Bubbles Give Rise to Optical Advance

FRAMINGHAM (03/30/2000) - A new tool for building all-optical networks is as simple as blowing bubbles.

Hewlett-Packard Co. spinoff Agilent Technologies has developed an optical switch fabric that relies on bubble-jet printer technology and a vapor bubble to switch light signals from one optical fiber to another without first converting the signals to electrical impulses.

The switch fabric can handle 32 input and 32 output fibers on a chip the size of a dime. These chips are already being designed into devices such as optical add-drop multiplexers and cross connects, according to Agilent. The company says it will put its switch fabric in the hands of vendors next year so they can build products around it.

The ability to switch light directly makes it possible for service pro-viders to sell entire high-bandwidth wavelengths to customers. Because the entire connection between points in the network will be optical, customers can put any traffic of any protocol at any bit rate onto a wavelength, says Christoper Nicoll, an analyst with Current Analysis in Sterling, Virginia.

With conventional services, such as frame relay and ATM, customers must package data in the proper protocol and send it at the rate of the service they have bought.

The tiny bubbles in Agilent's technology are an alternative to microscopic mirrors that Lucent uses in its all-optical switches.

Agilent's technology receives light from optical fibers and diverts it directly to any other fibers attached to the same switch fabric. Light from an incoming fiber is guided through the switch fabric along a waveguide, which is a glass channel that confines the light and directs it down a defined path (see graphic).

It is possible for the light to pass straight through the switch without being diverted. To do so, it traverses 32 fluid-filled trenches that cross the path of the waveguide. The light crosses the trenches unimpeded because the fluid has the same optical properties as the glass in the waveguide.

To switch the direction of the light signal, a bubble-jet heater wafer that makes up an adjacent layer of the switch fabric heats the fluid in the appropriate trench. The warmth vaporizes the fluid, creating a bubble in the trench that gives the trench a different refractive index than the glass.

As a result, the light bounces off the far side of the trench where the glass waveguide resumes, reflecting down a different waveguide. The second waveguide leads to the exit fiber. To work properly the switch temperature must be 65 degrees Celsius.

The switch has no moving parts, making it theoretically less susceptible to failure. Micromirror optical switching technology is still young, and it is not known how well it will stand up over the long term.

It is also unclear how well the bubble-jet heaters will stand up when used continuously for months or years, which could be the case in certain switch configurations.

The bubble-jet heaters can set up a switch path in less than 10 milliseconds.

That number is important because it beats the 50-millisecond limit carriers set on acceptable network failures. If the network can be restored within 50 milliseconds, the failure will go undetected by customer applications running across it. This means the switch could be used in a protection-switching scheme in which traffic is routed around a broken fiber along an alternate path.

With the appropriate peripherals built around it, the switch fabric could be the basis for a device that combines light signals from separate fibers onto a single fiber or vice versa.

The photonic switch could also be the basis for optical add-drop multiplexers.

Such a mux at a node of an optical network could pull off any light signal entering the node. At the same time, the switch fabric could be set to let signals destined for other nodes pass through.

The technology lends itself to remote provisioning. So if a new light path needs to be set up, the switch could be reconfigured via a remote management connection. That would dramatically reduce optical provisioning times, which can take days because each device in the network has to be set manually, depending on the equipment in use.

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