Why set compensation after connecting launch and receive fibers?
An OTDR launch fiber connects the instrument to the near end of the link under test; a receive fiber is attached at the far end to provide fiber beyond the last connection. Launch and receive compensation identifies the start and end of the link in the instrument, so reported distance and loss correspond to the agreed test span. Connecting the fibers alone does not guarantee that the boundaries have been identified correctly.
Fluke's compensation guidance explains that its DTX Compact OTDR marks the launch-fiber end and receive-fiber start on the trace and event table. Names, defaults and setup steps vary between instruments; use the manual for the actual model.
Connection arrangement and report boundary
| Location | Physical arrangement | Check in the report |
|---|---|---|
| Tester to near end | OTDR → launch fiber → link | Launch end corresponds to the link start |
| Link under test | Near connection, link fiber and far connection | Range used for distance, events and total loss |
| Far end to tail | Link → receive fiber | Receive start corresponds to the link end |
| End connections | Launch-to-link and link-to-receive mating points | Whether each end loss is included in budget and pass/fail |
The EXFO OTDR user guide describes defining the start by launch length or event number and the end using receive length or an event. Its defaults and treatment of end events are instrument-specific. The VIAVI 8100 manual also describes options for including the start and end connector losses in the budget. Agree those settings before comparing reports.
Review the events with a worked example
Suppose a setup has a nominal 500 m launch fiber, a link of roughly 2 km and a nominal 500 m receive fiber. Without appropriate boundary settings, the full trace can include the auxiliary fibers. Do not treat full-trace distance as link length or assume end-connection loss is included in the acceptance decision. These lengths are illustrative, not a universal recommendation.
Compare the trace events with physical connector labels. Follow the instrument manual to set the start and end by measured length or the correct events. Check both the event table and summary to establish whether the near and far connections are measurable and included under the agreed limits. If dead zones, noise or automatic event identification obscure a connection, resolve the test conditions; do not move markers merely to obtain a pass.
Procurement and acceptance checklist
- Identify the instrument, fiber type, connectors and polish at both ends, test wavelength, pulse width and launch/receive configuration.
- Select lengths for the link and instrument conditions, and verify access at the far end. Match interfaces to the drawings and equipment manuals.
- Agree link start and end, inclusion of end-connection losses in the budget and pass/fail, applicable limits and reporting format.
- Retain the event table, boundary settings, wavelength, pulse width, averaging time and raw trace. Identify instrument software and defaults separately.
- Follow site optical safety procedures; inspect end faces and clean and reinspect when needed. Correct and document any boundary mismatch without silently overwriting the original evidence.
Procurement FAQ
- Can a launch fiber alone measure the last connection? Do not assume so. Without an appropriate receive fiber the far-end connection loss may not be measurable; check the instrument and test plan.
- Does compensation remove end-connection loss? Compensation sets the analysis span. Inclusion of end losses depends on instrument settings and the acceptance plan.
- Is 500 m the right length for every project? No. The choice depends on the instrument, pulse width, event separation and fiber under test.
- Is a PDF screenshot sufficient? The contract decides; raw traces and boundary settings make review and repeat testing easier.
Related guidance
See OTDR launch fiber length selection, OTDR raw traces and acceptance data and OTDR launch cable products. Actual configuration is subject to the agreed specification.
Sources
Reviewed 2026-11-08: Fluke launch and receive compensation, EXFO OTDR user guide, and VIAVI 8100 user manual.
Sources checked: 2026-11-08