Insertion Loss Testing

Insertion loss refers to the loss of light, not so much in the cable assembly, but that loss caused due to the assembly's introduction. There may be very little light lost through the assembly itself, but the majority of losses related to optical cable assemblies relate to how well the fibres align with mating connectors when the assembly is introduced.

Insertion losses are measured in dB as per the formula

Insertion Loss = -10Log (P1/P0)

where P0 is the initial measured power and P1 is the measured power after the assembly under test is introduced. The following table shows typical dB losses represented as percentage loss.

Loss (dB)	Loss (%)
0.1	2.3
0.3	6.5
0.5	11

Fibrepulse performs and records insertion loss tests on 100% of all terminated connections as per IEC-61300-3-4 (method B). Procedure IEC-61300-3-4 (method C) is also used in some circumstances.

Insertion loss limits can depend on the customer's application. Generally, an insertion loss of 0.50dB per connection is acceptable. Fibrepulse has imposed a stricter 0.30dB max on all connector terminations. Doing this involves a high level of control of material selection and processes. The Diamond connector range which Fibrepulse terminate has a maximum insertion loss of 0.10dB. This is achieved by active core alignment.

Contributing factors of insertion loss are:

1. Misalignment.

In order for all the light to be successfully transmitted from one fibre to another the two cores need to be perfectly aligned. With singlemode fibres, because the core sizes are so small, a small misalignment can have a significant effect on loss. All the following tolerances effect alignment:

• Fibre diameter
• Core eccentricity
• Connector hole diameter
• Connector hole eccentricity

Fibrepulse combats these factors by selecting the highest quality components, and pre-selecting connectors for termination by tightest fit on the fibre. The offset caused by these factors can also be improved by optimising or tuning after termination. This is a method of keying the connectors so as to prevent eccentricities becoming accumulative. The Diamond "Core-aligned" assemblies, which Fibrepulse manufactures solves the problem by aligning the core into the centre of the ferrule during the assembly process. This has the effect of eliminating the effects of all the above tolerances, and insertion losses achieved are 0.1dB max.

2. Bending.

Bending a fibre is a sure-fire way of increasing attenuation. Fibre cables all have a minimum bend radius recommended. However, fibre connectors are spring loaded allowing the ferrule to push back inside the connector. This, particularly with soft buffer coatings, can cause bending. This is bending added on to the bending already there from threading the connector on in the first place. Fibrepulse has a patented process for reducing this bending and thus reducing stress on the fibre during mating. Bending inside the connector can be a big problem if cables are used at longer wavelengths or for DWDM. Bending can also occur throughout the cable (micro-bending). This can be also prevented by controlling production processes.

3. Stress.

If the fibre is subjected to excess stress, fractures may result causing light to leak into the cladding. Reducing stress applied to the fibre is a matter of proper production procedures. Potential sources of stress can come from the following processes:

• Reeling / De-reeling
• Coating removal (stripping)
• Connector insertion
• Oven cure temperature,/

These are all areas that Fibrepulse has imposed strict process controls.