The Optical Network: A Simplified View
Optical networks use high-speed pulses of light to transmit information over glass fibers. This technology enables optical networks to transmit more information over greater distances on a more cost-effective basis than electrical signals over copper wires. A typical optical network consists of six principal systems (see Figure I):
Sound, video and data signals are electronically digitized into a stream of on/off pulses, and converted into an equivalent electric signal. An optical transmitter takes the incoming electrical signal and converts it into an equivalent light signal. This process is accomplished by means of Light Amplification by Stimulated Emission of Radiation (or laser). Lasers are the devices that generate the modulated light signals and transmit them through the optical system by turning each laser device ON to represent a digital 1 and turning it OFF to represent a digital 0. Each optical signal has its own wavelength or frequency.
- Optical amplifier
A Typical Optical Network
Multiplexing refers to the aggregating of multiple wavelengths onto a single strand of fiber. This process, accomplished through the use of optical devices called Wavelength Division Multiplexers (or WDMs), is designed to increase the capacity of the optical network. Dense Wave Division Multiplexing (or DWDM) uses wavelengths that are spaced closely to each other, thereby allowing more wavelengths to be transmitted simultaneously on the same fiber. Coarse Wavelength Division Multiplexing (CWDM) uses fewer, more widely separated wavelengths. Along the way, the signal is monitored and protected by a range of optical monitoring, measurement, and testing equipment.
As light travels through fiber over a long distance, it becomes weaker. It must be amplified to compensate for loss of signal strength, or excessive attenuation. The Erbium Doped Fiber Amplifier (or EDFA) is the most widely deployed device used in the amplification of optical signals. The EDFA consists of silica fiber doped with ionized atoms of erbium. A pump laser supplies energy into the doped fiber and this energy is transferred to the signal, thereby amplifying it.
A communication network must incorporate multiple points of connectivity. Wavelengths must be added, dropped or redirected. The node acts as a junction between several segments of the network. In order to provide this connectivity, a network node contains switching elements and add/drop devices. Opto-mechanical (or Prism) switches and Micro Electro-Mechanical Systems (or MEMS) switches are two examples of devices that are commonly used to redirect optical signals. The Optical Add-Drop Multiplexer (or OADM) is a commonly used device to provide the entrance and exit ramps for the individual wavelengths in a multi-wavelength light signal.
De-multiplexers dis-aggregate the multi-wavelength optical signal by splitting it back into a series of the original wavelengths. After leaving de-multiplexers, these single-wavelength optical signals are sent to optical receiver units.
A receiver, which typically includes a photodetector and electronic circuitry, converts an incoming optical signal into an equivalent electrical signal. This signal is subsequently converted back into its original sound, video or data form.