Using Encircled Flux Testing Methods to Measure BIMMF Loss

In an earlier blog on this topic, we pointed out that loss measurement of a given cable assembly during a factory test won’t necessarily be the same as the loss measurement of the same cable assembly in actual field use. During field-verification testing, cable assembly loss depends on the mating and orientation of components, as well as the launch conditions. In factory testing, testing only occurs with the components that are part of the cable assembly.

Recently, ratifications were made to FOTP-171B (TIA/EIA-455-171B) for multimode fiber (MMF) cable assembly tests, including factory qualification and field verification testing. Here, we’ll discuss those changes and their impact.

Bend-Insensitive Multimode Fiber Loss Measurement

Encircled flux (EF) testing methods, which define the integral of power output of the fiber over the radius of the fiber, have been added to FOTP-171B (TIA/EIA-455-171B) as Method 4.

As a recommendation from the TIA standards committee, a reference quality launch cord should use non-bend-insensitive multimode fiber.

The industry has already moved toward bend-insensitive multimode fiber (BIMMF) to replace legacy non-BIMMF. Because many field testers were made to test non-BIMMF cable assemblies, we face practical challenges when conducting BIMMF cable assembly loss measurement.

Test Method Ratifications for FOTP-171B

FOTP-171B contains 16 qualification methods for testing connectorized cable assemblies to estimate expected attenuation performance when installed in actual fiber optic links.

Methods A1, B1, C1 and D1 allow mode filter to be used (non-BIMMF with five close-wound turns with minimal tension on a smooth, round mandrel) to create a restricted launch condition that removes transient conditions. These methods are for standardization purposes only, and don’t necessarily create equilibrium mode power distribution, nor guarantee encircled flux compliance.

As a consensus made by industrial experts, test methods A4, B4, C4, and D4 were added to specify launch conditions compliant with encircled flux. In Clause 3.6.1, the reference quality launch cable is specified as: “Multimode fiber reference quality launch cable shall have fiber of the same numerical aperture as core size as that used in the cable assemblies to be tested, and should have non-bend-insensitive multimode fiber.”

Because using BIMMF as a launch cable can potentially cause a high insertion-loss measurement, it’s more desirable to use non-BIMMF as a launch cable. The TIA TR-42.13 working group decided to make “non-BIMMF as a launch cable” as an informative recommendation rather than mandatory for these reasons:

BIMMF has dominated the market; as reference quality launch cords, they can be used with encircled-flux-complaint light sources in accordance with A4, B4, C4 and D4 methods without sharp bending, especially in a lab environment with encircled flux metric validation.

• Regarding A1, B1, C1 and D1, although it’s difficult to make effective mode filter with BIMMF, it’s still technically feasible with a smaller mandrel (e.g. 4 mm) in a lab environment.

Field Verification for BIMMF Cable Assemblies

In the field verification test, encircled-flux-compliant launching conditions shall be met; however, testers that are compliant with encircled flux can be considered costly investments. When encircled flux compliance is not a contractual requirement, mandrel wrapping with non-BIMMF is still often used as an alternative by installers and technicians.

In addition, test reference cords (TRCs) shall be used for certifying cabling systems to ANSI/TIA, ISO/IEC and IEEE standards. They are terminated with reference-grade connectors per ANSI/TIA-526-14-C and IEC 61280-4-2; these standards specify multimode connectors with a mated loss of ≤ 0.10 dB.

Below, different field verification methods are compared. Although not all of these methods comply with standards, they are still being used for different reasons.

As fiber optic cable, and active and passive optical components, continues to improve, we’ll keep you updated. Belden’s multimode fiber solutions optimize fiber for increased bandwidth by using multiple wavelengths in the lower end of the spectrum. Learn more about the experience and expertise we can bring to reduce costs, improve uptime, make the best use of space and ensure security.