Direct answer
An OTDR converts propagation time into distance using the configured group index. A helix correction may convert fiber distance to cable distance. Align wavelength, IOR, correction and span datums before comparing markings or assembly dimensions; a discrepancy alone does not establish a delivery shortage.
Identify the span being measured
An assembly drawing, cable-jacket marking difference and OTDR fiber distance may use three different datums. For preterminated trunks or test cords, define the comparison in the drawing and test plan before comparing numbers.
EXFO's AXS-100 manual explains time-of-flight conversion using IOR. Its MAX-700 manual distinguishes wavelength-specific group index from the helix factor accounting for fiber-to-cable length differences. These references establish principles and named-instrument settings, not universal software menus or correction algorithms.
How IOR affects distance
Before cable-length correction, distance can be understood as L = c × t / (2 × n_g). Here c is vacuum light speed, t is round-trip pulse time and n_g is group index at the test wavelength. The factor of two accounts for the round trip.
For unchanged flight time, a larger configured n_g gives a shorter distance. Obtain the fiber manufacturer's applicable value instead of making another fiber's default a contractual datum.
| Comparison | Conditions to align | First investigation |
|---|---|---|
| Two OTDR results | Wavelength, IOR, correction and span endpoints | Settings and reanalysis history |
| OTDR versus markings | Fiber-to-cable correction and marking locations | Cable construction and marking convention |
| OTDR versus assembly size | Mechanical datums and optical span | Connectors, branches and added test leads |
| Two fiber distances | Fiber type, settings and endpoints | Routing, excess fiber and splice construction |
A reproducible settings example
This is a calculation, not a measured product or recommended setting. Assume actual group index 1.468 at the chosen wavelength, configured index 1.460, identical endpoints and no helix correction.
| Item | Assumption or calculation | Interpretation |
|---|---|---|
| Actual fiber span | 1000 m | Given for this example |
| Displayed distance | 1000 × 1.468 / 1.460, approximately 1005.48 m | Conversion of the same flight time |
| Difference | Approximately 5.48 m or 0.548% | Group-index setting alone |
| Excluded effects | Endpoint selection, instrument accuracy and cable correction | Not a complete uncertainty budget |
The example explains a setting effect. It proves neither overdelivery nor shortage and does not establish a universal 0.548% tolerance.
Helix correction and service slack
The fiber path inside a cable can differ from the jacket path. A helix factor addresses that construction difference, using the applicable cable information and instrument definition. It does not automatically remove an external rack service loop. Do not adjust it merely to make an unexpected result match an order length.
Retain raw traces, IOR, wavelength, helix factor and software revision. Preserve both files and the reason when reanalysis changes distance. Consistent original settings make before-and-after comparisons meaningful.
Put dimensional acceptance in the purchase specification
For short jumpers, define connector measurement datums, the physical measurement state and length tolerance. Dimension trunk and branches separately. Review whether the OTDR can resolve those short spans; displayed decimal places alone do not establish capability.
Identify launch and receive leads and the tested span separately in the optical record. The drawing determines delivered mechanical dimensions; the agreed test plan determines optical-span acceptance.
Procurement FAQ
- Can factory-default IOR remain unchanged? Check it against the fiber manufacturer's model and wavelength values.
- Must 1310 nm and 1550 nm use identical IOR? Follow wavelength-specific fiber information and instrument requirements.
- Can a helix factor compensate for service slack? Do not mix construction correction with an externally reserved cable length.
- Do extra meters prove an inaccurate delivery? Align endpoints, added leads and settings before applying the dimensional acceptance method.
- Can OTDR replace a ruler for short jumpers? Review capability and contractual datums; display resolution does not establish dimensional accuracy.
Related products and reading
- OTDR pulse width and short-span resolution
- Fiber length and channel delay
- OTDR launch-cable configuration
- Preterminated breakout systems
Product pages describe configuration scopes. Supply, drawings and performance require project-specific confirmation.
Contact an engineer
Prepare both-end interfaces, drawings or existing test records and contact an engineer to discuss assembly requirements and verification boundaries.
Sources and review date
Sources checked on 2026-10-13. Third-party references explain technology and specific examples, not ARRANEX performance or certification. Purchasing tables are review recommendations; example assumptions apply only to the calculations.
Sources checked: 2026-10-13