Telecom Fibers

Telecom fibers are optical fibers which are used in optical fiber communications. This is a huge application field for fibers; several hundred million kilometers of telecom fibers are currently fabricated per year.

Mainly the following types of telecom fibers are used:

• Fused silica single-mode fibers with very low propagation losses (e.g. 0.2 dB/km) are used for long-haul data transmission (tens or hundreds of kilometers) with very high transmission capacity. Those which are suitable for high data rates are often spun fibers, exhibiting much reduced polarization mode dispersion.
• Fused silica multimode fibers, often in the form of graded-index fibers, are suitable for shorter distances of e.g. hundreds of meters (e.g. within storage area networks): They are less critical to handle, essentially due to much larger core areas, but intermodal dispersion limits the transmission distance.
• Few-mode fibers are optimized for mode division multiplexing, i.e., for transmission of multiple signal channels through different fiber modes.
• Plastic optical fibers (usually multimode), typically made of PMMA (polymethyl methacrylate, also called acrylic), represent the cheapest solution for short-range data transmission, e.g. in home networks.

A standard single-mode telecom fiber for the 1.3- or 1.5-μm wavelength region is the SMF-28 of Corning, and there is the enhanced version SMF-28e. The mode field diameter is ≈ 9.2 μm at 1310 nm (effective mode area = 67 μm²), or 10.4 μm at 1550 nm (85 μm²). The single-mode cut-off is at 1260 nm. The Lucent AllWave and the Alcatel ColorLock fibers have quite similar properties.

Other telecom fibers have somewhat modified properties, making them more suitable in certain areas:

• Corning offers Vascade fibers in different versions, e.g. the Vascade L1000 with an increased effective mode area, the non-zero dispersion-shifted Vascade LS+, and the Vascade LEAF fiber, a dispersion-shifted fiber with increased mode area.
• Lucent offers different versions of their TrueWave fiber, e.g. the TrueWave-RS with reduced dispersion slope and the TrueWave-XL with larger mode area.
• Alcatel developed the TeraLight fiber, another non-zero dispersion-shifted fiber. (Alcatel has later merged with Lucent to form the company Alcatel-Lucent.)

As glass fibers are not sufficiently robust for directly laying them down in a building or even in the ground, they are incorporated into optical fiber cables, where various polymer layers and sometimes even metallic armors provide additional protection. For flexible indoor use, fiber patch cables with standardized fiber connectors are suitable.

The International Telecommunications Union (ITU) has developed a number of standards for various types of fibers as used for optical fiber communications. Some of the most important of those standards are listed in Table 1.

Table 1: Important ITU standards concerning telecom fibers.

There are various other standards for telecom fibers, e.g. from ISO and IEC.

Many different properties of a telecom fiber can be relevant for the achievable performance (partly depending on details of the fiber-optic links) or concerning other aspects of use:

• The propagation losses (fiber attenuation) in decibels per kilometer (dB/km) are particularly important for long-distance links. For single-mode fibers operated in the 1.5-μm spectral region, they are often of the order of only 0.2 dB/km. Multimode fibers typically exhibit somewhat higher values in the 1.5-μm region, and substantially higher attenuation is obtained for operation at short wavelengths.
• The chromatic dispersion is particularly relevant for single-mode fibers in long-distance systems. It is not necessarily best to have a group velocity dispersion (GVD) as small as possible; certain dispersion management techniques work best for fibers with substantial GVD. The spectral slope of GDD (related to higher-order dispersion, particularly TOD) can also be very important for high-bandwidth wavelength division multiplexing. The zero-dispersion wavelength is sometimes of particular interest. (See also the article on dispersion-shifted fibers.)

Case Studies

Dispersion Engineering for Telecom Fibers

We explore different ways of optimizing refractive index profile for specific chromatic dispersion properties of telecom fibers, resulting in dispersion-shifted or dispersion-flattened fibers. This also involves automatic optimizations.

• For multimode fibers, the differential mode delay (DMD) is often the limiting factor for the achievable bandwidth–distance product. Carefully designed and fabricated parabolic-index fibers (e.g. of the OM4 class) promise the best performance.

Case Studies

Telecom Fiber With Parabolic Index Profile

We investigate how intermodal dispersion of a multimode fiber can be minimized with a parabolic doping profile.

• The strength of nonlinear optical effects in the fiber is mainly determined by the effective mode area, which can differ quite substantially between different telecom fibers, and less by the chemical composition of the fiber core. It is often an important parameter in a system design.
• For some applications, the sensitivity to bend losses is important. This is particularly the case for indoor installations, it may be impractical to avoid tight bending. A manufacturer may e.g. specify the induced macrobend loss in some wavelength range for a few turns of fiber wound up on a mandrel with given bend radius of e.g. 15 mm.
• The core–clad concentricity can be important for the possible quality (in terms of transmission losses) of fusion splices.
• Details of the protective fiber coating, such as its outer diameter and chemical composition, can also be relevant in various ways, e.g. when stripping it before mounting fiber connectors or concerning temperature resistance.
• There can be various environmental specifications, e.g. concerning the allowed temperature range, allowed tensile stress and increased transmission losses caused by high temperatures or water immersion.
• The available fiber length per spool (often many kilometers) may be limited. However, telecom fibers are mostly drawn from large preforms, each one delivering many kilometers of fiber.

While the optical fiber constitutes the transmission medium, the cable structure provides necessary mechanical protection and environmental isolation. Common designs include:

• Loose tube cables: The fibers are placed loosely within plastic tubes, often filled with a gel or water-blocking powder. This isolates the fiber from external mechanical forces and thermal expansion, making them ideal for outdoor and long-haul applications.
• Tight-buffered cables: A protective coating (buffer) is applied directly over the fiber coating (e.g., to 900 μm diameter). These are more robust for handling and termination, making them suitable for indoor LAN/WAN applications and patch cords.
• Ribbon cables: Fibers are aligned and bonded in flat ribbons. This allows for high fiber counts in a small diameter and enables mass fusion splicing, which is efficient for high-density networks.