3 Component selection 3.1 Elements It is important to recognize that a fibre optic termination, while appearing straightforward, is in fact a complex interaction of the constituent elem
General
This section of EN 4533 focuses on the termination aspects of fiber optic design in avionic installations Termination refers to the method used to connect one component, typically a fiber, to another This process is usually achieved through a connector that ensures precise alignment between the fiber and another component, often another connector, to maintain the uninterrupted transmission of optical signals within the operational envelope.
This section highlights the importance of high integrity terminations and addresses component selection challenges It also offers best practices for terminating fibers into connectors for high integrity applications For a comprehensive overview of the termination process, refer to Clause 4, which is structured to align with the typical steps of a termination procedure.
The wide variety of cable constructions and connectors makes it nearly impossible to establish a universal termination instruction for all combinations Due to the challenges in creating a generic termination guideline, this handbook focuses on outlining best practices for current and near-future fiber optic applications in aircraft.
Current studies focus on available 'avionic' silica fiber cables and adhesive-filled butt-coupled connectors, although the principles discussed are relevant to other termination techniques Additional termination methods are explored in the repair section of this handbook.
Need for high integrity terminations
To successfully implement a fibre optic system in aircraft, it is crucial to ensure that all system components function reliably throughout their lifespan A key factor in this reliability is the quality of interconnection components, particularly the cable to connector termination process Ensuring dependable light transmission through each optical connector is essential, necessitating a robust process that maintains high optical performance over the terminations' lifetime.
The in-service performance of optical connectors is influenced by various factors, including connector design, optical fiber selection, cable type, and the operating and maintenance environment However, a key determinant of connector performance is the quality of the termination between the cable and the connector.
The referenced documents are essential for the application of this document For dated references, only the specified edition is applicable, while for undated references, the most recent edition, including any amendments, is relevant.
EN 4533-002, Aerospace series – Fibre optic systems – Handbook – Part 002: Test and measurement
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Elements
A fibre optic termination may seem simple, but it involves a complex interplay of various components, including fibre coatings, connector design, cable strength member anchorage methods, adhesive types, and material properties Each of these factors significantly influences the termination's reliability, integrity, and overall process complexity.
Fibre optic cables
General
One of the main aspects to be addressed is the implication of choosing one cable construction over another
The market offers various types of fibre optic cables, including loose tube and tight jacket constructions, which can contain either a single fibre or multiple fibres However, as of this handbook's publication, aerospace users face a limited selection Most available cable types are primarily designed for telecommunication applications, as they lack the necessary environmental capabilities, leaving avionic solutions predominantly restricted to single fibre, tight jacket constructions.
Cable construction
The design of fibre optic cables for aircraft is generally consistent across manufacturers, yet significant differences exist, particularly in fibre coatings and cable strength member materials Each type presents unique advantages and disadvantages regarding termination procedures Avionic fibre optic cables are typically constructed in specific ways, as illustrated in Figures 1 and 2.
Figure 1 — Typical avionic fibre optic cable construction
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Figure 2 — Examples of typical avionic fibre optic cables
Fibre choice
When it comes to termination, small and large core optical fibers exhibit minimal differences The primary challenges in the termination process are linked to the materials used for cladding and primary coatings Additionally, the latest avionic fiber sizes are generally larger than the standard high-volume fibers found in the datacomm and telecomm markets, leading to increased costs and availability issues.
Cladding materials
Most avionic fibres utilize an "all silica" design, where both the core and cladding are composed of glass, functioning as a single glass filament Some variations incorporate non-glass materials for cladding, such as plastic (acrylate) or epoxy, known as Plastic Clad-Silica (PCS) and Hard Clad-Silica (HCS), respectively While these fibres have found use in various aircraft applications, their thermal endurance is limited, restricting them to less demanding environmental conditions The termination processes outlined in this handbook specifically address all-silica fibres.
