BIMMF is laser optimized for high-speed networks, used in applications that require a very tight loss budget – this doesn’t leave much room for loss due to installation issues. As a result, questions often arise about bend-insensitive multimode fiber (BIMMF) and how it stands up to testing given the tight bends it can withstand.

Test measurement practices for factory qualification and field verification have changed over the years as active and passive fiber optic cable and components have also changed. Conditions must ensure an accurate, tightly controlled test.

Here, we’re answering a few questions we’ve received as a result of two previous blogs we’ve published on the topic of testing BIMMF: Using Encircled Flux Testing Methods to Measure BIMMF Loss and MMF Cable Loss Measurement Tests: Light Sources & Launch Conditions.

Q: Do non-BIMMF and mandrel wrapping meet encircled flux (EF) compliance?

A: Light sources that produce overfilled uniform mode power distribution can be conditioned to be EF compliant; however, a fixed mandrel (e.g. five wraps on a 22 mm mandrel) with non-BIMMF cannot universally be applied across all legacy sources to achieve EF compliance. To achieve EF compliance with a specific legacy light source, a custom mandrel (mandrel diameter/number or wraps) and EF validation are necessary.

The field test equipment uses a single optical port that launches two wavelengths (850 nm and 1300 nm); a test cord conditioned with a mandrel may not allow alignment on the target for both wavelengths simultaneously.

Q: With the industry moving toward BIMMF in data centers and enterprise networks, can BIMMF and tighter mandrel wrapping meet EF compliance?

A: BIMMF is not recommended to be used within launch cords for several reasons:

BIMMF uses a trench layer with lower refractive index around the fiber core that reflects leaky modes back into the fiber core.

Using BIMMF as launch cord with a standard mandrel can render the result of loss measurement overly pessimistic, especially for short cable assemblies.

Although it’s technically feasible to use tighter mandrel wrapping to meet encircled flux launch (EFL) conditions, this is not good practice in actual field testing.

BIMMF cables are only optimized for 850 nm – not for 1300 nm. Bending characteristics are very different at 850 nm compared to 1300 nm.

When using BIMMF for reference launch cables that need modal conditioning, it’s wise to contact the fiber manufacturer and get its recommendations first.

Q: What are the risks and limitations associated with legacy testers and mandrel wrapping?

A: There are several things to think about here:

If an installer uses a light source that is slightly overfilled relative to the EF template target, loss readings will be slightly pessimistic; you could have high confidence in a “passing” result.

If an installer uses a source that was slightly underfilled relative to the EF template target, then loss readings will be slightly optimistic. There is a risk of link failure even if there’s a “passing” result.

If the fiber cabling link loss budget is tight, then the EF-compliant launch tester becomes more relevant as it reduces loss measurement uncertainty from ±40% to ±10% on a dB basis.


Mandrels for 22 mm non-BIMMF and 4mm BIMMF

Q: What other TIA insertion loss test standards should we know about?

A: TIA has published a few test-procedure documents for different measurement purposes. FOTP-34 and FOTP-171 are for factory qualification and field verification; ANSI/TIA-526-14 and ANSI/TIA-526-7 are mainly for actual field verification.

FOTP-34 (TIA/EIA-455-34) specifies the multimode fiber (MMF) and singlemdoe fiber (SMF) insertion loss test procedure for a complete connection (splice device, connector set, etc.) installed between two like fibers.

FOTP-171 (TIA/EIA-455-171) specifies attenuation test procedures for SMF and MMF cable assemblies. The results of this test method are not directly comparable to those of FOTP-34, which is the insertion loss test of an interconnection device.

ANSI/TIA-526-14-C-2015 provides optical loss measurement of installed MMF cable plant, as an adaption to IEC 61280-4-1 (with additional information found in the IEC document), which includes the EF metric for the launch condition for the 850 nm wavelength on 50 μm MMF.

ANSI/TIA-568.3-D specifies EFL conditions for testing multimode connector performance at 850 nm.

ANSI/TIA-526-7-A-2015 provides optical loss measurement of installed SMF cable plant, as an adaption to IEC 61280-4-2.

As BIMMF continues to be deployed, Belden can help you ensure that the fiber optic cabling and components you select have been tested for perfect performance after installation. Learn more about the experience and expertise we can bring to reduce costs, improve uptime, make the best use of space and ensure security in data centers.

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