We can all agree that it’s great having today’s high bandwidth optical networks to fulfill our needs within our daily lives. While the capabilities that these networks provide cannot be matched, their higher sensitivity to back reflection and attenuation can be a constant concern. As the internet takes over more and more of our daily regimens in business, entertainment, etc. this concern becomes elevated.

Because of these reasons, end users are ever more reliant on the constant reliability of these networks to support their day to day usages. This makes the selection and use of the proper test equipment so much more important today than it was, say a decade ago.

Within the industry there are two different levels or “tiers” for certifying a fiber optic network drawn up by the Telecommunications Industry Association (TIA): Tier 1 and Tier 2 testing. Depending which tier is required, specific test equipment is needed. Tier 1 is traditionally a simple test for attenuation, continuity, and polarity. Most times this testing can be achieved with a basic power meter and light source set, also known at an optical loss test set (OLTS).

Tier 1 Testing: Tier 1 testing is primarily done with a power meter and light source and involves using one or two reference cords. You typically would use one reference cord if you are either testing a short patch cord or a spool of cable. This method leaves the last connector under test that plugs into the power meter untested, so you have to flip the ends around and run the test a second time to get a proper reading of both terminating connectors.

Two reference cords are typically used when each end of the cable is in two difference locations. This way the meter and the source each have a cord to plug into the cable under test. With this test method, both terminating connectors are being characterized within one test. In order to perform a test for loss (dB) the user must set up a reference first by using one of these two methods. The procedure is below:

    1) Turn on the power meter and the light source. It’s important that you let the light source run for 1-2 minutes to stabilize so you get more consistent results.
    2) Note that the power meter when turned on will always default to be in dBm units (absolute power). You need this reading before a reference is taken.
    3) Take one or two reference cords for whichever type of fiber you are testing (SM, 62.5um MM, or 50um MM)
    4) Connect the ends of the reference cord(s) to your power meter and light source. If you are using two reference cords, make sure you clean and mate the two ends and mate them together. When done taking the reference you then would disconnect the two cords from each other and introduce the cable to be tested in the middle.
    5) If testing Multimode fiber, make sure to use the appropriate mandrel on your reference cord to strip out the higher modes of light that may exist in the cladding. SM does not need a mandrel.
    6) Now take a reference reading of the absolute power value (dBm) by pressing the REF button or ZERO button on the meter.
    7) Your reference has been stored and the meter now should read 0.00dB. It is now ready to measure loss relative to the cable you will be testing.
   8) If using one reference cord disconnect from the power meter side. Then connect the test cable onto the open end of the reference cord and plug the other end of the test cable into the power meter.
    9) Now, if using two reference cords, disconnect them from the mated connectors in the middle.
    10) Take the power meter with the open connector at the end of your reference cord to plug into terminated cable being tested. Also, take the light source with the open connector at the end of the other reference cable to plug into the opposite end of the terminated test cable.
    11) View dB loss reading…



Tier 2 is a bit more comprehensive. It involves an Optical Time Domain Reflectometer (OTDR) and in addition to measuring attenuation, it can measure back reflection, optical return loss (ORL) and give a pass/fail status for each event, whether reflective or non-reflective on the cable. This is what would need to be done to “certify” a network. The OTDR has the ability to characterize events and faults on the cable with several connections and shorter jumpers connecting patch panels. It will also certify the length of the cable allowing you to formulate a loss budget. Because most OTDRs come available with on board power meters, light sources, ORL readings, and distance readings; it usually makes this piece of test equipment an all in one tester for Tier 1 and 2 certification.



Formulating a loss budget:

Of course, all of this testing is irrelevant if you can’t determine what is considered a “passing” result or not. Whether you are a contractor going in to characterize a newly installed system, or a technician looking to verify or maintain your company’s own network. In order to ensure that the network is running as good as possible, there needs to be some verifiable number(s) to hold your test up against. Never rely on the “well it’s working so it must be good” mentality, do the job right.

To determine what level of attenuation is considered passing one must calculate a “loss budget” and it is dependent on a number of factors: frequency (wavelength) being used, the length of fiber, and the number of interconnects (mated connector pairs) in the cable run.

Each of the typical frequencies used in optical transmission has a unique rate of attenuation over a given distance. We characterize this value as its dB/km attenuation rate. For example, 850nm has a dB/km rate of 3.0, so it is expected, under perfect conditions that 850nm frequency will attenuate 3 dB over 1km.

The other variable to keep in mind is interconnects or mated connector pairs. Anytime you introduce a set of mated connectors into your line this will contribute to a small amount of loss into your run. According to the EIA/TIA standards a maximum of 0.75dB loss is accepted for mated connectors. So for each connector pair you must add 0.75dB to your loss budget for each connection. Typically 0.3dB is acceptable for multimode fusion splices and 0.05dB for singlemode fusion splices. Taking this formula into consideration, see below:

Testing 850nm 1km long cable with 2 connections and 1 fusion splice:
Distance loss= 3.0db
Connections (2) x 0.75dB = 1.5dB
MM Fusion Splice = 0.3dB
Total loss budget = 4.8dB



Connector End Face Inspection:

One other testing standard to be aware of is the International Electrotechnical Commission (IEC) standard for connector end face inspection. Within this standard are four distinct zones that have an allotment or tolerance for each. Zone “A”, which is the zone encompassing the core of the fiber cannot have any debris or defects in it for single mode and very few defects for multimode. Zone B which encompasses the cladding area is allowed a slight amount of debris or defects. Zone C encompasses the adhesive zone (typically epoxy) and Zone D is considered the “contact zone” where the rest of the ferrule will be making physical contact with the other ferrule. This zone has the highest tolerance because it is the farthest away from the core. All of these zones each have a specific tolerance and if any tolerance is exceeded the image will be marked failing. Please see below for more information on the different zones of which there are specific tolerances for multimode and single mode fiber.




There are many digital inspection probes that now come with software that overlays each zone on the image and analyzes it to the IEC standard. This requirement is sometimes needed by customers who request images of each connector end face and whether they passed or failed when measured up to the IEC when installed. It is also very popular in assembly houses where that can be an objective account to verify if a connector was properly polished. If you need any further assistance please feel free to contact FIS.



OTDR Directory

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