Testing Fibre Optics Do’s and Don’ts

Share post

Testing Fibre Optics is necessary to be able to know the quality of a fibre installation and to ensure reliable and on-going functionality of a fibre link.

In other words, testing is the only way to be able to confidently sign it off as ready for use.

When joining two fibres connectors together, the most important requirement is to ensure light passes from one fibre to the other fibre.

However, this light signal needs to be transmitted without excessive loss or back reflections.



The biggest challenge is trying to keep the connector end faces and adaptors clean. Any particle added into either of these components can cause significant issues. Such as insertion loss, back reflection, and sometimes even equipment damage.

To verify there is no dirt on the end face you need to inspect it using a fibre optic microscope.

So Do always use a microscope. This will show you any dust, dirt or damage you might be dealing with. Without a visual inspection, you are running blind and any amount of issues may arise. So Don’t do this.


blue tool


Another indispensable item is a cleaning pen or cartridge.  These items can clean the end face of a connector and the inside of an adaptor in no time.


Testing Fibre Optics – IEC61300-3-35

The IEC 61300-3-35 grades fibre cleanliness based on the quality and size of scratches and defects in each region of the end face.

This standard categorizes them into two groups – scratches and defects.

Certification to determine pass or fail is based on the number of scratches and defects found in each measurement region of the fibre end face.

Areas included are the core, cladding, adhesive zone and contact zone.

Having a software-based visual identifier can provide Pass/Fail information for the connector under inspection.

Helping to remove the human subjectivity this helps provide a more accurate and consistent test result.



When testing fibre optic installations a laser source can be used to see if the light is coming out at the other end.

This type of fibre test can only let you know if there is a break or a macro bend in the fibre run.

A Visible laser can also be used to see which fibre optics cabling is connected to a specific patch panel location.



Testing Fibre Optics – Light Source and Power Meter

  1. Individually tested cores (No Loop-Back)
  2. Unless otherwise specified- permanent link testing will be carried out.
  3. Test two wavelengths at a minimum.
  4. Do not average Optical loss test set (OLTS) results.
  5. Does not support two patch cord test method.

When testing, one or three patch cord referencing/zeroing method should be used .

The three patch cord method is the preferred standard.



Some things to consider when using reference cables in optic testers.

Are my reference cables good quality, low loss leads?

You can verify this by zeroing the meter, then reversing the reference cables.

Then check that the result obtained is consistent.

If your reference cables are not good enough your starting point is already compromised and results will be based on this.

· match the reference cables to the optical network.

The “3 reference cable” method is helpful here if the centre lead is the same as the system being tested.


· Check for Mismatched fibres in the installed system.

You might not be able to tell with the one lead method, but you will with the 3 reference cable method


Testing Fibre Optics – Length

The following can determine the length:

1. Sheath markings

 In one-meter intervals, length markings are printed on the cable sheath.


Testing Fibre Optics  – Continuity and Maintenance of Polarity

testing with a light source and power meter can be establish this


 • An OTDR with a tail cable can establish this.

To test fibre optical loss in a different way, an Optical Time Domain Reflectometer (OTDR) is used .

It emits a high-intensity laser light at a pre-defined pulse rate through a cable at one end of the fibre optic cabling.

The OTDR relies on the backscatter of light returning to the source to analyse the trace.

To see losses along the run and pinpoint the loss locations, the one-ended fibre test method should be used.


To sum things up, just because light is making it to the end of the fibre does not necessarily mean that the fibre is good.

Even a partially shattered fibre will pass light. It can be difficult to tell a bad fibre from a good one without the proper tools.

Checking for the appearance of light and measuring the actual amount of light or loss that is present using calibrated optic test equipment, is the only way to tell that the fibre is useable.

So to recap:-

– Use a Microscope to inspect the connector end face.

– A Visual Fault Locator to test the fibre for the appearance of light.

– And last but not least use a Light Source and Power meter and or an OTDR to test fiber for loss measurement.