Primary coating materials
Function
The primary role of the fibre buffer coating is to safeguard the fibre against abrasive and environmental damage Various materials have been utilized for the primary coating of optical fibres, with acrylate, polyimide, and silicone being the most recognized and commonly used Below is a brief overview of the advantages and disadvantages of each material.
Most fibers primarily utilize acrylate materials for their coatings, although other options like silicone, proprietary polymers, and specialized metals such as gold or aluminum may also be found.
Acrylate
The primary coating material for optical fibres is commonly a UV cured acrylate, which is translucent and matches the fibre's thickness This coating is relatively easy to remove with hand tools However, acrylates have a limited temperature tolerance, performing well only up to about 100 °C, necessitating the use of additional coatings for high-temperature applications.
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Polyimide
Polyimide coatings offer a higher temperature resistance than UV cured acrylates, functioning effectively at temperatures up to approximately 350 °C While they are advantageous for high-temperature applications, these coatings are challenging to remove and unsuitable for tool stripping They are commonly utilized in aircraft programs across the United States Fibers made with polyimide can be installed into connector ferrules without removing the primary coating, thanks to the precise control of core/cladding/primary coating concentricity and outer diameter tolerances This design minimizes handling of the fiber surface; however, the increased diameter of the polyimide is incompatible with standard connector bore dimensions, necessitating the use of non-standard ferrule bore sizes, which can incur additional costs and availability issues.
Silicone
The main benefits of silicone as a primary coating are the reduction of fibre micro-bend effects due to the
The soft primary coating layer provides a cushioning effect, demonstrating high temperature resistance up to 200 °C, excellent resilience to water absorption, and low flammability However, similar to acrylate, this material must be stripped before inserting optical fibers into fiber optic connectors, a process that can be quite challenging.
Aramid yarn versus fibreglass strength member
Most fiber optic cables incorporate a strength member layer to protect the internal fibers from external loads Kevlar®, a durable aramid yarn, is the most widely used material for this purpose, but fiberglass is also a significant alternative.
Fibreglass is more compatible with the thermal expansion characteristics of optical fibres compared to Kevlar ®, making it ideal for applications requiring dimensional stability at high temperatures exceeding 135 °C This is particularly important for cables exposed to rapid thermal cycling across a broad temperature range Nevertheless, Kevlar ® satisfies most current avionic temperature standards, which range from -55 °C to 135 °C.
To ensure effective optical fibre load isolation during termination, Kevlar ® and similar aramid yarns can be crimped directly onto connectors or termini, while fibreglass, due to its brittleness, requires bonding with adhesive or crimping through the cable's outer jacket.
Fibre optic connectors
Purpose
Fibre optic connectors are designed to align two optical fibres and maintain their position within strict physical constraints, ensuring a high-quality optical interface This alignment can be accomplished through various methods.
Connector types
A wide range of fibre optic connectors exists, from simple single-way "crimp and cleave" types to intricate multi-way "pot and polish" devices Understanding the distinctions between these connector types and their features is essential When selecting a fibre optic connector, it is important to specify the optical interfacing method, the fibre attachment method, and the number of fibres to be accommodated.
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Fibre optic system designers can choose between two types of optical connector interfaces: "butt-coupled" and "expanded beam." In a typical butt-coupled arrangement, the fibres physically connect by butting together at the junction.
Figure 3 — Butt-coupled fibre optic connector interface
The performance of this optical interface is primarily influenced by the quality of the fibre end-face, making cleanliness and proper polishing or cleaving essential Despite being the simpler option in terms of optical path elements, these stringent requirements are crucial for optimal functionality.
An alternative to butt-coupled interfaces is the use of lenses between fiber ends, known as 'Expanded Beam' connectors These lenses convert the small diameter, diverging output of the fiber into a larger, collimated beam of light, which improves tolerance to particulate contamination However, incorporating lenses increases connector insertion loss and adds cost and complexity to the termination process.
