Tag Archives: fiber testing tools

MPO Cables Testing Method

As the migration to 40G/100G Ethernet using parallel array transmission systems, the high-density MPO cables are widely used in the data center. In the connection, contamination even as small as 0.001 mm can cause the optical loss. Mating a contaminated connector to a clean connector will lead to poor performance and can damage the connection. So it’s important to test the link segments consisting of MPO array cabling and keep the cable clean. However, MPO cable testing and cleaning is full of challenges.

Why MPO cables testing and cleaning is not easy? First, MPO connectors are very sensitive to dirt and contamination. The ferrules are large and hard to clean and inspect. Most microscopes don’t have adapters for MPO connectors. Those microscopes with adapters for MPO connectors can only see a small section of the ferrule because they are adapted from single fiber microscopes. So you have to inspect the entire ferrule and every fiber. Second, cleaning is also a problem because of the designs of MPO connectors with pins and holes. Most dry cleaners can only clean the place between the pins. So dirt may increase around the pins or in the holes, which cause alignment problems. So remember often keep MPO connectors well covered and protected when not in use.

Calculate the Acceptable Attenuation

Calculate the acceptable total loss of the entire optical link so that you can find if the test result is good or bad. Do as the following steps with your link loss calculator:

  • 1. Select the fiber type and test wavelength combination;
  • 2. Select the unit of length in feet or meters;
  • 3. Enter the total link length under test;
  • 4. Enter the number of connections of each type (a pair of connectors counts as one connection);
  • 5. Enter the number of splices (each connection counts as a connection plus a splice).
MPO Trunk Cable Testing Procedures

In this case, MPO connectors can be directly connect to the test equipment. That requires 12 output sources and either 12 input ports or an MPO port with a detector which can accept the light from 12 or 24 fibers. But at present this is not available. In the laboratory or factory settings, there are test equipment that can achieve this. Then, engineers use an MPO to LC fan-out cord to separate the trunk into single fiber channels for testing.

There are five basic steps for an MPO trunk cable testing (see figure 1):

MPO testing

Figure 1. set reference (three pictures above) and test MPO cable (two pictures below)

  • 1. Find a test equipment where the input port can be changed to an LC connector or has an LC already.
  • 2. Set a reference and there are three methods. Insert the known good cords into relative input ports and run an autotest (the above three pictures). If the loss is fewer than 0.1 dB (usually the maximum loss of LC connection is 0.2 dB), then the reference cords are good. This is critical to the test.
Reference Method Reference Cable Connectors Included in Measurement Estimated Reduction in Measured Loss Estimated Increase in Errors
1-Cable Method(test equipment compatible with connectors being tested) 1, launch 0 0 dB 0 dB
2-Cable Method(single fiber ferrule connectors, test equipment not compatible with connectors being tested) 2, launch and receive 1 0.2-1 dB +/-0.2 dB
3-Cable Method(male/female or plug and jack connectors, test equipment compatible with connectors being tested) 3, launch, receive and “golden cable” 2 0.3-1.5 dB +/-0.25 dB

 

  • 3. After setting the reference, remove the middle test reference cords and connect fan-out cables with an MPO trunk cable.
  • 4. Measure and record the loss every pair of LC connectors at the left side.
  • 5. Measure and record the loss every pair of LC connectors at the right side.
Summary

Compare the test result with calculated acceptable attenuation, If the test result is not ideal,that may be caused by the contamination, defects in the cable plant, or improper test equipment usage. Then you should better check connector end-faces for dirt and defects, and check link segment for broken fiber, poor splices and tight bends. MPO connectors are very likely to be contaminated because of fibers, number of connections and tight loss budgets. To keep MPO cabling system perform well, frequent cleaning and inspection with one-push cleaner are required.

How Much Do You Know Fiber Optic Testing?

For every fiber optic cable plant, you need to test for continuity and polarity, end-to-end insertion loss, etc. If there were a problem, it must be fixed to keep the fiber optic cable plant working properly and ensure the communications equipment operate well.

fiber-optic-testing

Testing Tools

Fiber optic cable testing needs special tools and instruments. And they must be appropriate for the components or cable plants being tested. The following five kinds of fiber testing tools are needed for the testing work.

