Overview of Optical Networking
Getting inside the very fibers that shuttle data at light speed.
An optical network is a communication system that uses light signals, instead of electronic ones, to send information between two or more points. The points could be computers in an office, large urban centers or even nations in the global telecommunications system. Optical networks comprise optical transmitters and receivers, fiber optic cables, optical switches and other optical components. Optical and electronic networks can take several different forms. Point-to-point networks make permanent connections among two or more points so any pair of nodes can communicate with each other; point to multipoint networks broadcast the same signals simultaneously to many different nodes; switched networks like the telephone system include switches that make temporary connections among pairs of nodes. The basic building blocks of these networks are fiber-optic cables-the so-called “pipes”-which carry signals from node to node, with switches directing them to their destination.
An optical signal consists of a series of pulses produced by switching a laser beam off and on. Its speed depends on how fast the beam can be switched on and off, and how much the pulses spread in length during transmission, an effect called dispersion. The amount of dispersion depends on the type of fiber, the fiber length and the nature of the optical signal. The more dispersion, the more difficult it is to distinguish between adjacent pulses. With current technology, different types of fiber can be combined to reduce dispersion effects, allowing transmission at 10 gigabits per second for a few thousand kilometers. To achieve faster transmission speeds, researchers are exploring ways to actively compensate for dispersion.
A single fiber can transmit many separate signals simultaneously at different wavelengths of light, a technique called wavelength-division multiplexing. This is analogous to broadcasting many radio and television signals through the air at different frequencies.
Like the number of radio stations, the maximum number of optical channels is limited by the slice of spectrum used for each channel and the total amount of spectrum available. Devices called “demultiplexers” separate the optical channels and distribute them to separate optical receivers. Demultiplexers slice the spectrum into very narrow chunks, isolating each optical channel from adjacent ones.
Multiplying the number of optical channels by the data rate on each optical channel gives total transmission capacity of a fiber. Laboratory experiments have transmitted more than 10 trillion bits (10 terabits) per second through more than 100 kilometers of fiber. However, commercial transmission rates typically do not exceed a few hundred gigabits per second.
Achieving these high data rates and multiple channels requires sophisticated components. Semiconductor lasers-which generate the light pulses used in almost all fiber optic communications systems-must emit only a very narrow range of wavelengths to limit dispersion. Fibers also are designed to limit dispersion.
The clearest optical fibers can transmit signals more than 100 kilometers without amplification-much farther than copper wires. When the signal must span a longer distance, it is passed through an optical amplifier, which multiplies the strength of the optical signal. The most widely used optical amplifiers are fibers doped with atoms of erbium, a rare-earth element that absorbs light energy from an external pump laser. The erbium atoms then release that energy to amplify weak optical signals across the entire band of wavelengths that the laser transmits. With careful control, a string of dozens of optical fiber amplifiers can transmit signals thousands of kilometers across the ocean.
One challenge to optical networking is how to switch light signals. When a signal arrives at its destination, it must be separated from the rest of the channels. To drop one signal at an intermediate point, an optical filter separates the proper wavelength from the rest. Equipment at that point may also add a new signal to the now unoccupied wavelength.
Optical switches may operate on a single wavelength, or on all the wavelengths transmitted through a fiber. A fixed filter, like the one described above, could be replaced by a switch that selects one of several filters to divert the desired wavelength to the intermediate point. A third kind of switch separates the wavelengths into separate beams, and a moving mirror directs one or more of the wavelengths in a different direction. Other optical switches simultaneously switch all wavelengths passing through a fiber; one example is a mirror at the fiber output that could tilt between two different positions to reroute all optical channels in case of a fiber break.
The preceding examples are called “all-optical” switches because they operate on light signals. A different class of switches convert optical signals into an electronic form which can be switched electronically; the resulting electronic signal then feeds into an optical transmitter to generate a new optical signal. These are called opto-electro-optical switches.
As the technology continues to advance, optical networks will need to convert signals from one wavelength to another. This can be done now with opto-electro-optical wavelength converters that convert the input optical signal into electronic form to drive a transmitter at the second wavelength. All-optical wavelength converters have been demonstrated in the laboratory, but are not yet used in practical systems. Laser sources that can be tuned to many different wavelengths also will be needed; several types have been demonstrated, and some are in commercial production.
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