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Figure 4 — Expanded beam fibre optic connector interfaces
There are two primary methods for attaching fibre to connectors The first method, known as "pot and polish," involves bonding a fibre into a ferrule with adhesive, followed by a polishing process This technique typically requires adhesive, a heat source for curing, and various grades of polishing film and tools to ensure a high-quality finish on the fibre end.
The pot and polish process in aircraft environments differs significantly from conventional electrical wire termination methods, prompting extensive research into a mechanical, dry termination process that eliminates the need for heat when attaching fibers to connectors While some adhesive terminations can utilize specialized adhesives, such as anaerobic adhesives, without heat, their performance at elevated temperatures is inferior to that of heat-cured epoxies.
The "crimp and cleave" process addresses the demand for an electrical wire equivalent method by crimping a connector ferrule onto the fiber or its jacketing layers In this technique, the fiber end is prepared through cleaving instead of polishing.
While there are various connector termination processes, most are adaptations of the two primary techniques A notable exception is a fusion splicing variation, which is a fiber repair method that permanently joins two fibers In this process, a cleaved fiber is inserted into a connector and fused to a pre-installed, pre-polished fiber using an electric arc However, this technique is not commonly used due to significant safety concerns for on-aircraft applications.
Fibre attachment in the aerospace sector primarily relies on adhesive methods, whereas crimping is more common in telecommunications Currently, there are no mechanical termination processes suitable for silica fibres in aerospace applications, making adhesive terminations the focus of this document However, it is important to note that all-mechanical crimp and cleave terminations may be developed in the future.
When designing a fibre optic system, the designer must choose between using multiple single-way connectors or a multi-way connector to connect various optical fibres to equipment and each other.
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The single-way fibre optic connector is the most basic type, designed to connect a single tight jacket cable to another while providing integral strain relief and environmental sealing These connectors allow easy access to the fibre end-face for cleaning and inspection, either through disassembly or by utilizing exposed ferrules with a separate guide tube or adapter However, due to its design for individual fibre optic cables, it is not space-efficient for applications involving multiple cables.
3.5.2.6 Multi-way connectors and back-shells
Multi-way optical fibre connectors are designed to join multiple optical fibres simultaneously, utilizing common coupling hardware for various optical ferrules, which enhances space efficiency However, cleaning and inspection can be challenging unless a removable ferrule alignment assembly is included Unlike single-way connectors that isolate fibre load by terminating the cable strength member at the rear, multi-way connectors typically use a separate backshell for sealing and strain relief, which is screwed onto the connector's rear and serves as the anchor point for each cable's strength member.
Figure 5 — Typical multi-way fibre optic connector (with backshell fitted)
The advancement of fibre optic termination processes has resulted in a wide variety of tools for connecting optical fibres to optical connectors It is important to recognize that silica exhibits significantly different properties compared to copper.
Most tools for cable preparation are similar to those for electrical connectors/cables;
Appropriate choice of tools generally reduces the skill level requirements;
Optical termination differs from wire termination because final termination quality is not a simple function of tool quality, but is also dependent upon operator skill and judgement;
The optical termination process generally requires access to a power source (for epoxy curing processes);
When selecting tooling for operations involving silica fibre, it is crucial to exercise caution Even minor adjustments to the tool or its usage can significantly impact the strength of the fibre.
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General
The implementation of fibre optic systems in aerospace environments necessitates careful attention to health and safety, mirroring the standards required across various industries Key considerations arise from the processes and associated risks involved in these systems.
Chemicals
Adhesives and cleaning products contain various chemicals that necessitate careful attention to health and safety data It is essential to consider specific factors before using any chemicals during the assembly of optical or electrical harnesses.
This list is not intended to deter the use of fibre optic systems because the procedure is no different to that used for any other substance in Industry generally
Sharp edges can arise from both the fiber itself and the tooling used in fiber preparation during termination procedures Unprotected fiber ends may result from damage or breakage of the completed harness or occur at any stage of the manufacturing process.
