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W O R K S H O P

Shining Light on Optical Networking

August 19, 2002
By Darrin Woods

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Blinded by the Light

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	In this article
	Introduction
	Blinded by the Light

While the problem with optical transmissions is usually generating enough strength to get to the destination, sometimes the opposite is true. When too many photons get to the receiver, it is blinded--as though it's looking directly into the sun. Adding more photons is not a good thing for receivers. Multimode receivers are immune, for the most part, to being blinded, but single-mode receivers are not. This effect typically happens when the transmitter is mismatched to the distance that needs to be covered.

Single-mode transmitters come in a variety of strengths to transmit over different distances. The maximum distance varies by bandwidth and manufacturer. Although a transmitter and receiver might be designed for the long haul, it would be disastrous to assume that they could operate at shorter distances without the receiver burning out. The receiver design assumes a certain amount of photonic loss over the distance for which the transmitter is designed. When distance or power is mismatched you need a pair of fiber-optic sunglasses, or optical attenuators. A small sliver of shaded glass is placed in the path between two sections of fiber or at the receiver. The attenuator blocks a certain number of photons from going through. Attenuators are rated in decibels according to the amount of photonic loss they elicit.

Web Links

• "Fiber Optic Links Get New Lease On Life" (InformationWeek, May 13, 2002)

• LED Could Lead To Ultrasecure Communications" (InformationWeek, Dec. 14, 2001)

• "Grow Your Own Optical Components" (InformationWeek, Sept. 10, 2001)

Once the wavelengths get to the termination device they need to be demultiplexed and sent to the appropriate receiver. The easiest way to do this is by splitting the fiber and shunting the same signals to all the receivers. Then each receiver would look only at photons of a particular wavelength and ignore all the others. This form of narrowband receiver is very expensive and inefficient to manufacture. Instead, most receivers are wideband, allowing wavelengths across most of the laser spectrum to be seen. However, only photons from a given wavelength should be allowed to get to the receiver. One method of stripping the desired wavelengths from the unwanted ones involves Bragg grating (see diagram). Bragg grating can be thought of as small lines etched across the core of the fiber. With the lines etched at specific distances from one another, wavelengths of light can be reflected back toward the input. A blazed grating is also etched into the core at an angle that reflects the light coming back on the grating out of the core and onto a receiver. By placing several combinations of Bragg and blazed gratings in a fiber, you can separate all the wavelengths out to waiting receivers.

Care and Feeding

Data would never get from Point A to Point B without the fiber itself. Fiber-optic cable comes in a variety of lengths with different connector types. Connectors are probably the most difficult part of the fiber-optic setup. That's because the names of the various connectors--FC, SC, LC, ST and MT--can be confusing, and remembering which one is which can be difficult. Most people remember connector types by giving them names that reflect how they work: FC (finger cramp, because fingers are easily cramped while trying to screw the connector tight); SC (stab click--the connector is stabbed into the socket until it clicks); LC (little click--a smaller version of SC); ST (stab twist--the connector is stabbed into the socket and then turned about a quarter turn to lock); and MT (mighty tiny--the smallest of the connectors). For more on connector types, see workshop, "10 GIG Can't Wait to Interoperate").

While today's fiber is more resilient than its predecessors, care should still be taken when handling it. Fiber should never be tightly bent or curved, and though you're used to walking on the copper pairs strewn around your computer room, fiber-optic cable should be treated like glass--don't step on it. Any change in the diameter of the core can cause great changes in dispersion, resulting in the fiber losing its transmission qualities.

The fiber-optic speeds are incredible. With today's fiber systems, you can transmit the entire contents of a CD-ROM in about one half of a second. And the technology is not standing still. The Optical Internetworking Forum is working to complete the interface standards for OC-768, which will increase bandwidth to 40 Gbps. That's the entire content of eight CD-ROMs, or 5 GB of data, transmitted every second.

But even 40 Gbps is the tip of the iceberg. Alcatel continues to push the bandwidth on single fiber transmission by cramming 365 10-Gbps wavelengths over a single 6,800-km fiber. Designed for marine installations, this technology would transfer more than 3.5 Tbps, or enough bandwidth to carry 47 million simultaneous phone calls.

True optical switching is the next frontier, and companies like Opthos are already blazing a trail. Most switches today rely on OEO technology, with switching actually occurring within the electrical components. By switching at the optical layer, it occurs not only faster, but cooler and quieter as well.

Darrin Woods is a Network Computing contributing editor. He has worked as a WAN engineer for a telecom carrier. Send your comments on this article to him at dwoods@nwc.com.