Fiber Optic Cable Testing

Very similar in that special equipment is required.

Fiber Optic test equipment is available from many manufacturers, and we have some smarter than I here on what you are actually going to want to test for.

Meantime, go see what is available for equipment here.

Get those who installed it to test it.

fibre optic cables ,the core conducting media is glass with very high purity,there are two broad catagories...single mode and multimode.

you have to measure parameters usin a OTDR(optical time domain reflectometer)

the main criteria is attenuation and cromatic dispersion.

unlike metal conductors where megger etc are used to evalute insulation values as conductors are prone to shortings etc

regs

Murali Nambiar

The OTDR is probably an overkill and quite expensive bit of equipment. The OTDR will basically pinpoint the exact (or very close) location of the problem within the fiber. In other words, if you have many kilometers and you don't know where the actual fault is, the OTDR will be useful.

What you need is an optical power meter. They are fairly cheap (check Ebay)...and make sure that it has 850nm (multimode) and 1310nm, 1550nm (singlemode) wavelengths with the appropriate terminator connectors (ST, SC, FC, etc). Before you can make any useful readings/results you will need to know the operating wavelength of the transmitter side and adjust the optical power meter to that wavelength.

If you don't know the optical power the transmitter is outputing, you can use the optical power meter to measure the output - you actually should do this first even if you do know the rated optical power output as it is known that the laser/diode can fail and/or degrade over time. You may need a short fiber optic patchcord to do this (500mm - 2m).

Now that you have an ouput reading (measured in dBm), you take the optical power meter to the other end (at the receiver) and measure the optical power. You subtract the two figures and you will get an optical link loss.

Using multimode fiber at 850nm expect a loss of about 3.5dB/km

Using singlemode fiber at 1310nm expect a loss of about 0.4dB/km

Using singlemode fiber at 1550nm expect a loss of about 0.2dB/km

If your loss is more than the known length...there is a problem with the fiber. Believe it or not, most issues in loss is dirty fiber connectors. Using lint-free cloth and isopropyl alchohol usually will yeild the expected results.

Example. 1310nm single mode fiber. Optical output -10dBm. At receiver, -22dBm. A loss of 12dB is calculated (-22--10=-12). Length of fiber should be (calculated using the above figures) 30km. If you know that the distance is not about 30km then there is a problem. If you want to locate the problem, you will need an OTDR as you don't want to uproot 30km of fiber!

Hope it helps

Excellent answer.

Also when calculating loss, consideration must be taken in for splices, bends, terminations and environment. These incidences can be identified and measured with a good OTDR.

DWDM (Dense wavelength-division Multiplexing a.k.a. broadcasting multiple wavelengths/channels/frequencies at same on same fiber.) presents other problems. Tolerances are much tighter as more channels are broadcast on the same fiber. The optical meter must be able to receive and measure each frequency/channel that is being broadcast (as referenced above). The reason is that each frequency power will vary accordingly to their respective wavelength/frequency.Ex. You may have 8 channels on the fiber and only one may be out of specification.

Optical power is usually very stable. The best advice I can give when troubleshooting or checking power levels is clean,clean, and clean the fiber anytime you handle it. If you remove it to take a reading, take the reading to establish the working level, then clean it and take reading again and clean it before restoring the circuit. This little basic procedure can save hours of needless troubling shooting if fiber is dirty.

Thanks.

People usually accociate 'dirty' with something they can see...not so with fiber. Many times we got call outs to say the 'equipment does not work' when all the issue was was dirty fiber. I agree...clean, clean and then clean.

Jose John,

Before you take things for granted you should read your project specification where the requirements will be clearly mentioned and finding a skilled person or contractor who assures you by an agreement that he can test and cetify the cables as required by your project specification will be the ideal move. Some times even you need to obtain an approval from client for the type test equipment, approval of the company involved in testing and the accessoires used for installation.

If it is just for some temporary installation where you dont need any prior approval all you need is an equipped skilled guy and the accessories.

Good day!