Introducing Fiber-Optic Transmission

Fiber-optic technology is different in its operation than standard copper media because the transmissions are "digital" light pulses instead of electrical voltage transitions. Very simply, fiber-optic transmissions encode the ones and zeroes of a digital network transmission by turning on and off the light pulses of a laser light source, of a given wavelength, at very high frequencies. The light source is usually either a laser or some kind of light-emitting diode (LED). The light from the light source is flashed on and off in the pattern of the data being encoded. The light travels inside the fiber until the light signal gets to its intended destination and is read by an optical detector, as shown in Figure 1.

 
Figure 1: Reflection of a light signal within a fiber-optic cable

Fiber-optic cables are optimized for one or more wavelengths of light. The wavelength of a particular light source is the length, measured in nanometers (billionths of a meter, abbreviated nm), between wave peaks in a typical light wave from that light source (as shown in Figure 2). You can think of a wavelength as the color of the light, and it is equal to the speed of light divided by the frequency. In the case of single-mode fiber, many different wavelengths of light can be transmitted over the same optical fiber at any one time. This is useful for increasing the transmission capacity of the fiber-optic cable since each wavelength of light is a distinct signal. Therefore, many signals can be carried over the same strand of optical fiber. This requires multiple lasers and detectors and is referred to as wavelength-division multiplexing (WDM).

 
Figure 2: A typical light wave

Typically, optical fibers use wavelengths between 850 and 1550nm, depending on the light source. Specifically, multimode fiber is used at 850 or 1300nm and single-mode fiber is typically used at 1310, 1490, and 1550nm (and, in WDM systems, in wavelengths around these primary wavelengths). The latest technology is extending this to 1625nm for single-mode fiber that is being used for next-generation passive optical networks (PON) for FTTH (fiber-to-the-home) applications. Silica-based glass is most transparent at these wavelengths, and therefore the transmission is more efficient (there is less attenuation of the signal) in this range. For a reference, visible light (the light that you can see) has wavelengths in the range between 400 and 700nm. Most fiber-optic light sources operate within the near infrared range (between 750 and 2500nm). You can't see infrared light, but it is a very effective fiber-optic light source.

Note

Most traditional light sources can only operate within the visible wavelength spectrum and over a range of wavelengths, not at one specific wavelength. Lasers (light amplification by stimulated emission of radiation) and LEDs produce light in a more limited, even single-wavelength, spectrum.

Figure 3 shows the typical attenuation of single-mode and multimode fibers as a function of wavelength in this range. As you can see, the attenuation of these fibers is lower at longer wavelengths. As a result, longer distance communications tends to occur at 1310 and 1550nm wavelengths over single-mode fibers.

 
Figure 3: Attenuation of single-mode and multimode fibers

Notice that typical fibers have a larger attenuation at 1385nm. This water peak is a result of very small amounts (in the part-per-million range) of water incorporated during the manufacturing process. Specifically it is a terminal –OH (hydroxyl) molecule that happens to have its characteristic vibration at the 1385nm wavelength; thereby contributing to a high attenuation at this wavelength. Historically, communications systems operated on either side of this peak. However, in 1999 Lucent Technologies's optical fiber division (now OFS) created a zero water peak (ZWP) process whereby this water peak was eliminated by significantly reducing and then modifying the −OH molecule.

To help you understand, let's use a very simple spring and weight analogy: If you replace the hydrogen with deuterium (an isotope of hydrogen that weighs twice as much) the molecule would now have a characteristic vibration that is not at a frequency of 1385nm and therefore does not cause high attenuation—still there, but out of the way. This invention opened up this wavelength range to transmission systems and allowed the International Telecommunication Union (ITU) to create a new operating band referred to as the E-band. This type of fiber is commonly referred to as low water peak (LWP) and has been standardized in the industry as ITU-T G.652D fiber. Earlier fibers had much larger attenuations at 1385nm and are referred to as ITU-T G.652B fiber.

Warning

Laser light sources used with fiber-optic cables are extremely hazardous to your vision. Looking directly at the end of a live optical fiber can cause severe damage to your retinas. You could be made permanently blind. Never look at the end of a fiber-optic cable without first knowing that no light source is active.

When the light pulses reach the destination, a sensor picks up the presence or absence of the light signal and transforms the pulses of light back into electrical signals.

The more the light signal scatters or confronts boundaries, the greater the likelihood of signal loss (attenuation). Additionally, every fiber-optic connector between signal source and destination presents the possibility for signal loss. Thus, the connectors must be installed correctly at each connection.

Most LAN/WAN fiber transmission systems use one fiber for transmitting and one for reception. However, the latest technology allows a fiber-optic transmitter to transmit in two directions over the same fiber strand. The different wavelengths of light do not interfere with each other since the detectors are tuned to only read specific wavelengths. Therefore, the more wavelengths you send over a single strand of optical fiber, the more detectors you need.