Fibre-optic connectors: the make or break of a network

The phrase ‘fibre-optic technology’ conjures visions of information speeding around the world at the speed of light, ofhigh-tech data centres and previously unimagined capabilities in communications. What it does not evoke is visions of connectors. But without being able to connect fibres effectively, a network just wouldn’t exist.

Optical fibre connectors are used to join optical fibres where a ‘connect’ and ‘disconnect’ capability is needed. The connector terminates the end of the optical fibre and then mechanically couples with another connector, aligning the fibre cores so that light can pass through with little or no loss. Continuing research and development has resulted in the market release of over 100 differing fibre connectors since the mid-1980s, but they all contain four central elements.

The ‘ferrule’ is a long, thin cylinder through which the optical fibre is threaded.

In a connection, two ferrules physically touch, allowing the fibres to be accurately aligned and mated. Typically of metal or ceramic construction, the end-face of the ferrule can be polished to a convex shape that allows the fibres to physically touch when mated together. The end-face of the ferrule may also be polished with an 8° angle to further reduce reflection (high return loss).

The ferrule sits inside the connector housing, constructed of metal or plastic, which serves to make the connector easy to handle and keeps the connector firmly attached within a connection. The cable attaches to the connector housing and is the point of entry for the fibre. Typically, a strain relief boot is added over the junction between the cable and the connector body, providing extra strength and protection to the junction.

Optical fibre connectors do not use the male-female configuration common to electronic connectors. Instead, a coupling device such as alignment sleeve is used to mate the connectors. Similar devices may be installed in fibre-optic transmitters and receivers to allow these devices to be mated via a connector.

The original mass-utilised Straight Tip, or ST connectors, have a bayonet mount and a 2.5 mm ferrule, and mate via a mating adaptor. Subscriber Connectors (SC) also use a 2.5 mm ferrule but have a simple snap-in latch, and are widely accepted in multimode LAN applications.

Increasing demands for higher density applications drove the development of Small Form Factor (SFF) connectors, the most successful of which has been the Local Connector, known as LC. The LC uses a smaller ferrule of 1.25 mm, allowing more fibre ports per unit of rack space.

The high density demand soon resulted in the development of MPO, or multifibre push-on connectors. Based on the single ferrule technology, MPO connectors are used with ribbon or mini-round cable and can provide up to 24 fibre connections in a single connection point. MPO connectors are commonly used in data centre cabling systems in conjunction with LC fanouts.

Different connector types have different characteristics, different advantages and disadvantages, and different performance parameters. In general, a good connector should:

• cause little or no loss of optical power across a junction point (low insertion loss and high return loss);
• be easily and quickly installed;
• be able to be connected and disconnected many times without changes in loss;
• have a consistent and calculable level of loss.

While a well-engineered connector can minimise loss due to air gaps or misalignment, the reality is that a speck of dust can undo the designer’s good work. Dust, oil or dirt on the fibre can partially or even totally obscure an optical fibre connection. Dust caught between fibre end-faces as they are pressed together in mating can cause irreparable damage to both fibres. Keeping connectors clean is vitally important - best practice involves cleaning connectors prior to each connection and using dust caps to protect unused connectors and couplers.

Connectors certainly aren’t the most glamorous element of a fibre-optic network, but they can make or break it. So while you’re envisioning the magic that is fibre-optic technology, spare a thought for the humble connector, without which a fibre-optic network would just be a mass of glass cables.

Common connector types
Type	Typical loss (dB)	Applications
ST singlemode	0.3	Legacy industrial applications, patching and equipment interfaces
ST multimode	0.3	Legacy industrial applications, patching and equipment interfaces
SC singlemode	0.2	Telco, video, LANs and WANs, angled version for low RL (return loss)
SC multimode	0.2	LAN applications, patching etc new industry standard in multimode
LC	0.2	SFF - LAN,WAN, telco, high-density networking, data centres and storage networks
MU	0.2	SFF - telco applications
MPO/MTP	0.2	Multifibre/array - 40/100Gb ethernet, data centre, high-density LAN, storage or telecommunications x-connects
Less common connector types
SMA	1.0	Obsolete industrial LAN
FDDI	0.6	Obsoleted duplex connector used for direct interfacing to FDDI equipment
ESCON	0.6	Duplex connector purely used for direct interfacing into IBM equipment
FC	0.2	Singlemode telco and infrastructure networks. Angled version for low RL
DIN	0.5	Found on superseded European telecommunications equipment
VF45 Multimode	<0.75	No ferrule SFF connector, LAN, fibre to the desk
VF45 Singlemode	<0.75	No ferrule SFF connector, LAN, fibre to the desk
MTRJ	0.5	Superseded SFF LAN connector, LAN equipment interface and fibre to the desk
E2000 Singlemode	0.3	Optical interface found on European telecommunications equipment