Understand fiber optic fusion splicing, splice losses, cleaving, fiber distribution panels, LSPM/OLTS, OTDR, optical loss budget, documentation and acceptance criteria.
Check it out!
Fiber optic fusion splicing is the permanent joining of two fibers by controlled heating, normally produced by an electric arc in a fusion splicer. The objective is to create a splice with low optical loss, mechanical stability and suitable continuity for backbones, interbuilding networks, data centers, industrial networks and other optical infrastructures.
The value of fusion splicing is not merely “joining two fibers.” In a properly executed project, every splice must be accounted for in the loss budget, identified, protected in an appropriate tray, integrated into the fiber distribution panel or splice closure, and later validated by testing. The loss estimate shown by the fusion splicer is useful for process control, but it does not replace link acceptance measurement.
When fiber optic fusion splicing is used
Fusion splicing is appropriate when the network needs a permanent connection between fibers. Common applications include:
- terminating cables onto pigtails inside fiber distribution panels;
- providing cable continuity in campus or building backbones;
- repairing damaged optical cables;
- branching and organizing fibers in splice closures;
- industrial and mission-critical optical networks;
- outdoor and underground interconnections;
- PON networks when the architecture includes permanently spliced fibers and splitters.
The decision about where splices will be installed should appear in the design and installation documentation. Creating unplanned splices increases loss, maintenance effort and failure points and may consume optical-budget margin.
Fusion splice vs. mechanical splice vs. field connector
Defining splice locations only during construction often creates extra closures, consumes optical-budget margin and complicates maintenance. The optical-network design should define routes, fiber distribution panels, capacity, splices, connectors and test criteria before construction.
The three techniques serve different purposes.
| Technique | Nature | Advantage | Point of attention | Typical use |
| Fusion splice | Permanent | Low loss and high stability | Requires a splicer, preparation and splice protection | Backbone, fiber distribution panel, repair, critical infrastructure |
| Mechanical splice | No thermal fusion | Fast execution in specific situations | Greater dependence on alignment and component quality | Repairs or situations where fusion is not feasible |
| Field connector | Disconnectable termination | Enables direct connection | Must meet optical-performance and installation-quality requirements | Terminations and applications provided for by the system |
For backbones and permanent links, fusion splicing is often preferred when the design seeks low loss and stability. This does not make other techniques automatically unsuitable: the choice depends on architecture, maintenance, allowable loss, environment and system requirements.
How fiber optic fusion splicing is performed
The process must control preparation, cleaning, cleaving, alignment, arc and mechanical protection. A typical sequence is:
- identify the cable, tube and fiber to be spliced;
- prepare the cable and access the fibers without damaging them;
- remove the required coatings;
- clean the fiber with appropriate materials;
- cleave the fiber using the proper tool;
- position the fibers in the splicer;
- perform alignment and fusion;
- evaluate the splice and repeat if necessary;
- install the heat-shrink splice protector;
- place the splice in the tray while respecting bend radii;
- update identification and documentation;
- test the link according to the test plan.
Preparation, cleaning and cleaving
Many fusion-splicing problems begin before the electric arc. Dust, coating residue, angled cleaves, scratched fiber or improper positioning can produce an unstable splice or higher-than-expected loss.
Cleaning should use materials suitable for fiber optics and prevent recontamination after preparation. Cleaving must produce an end face suitable for splicer alignment; a worn or poorly adjusted cleaver increases process variability.
The technician must also maintain enough free fiber length for routing in the splice tray. Removing excessive coating or working with insufficient slack impairs future maintenance.
Core alignment and cladding alignment
Fusion splicers use different alignment methods. In general terms, equipment may align fibers based on cladding geometry or use optical systems to locate the core. The appropriate method depends on fiber type, required precision, equipment and the work being performed.
A fusion splice should not be evaluated only by the commercial name of the alignment method. Electrode condition, selected program, cleaning, cleaving, environmental conditions and operator skill continue to influence the result.
