3D Interferometer Testing for MPO Ferrules: Fiber Height, Coplanarity, Angle and Radius
An interferometer measures the 3D shape of a polished connector end face to the nanometer. On a single-fiber LC or SC ferrule it reports radius of curvature, apex offset and fiber height. On an MPO/MTP® ferrule, with up to 12 fibers in a row, it reports fiber height, the height difference between fibers (coplanarity), the X and Y angle of the ferrule surface and the surface radii. These numbers decide whether every fiber touches its partner under spring force. If even one fiber is too low, that lane loses physical contact, and its insertion loss and return loss suffer. A visual inspection cannot see these defects.
Scope of this guide: the 3D shape of the polished ferrule, measured in the factory. For how the results appear in shipping paperwork see factory test reports explained; for testing installed links see Tier 1 and Tier 2 fiber testing.
This guide explains each parameter, gives the published limits, and shows how to read a geometry report when you buy MPO assemblies.
Why Geometry Matters: Physical Contact
Fiber connectors work by pressing two glass end faces together so that no air gap remains. This is physical contact (PC). An air gap adds a reflection at each glass–air surface and extra loss. Polishing creates the shape that makes contact possible, and an interferometer verifies that shape.
End-face inspection (a microscope image) and end-face geometry (an interferometer map) answer different questions:
| Check | What it sees | What it misses |
|---|---|---|
| Microscope inspection | Scratches, pits, dirt, chips on the fiber | Height, angle and shape |
| Interferometer (3D geometry) | Fiber height, coplanarity, angle, radius, apex offset | Contamination added after polishing |
A clean fiber can still sit too low; a well-shaped one can still be dirty. In the field, inspection and cleaning remain essential each time you connect; see how to clean MPO/MTP connectors.
How an Interferometer Measures an End Face
A connector interferometer shines light on the end face and on a reference surface and records the interference fringes. The fringes are contour lines of height, and phase-shifting analysis turns them into a 3D surface map. Software then fits ideal shapes to defined regions of that map and reports the parameters.
The test methods are set by standards. IEC 61300-3-47 covers spherically polished single-fiber PC/APC ferrules. IEC 61300-3-30 covers rectangular multi-fiber ferrules such as the MT ferrule inside MPO and MTP® connectors. IEC 61755-3-31 calls on IEC 61300-3-30 for its geometry limits.
Single-Fiber Ferrules: Radius, Apex Offset, Fiber Height
LC and SC ferrules are polished into a dome. Three parameters describe that dome:
- Radius of curvature: the radius of the sphere fitted to the ferrule surface around the fiber. Too flat or too sharp a dome changes how the ferrule deforms under spring force.
- Apex offset: the distance between the top of the dome and the fiber center. A large offset means the highest point is not on the fiber, so the fibers may not meet.
- Fiber height: how far the fiber sits above (protrusion) or below (undercut) the fitted sphere, measured in nanometers.
The North American requirement document Telcordia GR-326-CORE gives these single-mode limits:
| Polish | Radius of curvature | Fiber height | Apex offset |
|---|---|---|---|
| PC (UPC) | 7 to 25 mm | +50 nm to −125 nm | <50 µm |
| APC | 7 to 12 mm | +100 nm to −100 nm | <50 µm |
A common fitting method fits the sphere to a ring between 140 µm and 250 µm in diameter around the fiber. Fiber height is then averaged over a central circle about 50 µm in diameter. The IEC 61755-3-1 limits are similar but not identical, so a report should state which document it follows.
MPO Ferrules: Why the Rules Change
An MT ferrule is not a dome. It is a flat rectangular face with up to 12 fibers per row on a 0.25 mm pitch. Two guide pins align the ferrules, and a spring pushes them together. Since the ferrule surface itself is nearly flat, contact depends on the fibers protruding slightly above it. The plastic ferrule and the fiber tips compress until every fiber in the row touches its partner.
That changes what you measure. Single-fiber fiber height is a few tens of nanometers around zero. MT fiber height is a deliberate protrusion of micrometers. The key question becomes how even the fibers are across the row.
The MPO Geometry Parameters
IEC 61755-3-31 sets the end-face geometry for single-mode angled-PC rectangular ferrules with one row of up to 12 fibers. These are the 8° angle polished (APC) MT ferrules used in single-mode MPO and MTP® assemblies. Its parameters are:
- Fiber height (H): how far each fiber protrudes above the ferrule surface. Too little and a fiber may not reach its partner. Too much and the tips take too much of the load.
- Adjacent fiber height differential (HA): the largest height difference between neighboring fibers.
- Minus coplanarity (CF): how far the fiber tips deviate from a best-fit line across the row. A fiber sitting below that line has to be reached by compressing its higher neighbors.
- Ferrule surface X-angle (SX): the tilt of the ferrule face across the fiber row. A tilt makes fibers at one end of the row lower than at the other.
- Ferrule surface Y-angle (SY): the tilt at right angles to the row. On an APC ferrule this is the 8° polish angle.
- Ferrule surface X- and Y-radius (RX, RY): the curvature of the ferrule face in each direction. The face should be close to flat across the row.
- Fiber tip spherical radius (RF): the curvature of each fiber tip.
- Geometry limit (GL): a combined figure that relates X-angle and coplanarity to the force needed to mate all fibers.
