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 single-mode interferometer, so testing multimode fiber (MMF) always involves a transition from single-mode fiber (SMF) at the front panel to the MMF under test. The mode population created at that transition determines how accurate your IL and RL 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.  Additionally, any flaws on connector end-faces that are near the edge of the multimode core may not be fully illuminated by the central modes and therefore may not show up as brightly as they really are.  

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 SM front panel and the MMF under test, fills the fiber's modes uniformly, matching how light couples in real-world LED- or VCSEL-driven links. This makes it more sensitive to loss mechanisms concentrated near the core-cladding boundary — macrobends, microbends, 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.