The loss shown by the splicer is not the final test
After the arc, the splicer normally displays an estimated loss calculated from images and internal parameters. This value is useful for detecting a visibly poor splice and for immediate execution control.
However, it does not measure the actual end-to-end attenuation of the link. Acceptance should use appropriate test instruments and methods — such as LSPM/OLTS for end-to-end loss and OTDR for event characterization — according to the project scope and NBR 16869-2.
A “0.00 dB” reading on the splicer does not prove that the complete link is within its optical budget.
How much loss can a fusion splice have?
Allowable loss should be treated at two levels: hardware/standard limit and design/acceptance criterion. NBR 14565 uses up to 0.30 dB per splice as an optical-hardware reference. This value should not be interpreted as the desired result for every fusion splice performed.
In a design with several splices, simply consuming 0.30 dB at each one can excessively reduce the available margin. Therefore, the optical budget should define the total allowable channel loss and may establish stricter workmanship criteria.
| Element | Design reference | Observation |
| Fiber | attenuation per km and wavelength | increases with link length |
| Mated connection | up to 0.75 dB as a hardware reference in NBR 14565 | the design may require better performance |
| Splice | up to 0.30 dB as a hardware reference in NBR 14565 | should not become an automatic workmanship target |
| Engineering margin | defined by the design | preserves operating tolerance and uncertainty |
Optical loss budget: every splice consumes margin
The loss budget adds the contribution of cable, connectors, mated connections, splices and other passive components and compares that value with the limits of the application and transceivers.
When construction adds unplanned splices — for example because the cable was too short, was damaged or the route changed — the designer should recalculate the budget. The solution of “adding another splice” may be technically acceptable, but it must fit within the available margin and be recorded in the As-Built.
Fiber distribution panels, pigtails and splice trays
In rack installations, the optical cable is commonly terminated by fusion-splicing its fibers to pigtails that reach the adapters in the fiber distribution panel. The tray protects splice sleeves and organizes fiber slack.
The capacity of the fiber distribution panel should consider the current number of fibers, spare capacity, trays, connector type, pigtails, identification, bend radius and expansion. Piling up splice protectors or winding fibers without control makes maintenance difficult and can create macrobends.
Good documentation associates each optical port with the fiber, tube, cable, origin, destination and splice tray. This reduces the risk of working on the wrong fiber during maintenance.
Bend radius and fiber routing
Even after a good splice, routing itself can degrade the link. Excessively tight bends, cover pressure, disorderly fiber crossings or poorly positioned slack can introduce additional loss.
NBR 16415 requires pathways and routing to respect the minimum radius defined by the manufacturer and, where no specification exists, provides general references for different cable types. Inside a distribution panel or closure, the practical rule is to preserve the geometry specified by the system manufacturer and maintain enough slack for future interventions.
Splices in single-mode and multimode fiber
Both single-mode and multimode fibers can be fusion-spliced, but testing and interpretation of results must consider the fiber type and applicable wavelengths.
According to NBR 16869-2, multimode cabling measurements use 850 nm and 1,300 nm; single-mode measurements use 1,310 nm and 1,550 nm. Compatibility between fibers must also be verified: mixing categories, constructions or optical characteristics without analysis can produce unexpected behavior.
LSPM/OLTS vs. OTDR after fusion splicing
The estimate displayed by the fusion splicer is not link acceptance. To verify the infrastructure, attenuation must be measured and, when required, events must be characterized using calibrated equipment, a defined methodology and traceable reports.
The two methods answer different questions.
| Method | Main question answered | Role in acceptance |
| LSPM / OLTS | What is the total attenuation of the link? | Compares end-to-end loss with the limit |
| OTDR | Where are events, splices, connectors and anomalies? | Characterizes the link and locates events |
| Optical inspection | Is the connector end face clean and intact? | Prevents false results and interface damage |
NBR 16869-2 requires inspection and cleaning of the test-system and cabling interfaces. Contamination can alter results and can even damage a connection. A dirty connector should be cleaned and reinspected before measurement.
