Testing Fibre Optics Do’s and Don’ts.
To be able to know the quality of a fibre installation and to ensure reliable and on-going functionality of a fibre link some fibre testing will need to be done.
This 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.
This transmission of light needs to be without excessive loss or back reflections.
The biggest challenge here is trying to keep the connector end faces and adaptors clean.
Any particle added into either of these components can cause significant insertion loss, back reflection, and even equipment damage.
The only way to know if you have a dirty end face is to inspect it using a microscope.
So Do always use a microscope as 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.
Another indispensable item is a cleaning pen which can clean the end face of a connector and the inside of an adaptor in no time.
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.
This includes the core, cladding, adhesive zone and contact zone, as well as the quantity and size of the scratches and defects.
Having a software-based visual identifier can provide Pass/Fail information for the connector under inspection.
This removes the human subjectivity, providing more accurate and consistent results.
When testing a fibre network link 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 optic cable is connected to a specific patch panel location.
Visible Fault Identifier
This, however, does not actually test that the fibre link is in working condition it just lets you know there is some actual light being emitted.
We still need a measurement to tell us the actual amount of light or loss that is present.
AS/NZS3080:2003 and AZS/NZS/ISO/IEC 14763-3 2007
In AS/NZS3080:2003 and AZS/NZS/ISO/IEC 14763-3 2007 some of the requirements for testing are:
3.Continuity and maintenance of polarity
Since procedures are subject to occasional updates we recommended that where conformance is required the user contact the relevant standards body. For Australia www.standards.org.au
As the light travels down the fibre, power levels decrease.
This decrease in power level is called optical loss and is expressed in Decibels (dB).
The recommended way to measure optical loss in a fibre is to inject a known level of light into one end and measure the level of light transmitted out of the other.
This measurement is performed using a light source and a power meter.
Testing Requirements for light source and power meter are:-
• All cores are to be individually tested (No Loop-Back)
• Unless otherwise specified- permanent link testing will be carried out
• Testing shall be carried out in both directions and a minimum of two wavelengths- one of which should be the operational wavelength.
• Light Source and Power Meter tests results cannot be averaged.
• The two patch cord test method is not supported.
Testing should be carried out using a one or three patch cord referencing/zeroing method.
The three patch cord method is the preferred standard.
Some things to consider when using reference cables.
·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.
·Do my reference cables match the system being tested?
This is where the “3 reference cable” method is helpful, as long as the centre lead is the same as the system being tested.
·Are there any 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
Length can be determined by any of the following:
Continuity and Maintenance of Polarity
• Deemed to be established if testing carried out with a light source and power meter
• Can be established by the use of an OTDR with a tail cable.
An Optical Time Domain Reflectometer (OTDR) is used to test fibre optical loss in a different way.
It emits a high-intensity laser light at a pre-defined pulse rate through a cable at one end of the fibre optic cable run.
The OTDR relies on the backscatter of light returning to the source to analyse the trace.
This one-ended fibre test method can be used to see any losses along the run and can also pinpoint these loss locations.
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 and 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 is the only way to tell that the fibre is useable.
So to recap:-
Use a Microscope to inspect the fibre end face.
A Visual Identifier 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 measure your losses.
Doing none of the above will leave you hoping for the best, doing all of the above will leave you knowing you have done well!