The end face geometry of multi-fiber (MPO) connectors is a key factor in controlling connector performance, directly affecting insertion loss (IL) and return loss (RL). Measuring end-face 3D parameters such as ferrule X/Y-angle (Sx/Sy), fiber height (H), minus coplanarity (CF), ferrule surface X/Y-radius (Rx/Ry), fiber tip spherical radius (RF), and core dip (CD) provides both quality control and quality assurance, especially in high-density data center applications such as 800G and 1.6T.
IEC standards define different 3D geometry requirements for various fiber counts, including single-fiber, 4-fiber, 8-fiber, and 12-fiber interfaces. In this article, a 12-fiber MPO ferrule is used as the example.
The optical interface coordinate system is defined as follows: the X-axis is the line passing through the centers of the two guide holes; the Y-axis is perpendicular to the X-axis and passes through the midpoint between the two guide holes; and the Z-axis is orthogonal to both the X- and Y-axes, pointing away from the ferrule.
End face angle refers to the inclination of the MPO end face relative to the best-fit plane along the X- and Y-axes, and is used to characterize whether the overall end face geometry meets the requirements for physical fiber contact.
According to IEC 61755-3-31:
Ferrule Surface X-Angle (Sx) represents the tilt angle of the best-fit plane about the X-axis (long axis).
Ferrule Surface Y-Angle (Sy) represents the tilt angle of the best-fit plane about the Y-axis (short axis).
For a 12-fiber MPO ferrule, the standard tolerance range for Sx is -0.15° to +0.15°. For APC end faces, the standard tolerance range for Sy is 7.8° to 8.2°, as the APC interface itself is designed with an 8° angled end face.
Angular deviation can cause misalignment during physical contact and uneven contact pressure distribution, directly leading to increased IL and RL. Over long-term use, it may result in unstable connections or intermittent signal transmission. Severe angular misalignment can also accelerate end face wear and reduce connector lifetime.
Fiber Height (H) , where a positive value indicates protrusion, is a planar height parameter defined as the distance between the fiber end face and the best fit plane. In MPO/MT structures, the fibers are typically required to maintain a positive protrusion condition.
According to IEC 61755-3-31, the fiber protrusion range for PC/APC ferrules is +1.0 μm to +3.5 μm. Positive protrusion ensures physical contact during mating and minimizes IL. Insufficient protrusion or fiber recession can result in poor physical contact and significantly increased loss. Excessive protrusion, however, may concentrate mating pressure on the fiber tips, leading to fiber chipping, end face scratches, and excessive mechanical stress.
Adjacent Fiber Height Differential (HA) refers to the maximum height difference between one fiber and its adjacent fibers — typically the two neighboring fibers in a single-row ferrule or the four neighboring fibers in a multi-row ferrule.
The standard requirement for HA is ≤ 0.3 μm. The smaller the height differential, the better the coplanarity of the fiber end faces, helping to prevent performance degradation caused by poor physical contact and ensuring uniformity and long-term stability in multi-channel connections.
This leads to the concept of coplanarity. In coplanarity evaluation, Minus Coplanarity (CF) is a key parameter used to characterize the distribution consistency of fiber protrusion heights.
It is defined as the greatest parallel offset between the best fit plane (or line in the case of single row ferrules) of the fiber ends (the average elevation of the averaging regions) and a fiber end below that plane (or line). The standard requirement for minus coplanarity is ≤ 0.4 μm.
If minus coplanarity is excessive (i.e., the fiber is recessed too deeply), the affected channel may fail to achieve effective physical contact, resulting in an air gap that significantly increases insertion loss and degrades return loss, severely impacting link reliability. Excessive minus coplanarity can also disrupt the overall coplanarity of the fiber array, leading to inconsistent multi-channel performance and making it difficult to meet the stringent requirements of high-speed optical interconnect systems.
Ferrule Surface Radius refers to the curvature radius of the ferrule end face profile. The Ferrule Surface X-Radius (Rx), measured along the long axis of the end face, is specified to be ≥ 2000 mm, resulting in an almost flat surface profile.
The Ferrule Surface Y-Radius (Ry), measured along the short axis of the end face, is specified to be ≥ 5 mm. An appropriate Ry curvature helps ensure uniform physical contact across the mating surface, thereby improving connection reliability and transmission performance.
Fiber Tip Spherical Radii (RF) is the radius of curvature of the fitted sphere. The standard requirement for RF is ≥ 1 mm. A smooth spherical end face helps distribute contact stress more evenly, reducing the risk of fiber chipping and end face damage.
Core Dip (CD) is a measurement of the fiber core elevation compared to the cladding elevation at its end face. Core dip is of more relevance to multimode fibers because the large core is softer than the cladding of the fiber and tends to polish away faster.
For multimode fibers, the standard requirement for core dip is typically < 120 nm. Excessive core dip can create microscopic air gaps during mating, significantly increasing insertion loss and degrading return loss, making it a critical parameter affecting stable multi-channel physical contact.
These core parameters collectively define the 3D geometry profile of the end face and directly affect the physical contact performance during fiber mating.