When terminating fibers, it is crucial to safely dispose of the trimmed ends and take necessary precautions to protect the operator from potential hazards, such as skin and eye penetration or inhalation of glass fragments.
The list of factors for chemicals above can be used to classify the risks involved in handling “sharps” in a similar manner
Given the diverse chemicals in adhesives and cleaning agents, as well as the sharp tools used in manufacturing fiber optic harnesses, it is essential to consult the local or company health and safety representative for guidance on regulations and safe handling practices.
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Objective
The primary goal of the fibre optic termination procedure is to securely position an optical fibre within an alignment element, typically the ferrule of a connector, ensuring stability during manufacturing, installation, and operational phases To achieve optimal fibre core alignment, it is essential to pre-select the fibre, alignment element, and their dimensional tolerances High precision and tight tolerance bores in the ferrule facilitate accurate fibre core alignment, while maintaining a suitable fibre end-face quality is crucial for effective optical transmission.
Achieving this requires a series of distinct, sequential processes, which may differ between manufacturers' termination procedures Nonetheless, most procedures will encompass a substantial portion of the stages outlined in the following sections.
Cable preparation
General
To access the fiber in a fiber optic cable, it is essential to prepare the cable before connecting it to the ferrule This preparation includes stripping the outer jacket and buffer layer to precise dimensions and trimming the strength member to the appropriate length.
Cutting to length
The goal of this activity is to accurately cut through a designated fibre optic cable, including the fibre, strength member, and any protective layers, while preventing damage that could complicate future preparation steps Additionally, the process aims to enhance tool longevity and user convenience.
Cutting fibre optic cables to length may appear simple, but it poses challenges due to the materials involved Avionic specification fibre optic cables consist of silica optical fibres and aramid yarn strength members, which are both hard and tough This combination can quickly damage conventional hardened steel wire cutters, often resulting in blade damage after just one cut, as illustrated in Figure 6.
Figure 6 — Hardened steel tool damage after cutting through silica optical fibre
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Piano wire cutters equipped with tungsten carbide blades are the optimal choice for cutting fibre optic cable It is essential to use butting blades instead of shearing blades, as the latter can exert excessive pressure on the tool's hinge joint Tools with tungsten carbide blades are durable and can perform thousands of cuts with minimal wear.
Cutting the cable can result in a splayed or "chiselled" end, especially in fibre optic cables with polyimide primary coated fibre, as noted in EN 4533-002 If this issue is not addressed by trimming the affected fibre end, it may hinder the ability to insert the fibre into a connector's ferrule.
Figure 7 — Splayed out polyimide fibre end
Removal of outer jacket
The goal of this activity is to accurately strip a specified length of cable jacketing material without harming the underlying cable layers It is essential that the cut edge of the jacket remains perpendicular to the fiber and is smooth, free from any rough edges or "ears." Additionally, the process should prioritize long tool life and user-friendliness.
When selecting an outer jacket stripping tool, personal preference and functional performance play significant roles, given the wide variety of available options It's essential to consider several key factors when making this choice.
5.2.3.2 Blade type and wear characteristics
Butting knife type blades feature a small, sharp contact area that wears down quickly, leading to a reduction in the stripping hole's size and potential damage to sub-layers Despite this wear, these tools deliver exceptional strip results when they are new.
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Figure 8 — Butting knife type blades
Butting die-type blades feature a large contact area during closure, resulting in minimal wear on the contact surface As a result, the precision ground cutting blades are the primary area of wear, increasing in diameter over time This wear leads to a decline in stripping performance, eventually causing the tool to fail in stripping the cable's outer jacket This failure mode is considered relatively safe.
Shearing knife blades, characterized by their scissor-like design, feature sharpened V-shaped apertures that effectively cut through outer jackets These blades function similarly to household electrical wire strippers, although their performance can vary.
