Ten things to remember about fibre optics

1. Pair your single strand devices correctly

You wouldn’t buy two left shoes, but people often make a similar mistake when they’re working with Single Strand Fibre (SSF). SSF technology allows for the use of two independent wavelengths, often 1310 and 1550nm, on the same piece of cable. But the transmitters and receivers must be matched correctly. One unit must be a 1310/1550 device (transmitting at 1310nm, receiving at 1550nm) and the other must be a 1550/1310 device (transmitting at 1550nm, receiving at 1310nm).

The 1550nm SSF transmitters are more expensive than the 1310nm transmitters, thanks to the cost of their more powerful lasers. So network administrators may hope to save money by installing a pair of 1310/1550 devices. But, like mismatched shoes, it doesn’t work.

SSF equipment is also more expensive than standard single-mode. But there are situations in which SSF is well worth the investment. For example, if a strand of fibre is available in an existing bundle, deploying SSF products would be cheaper than digging up the cable run to add another bundle of fibre. And in applications where all of the available single-mode fibre capacity is already in use you can double your fibre capacity by switching to SSF products.

2. Do not mix single-mode and multi-mode fibre products

It may be tempting to try to make use of legacy cabling or equipment from an older fibre installation. But don’t even think about it unless you know exactly what you’ve got, and what all the pieces are. Single-mode and multi-mode fibre products are normally incompatible. Some of the major differences would include the wavelengths that the fibre cable can accommodate and the light sources that are being used.

Multi-mode fibre uses cable with a relatively large core size, typically 62.5µm, with 50µm still used in some installations. The larger core size simplifies connections and allows for the use of less powerful, less expensive light sources. But it also lends itself to modal dispersion – the light tends to bounce around inside the core. That limits multi-mode’s useful range to about 2km.

Single-mode fibre combines powerful lasers and cabling with a narrow core size of 9/125µm to keep the light focused. It has a range of up to 130km, but it is also more expensive.

So let’s say you tried to use single-mode products over a multi-mode cable run. The core size of the fibre cable would be far too large. You’d get dropped packets and CRC errors, and your customers would not be happy. Single and multi-mode fibre may have many similarities – but they’re not the same thing.

3. Be prepared to use attenuation with short single-mode cable runs

Networks designers don’t always specify multi-mode for shorter fibre cable runs. They may choose single-mode instead, and their reasoning may be based upon issues like bandwidth or interoperability with large, complex systems. Depending upon the distances involved, a single-mode fibre laser can sometimes be too “hot”, meaning that it has a very powerful signal strength relative to the distance that is being covered. This can lead to oversaturation of the receiver at the other end of the connection.

The solution is attenuation: deliberately reducing the signal strength without impacting the integrity of the line or the throughput of data. Attenuators can take numerous forms. Examples include an O ring type that slips onto an ST connector interface, a coupler on the fibre line itself, and connector caps that clip onto a fibre interface connector. They are available with various dB values. Folks who don’t want to perform heavy mathematical calculations often start with 10dB.

4. Don’t assume that all fibre products are Class 1, eye-safe

Fibre media converters normally use Class I, eye-safe lasers. And vendors will normally list all of the relevant specifications in their documentation or on their web sites. But why take any chances? More powerful lasers are often used in “Fibre Channel”, medical and military applications, so it’s best to avoid letting any lasers shine directly into your eyes at any time, even if it seems unlikely that they would be powerful enough to be dangerous. Better safe than sorry.

5. Make sure you have the right connectors

Fibre transceivers use a variety of connectors, so familiarise yourself with them before you begin ordering products for a fibre installation. SC, “stick and click”, is a square connector. ST, “stick and twist”, is a round, bayonet-type. LC, or the “Lucent Connector”, was developed by Lucent Technologies to address complaints that ST and SC were too bulky and too easy to dislodge. (LC connectors look like a compact version of the SC connector.) Small Form Pluggable (SFP) transceivers use LC. Less common connectors include MT-RJ and VF.

