By William Graham
For those technicians that think you can grab a pair of meters and patch cords and plug things in and get a valid reading just like that, I humbly suggest their days will be a days of confusion, frustration, bad judgments and poor, invalid readings. Maybe some upset supervisors and unhappy customers.
People sometimes say that preparation is 90% of the job. Nothing could be more true in Fiber Optic Testing.
A clean testing area is the first requirement. A table top, a cardboard box turned upside down or any surface available will do. We don’t usually have ideal conditions but wherever we test the surface must be clean. Sometimes a dollar store table cloth solves a lot of problems.
Clean the surface with alcohol and wipes, preferably lint free wipes.
Blow any lint or dust off with your air bulb. We prefer to use an air bulb because it doesn’t leave a residue and also doesn’t need to be refilled.
Make sure the meters you set on this clean surface are clean. Hopefully they were in a sealed case. Wipe them if in doubt. Be especially sure they are clean and dust free around the connector connection points.
Make sure your reference cords are clean before you set them down. Ideally they will have been in a clean ziplock bag.
You need some type of microscope to view the end of the connector.
Be sure your meters have enough battery life. Anything less than 9 Volts is not sufficient, especially on your light source.
Ensure that your meter were capped. If not, expect that they might be dirty and give erratic readings.
Be sure that both meters will test at the wavelengths (λ) you wish.
Do your meters work? If in doubt, turn both on, set to the same wavelength and connect a clean patchcord between them. Put your power meter on dBm and you will read the power from the light source. How much it puts out will depend on the mode and power of the meter. For example; a 1550 Laser source might put out 95 uW of power. Know your meters and know what to expect.
Test Cords (Reference Cords):
You should have two and ideally three of the reference cords you will need for each type of fiber you are to test.
Reference cords should be 1 – 3 meters long. They should be the same size and type as the fiber we are testing. The closer they are the smaller will be the variable.
Check each reference cord to be sure they are clean.
Remember, if you plug a dirty connector into a clean connector you now have two dirty connectors. If they are dirty, clean them with Alcohol and a dry wipe or any other effective method. Check them again and cap them if they are clean.
Are the dust caps clean. Often they are not. We can clean them with a polyester pipe cleaner. Clean it first with a pipe cleaner dipped in alcohol. Next, dry it out with the other end of the pipe cleaner. We can also use the polyester pipe cleaner to clean out a dirty coupling.
You need couplings to join you cords. They might be SC/SC, ST/ST, LC/LC, LC/SC or many other combinations. Be sure they are clean. If in doubt, clean them with alcohol and blow the dust our with your Air Bulb. For all testing, pick good quality, Single-Mode couplings with Ceramic inserts for greater accuracy. Keeping your test couplings in a zip lock bag is an inexpensive way to keep them clean.
Congratulations, now you are ready to test. Guess what? All that you did to this point was necessary and will save you time when you start testing. It will also help ensure your readings are correct. No guarantees that they will, but you have removed a lot of variables.
Method One – Single Ended Testing:
This test will give us the loss on the cable being tested including the loss of the connector connected to the reference connector. If the fiber cable is a few meters the loss is negligible.
What is needed:
Optical Test set including the appropriate light source and power meter
Reference (Launch) cable of the same type and size fiber as the system we are testing, with connectors compatible to the test meters.
Adaptors to adapt to the connectors on the cable being tested and the power meter.
Ensure all connectors, mating adaptors and work area is clean.
Turn on equipment allowing time to stabilize and reach room temperature.
Attach launch cable to light source and leave it connected for the duration of the test.
Set the Zero dB reference on the power meter. Allow it to stabilize.
Meter should read “0 dB”
Attach the cable being tested between the power meter and the reference (launch) cable.
Reverse the cable and repeat the test.
Add both readings together, divide by two and you have “Average Link Loss”