Back reflection and insertion loss can severely decrease the performance of your data center cabling. So what are they, and what can you do to minimize the effects?
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 a fiber and return to the source. This occurs most often at connector interfaces. The diagram below shows light (the orange arrows) bouncing back to its source in a fiber cable:
Optical return loss must be minimized to ensure maximum data center performance, particularly when running a network on 10, 40 and 100G Ethernet speeds. One way to do this 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?
There are two things you can do to greatly decrease back reflection. The first is to make sure your fiber cable end-faces are properly polished before installation. The second is to make sure cables are aligned correctly. Doing both of these things will ensure proper fiber contact and minimize back reflection.
Back reflection is measured in dB. A higher return loss results in better performance. Listed below is the amount of light that may return to the source without altering the quality of the light signal:
- Physical Contact (PC): 40dB or higher
- Ultra Physical Contact (UPC): 50dB or higher
- Angled Physical Contact (APC): 60dB or higher
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.
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 CABLExpress® Cable Talk™#34 – Fiber End Face Polish Types.