| Full title | Fibre optic interconnecting devices and passive components. Basic test and measurement procedures. Part 3-35: Examinations and measurements. Visual inspection of fibre optic connectors and fibre-stub transceivers |
| Publisher | IEC, subcommittee SC 86B |
| Current edition | Edition 3.0, 2022-09. Replaces Edition 2.0 (2015) |
| European version | EN IEC 61300-3-35:2022 |
| Status | Current. IEC stability date 2031. An Amendment 1 is in draft |
| Related standards | IEC TR 62627-01, IEC TR 62627-05, IEC TR 62572-4, IEC 60825-2, IEC 61280-4-1, IEC 61280-4-2 |
What it covers
IEC 61300-3-35 covers the observation and classification of debris, scratches and defects on fibre optic connector end faces and fibre-stub transceivers. It sets the minimum microscope performance, the inspection procedure and the quantitative pass/fail criteria.
Visual inspection is in addition to optical testing. It does not replace attenuation or return loss measurement. Edition 3 states that a connector cannot be rejected on visual inspection alone. If a clean connector still fails inspection, it is tested against its optical performance specification. If it meets that, it is fit for use.
Inspection methods and microscopes
| Method | Type | Notes |
|---|---|---|
| A | Direct view optical microscope | Needs a built-in laser safety filter. The user confirms no laser is active on the link first |
| B | Video microscope | Image shown on a display. A transparent overlay helps estimate sizes |
| C | Automated analysis microscope | Software grades the image against programmed criteria. Less operator dependent |
All three methods must be validated with a traceable artefact (Annex C). The standard warns that all methods show variability. Methods A and B depend on the operator. Method C has inherent uncertainty for features close to the limits.
Minimum microscope performance
The standard sets field of view and detection capability, not a fixed magnification figure.
| Microscope | Field of view | Must detect |
|---|---|---|
| Standard field of view | At least 250 µm | Defects of 2 µm diameter and scratches 3 µm wide |
| Large field of view (array ferrules such as MPO) | At least 6.4 mm × 2.5 mm | Debris of 10 µm diameter |
Inspection procedure
- Inspect for cleanliness first. On cylindrical ferrules, inspect at least the central 250 µm diameter. On rectangular ferrules, inspect the whole 6.4 mm × 2.5 mm surface.
- Remove all loose particles. Debris left after cleaning counts as a defect.
- Then inspect zones A and B of each fibre against the relevant table in Annex A.
- A defect that crosses zones is judged in each zone only for the part inside it.
- Defect size is the diameter of the smallest circle that encloses it. Scratches are graded by width.
Recommended cleanliness limits
| Ferrule | Recommended limit |
|---|---|
| Cylindrical, ring 135 µm to 250 µm | Maximum 10 particles of 5 µm to 10 µm. No limit below 5 µm. None above 10 µm |
| Rectangular, whole surface | Maximum 10 particles of 10 µm to 25 µm. No limit below 10 µm. None above 25 µm |
Pass/fail criteria (Annex A extracts)
Multimode zone A is set at 65 µm to cover both 50 µm and 62.5 µm cores. For multi-fibre ferrules, the criteria apply to every fibre.
Multimode PC and APC (Table A.1)
| Zone | Defects | Scratches |
|---|---|---|
| A: core, 0 to 65 µm | Below 2 µm: no limit. 2 to 5 µm: max 4. Above 5 µm: none | Below 3 µm: no limit. 3 to 4 µm: max 4. Above 4 µm: none |
| B: cladding, 65 to 110 µm | Up to 25 µm: no limit. Above 25 µm: none | No limit |
Single-mode PC, RL ≥ 26 dB, and fibre-stub transceivers (Table A.2)
| Zone | Defects | Scratches |
|---|---|---|
| A: core, 0 to 25 µm | Below 2 µm: no limit. 2 to 3 µm: max 1. Above 3 µm: none | Below 3 µm: no limit. 3 to 4 µm: max 2. Above 4 µm: none |
| B: cladding, 25 to 110 µm | Up to 25 µm: no limit. Above 25 µm: none | No limit |
Single-mode PC, RL ≥ 35 dB (Table A.3, new in Ed. 3)
| Zone | Defects | Scratches |
|---|---|---|
| A: core, 0 to 25 µm | Below 2 µm: no limit. 2 to 3 µm: max 1. Above 3 µm: none | Below 3 µm: no limit. 3 to 4 µm: max 1. Above 4 µm: none |
| B: cladding, 25 to 110 µm | Up to 25 µm: no limit. Above 25 µm: none | No limit |
Annex A also gives separate tables for single-mode PC connectors with RL ≥ 45 dB (Table A.4) and single-mode angle polished APC connectors (Table A.5). Values for those two grades are not reproduced here.
Main changes in Edition 3
- Pass/fail grading of zones C (adhesive) and D (contact) removed. A general cleanliness check of the 250 µm area, or the whole rectangular ferrule, replaces them.
- Outer edge of zone B reduced from 115 µm to 110 µm.
- New criteria for single-mode 35 dB connectors.
- Defects partly in the core are judged only for the part in the core.
- Cleaning recommendations added in Annex D.
- Visual inspection stated as not a substitute for optical testing.
What it means in practice
- Use a validated microscope that meets the field of view and detection minimums.
- Pick the Annex A table that matches the connector type and return loss grade.
- Inspect both sides of every mated pair, including bulkhead ports and test cords.
- Clean, then re-inspect. Record failures, then check optical performance before rejecting a connector.
- IEC 61280-4-1 and IEC 61280-4-2 both require end face inspection to this standard before loss testing.
Related reading: Why fibre connectors need cleaning and how to do it · Fibre optic connectors explained: LC vs SC, UPC vs APC.