For OTDR testing, a launch fiber and receive cord are used to characterize the initial and final interfaces. Tests in both directions may be necessary when an appropriate quantitative measurement of events and splices is required, because backscatter differences between fibers can distort a unidirectional reading.
Marginal results should not be accepted as compliant
NBR 16869-2 states that marginal results are not permitted in optical-cabling testing. The result must meet the requirement considering measurement uncertainty or be treated as noncompliant.
In practice, this prevents an installation from being accepted with reasoning such as “it was practically at the limit.” When there is doubt, the cause should be investigated, the link corrected when necessary and the test repeated.
How to interpret a splice on an OTDR trace
On an OTDR trace, a splice usually appears as a non-reflective event associated with a change in backscatter level. The apparent value can vary according to test direction and the characteristics of the connected fibers.
Therefore, in work requiring quantitative splice evaluation, bidirectional measurements and averaging of both directions reduce the influence of the so-called gainer — a condition in which a splice may appear to show gain in one direction because of a difference in backscatter, even though no real signal amplification occurs.
The OTDR also helps locate macrobends, breaks, connectors and unexpected events along the route.
Quality control during execution
Waiting until hundreds of splices are complete to discover a systemic problem increases rework. The quality plan may include:
- inspection of materials and pigtails;
- verification of the selected fusion program;
- checking cleaver and electrodes;
- supervised initial samples;
- periodic review of splicer estimates;
- inspection of trays and slack;
- intermediate testing by segment;
- consolidation of failures by crew or location;
- retesting after corrections.
If several splices from the same work front show high loss, the problem may be cleaning, cleaving, equipment configuration or material — not isolated defects in each fiber.
Optical-cable repairs and restoration
After a cable break, the objective is not merely to restore continuity. The repair must consider closure location, available slack, the need to insert a new cable section, the additional number of splices, mechanical protection, sealing and documentation updates.
An improvised restoration can recover service while creating a poor maintenance point or consuming excessive optical budget. For critical networks, the repair plan should provide materials, slack and a contingency strategy before a failure occurs.
Fusion splicing in outdoor closures and industrial environments
Splicing outside technical rooms requires attention to moisture, dust, vibration, temperature, sealing, mechanical protection and maintenance access. The closure and cable types must be compatible with the environment.
In industrial plants, substations and outdoor areas, the design should coordinate fiber with pathways, closures, grounding of metallic elements where present, mechanical protection and segregation from other systems. A high-quality splice does not compensate for a closure that is unsuitable for the environment.
Procurement: how to contract fiber splicing technically
The contracting scope should go beyond “price per splice.” A verifiable specification may include:
| Item | Requirement to define |
| Fibers | type, category, quantity and identification |
| Splicer | compatible method/capability and adequate maintenance |
| Cleaving and cleaning | appropriate tools and consumables |
| Splice protector | type and tray compatibility |
| Distribution panel/closure | capacity, protection and organization |
| Loss criterion | design/acceptance limit |
| Testing | LSPM/OLTS, OTDR and applicable wavelengths |
| Calibration | valid calibration certificates for measurement equipment |
| Documentation | fiber matrix, reports and As-Built |
| Retesting | procedure for failures and corrections |
The contractor should deliver enough traceability to relate every result to the fibers actually installed.
Minimum documentation for splices and tests
NBR 16869-2 requires test documentation to report parameters, configuration, equipment, serial number, calibration, wavelengths, fiber category, connectors, reference used, measurement direction, date, operator and result.
For project governance, it is advisable to add:
- cable code;
- tube and fiber number;
- origin and destination;
- distribution panel/closure and tray;
- port or adapter;
- splice identification;
- physical route;
- repair history;
- As-Built version.
Commissioning and acceptance of the optical network
Acceptance should combine physical inspection, documentation and measurement. It is not enough to receive OTDR files without verifying that they correspond to the installed fibers.
A robust sequence includes checking distribution panels and closures, identification, slack, bend radii, cleanliness, polarity, LSPM/OLTS results, OTDR traces when required, and reconciliation with the As-Built. In redundant networks, it also verifies whether the installed fibers and routes correspond to the designed architecture.
