Fiber Optic Pigtails: Design, Selection, and Use in Splice Boxes

What is a fiber optic pigtail and how is it constructed?

"LWL" is the German abbreviation for fiber optic cables. On a pigtail, one side carries a pre-assembled and tested fiber optic connector. On the other side, the fiber remains free for further processing. The construction includes the glass fiber with its core and 125 µm cladding, protective coatings or buffering, and the connector with ferrule and strain relief. The exact design depends on the product.

Unlike a more robust installation or patch cable, a typical pigtail is intended for protected storage within a housing. It provides the transition from the permanently spliced installation fiber to the detachable plug connection on the front of a distribution panel.

What is the purpose of a pigtail in a splice box and distribution panel?

The installation cable is anchored in the splice box, and its fibers are led to the splice cassette. There, each required fiber is connected to the free end of a suitable pigtail. The pigtail's connector is seated in an adapter on the front panel. A patch cable can then be plugged in from the outside and replaced if necessary without opening the permanent splice.

Pigtail or Patch Cable?

What happens during fusion splicing?

During fusion splicing, the free pigtail end and the installation cable fiber are prepared. Qualified personnel remove the buffer and coating to the required length, clean the glass fibers, and cut them using a precision tool called a cleaver. The fusion splicer aligns the prepared fiber ends and fuses the glass using an electric arc. Afterwards, the connection is fitted with a splice protector and stored in the cassette without tension.

Finally, a documented visual and performance inspection must be carried out according to the procedure defined for the project. The attenuation estimated by the fusion splicer does not replace the required link measurement in every project. Splicing, cleaning, and testing must be performed by appropriately trained personnel.

Fiber types: Correctly assigning OS2, OM3, and OM4

The pigtail fiber type must match the installation fiber, the rest of the passive link, and the active optics. OS2 designates a single-mode category. The short notation "9/125 µm" is commonly used for single-mode fiber with a core diameter of approximately 9 µm (or corresponding mode field specifications) and a 125 µm cladding. The specific fiber standard and data sheet remain decisive.

OM3 and OM4 are laser-optimized multi-mode categories with 50/125 µm. Both nominally have a 50 µm core and 125 µm cladding diameter; OM4 has higher requirements for modal bandwidth than OM3. It does not follow that OM4 should be inserted into any existing OM3 link without testing. For a termination, the existing category must be documented throughout.

OS2 describes a cabling or cable category, not every property of the fiber used therein. For pigtails, for example, fibers according to ITU-T G.652.D or bend-insensitive versions according to ITU-T G.657.A1 or G.657.A2 may be relevant. Especially with tight storage radii and splices to existing fibers, the specific fiber specification, the product data sheet, and project requirements must be checked.

A more detailed classification can be found in the guide Selecting Fiber Optic Cables by Fiber Type.

LC, SC, and other connectors, as well as UPC or APC

LC and SC are commonly found in current distribution panels. LC is more compact and works with a 1.25 mm ferrule; SC uses a 2.5 mm ferrule. Other designs such as E-2000/LSH, ST, or FC may be relevant in legacy systems and special projects. The selection depends on the adapter and counterpart actually present.

An in-depth classification of interfaces can be found in the guide Fiber Optic Connectors, Designs, and Polishes Overview.

UPC and APC denote end-face geometry

With UPC, the ferrule end face is polished convex for physical contact and not angled relative to the fiber axis. APC has an angled contact surface, usually at an 8° angle. The angular geometry directs back reflections differently than UPC. The required version depends on the counterpart and the optical system.

UPC and APC must not be connected directly to each other. The surfaces do not meet geometrically correctly. This can degrade insertion loss and reflection and damage the end faces upon contact. Green is often used for APC and blue often for UPC; however, color alone is not sufficient proof of compatibility. The designation, data sheet, adapter, device, and measurement adapter must match.

What do 900 µm, length, and pigtail sets mean?

A 900 µm version describes the outer diameter of the buffered individual fiber, not the glass fiber core. Within this dimension lie the 125 µm glass fiber and its coatings. Whether it is a tight-buffered or other buffer construction and how it is stripped can be found in the product data sheet.

Individually packaged pigtails make sense if only a few fibers are needed, special combinations occur, or spare parts are stored separately. Color-coded sets make clear assignment easier when dealing with multiple fibers, provided the color sequence and project documentation match. A set does not replace allocation planning.

The length must be sufficient for the path from the adapter through the cassette guide to the splice point, plus a neat reserve. Pigtails that are too short create tension; unnecessary excess length makes storage difficult. The bend radius and storage requirements of the specific component must be observed. Fundamentals on this can be found in the guide Bend Radius in Fiber Optics.

Selection matrix for fiber type, connector, and polish

Selecting a pigtail to match the splice box

1. Survey existing conditions: Check installation fiber, active optics, existing adapters, and documentation.
2. Determine fiber type: Specify OS2, OM3, or OM4 to match the entire link; do not just note "single-mode" or "multi-mode."
3. Identify connector: Choose LC, SC, or another design to match the front panel and adapter.
4. Check polish throughout: Align UPC or APC on pigtail, adapter, patch cable, device, and measurement equipment.
5. Count fibers: Document occupied fibers and planned reserve separately; define packaging and color sequence.
6. Check length and buffering: Clarify cassette path, reserve storage, bend radius, and the required 900 µm construction.
7. Confirm scope of delivery: For a splice-ready box, check which pigtails, adapters, and splice protectors are actually included.

You can check existing versions in the category Fiber Optic Splice Boxes and Assemblies. For pigtails in project-specific versions, the pigtail inquiry at abatec is available.

Typical planning, ordering, and installation errors

• Fiber type and class are assumed based only on color.
• LC or SC is correct, but UPC and APC are confused.
• The pigtail matches the adapter but not the patch cable or counterpart.
• The number of duplex adapters is equated with the fiber count.
• Pigtails are ordered too short or planned without storage path and reserve.
• The meaning of 900 µm is confused with the core or cladding diameter.
• Color-coded sets are used without documented color and port assignment.
• Fiber ends or ferrules are left uncovered, touched, or plugged in uncleaned.
• Splice protector, bend radius, strain relief, or measurement concept are missing from the work plan.

Checklist for ordering or project inquiry

• Location and splice box or distributor identified
• Fiber type and class: OS2, OM3, OM4, or others clearly documented
• Connector and adapter: e.g., LC or SC
• Polish: UPC or APC
• Number of fibers to be terminated and desired reserve
• Pigtail length and buffer construction, e.g., 900 µm
• Individual packaging or color-coded set including color sequence
• Existing counterparts, patch cables, and active components
• Desired testing and documentation requirements
• Required adapters, splice protectors, and other scope of supply
• Quantity, deadline, and project-specific labeling