Optical Fiber Selection for Network Interconnection

The emergence of Data Centers, Storage Area Networks and other computing applications drives the needs for ultra-high speed data interconnections and structured cabling. The interconnect media choices include wireless technology, copper cable and optical fiber cable. Fiber cable offers the highest bandwidth and supports the highest data rates. There are single-mode and multimode fiber types. Different types of fiber connect with fiber optic transceivers resulting in different performances and costs. So it’s important for the network designers to understand the fiber types and select the right fiber and corresponding fiber optic transceivers for network interconnection.

Optical Fiber Types

There are three main types of optical fiber suitable for network interconnection use:
9/125μm Single-mode fiber
50/125μm multimode fiber
62.5/125μm multimode fiber

The above numbers respectively mean the diameter of the glass core where the light travels and outside glass cladding diameter which is almost the same to most fiber types. So the difference of each fiber type is caused by the core diameter. It has great impact on system performance and system cost when balanced against network application needs. Two primary affected factors are attenuation and bandwidth.

Factors Affected by the Fiber Core Diameter

Attenuation is the reduction of signal power, or loss, as light travels through an optical fiber. Fiber attenuation is measured in decibels per kilometer (dB/km). The higher the attenuation, the higher rate of signal loss of a given fiber length. Single-mode fibers generally operate at 1310 nm (for short range) while multimode fibers operate at 850 nm or 1300 nm. Attenuation is not usually considered to be the main limiting factor in short rang transmissions. But it can cause big differences in high speed network such as 100Gb/s.

Bandwidth means the carrying capacity of fiber. For single-mode fiber, the modal dispersion can be ignored since its small core diameter. Bandwidth behavior of multimode fibers is caused by multi-modal dispersion during the light traveling along different paths in the core of the fiber. It has an influence on the system performance and data rate handling. Multimode fiber uses a graded index profile to minimize modal dispersion. This design maximizes bandwidth while maintaining larger core diameters for simplified assembly, connectivity and low cost. So manufacturers start to develop higher-performance multimode fiber systems with higher bandwidth.

System Costs: Single-mode and Multimode Fibers

A fiber optic transceiver usually consists the optical light sources, typically LED–light emitting diode and optical receivers. Since the core diameter size and primary operating wavelengths of single-mode fiber and multimode fiber are different, the associated transceiver technology and connectivity will also be different. So is the system cost.

To utilize the single-mode fibers generally for long distance applications (multi-kilometer reach), transceivers with lasers such as SFPP-10GE-LR (an SFP+ 1310nm 10 km transceiver supporting single-mode fibers) that operate at longer wavelengths with smaller spot-size and narrower spectral width. But these kinds of transceivers need higher precision alignment and tighter connector tolerance to smaller core diameters. Thus, it causes higher costs for single-mode fiber interconnections. To lower the cost, manufacturers produce transceivers based on VCSEL (vertical cavity surface emitting laser), for example, 10G-SFPP-SR (an SFP+ 850nm 300m transceiver supporting multimode fibers), which are optimized for use with multimode fibers. Transceivers applying low cost VCSEL technology to develop for 50/125μm multimode fibers, take advantage of the larger core diameter to gain high coupling efficiency and wider geometrical tolerances. OM3 and OM4 multimode fibers offer high bandwidth to support data rates from 10Mb/s to 100Gb/s.

Conclusion

Optical fiber is an easily-installed medium that is immune to electromagnetic interface and is also more efficient in terms of power consumption. What’s more, fiber optic cable can save space and cost with higher cabling density and port density over copper cabling. For single-mode fiber and multimode fiber, each one has its advantages and disadvantages. Network designers should better select the right fiber type and related fiber optic transceivers according to specific situations for higher system performance. Of course, cost is another important factor to be considered.

Originally published at www.fiber-optic-equipment.com