWDM systems. However, the recent rapid growth in network capacity has meant that even higher capacity transmission is required in DWDM systems. To meet the growing capacity demands, it is necessary to continue increasing the channel counts of these AWGs as far as possible, i.e. decreasing their channel spacing going down to 10 GHz or less. Such AWGs play a key role in the very high density WDM applications.

The optical signals transmitted can have different shapes. The most common is the Gaussian passband (or Gaussian shape) which features very low insertion loss. In contrast to this, the flat-top passband suffers far higher insertion losses but features much better detection conditions. Somewhere between these two shapes lies so-called semi-flat passband, this is also often used in DWDM systems.

A special part of the AWG family creates so-called “cyclic” or “colorless” AWG with an usual 100 GHz or 50 GHz channel spacing and 8 (or 16) output channels. Here applying a special design such AWG will repeat its orders and can work in any predefined channel band. In other words, the same colorless AWG can work on channels 1 to 8 or 9 to 16 or 17 to 24, and so on.

Temperature-Insensitive (Athermal) AWG vs Thermal AWG
In order to use AWG devices in practical optical communications’ applications, precise wavelength control and long-term wavelength stability are needed. However, if the temperature of an AWG fluctuates, the channel wavelength will change according to the thermal coefficient of the material used. Using the thermo-optic effect, a temperature controller can be build into the AWG to control and tune the device to the ITU grid or any other desired wavelength. With the technology developed, there is launching a new type of AWG, which called Atermal AWG. This kind of AWG is based on the silica on silicon technology and no electrical power is required. Here is a comparison between Athermal AWG and Thermal AWG.

AWG Advantages & Applications
Advantage
The key advantage of the AWG is that its cost is notdependent on wavelength count as in the dielectric filter solution. Therefore it suits metropolitan applications thatrequire the cost-effective of large wavelength counts. Other advantage of the AWG is the flexibility of selectingits channel number and channel spacing and, as a result,various kinds of AWG’s can be fabricated in a similar manner.

Applications
You may have a question that where can I use AWGs in my optical network. Generally AWG devices serve as multiplexers, demultiplexers, filters, and add-drop devices in optical WDM and DWDM applications: