How Fiber Optic Cable is Used

Uses for Digital Fiber-Optic Cable

This article will not only focus on the fiber-optic cable used for communications, but also on diameters and the wavelengths of light "pumped" down them.

If you've read my first article in this series you know that fiber-optic communications cable is made in a variety of sizes. This is not by accident.

Core Diameter
The core, the part of the cable that actually carries the light, can be from eight to nine (8~9µm) micrometers in diameter to sixty-two point five (62.5µm) micrometers in diameter. A micrometer is one millionth of a meter or sometimes called a micron¹.

It is no accident that there are three primary core diameters for fiber-optic cable. Over time physicists and materials scientists have determined that each type of data transmission via light has an ideal core diameter for that transmission type.

Single-Mode for Distance
The core diameter for single-mode fiber-optic cable is 8~10µm¹. The ideal size is 8.3µm. This cable is also designated OM1².

For example the best carrier for voice communications is single-mode (SM) fiber though CATV (Cable TV) also uses this fiber type for long distance runs. Single-mode fiber works best with near-infrared³ wavelengths of light in the 1,300 to 1,550 nm (nanometer) ranges. In other words (see image right) the distance of the light wave from peak-to-peak is from 1,300 to 1,550 nanometers.

Longer wavelengths are possible with single-mode fiber up to and including 4,500nm. Roughly speaking, the higher the wavelength number (in nanometers) the greater the distance. Because these light cables have a narrow core and are typically used to transmit one wavelength of light the direction of transmission is one-way. For that reason single-mode fiber is generally installed with two fibers. One for each direction of transmission.

These longer wavelengths are what enable this fiber type to be used for such long distances. In fact, single-mode fiber can accurately transmit data up to fifty (50) times farther than multimode cable.

Because the near-infrared light generated for these cables is in the higher nanometer ranges and the core is so narrow the light-driver is typically a laser not an LED. (see last two images at right) A laser can produce a coherent beam or one that fits precisely into the narrow core without any of that light spilling over to the cladding.

Multimode for Multiplexing
The ideal diameter for multimode cable is 50µm; this is a drop from 62.5µm. There is still quite a bit of the larger core multimode cable out there, but the ideal size is now 50µm; this is for OM3 or OM4² cable. Multimode cable includes OM2 (62.5µm), OM3 (50µm) and OM4 (50µm laser optimized).

Multiplexing means that more than one signal is transmitted at the same time.

Multi-mode (MM), with it's larger core, allows for multiple wavelengths of light to be transmitted at the same time down the same cable. The light is "multiplexed" or joined together at the sending end and "demultiplexed" at the receiving end. MM cable typically carries 850, 1,310, and 1,550nm wavelength light.

These wavelengths are typically used for specific types of data transmission where 850nm is used for voice telephone, 1,300 ~ 1,310nm for video broadcast signals, and 1,500 ~ 1,510nm for Internet data transmission. The 1,550nm range is used to return transmission from the home/office to the central office.

In the consumer data transmission field using all three nanometer ranges for data transmission is called "triple-play."

To carry multiple signals the light pumped into this cable type is converted from the three wavelengths to a combined light signal via Wave Division Multiplexing (WDM).

Once the signal gets to the other end of the cable a de-multiplexer is employed to separate out the three original signals or wavelengths of light.

This is typically nothing more than a prism. The prism de-multiplexes the light at the receiving end of the cable and each beam of de-multiplexed light is directed to it's own media converter⁴, changing the light signal into three different wavelengths and then into three distinct electrical signals.

Mind the Gap
Notice in the figures above there are very definite gaps in the bandwidths. For example you do not see 1,100nm or 1,400 nm wavelengths. This is because the optical-fiber will absorb these two wavelengths and not transmit them.

Light Danger
Typically the light pumped into a fiber is beyond the range of human vision, just past the red end of the visible spectrum. (see second image right)

This does not make it less dangerous than visible light. Staring into the end of an active fiber optic cable cold cause retinal damage, so it is advisable to leave these cables well alone unless you have some assurance that the fiber you are examining is not active.

Because single-mode is so good at covering long distances it is also quite good at huge bandwidths. In other words, single-mode is good not only for transmitting long distances, it is also ideal for transmitting large amounts of data in short periods of time.

Currently single-mode is capable of transmitting data in the ten gigabit per second (10Gbp/s) range with work steadily advancing toward the tera (million) bit ranges. In fact many installations of single-mode fiber are referred to as 10 Gigabit Ethernet. Though the IEEE (Institute of Electrical and Electronic Engineers) standard does not spell out fiber-optics in the 802.3 specification, communications industry companies quickly adopted fiber-optic due to its ideal properties in high speed data transmission.

Single-Mode for Distance
When cable runs are over three hundred (300m) meters, single-mode fiber is preferred. The data transmitted in single-mode is encoded with addressing and data type as part of the data stream. When the reaches the central office the data is converted to multi-mode fiber.

Multi-Mode for Multiple Channels
Because multi-mode can handle multiple wavelengths of light simultaneously it is better suited for the "triple-play" service provided to customers. With cheap wave division multiplexers and de-multiplexers it is possible to send all three data types (voice, television, and Internet data) to the home. Typically this fiber run is FTTC⁵ (Fiber to the Curb) with the remaining distance covered by copper wire (twisted pair or coaxial cable).

FTTH (Fiber to the Home) is quite possibly the next step in this progression especially in light of the Federal Broadband Initiative. In fact, it is hard to imagine "wired" technology being able to meet the Megabit per second (100Mbp/s) provision of this national plan.

With the "dead-zones" well defined (see above) physicists and materials scientists are already experimenting with light transmission at even longer wavelengths. This means that data rates could conceivably reach the gigabit (billion bit) and terabit (trillion bit) ranges.

The drawback to this area of research is that new infrastructure would have to be implemented to reach these speeds. This could involve new fiber plants⁶, new multiplexers, new media converters, and new Ethernet hardware at both the sender and receiver ends.

Of course, all this new equipment is going to cost more money; at both ends of the fiber. Still, a great deal of existing fiber installations are already capable of ten gigabit speeds. The equipment at either end of the fiber needs to be updated however.

¹ Micron is an archaic term for micrometer. A micron is one millionth of a meter.

² OM3 and OM4 are designations for mutlimode cable. OM3 and OM4 are both 50µm cable but the newer designation (OM4) is laser optimized.

³ Near-infrared is just outside the far end of the visible spectrum of light. It rests comfortably between visible light and infrared light.

⁴ A media converter translates light signals into electrical signals or electrical signals into light signals.

⁵ Fiber to the Curb (FTTC) means that the fiber signal is sent to a box or under-ground vault where the light signal is converted to an electrical signal. From there the electrical signal makes the final few hundred meters to the home or business over wire. Fiber to the Home (FTTH), where the light signal is converted to electrical in the home, may well be the next step in fiber-optic data transmission.

⁶ A plant is where the cable is laid. Think of planting a flower or tree, but in this case it's cable. If the cable is installed indoors it's an "inside plant." If the cable is run out of doors the installation is called "outside plant."