Measuring Multimode Fiber with the LWA or OBR

Luna's Lightwave Analyzer (LWA) and Optical Backscatter Reflectometer (OBR) platforms perform OFDR-based measurements using an internal singlemode interferometer, so testing multimode fiber (MMF) always involves a transition from singlemode fiber (SMF) at the front panel. The mode population created at that transition determines how accurate your insertion loss results are and how well the scan can resolve closely spaced events, so the launch method has to match what you're trying to measure.

The two launch methods below have been tested by Luna and are known to produce stable, repeatable results. Which one to use depends on the application.

1. Direct launch for the best resolution and offset-splice sensitivity

A direct launch splices or connects SMF straight to the MMF under test with no additional hardware in between. Because the numerical aperture of SMF is much lower than MMF, this only excites the low-order modes near the fiber's center, which preserves spatial resolution since fewer modes means less modal dispersion.

The same low-order modes filter the return light too, so a direct launch is more sensitive to loss events that shift the mode profile, such as an offset splice, though it can under-report loss from macrobends or splices that mainly scatter light out of the high-order modes near the core-cladding boundary. For the same reason, reflective features near the edge of the multimode core are not strongly illuminated by a direct launch and can read lower than they would under a full mode fill.

However, this direct launch method helps maintain accuracy of length measurements, as the lower modal dispersion will show any connectors along the fiber path more sharply. Direct launch is also preferred if you need to resolve closely spaced events.

2. Mode conditioner launch to catch high-order-mode loss

A mode conditioner (also called a mode converter or mode scrambler), placed between the singlemode front panel and the MMF under test, fills the fiber's modes uniformly, producing the mode profile called for by multimode cable testing standards. This makes it more sensitive to loss mechanisms concentrated near the core-cladding boundary, such as macrobends and splices with waists or cladding bubbles, and its IL results track a multimode power meter more closely than a direct launch does.

Since neither launch method alone is guaranteed to catch every defect type, you may need to use both methods when troubleshooting a suspect link.

Transition from SMF to MMF before the connector under test

The Rayleigh scatter level can change at the SMF-to-MMF junction, which can look like an IL event (either a loss or a gain) even though it's simply due to the different Rayleigh scatter levels of the dissimilar fiber types. To get an accurate insertion loss reading on an MMF connector, make sure the SMF-to-MMF transition happens before the first connector under test, not at it. Where the transition cannot be moved, the scatter level shift can be measured directly and subtracted instead, using the method in Section 5.2 of the engineering note below.

The engineering note below, EN-FY1301, was written for the OBR. The measurement principles apply equally to the LWA, though software control names and available wavelengths may differ.