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Figure 10 — Shearing knife type blades
Precision wire strippers combine features of both butting die and shearing knife blades, with the contact point at the tool's tip remaining largely unaffected by wear As the tool wears, the precision stripping hole may enlarge, particularly at the junction of the shearing knife edges, where fine unsupported edges can break, leading to "ears" on the stripped cable ends due to gaps between the blades Similar wear patterns are observed in primary coating removal tools It's important to note that the aperture sizes of these tools, often specified in SWG or AWG, can vary significantly; for instance, a 22 AWG stripping hole on one tool may not match the size of a 22 AWG hole on another tool.
Figure 11 — Precision electrical wire strippers
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When selecting a fibre optic stripping tool for aircraft applications, it is essential to impose conditions similar to those for electrical cable stripping The primary focus should be on ensuring strict control over the tool's performance while minimizing the potential for misuse.
Avoid tools that allow manual adjustment of blade cutting depth to compensate for wear, unless precise control over the adjustment can be ensured, as this may result in improperly set blades and potential damage to cable sub-layers.
5.2.3.4 Cable handling tools (Gripping the cable)
Cable guiding and support can be accomplished through various methods, including manual handling, integration within stripping tools, or the use of dedicated tools Certain tools feature small v-groove sections that assist in aligning the cable with the cutters and prevent small diameter cables from shifting laterally when clamped.
Gripping avionic fibre optic cables is crucial due to their design for high-temperature environments and exposure to aggressive chemicals, typically featuring an ETFE outer jacket This material has a low coefficient of friction, making it challenging to grip Additionally, sharp ribbed gripping mechanisms, often found on electrical wire strippers, can easily deform these cables.
The optimal cable gripper for ETFE is a cable clamp featuring either high-friction rubber or a gritty surface finish, as the latter minimizes fiber damage compared to hard knurled or fine ridge types Ridge-type cable grippers can severely deform avionic fiber optic cables and should be avoided.
Accurate cable handling is enhanced by tools designed with pre-set detent points, allowing for precise strip lengths without the need for a marker pen and ruler These tools enable the cable to be securely clamped, ensuring that stripping tools are accurately positioned for each step This functionality is particularly beneficial when aiming for a specific exposed length of the strength member.
Figure 12 — Fibre optic cable handling tool
Strength member trimming/removal
Termination procedures for fibre optic cables with Kevlar ® strength members necessitate cutting the strength members to a precise length The length of the exposed strength member is crucial, as it affects the surface area available for trapping during the crimping process, which ultimately influences the pull-off strength between the cable and the termination.
The strength of the bond between the fibre and ferrule epoxy is not addressed here, as the attachment method for the strength member is intended to protect the fibre/ferrule assembly from external tensile loads.
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Achieving precise tolerance on exposed strip length can be challenging; therefore, a two-stage "cut and strip" method is recommended This involves initially stripping the outer jacket and removing the strength member as completely as possible Subsequently, the outer jacket is stripped further to reveal the desired length of the strength member.
Processing fibre optic cables in this manner guarantees that the strength member strands maintain a consistent length around the fibre's circumference Exposing the entire length of the strength member at once complicates the trimming of its ends, making it challenging to achieve both the desired length and uniformity.
For trimming strength members made of Kevlar®, it is advisable to use tungsten carbide bladed cutters due to the material's hardness and toughness While ceramic scissors can also cut Kevlar®, they are typically too large for precise work near the fibers, as they are intended for cutting longer lengths of the material.
Fibreglass is naturally brittle and does not require additional safety measures beyond those for Kevlar ® It is essential to wear appropriate protective gear when handling fibreglass or aramid yarns like Kevlar ®, as they can lead to skin irritations.
Removal of secondary coating(s)
The selection of tools is influenced by whether the secondary coating will be removed independently or simultaneously with the primary coating If both coatings are to be stripped at the same time, the precision of the tools must align with the requirements for removing the primary coating, necessitating the use of primary coating stripping tools.
