New Variable Optical Attenuator Offers Inexpensive, Reliable Solution

Breck Hitz

Variable optical attenuators are important components in wavelength division multiplexed telecom systems, in which they balance power levels among channels for reliable transmission. Dozens of designs have been produced and implemented in commercial systems, but many of them are not well-suited for large-scale optical integration. Besides good performance and reliability, low power consumption and high fabrication yields are of paramount importance in large-scale integration. Recently, researchers at the National Research Council's Photonics Systems Group in Ottawa proposed, analyzed and built a variable optical attenuator that they believe offers all the parameters required for large-scale integration applications.

The operating principle of the device is straightforward. It's a simple planar waveguide, with a silica core surrounded on two sides by a polymer material. At room temperature, the refractive index of the polymer is the same as or greater than that of the silica core, so waveguiding in the core does not take place. But because the temperature dependence of the polymer's index (dn/dT) is an order of magnitude greater than that of the silica, it decreases more rapidly as the device is heated, and the core starts to become a waveguide. In other words, as the device is heated, it changes from a state of high attenuation to one of low attenuation.

Figure 1. The fabrication of the variable optical attenuator is straightforward, resulting in high yields.

The simple technique illustrated in Figure 1 assures a high fabrication yield. First, a waveguide is fabricated in pure silica with conventional silica-on-silicon technology (Figure 1a). A thin-film heater is added to the top of the structure (Figure 1b), and deep grooves are etched on either side of the core (Figure 1c). Finally, the grooves are filled with the polymer material (Figure 1d).

Using commercial optical-simulation software, the researchers analyzed the performance of their proposed variable optical attenuator. They found that the attenuation could be tuned from 0 to 20 dB by changing the temperature by as little as 10 °C, if the temperature dependence of the polymer's index were great enough (Figure 2).

Figure 2. A 400-µm-long variable optical attenuator could have a range of greater than 20 dB with a temperature change of less than 10 °C, according to the researchers' simulation.

To demonstrate the feasibility of the device, they built several versions, using a different type of polymer in each. They saw the best results with a special vinyl dimethyldiphenylsiloxane copolymer they developed for this application. Using the material, they observed an attenuation range of greater than 20 dB at 1550 nm as they tuned the temperature from 73 to 80 °C, a result consistent with their analysis.

A drawback to their approach, they note, is that the variable attenuator must be kept in a constant-temperature environment. However, this probably would not be a disadvantage in a large-scale integration in which the device would be integrated with an arrayed waveguide grating, which also requires a constant-temperature environment.

In fact, the attenuator is perfectly suited for integration as the input waveguide to or the output waveguide from an arrayed waveguide grating.