6. Minimise connectors and splices

Although single-mode fibre suffers from less signal loss per km than multi-mode, all fibre performance is impacted by connectors and splices. The signal loss at a single connector or splice may seem insignificant. But as connectors and splices become more numerous signal loss will steadily increase. Typical loss factors would include 0.75dB per connector, 1dB per splice, 0.22dB attenuation per km for single-mode fibre and 0.35dB attenuation per km for multi-mode fibre. Add a 3dB margin for safety. The more splices and connectors you have in a segment, the greater the loss on the line.

If a line is too degraded there are some things you can do to try to resolve the problem. Installing a more powerful fibre optic device may compensate for signal loss. You may be able to shorten the length of the cable run. You may be in a position to reduce the speed, from gigabit to Fast Ethernet. (Lowering the speed extends range.) If none of these remedies succeed you may have to pull a new bundle of fibre, which is expensive and time-consuming.

7. Use a power meter

In theory, subtracting minimum receive sensitivity from minimum transmit power should give you your power budget, the amount of light that is available to transmit signals over a specified distance. (Transmit Power - Receive Sensitivity = Power Budget.) And the vendor’s spec sheet or web site should provide you with the relevant values. But splices, connectors and distance all affect the power budget as well, and there may be subtle differences between devices provided by different vendors. Use a power meter to verify and troubleshoot each segment of your installation.

8. Use the correct ferule

Fibre connections may use Angle Polished Connectors (APC) or Ultra Polished Connectors (UPC), and they are not interchangeable. There are physical differences in the ferules at the end of the terminated fibre within the cable. An APC ferrule end-face is polished at an 8° angle, while the UPC is polished at a 0° angle. If the angles are different, some of the light will fail to propagate, becoming connector or splice loss.

UPC connectors are common in Ethernet network equipment like media converters, serial devices and fibre-based switches. APC connectors are typical for FTTX and PON connections. ISPs are increasingly using APC.

9. Become familiar with transceiver acronyms

SFP transceivers are more expensive than fixed transceivers. But they are hot swappable and their small form factor gives them additional flexibility. They’ll work with cages designed for any fibre type and their prices are steadily dropping. So they have become very popular.

Standard SFPs typically support speeds of 100Mbps or 1Gbps. (One can also find a T1 SFP, although they are rare.)

XFP and SFP+ support 10Gbps connections. SFP+ is the smaller of the two and allows for greater port density. But even if the modules were all the same size, you couldn’t successfully connect XFP or SFP+ to a device that only supports 1Gbps.

10. Speeds and protocols make a difference

Most fibre-based devices are defined by speed and protocol. If a gigabit network switch has a fibre interface on it, and it is labelled a GBIC, it must be connected to an Ethernet device that is capable of gigabit speed as well. Fibre devices are normally incompatible with equipment that is designed to communicate at a different speed.

Protocols matter, too. An Ethernet gigabit network device, for example, won’t be able to communicate with a Gigabit Passive Optical Network (GPON) device, as the GPON is a point-to-multipoint access mechanism. It uses passive splitters and encryption to serve multiple locations. Both devices may be gigabit, but they won’t be able to communicate effectively.

Mode converters can convert one type of fibre to another, and they are not confined by a protocol. But they are limited by a native speed range. A mode converter that supports up to 155Mbps can’t be connected to a gigabit mode converter. There are, however, fibre technologies like WDM (two channel fibre, passive optics), that can be connected at any speed up to 10Gbps.

Author profile: Susan Stanley is the training manager at B&B Electronics. She has spent the last 18 years in engineering and customer service at technology-related companies. Stanley’s working experience encompasses many technologies, including Operating Systems, Ethernet Networking and Fibre Optics. She has developed Intranets and comprehensive FAQ systems to support internal staff. Certified as a technical trainer, Stanley has trained new employees for product knowledge. She is currently developing technology and application content for use with customers and other end users. Stanley’s ability to convey technical information in layman’s terms is a critical element in quickly resolving end-users’ product issues and questions. Training is her passion and she takes great pride in making the learning process meaningful but fun. Stanley earned a bachelor of history with a minor in English from California State University at Fullerton.