Insertion Loss - Lab

2 - Review: Insertion Loss Testing - Singlemode

Testing Attenuation In A Fiber Optic Communications Link

fiber optic link attenuation
In a fiber optic link, light from a transmitter is coupled into a fiber in the cable plant and transmitted to a receiver on the other end of the link. As the light travels down the fiber, its optical power is attenuated by the losses in the fiber (scattering and absorption) and losses at connectors or splices.

To test the loss of a signal in a fiber optic link in a way that mimics the way the link transmits data, we use an insertion loss test. We use a test source that is similar to the source in the transmitter and a calibrated power meter to simulate the receiver. Launch and receive reference cables are used to connect our source and meter to the cable plant - think of them as the patchcords you use to connect equipment.

Here is a diagram of how we make an insertion loss test. See how it simulates the actual fiber optic data link?

test insertion loss

We need these test instruments and components
  • Test Light Source: portable, stable source using a LED for multimode or laser for singlemode fiber at the proper wavelengths.
  • Optical Power Meter: A fiber optic light meter calibrated in the proper wavelengths for multimode and singlemode fiber.
  • Connector Inspection Microscope: To examine the condition of the connectors being used in the test - both on reference cables and the cable plant being tested.
  • Reference Cables: Matching the fiber type (multimode or singlemode) with connectors compatible with the test instruments and the connectors on the cable plant to be tested.
  • Connector Mating Adapters: To allow connecting reference cables to each other and the cable plant being tested.
  • Connector Cleaner Kit: Wet/dry cleaner for connectors.

The loss is measured in dB - a relative measurement on the meter. Remember dBm is absolute optical power used for measuring the output of a transmitter source or input of a receiver, while dB is a difference between two measurements in dBm. So if the output of the
transmitter source is SdBm and the input at the receiver is RdBm, the difference between the two: SdBm-RdBm = LossdB or the loss in the link.

Since loss is a relative measurement, we need a reference point for the power of the test source to set a "0 dB" reference. For most tests, we use the output of the launch cable attached to the source. We simply:
  • Clean both connectors on one of the reference cables
  • Connect the reference cable to the source to be the launch cable
  • Connect the reference cable to the power meter
  • Turn on the source and let it stabilize for a minute. You can watch the power meter display to see if the source output varies as it warms up.

  Insertion loss

With most power meters, when measuring in the dB measurement range, we have a "0dB" set button. Push that button and we can set the meter to read "0 dB" at any power level, making it convenient because it will read loss directly - it's ready to measure loss.

Once we set the 0dB reference with the launch cable, we can test our reference cables. If they aren't good, all our tests will be bad.
Disconnect the launch cable from the meter - don't touch it at the source once you set the 0dB reference level as disconnecting/reconnecting it may change the power level.
  • Attach the receive cable to the launch cable with a mating adapter.
  • Attache the other end of the receive cable to the power meter.
  • The loss reading on the meter is the connection between the launch and receive cable. The fibers are too short to make a difference.

test ref cables
Here we are testing the reference cables and the connection loss is 0.5dB - that's not great but acceptable. Less loss here indicates the connectors are better and will give better test data.

Don't touch those meter settings! Some people are tempted to reset the 0dB reference at this step, but that would lose our 0dB reference which is the output of the launch cable. )We'll talk about other ways to set the 0dB reference later.) This step just tests the reference cables.

Now we're ready to test our cable we want to test. Here are the steps:
  • Disconnect the connection between the launch and receive cable. You can leave the mating adapter on one of the cables, as you will need it plus another.
  • Connect the cable to test - one end to the launch cable and the other end to the receive cable, using two mating adapters.
  • Now the meter reading will be loss of the cable under test - its connection to the launch cable, all the loss in the cable itself - fiber, splices and connectors - and the connection to the receive cable.

See if you can follow those steps in this animation - you will need to watch it several times to get all the steps of the sequence:

insertion loss animation


To test insertion loss, we need;
  • Source similar to the source used in a data link: 850nm LED for multimode, 1310 & 1550nm laser for singlemode
  • Power meter set to the same wavelength as the source
  • Launch and receive cables to match the cable plant to be tested, plus mating adapters.

And the procedure is:
  • Set a "0dB" reference level on the output of the launch cable
  • Check the connectors on the reference cables
  • Connect the cable to test between the launch and receive cable and measure the loss.

Got that? OK let's start testing.



Table of Contents: The FOA Reference Guide To Fiber Optics

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