Since most of the information in our explanation of fiber optical communication is fairly old (mid-1980s), here's a few of the more recent advances in fiber optic technology. All of the changes have been in how to more efficiently send light waves, what manufacturing processes to use, or what material works best so the basic concepts in the explanation will still hold.
One of the best methods for sending data through a fiber optic cable is with the Synchronous Optical Network (SONET) protocol. The primary goal of SONET is survivability - the network should be able to recognize a fiber cut and reroute traffic before service is interrupted. SONET is at the lowest level of the OSI protocol stack (see figure 1). Each frame is a two-dimensional array consisting of nine rows by 90 columns of bytes (meaning 6480 bites per frame). Groups of transport frames may also be byte interleaved (see figure 2).
Before the signal reaches the fiber, there is a translation to light-based signals that needs to be done. Traditionally, this has been done by Time Division Multiplexing (TDM) equipment that basically translates a signal of ones and zeros into lights and darks on a given light wavelength. A recent advancement, Dense Wave Division Multiplexing (DWDM), uses a composite optical signal carrying multiple information streams, each transmitted at a distinct optical wavelength. Parallel wavelengths are densely packed and integrated into the transmission system.
The primary methods of actually sending the signal through the fiber are light-emitting diodes (LEDs) and semiconductor lasers. The latter are newer (only made to work at room temperature as of 1962) and much more expensive but also much more precise in the signal they send. They are used more in single-mode fiber in order to get that single wavelength going. LEDs primary advantage is that it's cheap but, as superconductor lasers' price drops, it is becoming less used.
Most of the reception done on fiber optic networks has been done by photodiodes. A recent advance in making photodiodes more effective is Bragg grating. It consists of a length of fiber prior to the reception site who's refractive index has been modified in a periodic fashion. This results in that length of fiber behaving as a wavelength-dependent reflector. This is useful to the photodiode for precise wavelength separation prior to processing into electrical signals.
One tactic for speeding up transmission along any wavelength is what is called "doping" the fiber. It basically is just injecting particular ions into the core of the fiber to amplify the signal. Early experimentation was done with all sorts of metallic ions but titanium doping was found to be the best until the early 1990s when erbium and germanium doping became commercially feasible.
Another way to speed up transmission is to optimize the cladding for low index of refraction to reduce attenuation. Recent work at Lucent Technologies (of Bell Labs) has resulted in Depressed Cladding Single Mode Fiber. The idea is to place a germanium-doped core within two concentric silica cladding layers (rather than just one). The inner layer is doped with fluorine to reduce its index of refraction to below the outer layer. This is a big step up from simple glass cladding surrounded by an oatmeal carton that Curtiss used in 1956 (see figure 3).
One of the bottlenecks in fiber-optic communication is routing between different networks or to a host computer. In the former category at least, the industry has been advancing towards smaller connectors (Comparing the new-generation fiber-optic connectors, Richard Akins).
One specific recent advance is the Optical Add/Drop Multiplexer (OADM). It allows up to four channels of data (consisting of multiple data lines each) can be added or dropped at each multiplexer. Additionally, this allowing remultiplexing en route if the signal breaks down and is resent without multiplexing.
An additional new type of connector without multiplexing is polymer-ferrule connectors. While most of the industry still uses ceramic ferrules (called ST-type and duplex SC), as the fiber optic cabling industry becomes aware of polymer-ferrule, it will be more widely used as it is lighter and there is no mess in applying epoxy, the substance used to hook up the connector (Next-generation fiber optics, Tony Beam).
A final note in network management concerns increased portability and network flexibility: the Intermountain Network and Scientific Computation Center (INSEC) has been using compressed air or nitrogen to "blow" lightweight optical-fiber bundles through predefined routes in their buildings. The infrastructure comprises tube cables containing up to 19 coded tube cells. The cells are joined in tube distribution units by using push-fit connectors to achieve a route between the network hub and the application (Fiber delivers bandwidth and flexibility, Terri Dixon).
Last Modified: 3/15/00
By Brian Patterson and
Erin Quealy