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Fiber 101

What is Optical Fiber?
Optical fiber, in its many forms, cable configurations and connectorizations, has become the standard building block for advanced communications networks. Essentially, optical fiber functions as a "light pipe," carrying light generated by lasers and other signal transmission sources to its destination. Glass optical fiber provides high-capacity, economical transmission of voice, data and video signals from short to very long distances. It does this at very high data rates and with very low signal loss.

Optical fibers are hair-thin structures created by forming preforms, which are glass rods drawn into fine threads of glass protected by a plastic coating. Fiber manufacturers use various processes to make the preforms, such as Modified Chemical Vapor Deposition (invented by Bell Laboratories), Vapor-phase Axial Deposition and Outside Vapor Deposition. The fibers drawn from these preforms are then typically packaged into cable configurations, which are then placed into an operating environment for decades of reliable performance.

Basic Fiber Mechanics
Optical fiber uses a construction of concentric layers for optical and mechanical advantages. The "core" is made of silica glass, sometimes treated (or "doped") with another element to change its refractive index (velocity of light down the fiber). This core is completely surrounded by a "cladding," which acts as a guide to the light waves, preventing light from leaking out of the core. Cladding keeps light traveling in the proper direction down the length of the fiber to its destination. Surrounding the cladding is usually another layer, called a "coating," that typically consists of protective polymer layers applied during the fiber drawing process, before the fiber contacts any surface. "Buffers" are further protective layers applied on top of the coating(s).

Figure 1. The basic design of a single-mode optical fiber (a high index core surrounded by a lower index cladding and covered with a protective coating)

Fiber Parameters
A number of key parameters impact how optical fibers perform in transmission systems. The specifications for each parameter will vary by fiber type, depending upon the intended application. Two of the more important fiber parameters are attenuation and dispersion.
Attenuation is the reduction in optical power as it passes from one point to another. In optical fibers, power loss results from absorption and scattering and is generally expressed in decibels (dB) for a given length of fiber, or per unit length (dB/km) at a specific transmission wavelength. High attenuation limits the distance a signal can be sent through a network without adding costly electronics to the system.

Dispersion is inversely related to bandwidth, which is the information-carrying capacity of a fiber, and indicates the fiber's pulse-spreading limitations. Chromatic dispersion causes pulse spreading because of the various colors of light traveling in the fiber at different speeds, causing a transmitted pulse to spread as it travels down the fiber. When pulses spread too far, the signal cannot be properly detected at the receiving end of the network.

Types of Fiber
Fibers come in several different configurations, each ideally suited to a different use or application. Early fiber designs that are still used today include single-mode and multimode fiber. Since Bell Laboratories invented the concept of application-specific fibers in the mid-1990's, fiber designs for specific network applications have been introduced. These new fiber designs - used primarily for the transmission of communication signals - include Non-zero Dispersion Fiber (NZDF), Zero Water Peak Fiber (ZWPF), 10 Gbps laser optimized multimode fiber and fibers designed specifically for transoceanic applications. Specialty fiber designs, such as dispersion compensating fibers and erbium doped fibers, perform functions that complement the transmission fibers.

The differences among the different transmission fiber types result in variations in the range and the number of different wavelengths or channels at which the light is transmitted/received, the distances those signals can travel without being regenerated or amplified, and the speeds at which those signals can travel.

Single-mode fibers (Fig. 2) have a small core size (< 10 Ám) which permits only one mode or ray of light to be transmitted. Single-mode fibers have low attenuation and zero dispersion at 1310 nm. This fiber is a general-purpose fiber for systems of moderate distance, transmission rates and channel count.

Figure 2. Single-mode fiber

Multimode fibers (Fig. 3) have larger cores that guide many modes or rays simultaneously. When one pulse of a signal is generated into a multimode fiber, the multiple modes enter the fiber core from different angles and each mode propagates at a different speed. This causes pulse broadening (modal dispersion), limiting the speed at which subsequent pulses may be generated without overlapping. Multimode fibers are generally used for short distance applications, such as within buildings.

Figure 3. Graded-index multimode fiber

Non-Zero Dispersion fibers were designed specifically to address the needs of long-haul optical networks - the ability to move a large amount of information over a long distance cost effectively. To accomplish this, the dispersion characteristics are modified from conventional single-mode fiber such that it has low, but non-zero, dispersion around 1550 nm, where attenuation is lowest (Fig. 4). It also has low dispersion slope, which means it has similar transmission characteristics across a wide range of wavelengths - perfect for handling many transmission channels on one fiber.

Zero Water Peak fibers are single-mode fibers in which the water absorption characteristics on conventional single-mode fibers, which impact signal loss, have been adjusted to make more wavelengths available for high quality transmission (Fig. 5). These fibers allow 50% more channels by opening up the wavelength range around 1400 nm, previously unavailable for transmission. This capability is a perfect match for the requirements of metropolitan networks.

Figure 4. Dispersion

Figure 5. Attenuation
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