TESTING MULTIMODE FIBER Fiber Optics TESTING MULTIMODE FIBER

Fiber Optics One of the first things a technician should do is test the fiber reel for light continuity before installing the fiber optic cable. This is done by simply shining a flash light thru all the strands to make sure there are no dark fibers. THIS END IS MADE ACCESSABLE BY THE MANUFACTURER FOR THE PURPOSE OF TESTING THE REEL

Fiber Optics We can also use a flashlight to trace an unidentified fiber strand. There is a device called a visual fault locator (VFL) that is a laser (like a laser pointer) that accomplishes the same task.

Fiber Optics A visual fault locator (VFL) is also used to find which end of a terminated fiber strand is bad. The laser light will bleed out of a bad connector or broken strand.

Fiber Optics After the terminations are done the next step is to test your fiber terminations for ATTENUATION. WHAT IS ATTENUATION AS IT APPLIES TO FIBER? HOPEFULLY YOUR CONNECTORS LOOK THIS GOOD.

Fiber Optics Attenuation is a measure of signal loss. When we use it in conjunction with fiber we are measuring optical signal loss. This loss is measured in dBm, decibels in reference to mill watts.

Fiber Optics Acceptable loss limits for any field terminated connector is; 0.75dBm for a multimode connector measured at 850nm. 0.5dBm for a multimode connector measured at 1300nm.

Fiber Optics In the field it is industry practice and is expected that all field terminated fiber connectors will measure; < 0.75dBm and < 0.5dBm respectively. This insures that the total link loss budget for the network will be less than the budgeted loss.

Fiber Optics Once the fiber is placed and terminated it is time to test for optical signal loss. For multimode backbones this is done using a power source and light meter. MM POWER SOURCE AND LIGHT METER

Fiber Optics When testing using a source and meter it is important to use new factory made test cords to ensure accurate readings. You will need a launch and receive cable to test a fiber cable plant. These test jumpers are simplex fiber cords (only 1 strand).

Fiber Optics THE METER WILL REQUIRE AN ADAPTER FOR EITHER ST, LC OR SC CONNECTORS.

Fiber Optics Looking at the controls of the light meter; λ = CHOOSE THE WAVELENGTH d/B or d/Bm BACK LIGHT ZERO REFERENCE ON/OFF

Fiber Optics Looking at the controls of the power source; SELECTOR SWITCH FOR EITHER 850NM OR 1300NM. MOVE SWITCH IN DIRECTION OF DESIRED SETTING

Fiber Optics

Fiber Optics Setting up for the test, you will need known good fiber jumpers and alcohol to wipe the connectors off before testing.

Fiber Optics Insertion Loss Per TIA OFSTP-14 (Multimode) and OFSTP-7 (Singlemode) (and similar international standards Here the launch reference cable is attached to the source, the receive reference cable to the meter, then the two cables are mated to set the reference.

Fiber Optics Insertion loss testing with a test source and power meter simulates the way the cable plant will be used with an actual link. The test source mimics the transmitter, the power meter and the receiver. But insertion loss testing requires reference cables attached to the source and meter to connect to the cable under test.

Fiber Optics This insertion loss test can use 1, 2 or 3 reference cables to set the “zero dB loss” reference for testing. Each way of setting the reference gives a different loss. Generally network standards prefer the 1 reference cable loss method, but it requires that the test equipment uses the same fiber optic connector types as the cables under test.

Fiber Optics If the cable has different connectors than the test equipment (e.g. LCs on the cable and SCs on the tester), it may be necessary to use a 2 cable reference, which will give a lower loss since connector loss is included in the reference and will be subtracted from the total loss measurement.

Fiber Optics Once the reference is set, the launch reference cable should not be removed from the source, as it may have a different coupled power when reattached. WE CAN ADJUST THE REFERENCE LOSS TO THE NEXT WHOLE NUMBER, IN THIS CASE WE WOULD TURN THE CALIBRATION SCREW UNTIL THE READING EQUALLS 13dBm. WE THEN SUBTRACT 13dBm FROM OUR READING ON A FIBER LINK

Fiber Optics If we reference our meter to 13dBm and we get a reading of 14.2dBm when testing a link we would then subtract 13 from 14.2 and the link loss would equal 1.2dBm. ANYTHING BELOW A 2dB LOSS IS CONSIDERED GOOD.

Fiber Optics Some meters will allow you to automatically set the reference to zero by pressing the reference button. The meter will calculate the loss taking into account the launch and receive cable, no need to do the math. When testing multimode we test at both the 850nm and 1300nm wavelengths.

Fiber Optics Most job specifications require that the fiber link needs to be tested in both directions. This means that we would have to take the power source to the other end and perform the test again using both wavelengths. The good news is most tester can now do this without have to move to the other end, in essence todays testers are both a source and meter in both units.

Fiber Optics The key to successful testing is to always clean the connector ends before inserting them into the testers. Never blow on the ends of the connectors. Keep the launch and receive cable as straight as possible while testing. Make sure the connectors are firmly seated in the test sockets.

Fiber Optics If we have connectors with high dB losses then we will have to determine which end is bad, neither the source or meter will determine this. So how do we determine which end is bad? FLIP A COIN OR USE A VFL

Fiber Optics By using a visual fault locator we can determine which connector is bad. The laser light will cause a lot of light to escape from the back of the connector which should be obvious. You can compare the amount of light bleeding from one connector to the connector on the other end, the one that allows the most laser light out is the bad connector.