Understanding Fibre Optic Patch Leads

Fibre Size 

When specifying the fibre optic within the patch lead, we want to match it best with the fibre type within the trunk or fixed cable installed. So, before ordering, it is always best to refer to cable specifications or the cable jacket print on the trunk cable.

Fibre size is often referred to in relation to the core and cladding, for example 9/125. The first number in the sequence refers to the core size; 62.5/, 50/ or 9/ are more commonly seen.

The second number refers to the cladding which in most applications within the data/telecommunications arena will remain a constant. This is shown as’/125’ and is the outer diameter of the cladding, in this case 125µm or microns. None of these fibres with a different core size can be effectively matched due to size differences, at least not without significant loss of signal.

There may be some small differences in core sizes quoted by manufacturers such as 8/125, 9/125 & 10/125. Generally, these are so small differences that the effect is negligible for most applications.

Fibre type is often referred to by Multimode (MM, MMF) and Single-mode (SM, SMF, mono-mode). This is shown in the table to the left. With fibre types, it is always best practice to match like-for like-fibre, though compatibility between manufacturers should not pose a problem as they should all conform to the same international standards.

A note on OS1 & OS2 – reputable fibre manufacturers produce single-mode fibre conforming to OS2 specifications. This covers G.652.D, G.657.A1 & A2, the bend insensitive variety. OS1 produces higher attenuation than OS2, which at longer telecom lengths may not be acceptable to the network budget.

Cable manufacturers will often standardise on a fibre type that can be used within multiple cable designs. So, a tight buffered patch lead will contain the same fibre as that of a loose tube cable design. The construction of Tight Buffered cable, however, applies a squeeze on the fibre compared to Loose-Tube cables. On very long cable runs, tight buffered cable attenuation values may sit outside of OS2 specifications. Tight buffered cables though are primarily used on shorter cable runs which normally do not present an attenuation problem.

Cable selection for patch leads is important. Now we know the fibre size/type and the fibre count, we arrive at the cable construction and physical parameters.

Simplex patch leads have one fibre connector at each end. There are, however, several formats. The Mini-Simplex range includes 1.2mm, 1.6mm, 1.8mm and 2.0mm. These cables are used to maximise space saving and more often contain bend insensitive fibres such as OM3, OM4 and G.657.A1 & A2.

Larger diameter cables are referred to as Simplex cables, ranging from 2.3mm - 5.0mm. They tend to be selected for industrial or harsher environment applications. However, it’s worth noting the mechanical strength properties between a 2mm and 3mm is negligible so selection may be on personal preference or historical use.

Duplex patch leads have two fibre connectors at each end. The format is like Simplex but the range tends to be reduced. Duplex patch leads are most commonly a Zip Twin or Shot-gun style cable. The Mini-Zip range is typically 1.6mm, 1.8mm, 2.0mm and the Zip Twin range is 2.5mm and 3.0mm.

Duplex patch leads are the most common assemblies as they cater for the widest range of applications. The dual fibre offers Transmit (TX) & Receive (RX) within the same assembly so makes for ease of patching.

Flat Twin patch leads are also known as Ruggedised patch leads. It’s easier to visualise two Simplex cables surrounded with an outer jacket, so becomes an oval shaped cable. There are two sizes; Mini-Flat is 2.0mm inner tubes and Flat Twin is 3.0mm inners. They are highly suitable for longer run lengths as they have superior crush and tensile strength when comparing to Duplex cables.