The Beyondtech Guide for Fiber Optics Testing (PART II): Insertion Loss

So, you may be walking by the street and stumble on some random Fiber Optic cable somebody lost and now all of a sudden, you’re thinking about Loss or as it is also known “dB Loss”, “attenuation”, as well as “Insertion Loss”.

Insertion Loss is the measurement that you are required to do, as said by the TIA-568B (a set of telecommunications standards from the Telecommunications Industry Association), because it could be caused by poor practices of installation such as an inadequate polished or dirty connector and arise high measurement of Loss, or a cable that was installed to go around a corner that didn’t meet the minimum bend radius.

The standards by the TIA (Telecommunications Industry Association) say we could use an OPM (Optical Power Meter) with the correct Light Source to run an Insertion Loss Test in order to get a certification of an Optical Fiber Cable. Just remember, we use an LED source when it’s a Multimode fiber loss measurement and we will require a laser source with of Single-mode fiber loss.

Light sources and optical power meters are available as budget or low-cost separate units (Figure 1), or they may be integrated into all-in-one “smart” optical loss test sets (Figure 2), which will give you additional features like dual-fiber testing, pass/fail analysis and length measurement. So, if you use the proper reference settings and the correct cleaning methods you will surely get a precise standards-compatible result.

There is a peak measure for length in Fiber Optic Cables that we can find in the TIA-standards, which for the certification process you must test. Nevertheless, you are not obligated to test the length of a cable. To conclude: you are going to need an OPM and the correct Light source or an OLTS as shown in Figure 2. It wouldn’t hurt to use other equipment but this is the only thing you are demanded to do.

We have a small example to help us understand what we are talking about, consider a 50mtrs (164 ft) backbone cable with the following component loss values:

• Splice (0.05 dB)
• Fiber (0.05 dB)
• ERC (Equipment Room Connection) (0.6 dB)
• Telecom closet connection (0.15 dB)

The top suitable loss for this cable is 0.75 dB of connection loss (0.32 dB per connection) plus 0.05 dB of fiber loss (1 dB / km) and 0.15 dB of splice for a result of 0.95 dB (check the TIA-standards for this data in tiaonline.org). Given this cable has an altogether insertion loss of 0.85 dB, we can be certain that it will have its certification.

Aside from all this, contamination like dirt or maybe dust can be the origin of your signal loss; with these cables we have a very low loss of signal for how fast a connection they provide, but signal loss can be a very big trouble for your network and small details like contamination in one connection could create a weak link in your network.

 So, let’s show how we should approach Inspecting and Cleaning:

• First, in every occasion we should assume connectors aren’t clean, so, get the proper equipment for cleaning them.
• Static is your enemy, for this, there are cleaning fluids you can use to prevent it
• Cleaning one end of the cable and connecting it to a dirty connector will make you lose your time, so, always clean both ends of the cable in your network, this is part of doing a proper installation.
• Always use quality optic fiber cleaning supply, this should be something where you shouldn’t spare any expenses consequences could be more expensive.
• A fiber optic microscope to check all connectors, some microscopes have a eye protection to make your job safer.

The combination of a bad installation, wrong handling of cables or supplies could lead you to hours lost in cleaning, checking and reconnecting everything in your network. But the good thing is that, if you’re already in the field and you’re meeting a network with some contamination in its connection the adequate cleaning supplies and good test techniques allow experienced technicians to get that network back to its rightful glory. 

On the next entry in Beyondtech’s Reference for Fiber Optics Testing we’ll talk about the OTDR (Optical Time Domain Reflectometer), what it is, why we need one and how it can be used in the field, so keep your eyes peeled for our next entry and remember to follow us on twitter at @beyondtechinc.