When a splice or link fails, the correction should be recorded and the test repeated. The final product is a verified and traceable optical infrastructure, not merely a set of physically completed splices.
Common errors in fusion-splicing services
Recurring problems include:
- accepting the splicer’s estimate as certification;
- failing to clean the fiber adequately;
- using a cleaver with a worn blade;
- performing splices without prior identification;
- mixing incompatible fibers without analysis;
- routing slack below the appropriate bend radius;
- exceeding tray capacity;
- leaving splice protectors pressed by the cover;
- adding splices without recalculating the budget;
- testing only one wavelength when the plan requires two;
- not using a launch fiber/receive cord with OTDR;
- delivering PDFs without sufficient native files and traceability;
- accepting a marginal result.
Final considerations
Fiber optic fusion splicing is a network-construction step, but its quality is demonstrated only when design, execution, protection, identification and testing are treated as one process.
Splices should be included in the optical budget, performed with controlled preparation, protected and organized in suitable distribution panels or closures, and verified by coherent test methods. In critical infrastructure, engineering must also ensure that results reconcile with routes, documentation, redundancy and formal acceptance criteria.
When the optical network is part of an engineering deliverable, test results must reconcile with distribution panels, identification, routes, As-Built documentation and formal acceptance criteria. Commissioning turns isolated tests into evidence of system acceptance.
Technical references
[1] ABNT. ABNT NBR 14565:2019 — Structured cabling for commercial buildings. ABNT, 2019. Available at: https://www.abntcatalogo.com.br/.
[2] ABNT. ABNT NBR 16415:2021 — Pathways and spaces for structured cabling. ABNT, 2021. Available at: https://www.abntcatalogo.com.br/.
[3] ABNT. ABNT NBR 16869-1:2020 — Structured cabling — Part 1: Planning and installation. ABNT, 2020. Available at: https://www.abntcatalogo.com.br/.
[4] ABNT. ABNT NBR 16869-2:2021 — Structured cabling — Part 2: Optical cabling testing. ABNT, 2021. Available at: https://www.abntcatalogo.com.br/.
[5] ISO/IEC. ISO/IEC 14763-3 — Implementation and operation of customer premises cabling — Testing of optical fibre cabling. Available at: https://www.iso.org/standards.html.
[6] TIA. ANSI/TIA-568.3-E — Optical Fiber Cabling and Components Standard. Available at: https://tiaonline.org/standards/.
[7] IEC. IEC 61300-3-35 — Fibre optic interconnecting devices and passive components — Visual inspection of fibre optic connectors. Available at: https://webstore.iec.ch/.
Frequently asked questions
It is the permanent joining of two fibers by controlled heating, normally using an electric arc, to form a low-loss optical splice.
NBR 14565 uses 0.30 dB as a maximum hardware reference for a splice, but the design may define stricter workmanship limits to preserve the optical budget.
No. It is an equipment estimate and does not replace link measurement with appropriate methods such as LSPM/OLTS and OTDR according to the test plan.
LSPM/OLTS measures total end-to-end link loss. OTDR characterizes events and helps locate splices, connectors, bends and anomalies along the fiber.
NBR 16869-2 specifies 850 and 1300 nm for multimode and 1310 and 1550 nm for single-mode in the applicable methods.
It moves the first event away from the OTDR dead zone and allows the initial link interface to be evaluated properly; a receive cord also allows characterization of the remote end.
It depends on the application. In backbones and permanent links, fusion is often preferred for stability and low loss; other techniques may be valid when specified by the design.
They are normally protected by heat-shrink splice sleeves and routed in splice trays, with controlled slack and bend radii.
NBR 16869-2 does not allow marginal results in optical-cabling testing; the result must meet the criterion considering measurement uncertainty.
Complementary technical materials
Related services
- Fiber Optic and Optical Network Design
- Technical Testing and Verification
- Engineering Commissioning
- Telecommunications Design
- Structured Cabling Design
