The emergence of Data Centres, Setorage 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 fibre cable. Fibre cable offers the highest bandwidth and supports the highest data rates. There are single-mode and multimode fibre types. Different types of fibre connect with fibre optic transceivers resulting in different performances and costs. So it’s important for the network designers to understand the fibre types and select the right fibre and corresponding fibre optic transceivers for network interconnection.
There are three main types of optical efibre suitable for network interconnection use:
9/125μm Single-mode fibre
50/125μm multimode fibre
62.5/125μm multimode fibre
The above numbers respectively mean the diametre of the glass core where the light travels and outside glass cladding diamere which is almost the same to most fibre types. So the difference of each fibre type is caused by the core diametre. It has great impact on system performance and system cost when balanced against network application needs. Two primary affected factors are attenuation and bandwidth.
Attenuation is the reduction of signal power, or loss, as light travels through an optical fibre. Fibre attenuation is measured in decibels per kilometre (dB/km). The higher the attenuation, the higher rate of signal loss of a given fibre length. Single-mode fibres generally operate at 1310 nm (for short range) while multimode fibres 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 fibre. For single-mode fibre, the modal dispersion can be ignored since its small core diametre. Bandwidth behavior of multimode fibres is caused by multi-modal dispersion during the light traveling along different paths in the core of the fibre. It has an influence on the system performance and data rate handling. Multimode fibre uses a graded index profile to minimize modal dispersion. This design maximizes bandwidth while maintaining larger core diametres for simplified assembly, connectivity and low cost. So manufacturers start to develop higher-performance multimode fibre systems with higher bandwidth.
A fibre optic transceiver usually consists the optical light sources, typically LED–light emitting diode and optical receivers. Since the core diametre size and primary operating wavelengths of single-mode fibre and multimode fibre are different, the associated transceiver technology and connectivity will also be different. So is the system cost.
To utilize the single-mode fibres generally for long distance applications (multi-kilometre reach), transceivers with lasers such as SFPP-10GE-LR (an SFP+ 1310nm 10 km transceiver supporting single-mode fibres) 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 diametres. Thus, it causes higher costs for single-mode fibre 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 fibres), which are optimised for use with multimode fibres. Transceivers applying low cost VCSEL technology to develop for 50/125μm multimode fibres, take advantage of the larger core diametre to gain high coupling efficiency and wider geometrical tolerances. OM3 and OM4 multimode fibres offer high bandwidth to support data rates from 10Mb/s to 100Gb/s.
Optical fibre is an easily-installed medium that is immune to electromagnetic interface and is also more efficient in terms of power consumption. What’s more, fibre optic cable can save space and cost with higher cabling density and port density over copper cabling. For single-mode fibre and multimode fibre, each one has its advantages and disadvantages. Network designers should better select the right fibre type and related fibre optic transceivers according to specific situations for higher system performance. Of course, cost is another important factor to be considered.