Fiber optic cables, like any other cable type, require testing to ensure that they meet the applicable performance level for the application they will be used in. These tests are generally easy to perform, however some tests do require specialized test equipment.


Types Of Tests

There are three basic tests which are commonly employed to test fiber optic cable. These tests range from very basic to very advanced. In order of simplest to most complex, the common tests performed on fiber optic cabling are:

"Flashlight" Test
Attenuation (Loss) Test
Optical Time Domain Reflectometer Test


" Flashlight" Test

This test is the most basic test possible for a fiber optic cable assembly. It simply checks to see if the cable will carry any light from one end to the other. It does not tell how much light is lost in the cable, nor will it detect where in the cable any fault is located. To perform the test, simply disconnect both ends of the cable from any equipment and shine the beam of a flashlight or other visible light source in to one end. Next, go to the other end of the cable and look in to the connector. If the cable is able to carry light, you will see a small point of light in the center of the connector. If the center of the connector is dark, then there is a break somewhere in the cable.

This test may seem too simple, however it will detect around 90% of the type of faults which cause fiber optic runs to not work. Also, because it does not require any special equipment to perform, it makes a quick test possible in situations where more advanced testing would be difficult or impossible to perform.

WARNING!

To avoid possible eye damage, NEVER look in to the end of any fiber optic cable connected to any type of equipment!


Attenuation Test

Attenuation testing is the next step up in sophistication from a "Flashlight" test. This test tells exactly how much light is lost within a fiber optic cable assembly. It requires the use of a Light Source and Optical Power Meter and a reference cable.

An attenuation test basically consists of transmitting light at a known intensity into one end of a cable, and measuring exactly how much comes out the other end. The difference is the loss, or attenuation, of the cable.

The exact testing procedure will vary somewhat depending on the specific make and model of test equipment is being used. However, the basic steps for an attenuation test are:

1. Obtain an Optical Power Meter and Light Source. These are included in our Fiber Optic Test Kit and Deluxe Fiber Optic Test Kit.

2. Using alcohol and a clean lint-free cloth, clean the end of each connector.

3. Using a pair short cables with a known amount of attenuation ("Reference Cables") coupled together with a Fiber Optic Coupler, connect the Optical Power Meter to the Light Source (see Figure One (A), below).

4. Read the power level indicated on the Optical Power Meter. This number is our baseline transmit level.

NOTE: Some Optical Power Meters have an "Auto Zero" feature which calibrates the Optical Power Meter to the Light Source - Reference Cable combination. This feature sets a reading of zero ("0") to the combination of reference cable and light source transmit level. The result is that the display of the Optical Power Meter will show the actual loss of the cable being tested without having to manually compensate for the transmit level and reference cable losses. This results in faster testing with less chance of error, and should be used whenever it is available. See Figure Two below.

5. Disconnect the coupler and insert the cable you want to test between the two Reference Cables. Read the display on the Optical Power Meter. See Figure One (B) below.

6. Subtract transmit level determined in Step Two from the reading on the Optical Power Meter. This number is the attenuation of the cable. If the test is performed using a Optical Power Meter calibrated with an "Auto Zero" feature disregard this step. In such a case, the display of the Optical Power Meter will be the loss of the cable. This is shown in Figure Two (B) below.

An example of the math used is shown below:

Result of Step Two = -0.6 dBm
Result of Step Six = -2.9 dBm

Subtract Step Six from Step Two: -2.9 - (-0.6) = -2.9 + 0.6 = -2.3 dB

This cable has a loss of 2.3 dB.

Figure One
Attenuation Test Without Auto-Zero

Figure Two
Attenuation Test With Auto-Zero

The results of an Attenuation Test can be useful in troubleshooting a pair of fiber optic devices which will not communicate, but the fiber passes a "Flashlight" Test. Each fiber optic device will usually contain in its instruction manual the transmit level and the minimum recieve level it can accept. If we take the transmit level of the device at one end of the cable and subtract from it the loss introduced by the cable run, the result is the signal level being presented to the reciever. This level must be greater than or equal to the reciever sensitivity of the unit being connected. If it happens to be less, then the equipment will not communicate.

For example, assume that we have a pair of fiber optic modems separated by a fiber optic cable run. The manual of the modems states the units have a transmit level of -15 dBm and a reciever sensitivity of -28 dBm. An attenuation test of the cable run between the units reveals that the cable has 16 db of attenuation. Quickly doing our math reveals that:

-15 - 16 = -31 dBm at reciever

Reciever requires -28 dBm or more power. -31 dBm is less than -28 dBm, so this setup is not likely to work properly.


Optical Time Domain Reflectometer (OTDR) Testing

The most sophisticated test equipment in general use for fiber optic cable is a device called an Optical Time Domain Reflectometer, or OTDR for short. Although the name may seem intimidating, an OTDR actually uses a simple concept for testing. A "reflectometer" is simply a device which sends a short pulse (usually 1 nanosecond long) of energy in to a cable and measures how much of that energy is reflected back to it. A "Time Domain" Reflectometer is a reflectometer which displays the result of its tests relative to the amount of time elapsed between when it sends the pulse and when it recieves any reflections. Finally, an "Optical" Time Domain Reflectometer is a TDR which works with fiber optic cable.

OTDR Screen

Figure Three
Sample OTDR Display

Figure Three above shows a sample trace from the screen of an OTDR. This display shows the relative reflected light recieved by the unit over time. Each vertical grid line represents a 0.1 dB difference in power, and each horizontal grid line represents 1 nanosecond of time. Note that the screen has been annotated with the letters A, B, and C. These letters are not part of the display of the OTDR, rather they are simply markers added to make explaining the trace easier.

At A, we have just sent a pulse of light in to the cable and are recieving some light back through the glass. Between A and B we see only the normal backscatter of the light as it travels through the glass. This shows as a smooth, gradually decreasing curve. At point B we see a "spike" in the trace. This spike is caused by a sudden increase in the amount of light recieved and is usually caused by a break, microbend, or splice in the cable, and if the result of an attenuation test shows a high loss in the cable, that is the point where it is most likely occuring. Finally, at point C we reach the end of the cable and see another spike in the trace caused by light reflecting off the mirror-like polished end of the fiber.

The information shown in the above OTDR trace can be invaluable in troubleshooting a long fiber optic cable run because it not only shows how much of a fault is in the cable, but also where it is. In the example above, we have a rather major break at a point 12 nS away from the source. Since the propagation speed of light in glass is about 0.3 nS per meter, we know that the fault is at a point roughly 3.6 meters from the end we tested it at. We also know the approximate length of the cable run by the location of point C is about 4.4 meters. Therefore, it would be a good idea to go 3.6 meters away from the OTDR (or 0.82 meters from the other end of the cable) and begin physically examining the cable for splices, breaks, crushes, kinks, or other damage.

The above is a very basic description of OTDR usage. It is beyond the scope of this document to go in to exact detail as to how to operate an OTDR, as each model is different. Therefore, for specific information on a particular OTDR, it is best to consult the manufacturer or instruction manual of the unit in question.


Summary

Fiber optic cables require testing to ensure that they work properly. These tests can be as simple as shining a light in one end and looking for it at the other end to scanning the cable with an OTDR. Simple tests only give very basic information, and more advanced tests give more information, but the cost and difficulty of use of the equipment increases rapidly with the sophistication of the test. An OTDR is required to find the location of faults within a cable, and a light source/optical power meter combination are required to perform Attenuation Tests.