HOML as a Source Qualification Test
TIA adopted IEC 61280-4-1, the standard defining attenuation measurements of installed multimode fiber cabling, as ANSI/TIA 526-14-C.
In IEC 61280-4-1, the encircled flux launch is defined as normative (required) for these cases and there is no intention to change requirements in the next edition:
Case 1 - 50 μm core fiber at 850 nm,
Case 2 - 50 μm core fiber at 1300 nm,
Case 3 - 62.5 μm core fiber at 850 nm,
Case 4 - 62.5 μm core fiber at 1300 nm.
For the TIA adaption, only case 1 is required (50 μm core fiber at 850 nm). For the other three cases the encircled flux launch is recommended for applications with loss budgets less than 3 dB. When the launch condition is not known to be encircled flux compliant, the source must be qualified using a High Order Mode Loss (HOML) test. As noted in the TIA standard, the HOML test is not a substitute for encircled flux compliant sources.
The HOML qualification test may have led to confusion about what an acceptable light source might look like and how HOML should be used. In practice, HOML is intended to disqualify sources that are considered too “under-filled” which would result in overly optimistic test results. An example of a source that is under-filled is a VCSEL.
Prior to the wide-spread use and implementation of the encircled flux launch for light sources, mandrels were routinely used during testing. A typical prescription required 5 turns around a mandrel having a specified outside diameter. The HOML qualification test found in ANSI/TIA 526-14-C uses similar mandrel prescriptions but accounts for variations in test cords (e.g., 900 µm vs. 3 mm cable).
The HOML qualification test for a source and launch cord combination requires a four step process. First, the optical power is measured at the output of the test cord with no mandrel applied. Second, a mandrel is applied to the test cord. Third, the optical power is measured again. As the fourth step, a decision is made to do one of three things depending on the loss value.
Should the loss be greater than 0.6 dB, the cable plant should be measured with the HOML test mandrel left in place. Should the loss fall between 0.1 dB and 0.6 dB, the cable plant should be measured with the HOML test mandrel removed from the test cord. Should the loss be less than 0.1 dB, the source and launch cord combination should be disqualified for measuring cable plant loss.
While the HOML test may be adequate to disqualify a light source, it may not be sufficient in the field for the following practical reasons.
- A known good launch cord with reference grade connections, a core diameter tolerance of ±0.7 µm and numerical aperture of ±0.004 must be used.
- The mandrel diameters are based on nominal values of 20 mm (for 62.5 µm) and 25 mm (for 50 µm) but reduced or increased depending on the cord diameter.
- No bend insensitive multimode fiber test cord can be used.
- The method cannot be used on a test cord already employing some type of mandrel or mode conditioner (i.e. an encircled flux device).
- It may mistakenly disqualify an encircled flux compliant light source.