Why control the launch when testing bend-insensitive multimode fiber?
Bend-insensitive multimode fiber (BIMMF) is designed to reduce optical loss under specified bending conditions. It is useful in dense patching and constrained routes. The distribution of light launched into a multimode fiber can affect the measured link loss. Using the same fiber type or setting a power reference alone does not guarantee identical results from different testers.
Encircled flux (EF) describes the launch distribution of a multimode source. Fluke's EF explanation emphasizes maintaining the required launch condition at the specified reference-cord output. Follow the project's adopted standard edition, the instrument manual and cable supplier requirements for the actual setup.
What should the test plan record?
| Item | Record | Do not assume |
|---|---|---|
| Link under test | Fiber category, interfaces, length and wavelengths | A fiber label specifies the whole test |
| Launch condition | Instrument, cord and applicable EF verification | A suitable source alone proves the complete setup |
| Reference | Method, adapters and connection diagram | Every method includes the same end connections |
| Reference cords | Core, interface, end face and supported part number | Any ordinary patch cord is a test reference cord |
| Report | Both end configurations, channel results, limits and exceptions | Numbers measured with unspecified conditions are directly comparable |
Fluke's BIMMF testing note discusses loss differences from launch conditions and describes its recommended setup. It also notes the importance of matching receive-cord and link fiber characteristics. Choose cords against the actual tester's supported combinations and the agreed method; one manufacturer's configuration is not a universal rule for every device.
What does a one-cord reference include?
With a one-cord reference, the launch reference cord establishes the reference before the link and any required receive cord are connected using the instrument's procedure. In common end-to-end link tests this can include both end-connection losses; some interfaces or instruments require another approved method. Draw the reference and measurement connections first, identify which mating points are included, then apply the appropriate limits. Fluke's test strategy compares common boundaries for one-, two- and three-cord references.
Follow site optical safety procedures; inspect end faces and clean and reinspect when needed. Set and verify references and handle exceptions according to the tester manufacturer's instructions. Before comparing results from different days or instruments, check wavelength, launch condition, reference method, link endpoints, connectors and routing—not merely the two dB values.
Procurement and acceptance checklist
- Supply the multimode category, core size, connectors, wavelengths, channel count and link diagram.
- Identify the OLTS model, both ends of each reference cord, whether an EF conditioner is needed and the complete supported configuration.
- Agree reference method, cord inspection and verification, inclusion of end connections, and limits for each wavelength and channel.
- Preserve original results, instrument and cord IDs, setup time, test direction, connection diagram and cleaning inspection records for repeat tests or disputes.
- When fiber, connectors, reference cords or tester change, review whether the combination still meets the project test plan.
Procurement FAQ
- Can bend-insensitive fiber ignore cable bend limits? No. Follow the specific finished cable, connector and support requirements during installation and operation.
- Is EF an insertion-loss limit? No. It describes launch conditions; the project must still define loss criteria.
- Can an ordinary patch cord be used as a reference? Do not assume so. Check the interface, reference-grade performance and instrument compatibility.
- Does one-cord reference work with every interface and instrument? Not necessarily. Confirm the approved method and boundary in the tester manual and project procedure.
Related guidance
Read fiber cable bend radius, fiber loss reference validity and test reference cables. Actual configurations and test capabilities are subject to the agreed specification.
Sources
Reviewed 2026-11-09: Fluke BIMMF test guidance, Fluke EF and repeatability, and Fluke OLTS testing strategy.
Sources checked: 2026-11-09