For the 4-fiber interface variant (Table 5 of the standard), the limits are:
| Parameter | Limit |
|---|---|
| Fiber height H | 1 to 3.5 µm |
| Adjacent fiber height differential HA | 0 to 0.3 µm |
| Minus coplanarity CF | ≤0.4 µm |
| X-angle SX | −0.2° to +0.2° |
| Y-angle SY | 7.8° to 8.2° |
| Fiber tip radius RF | ≥1 mm |
| X-radius RX | ≥2,000 mm convex or ≥10,000 mm concave |
| Y-radius RY | ≥5 mm |
| Geometry limit GL | ≤22.6 |
The standard gives separate tables for 8-fiber and 12-fiber variants. Use the table for your fiber count and do not apply the 4-fiber numbers to a 12-fiber ferrule. These limits apply to ferrules of polyphenylene sulphide (PPS) under 7.8 N to 11.8 N of compression force. They cover performance grades B, C and D.
Reading a Geometry Report
A typical MPO geometry report shows a 3D height map of the ferrule face, a bar chart of fiber heights (one bar per fiber) and a table of parameters with pass/fail flags. Read it in this order:
- Standard and fiber count. Confirm the report names the standard (for example IEC 61755-3-31) and the table for your fiber count.
- Fiber height bars. All bars should fall inside the height window. A row that slopes steadily from one end to the other points to X-angle. One short bar in an even row points to a single recessed fiber.
- Adjacent differential and coplanarity. These show how even the row is. They matter more than the absolute height, because the lowest fiber decides whether all fibers make contact.
- X-angle and Y-angle. For APC, Y-angle should sit near 8°. X-angle should be near zero.
- Radii. Check that the surface is within the flatness limits.
- Serial number. Make sure the report matches the connector label on your cable.
Common Geometry Failures and Their Causes
| Symptom on the report | Likely cause | Effect after mating |
|---|---|---|
| Heights slope across the row | X-angle from uneven polishing pressure | Fibers at the low end lose contact first |
| One fiber much lower | Fiber pulled back or over-polished | That lane shows high loss and low return loss |
| All fibers too low | Over-polished or wrong film sequence | Poor contact on many lanes |
| All fibers too high | Under-polished | Tips carry the load; risk of damage |
| Y-angle off 8° | Fixture or polishing angle error | Reduced return loss on APC |
These faults are fixed by repolishing or reterminating. Field cleaning will not fix them, which is why geometry is checked at the factory.
Why It Matters More at 400G and 800G
Parallel optics run every lane at once. An 800GBASE-SR8 link uses 16 fibers, and an 800GBASE-DR8 link uses 16 single-mode fibers with angle polish. If a single fiber in the row loses contact, the link carries a bad lane. Tight geometry keeps loss even across all fibers, so a channel designed with the maximum loss per connection works in practice. For budgeting with connector grades, see low-loss vs standard MPO and the MPO insertion loss budget guide. For the APC requirement on single-mode MPO, see APC vs UPC MPO connectors.
What to Ask Your Supplier
- Which standard and limits do you test end-face geometry to, and for which fiber count?
- Is geometry tested on every connector or on samples? (At Huawellux, interferometric geometry is checked by sample-based testing in standard production; 100% interferometer testing of a batch is available on request.)
- Can the geometry report be supplied with the insertion loss and return loss report?
- What are your insertion loss and return loss limits per connection?
Frequently Asked Questions
What does an interferometer measure on an MPO connector?
It measures the 3D shape of the MT ferrule face. It reports fiber height for each fiber, the height difference between adjacent fibers, coplanarity across the row, the X and Y angle of the ferrule surface, the surface radii and the fiber tip radius.
What is fiber height on an MPO ferrule?
It is how far each fiber protrudes above the flat ferrule face. IEC 61755-3-31 sets 1 to 3.5 µm for its 4-fiber single-mode APC variant. The protrusion lets every fiber touch its partner when the springs compress the ferrules.
What is coplanarity in MPO end-face geometry?
It describes how evenly the fiber tips line up across the row. Minus coplanarity measures how far fibers sit below the best-fit line. If one fiber is too low, it may not make contact, and that lane shows higher loss and lower return loss.
Is end-face inspection the same as interferometer testing?
No. A microscope inspection finds scratches, pits and dirt. An interferometer measures height, angle and shape. A connector can pass one and fail the other, so production needs both.
Does Huawellux test every MPO connector on an interferometer?
In standard production, interferometric end-face geometry is checked by sample-based testing. If your project needs it, we can run 100% interferometer testing for a batch on request. Every product still receives end-face visual inspection under a 400x video microscope and is tested for insertion loss (IL) and return loss (RL).
Can cleaning fix a geometry failure?
No. Cleaning removes contamination, but geometry comes from polishing. A ferrule with a recessed fiber or the wrong angle must be repolished or reterminated.
Huawellux Specification and Ordering
Huawellux's specification for MPO and MTP® assemblies (identical for both) is insertion loss ≤0.35 dB for single-mode and multimode, with return loss >60 dB single-mode (OS2, APC) and >30 dB multimode (OM3/OM4/OM5). LC and SC patch cords are listed at insertion loss ≤0.35 dB and return loss ≥50 dB (single-mode UPC), ≥60 dB (single-mode APC) and >30 dB (multimode). Every Huawellux product receives end-face visual inspection under a 400x video microscope and is tested for insertion loss (IL) and return loss (RL) before it ships. Interferometric end-face geometry (fiber height, coplanarity, end-face angle and radius) is checked by sample-based testing in standard production, and 100% interferometer testing of a batch is available on request. Dust caps are fitted on every connector, and a formal test report is available on request.
Browse MPO trunk cables, MPO patch cables and breakout harness cables, or request a quote for an assembly made to order. To learn more about the connector, read what is an MTP/MPO cable.
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Huawellux replies to inquiries within 24 hours. Custom orders typically take 1–5 business days to complete production, with final timing confirmed case by case in the quotation. For special/custom requirements, MOQ is 2 km of fiber length. Standard listed patch cords have no formal MOQ. The sample fee is credited against the bulk order.
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