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  • Stargazing for Better Communications

Photonics Spectra
Sep 2001
Brent D. Johnson, Senior News Editor

For many reasons, it is not a trivial task to wire older buildings with optical fiber. Economic and physical obstacles combine to prevent many businesses from having access to high-speed broadband communications networks.

A popular astronomical telescope is the optical core of a free-space communications system that brings broadband to buildings that optical fiber has not yet reached.

Terabeam Corp. gets around these problems by beaming communications signals from a main communication line through the air -- so-called free-space optical communications -- to a business's local area network hub site 300 m to 2 km away. But as engineers were designing the technology, they had to figure out how to collect the light. The surprising solution turned out to be a popular astronomical telescope from Meade Instruments.

The Terabeam fiberless optic system consists of two parts, one of which is the customer access subcomponent. Within that component is a Meade Instruments model 125 5-in. telescope that collects light on the receiver side of a transceiver and focuses the light onto a detector, producing a data stream.

Larry Stapleton, Terabeam's vice president of product development, said the company chose Meade because its quality optics are reasonably priced and the telescopes can be customized. "We want to collect as many photons as we can," said Stapleton, whose motto is, "Every photon is sacred." This requires high-quality optics that can gather distant light in an urban environment.

The customization was critical for several reasons. "Focusing in the near field was an issue," he said.

Motion control

The problem with an astronomical telescope is that it is designed to resolve a star field, said Robert Davis, Terabeam's vice president of business development. In free-space optical communications, the tracking area is much narrower than astronomical applications demand.

Designers had to make some modifications to the optical system to cover the 50- to 200-m area.

A second problem was one of motion. Stapleton explained that, for a telescope to follow a star field over time, it must move as the Earth rotates, but that movement is always in the same direction. Buildings, however, tend to move back and forth in a sinusoidal path. The designers corrected this problem by altering the telescope gimbal and making some other changes in the telescope to insulate it from vibration

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