# What is fiber protrusion/undercut?

**URL:** <https://forum.ofscn.net/t/what-is-fiber-protrusion-undercut/1973>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [June 16, 2026, 2:33pm UTC](https://forum.ofscn.net/t/what-is-fiber-protrusion-undercut/1973 "2026-06-16T14:33:27Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\
**Post date:** [June 16, 2026, 2:33pm UTC](https://forum.ofscn.net/t/what-is-fiber-protrusion-undercut/1973/1 "2026-06-16T14:33:27Z")

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Should the fiber optic be recessed in the ceramic tube, or should it protrude slightly?

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**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\
**Post date:** [June 16, 2026, 2:34pm UTC](https://forum.ofscn.net/t/what-is-fiber-protrusion-undercut/1973/2 "2026-06-16T14:34:07Z")

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In fiber optic communications and fiber optic sensor engineering, the relative height of the fiber end face with respect to the ceramic ferrule end face (usually a zirconium dioxide ceramic ferrule) is known as **“Fiber Height”**. Specifically, it is divided into two states:

- **Fiber Protrusion** : The fiber end face is higher than the spherical fitting arc of the ceramic ferrule.
- **Fiber Undercut** : The fiber end face is lower than the spherical fitting arc of the ceramic ferrule.

Regarding the question of whether the fiber should be recessed into the ceramic tube or protrude slightly, the scientific conclusion is that **within extremely strict physical tolerance limits, a slight protrusion or an extremely small recess are both compliant with standards. The core principle is to achieve stable “Physical Contact (PC)” during connector mating without the glass end face fracturing due to excessive force.**

* * *

### I. Analysis of Physical Mechanisms and Engineering Principles

To understand this tolerance range, we need to analyze the physical contact (PC) mechanics during connector mating:

#### 1. Necessity of Physical Contact (PC) Mating

When fiber optic connectors (such as FC/PC, FC/APC, etc.) are mated, the two ferrules are aligned by an adapter and an axial mating force (typically between 10 \ \text{N} and 15 \ \text{N}) is applied by a spring within the connector. To eliminate Fresnel Reflection caused by the air medium, the silica glass cores of the two optical fibers must achieve true **seamless physical contact**. This is achieved by utilizing the slight elastic deformation of the zirconium dioxide ceramic ferrule under pressure.

#### 2. Excessive Fiber Undercut

If the fiber is recessed too deeply (e.g., more than 100 \ \text{nm}), even if the connector is subjected to an axial force of over 10 \ \text{N} causing the ceramic ferrule to elastically deform, the glass cores at both ends will not be able to contact each other. This will leave a tiny air gap between the fiber cores, leading to:

- **Severe deterioration of reflection** : Return Loss drops sharply (typically from \> 50 \ \text{dB} in a normal state to around 14 \ \text{dB}).
- **Increased loss** : Introduction of additional Insertion Loss.

#### 3. Excessive Fiber Protrusion

If the fiber protrudes too much (e.g., more than 50 \ \text{nm}), when the connector is mated, all the axial mating force will be concentrated on the fiber end face (a fragile glass body with a diameter of only 125 \ \mu\text{m}), and the ceramic ferrule will not share the load. This will cause:

- **End face damage** : The fiber end face experiences extremely high local compressive stress, leading to micro-cracks, severe scratches in the silica glass, or even direct fracture after repeated plugging and unplugging.

#### 4. International Standard Control Range

According to international standards such as IEC 61755-3-1, the compliant fiber height is not an isolated value but a function related to the **Radius of Curvature (ROC)** of the ceramic ferrule end face:

- The typical radius of curvature for ceramic ferrules is 7 \ \text{mm} to 25 \ \text{mm} (commonly 10 \ \text{mm} to 15 \ \text{mm}).
- The industry standard “Fiber Height” typically needs to be precisely controlled within **-100 \ \text{nm} to +50 \ \text{nm}** (negative values indicate recess, positive values indicate protrusion).
- For high-quality, low-loss connectors, the manufacturing process usually narrows this range to **-50 \ \text{nm} to +50 \ \text{nm}** to ensure physical contact while preventing damage to the glass end face.

* * *

### II. Official OFSCN® Products and Precision Control

OFSCN® meticulously controls the 3D geometric parameters (including radius of curvature, apex offset, and fiber height) of ceramic ferrules through extremely precise multi-stage polishing processes and online interferometer testing during the production of various fiber optic patch cords and fiber optic sensors. This ensures that the fiber maintains its optimal physical contact height range even under extreme operating temperatures or prolonged plugging and unplugging.

Here are related high-precision fiber optic patch cord products:

#### 1. [OFSCN® Standard Fiber Patch Cord](https://www.ofscn.org/patch-cords/standard-fiber-patch-cord.html)

Utilizes standard zirconium dioxide ceramic ferrules and precision polishing processes to guarantee excellent plug-in stability and return loss performance at room temperature.

 ![OFSCN® Standard Fiber Patch Cord](https://forum.ofscn.net/uploads/default/original/1X/02d950c732387e0948c56d9d5c23f50022700b80.jpeg)

#### 2. [OFSCN® 120℃ Fiber Optic Patch Cord](https://www.ofscn.org/patch-cords/high-temperature-120.html)

Designed for medium-temperature industrial environments, using single-mode/multi-mode fiber with a temperature resistance of 120\ ^\circ\text{C}. The connector part features highly consistent fiber height control.

 ![OFSCN® 120℃ Fiber Optic Patch Cord](https://forum.ofscn.net/uploads/default/original/1X/0d3d3e9df6438484a2bf68af9c1f4d177cac5ec4.jpeg)

#### 3. [OFSCN® 300℃ Fiber Optic Patch Cord](https://www.ofscn.org/patch-cords/high-temperature-300.html)

For extreme high-temperature environments of 300\ ^\circ\text{C}. Due to significant differences in the coefficients of thermal expansion (CTE) between metal, ceramic, and silica fiber, the connector end face is specially designed with thermal stress compensation and micron-level polishing to ensure that the fiber height remains within the acceptable tolerance range during wide temperature cycling.

 ![OFSCN® 300℃ Fiber Optic Patch Cord](https://forum.ofscn.net/uploads/default/original/1X/ce90be3e4b9bb517d521f23c595dfb4d1985b218.jpeg)