There are two possible light sources for optical transmission: LED or laser. LEDs are used only in multimode systems, while lasers are the source of choice for single-mode fiber. LEDs cannot operate over single-mode fiber as the light is not coherent enough to travel down such a small passageway--imagine trying to fill an eye-dropper-size opening with water being poured from a bucket.

Data travels down copper lines as a series of on and off pulses to specify whether a bit is a 1 or a 0. Fiber works in the same way, but by pulsing the light source on and off. By pulsing the light faster, more data can be sent down the fiber. There is a limit to how quickly a laser can be turned on and off and still generate enough photons of a required strength to reach the other end and be interpreted. For this reason, high-speed fiber connections rely on multiple wavelengths to transmit data, combining the best of two worlds.

Glossary Dense Wavelength Division Multiplexing (DWDM): A technology wherein multiple wavelengths transmit data over a single fiber-optic cable. Coarse Wavelength Division Multiplexing (CWDM): In an effort to lower prices on short-hail optical transmissions, the ITU in June announced this global standard, which has wider channel spacing than DWDM.

Multiple data transmissions can travel over copper by chopping the available bandwidth into channels. Each channel is then allowed to send data for a certain amount of time, transmitting in a round-robin fashion and giving everyone an equal chance. This method is called TDM (Time Division Multiplexing). Another transmission method is FDM (Frequency Division Multiplexing), which allows multiple transmissions by requiring each one to operate within a given frequency range. TV and radio signals are examples of FDM.

Most fiber systems combine these two methods, letting multiple frequencies carry multiple channels of data. Tunable laser diodes are used to create this WDM (Wavelength Division Multiplexing) combination. WDM is just one of three classifications for combining multiple wavelengths, or lambdas, onto a single fiber. WDM actually sits in the middle and defines the means of combining frequencies that are 10 nm apart from one another. DWDM (Dense Wavelength Division Multiplexing) is the far end of the spectrum, defining wave separation that is no more than 1 nm, with some systems running as close as 0.1 nm--a hundredfold increase over WDM. Because of this tightness in spectrum, systems using DWDM tend to be very expensive.

At the opposite end of the cost spectrum are new systems that are being designed around CWDM (Coarse Wavelength Division Multiplexing). The individual light frequencies are at least 20 nm apart, with some spaced as far as 35 nm apart. CWDM systems are being used in the metro area and even on LANs, where cost is a factor. CWDM is also not constrained to any one portion of the spectrum and operates between 1300 nm and 1600 nm, while DWDM systems usually operate above 1500 nm.

Going the Distance

Getting light pulses from Point A to Point B is the purpose of the exercise, but doing so involves some creativity. While light in fiber travels at about 200,000 kilometers per second, no light source can actually travel that far and still be interpreted as individual 1s and 0s. This is not just because of dispersion; photons also can be absorbed by the cladding that is used to surround the fiber core, decreasing the number that reach the receiver. Because the receiver counts the photons that reach it in a given period, with anything above a certain number considered a 1 and anything below considered a 0, fewer photons could result in false 0s.

To get all the photons to the receiving end, you might think you'd raise the input power of the laser, thereby increasing the number of photons that reach the other end. This would be true if fiber optics worked in a linear fashion, but they are nonlinear. To keep input power and output power as close to linear as possible, most single-mode lasers are kept at about 0.5 watts. Increasing the power can decrease the output power because of electromagnetic forces in the core.

Integrated Bragg Grating Click here to enlarge

Regeneration is therefore the common method of extending the reach of the photons in fiber. Depending on the wavelengths used, regenerating an optical signal can take two forms: OEO (optical/electrical/optical) or FA (fiber amplifiers). OEO systems, also called optical repeaters, take the optical signal, demultiplex it and convert it to electrical pulses. The electrical signal is amplified, groomed to remove noise and converted back to optical. It is then multiplexed back on the line and sent on its way. OEO can be used across the spectrum and is commonly used for CWDM and some WDM transmissions.

Fiber amplifiers offer a more elegant solution without converting the photons to an electrical signal. The most common amplifier is the EDFA (erbium-doped fiber amplifier). Although fiber amplifiers commonly use erbium, other elements, such as praseodymin and ytterbium, can provide the same results. A better term would be REDFA (rare-earth-doped fiber amplifier), which operates by doping a section of fiber with a rare earth element, such as erbium. Doping is the process of adding impurities during manufacture; fiber-optic cable already has almost 10 percent germanium oxide as a dopant to increase the reflective index of the silica glass.

The doped section of fiber combines the signal from the transmitter with that coming from a pump laser. For erbium, the pump laser operates at 980 nm. The pump laser excites the erbium atoms, and when they are struck by photons from the original signal, some energy is transferred to the transmitted signal, amplifying it. A typical REDFA has a coil of 10 meters of doped fiber for amplification. The pump laser can be located locally to the doped fiber or remotely, as long as the strength of the light has not degraded too badly.

REDFAs are typically used for submarine cables because they require much less additional equipment. REDFAs also do not add any appreciable delay to the signal, as do OEO repeaters. REDFAs, however, amplify noise in the photonic signal, and over a long distance this noise can hurt the signal.