OLTS—Optical loss test set (OLTS) with optical ratings matching the specifications of the installed system (fiber type and transmitter wavelength and type) and proper connector adapters. Power meter and source are also needed for testing transmitter and receiver power for the system testing.
Reference test cable—This cable should be with proper sized fiber and connectors and compatible mating adapters of known good quality. And the connector loss is less than 0.5 dB.
VFL—Visual fiber tracer or visual fault locator (VFL)
Microscope—Connector inspection microscope with magnification of 100-200X, video microscopes recommended.
Cleaning Materials—Cleaning materials intended specifically for the cleaning of fiber optic connectors, such as dry cleaning kits or lint free cleaning wipes and pure alcohol.

Notes Before Testing
Cleaning Issue

Before testing, it’s very important to keep connector clean so that there is no dirt present on the end face of the connector ferrule as the dirt will cause high loss and reflectance. For example, the dust caps which is used to keep connectors clean usually contain dust. So it may leave residue or cause harm to the connectors to use cleaning tools with dirt.

Eye Protection

Connector inspection microscopes focus all the light into the eye and can increase the danger. Some DWDM and CATV systems have very high power and they could be harmful. Though fiber optic testing sources are too low in power to cause eye damage, it’s still suggested to check connectors with a power meter before looking it. As most fiber optic sources are at infrared wavelengths that are invisible to the eye, making them more dangerous. So better protect your eyes from these potential harms.

Loss Budget

Before testing, you should clearly know the loss budget as reference loss values for the cable plant to be tested. Here are some guidelines:

    • For connectors, 0.3-0.5 dB loss; for adhesive/polish connectors, 0.75 dB loss; for prepolished/splice connectors (0.75 max from TIA-568)
    • For single-mode fiber, 0.5 dB/km for 1300 nm, 0.4 dB/km for 1550 nm. It means a loss of 0.1 dB per 600 feet for 1300 nm, 0.1 dB per 750 feet for 1550 nm.
    • For each splice, 0.2 dB
    • For multimode fiber, the loss is about 3 dB/km for 850 nm, 1 dB/km for 1300 nm. It means a loss of 0.1 dB per 100 feet for 850 nm, 0.1 dB per 300 feet for 1300 nm.

So for the loss of a cable plant will calculated as (0.5 dB X # connectors) + (0.2 dB x # splices) + fiber loss on the total length of cable.

Fiber Optic Loss Testing

Before installation, it’s necessary to inspect all cables as received on the reel for continuity using a visual tracer or fault locator. An OTDR is needed to test if cables are damaged during the shipment. Any cable showing damage should not be installed.

After installation, all cables should be tested for insertion loss using a meter of OLTS according to standards OFSTP-14 for multimode fiber and OFSTP-7 for single-mode fiber. Usually cables are tested individually (connector to connector for each terminated section of cable and then a complete concatenated cable plant is tested “end-to-end”, excluding the patch cords that will be used to connect the communications equipment which are tested separately. Insertion loss testing should be done at the wavelengths of 850/1300 nm with LEDs for multimode fiber, 1310/1550 nm with lasers for single-mode fiber, 1490 for FTTH. Keep the data on insertion loss for future comparisons if problems arise or restoration becomes necessary. Long cables with splices may be tested with an OTDR to confirm splice quality and detect any problems caused during installation, but insertion loss testing with an OLTS (light source and power meter) is still required to confirm end-to-end loss.

Testing Results and Methods

If the cable plant loss is tested within the loss budget, the communication link should work properly.

If the loss is higher than the loss budget, first you need to test in the opposite direction using the single-ended method. Since this method can only test the connector on one end, you can isolate a bad connector. If the tested losses are the same on both directions, you need to test each segment separately to isolate the bad segment or use an OTDR if it is long enough.

If there is no light through the cable and only darkness when tested with your visual tracer, there must be very high loss. Then you need to cut the connector on one end (maybe the wrong one) by your decision.

Originally published at http://www.articlesfactory.com/articles/communication/how-much-do-you-know-fiber-optic-testing.html