How Back Reflection and Insertion Loss Affects Data Center Performance

What is Back Reflection?

Back reflection (also referred to as optical return loss or ORL) is an undesirable characteristic that is primarily a concern for singlemode fiber. It is expressed in negative dB and used to describe any process that causes light to change direction in fiber and return to the source. This occurs most often at connector interfaces.

Optical return loss must be minimized to ensure maximum data center performance, particularly when running higher data transmission speeds. It could even damage a transceiver.

One way to minimize back reflection is to ensure that the fibers are touching or in physical contact (PC). On singlemode fiber, PC finishes work even better at a slight angle. These are called APC or angled physical contact connectors.

How can I minimize the negative effects of back reflection?

Make sure your fiber cable end-faces are properly polished before installation. Polishing is a process of the manufacturing phase in where the fiber end-faces are polished to very specific criteria that greatly affects performance.

Since you are probably not building the cables yourself, you can manage this by obtaining the back reflection specification from the supplier you plan to purchase from. Use the below dB specifications as a benchmark for purchasing.

Listed below is the amount of light that may return to the source without altering the quality of the light signal (benchmarks for back reflection):

• Physical Contact (PC): 40dB or higher
• Ultra Physical Contact (UPC): 50dB or higher
• Angled Physical Contact (APC): 60dB or higher

*Hardware/optics dictate what type of connector polish is required.

What is insertion loss?

In any fiber optic interconnection, some loss occurs. Insertion loss (IL) is the total optical power loss (measured in dB) caused by the insertion of a component (a connector or coupler).

Some common causes of insertion loss are:

• The misalignment of ferrules
• An air gap between mating ferrules
• Absorption loss from scratches, oils, and contamination

How can I minimize the negative effects of insertion loss?

Insertion loss can be minimized with the proper selection of materials, polishing the end-face of the connector, and safe termination practices. Decreasing insertion loss improves cable performance immensely.

Just as with back reflection, review the specifications provided by the supplier of the fiber assemblies and use the below benchmarks for purchasing.

Listed below are the recommended maximum dB loss amount for data center fiber optic cabling:

• LC multi-mode connector: .15dB maximum
• LC singlemode connector: .15dB maximum
• MPO (MTP) multi-mode connector: .20dB maximum
• MPO (MTP) singlemode connector: .30dB maximum

To ensure you're ordering quality cables, ask the manufacturer if return loss and insertion loss are tested during the manufacturing process. For more information on back reflection and fiber end-face polish types, see our tutorial video - Fiber End Face Polish Types.