If the secondary coating is to be removed independently then the choice of tooling is less critical
The Fibre Optic Harness Study indicates that stripping both secondary and primary coatings simultaneously is less harmful to the underlying glass fibre compared to stripping them individually Furthermore, this method of removing multiple buffers results in reduced debris on the fibre, which is crucial since debris can interfere with bonding to the ferrule and may require additional cleaning of the fibre.
Removal of primary coating
General
The term ‘primary coating’ usually refers to the buffer coating immediately adjacent to the fibre cladding For example, common telecommunications fibre with a 125 àm glass cladding diameter often has a standard
Fibre optic cables feature a 250 µm diameter acrylate primary coating, which serves as a crucial protective layer before the glass fibre surface This primary coating is complemented by secondary buffer coatings, strength members, and an outer jacket, ensuring optimal durability and performance.
To prepare a termination or splice, it is essential to carefully remove the primary coating to allow the fibre to fit into a connector ferrule or alignment structure Once the primary coating is removed, the glass cladding surface becomes exposed and is vulnerable to damage from handling and contact with particles Although silica fibre is strong, even minor flaws can significantly weaken it under stress Therefore, best practices dictate that the primary coating should be removed with utmost care, utilizing tools designed to minimize damage to the glass surface.
Coatings on fibres vary in hardness, affecting their removal ease Some fibres feature a protective polyimide coating for harsh environments, with connectors designed to fit this layer Unlike acrylate primary buffers, which can be easily removed with hand tools, polyimide coatings necessitate aggressive and hazardous removal methods, such as hot concentrated sulphuric acid or flame techniques.
Possible techniques for removing the primary buffer from fibres follows
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Mechanical techniques for primary coating removal
To effectively remove soft, acrylate-type primary buffers, the preferred method is utilizing a handheld fiber optic stripping tool These tools resemble electrical wire strippers and feature a precise blade aperture that is slightly larger than the diameter of the optical fiber cladding.
Mechanical hand tools are advantageous for their ability to operate in confined spaces without the need for power or extensive safety measures, making them ideal for production and repair settings, such as in airframe applications These tools function by closing their jaws around the buffer and stripping the coating along the fibre axis While some hand tools are designed to be held perpendicular to the fibre axis, others strip the fibre by closing the blades and pulling the tool along the fibre More advanced tools feature alignment guides to ensure proper fibre centralization and reduce bending It is essential to read the manufacturer's operating instructions before using mechanical stripping tools, along with adhering to best practice recommendations.
Choosing the right stripping tool for a specific fibre is crucial, as not all tools are the same Key factors to consider include the stripping aperture size and the guide tube arrangement The stripping aperture should be slightly larger than the fibre cladding; for instance, a tool with a 305 µm blade aperture is suitable for stripping a 280 µm fibre, with a recommended clearance of at least 20 µm Most tools indicate the blade aperture size on their body or use color coding If the aperture size is too close to the fibre size, there is a significant risk of damaging the fibre Some tools feature guide tubes to help centralize the fibre, which should be just large enough to allow the buffer layers to pass through An oversized guide tube can lead to improper centralization, causing the fibre to bend during stripping and potentially resulting in damage and a reduced lifespan.
Whatever the tool, it is recommended that the buffer is removed in manageable segments no larger than
10 mm at a time Attempting to strip the buffer in larger sections, or even in one go, can unduly stress the fibre, with the potential for damage
To prevent bending of the fiber during stripping, grip the jacketed fiber with one hand near the end to be stripped while using the other hand to operate the tool Ensure the fiber cable remains straight, and close the tool's jaws on the coated fiber to score it before removing the coating.
Stripping should be conducted with a smooth and positive motion, avoiding repeated scraping of the tool on the exposed fiber surface It is preferable to clean small amounts of coating debris rather than relying on repeated tool actions.
To ensure optimal performance when stripping, the blade aperture of the tool must remain perpendicular to the fibre axis, as any angling can transform a round cutting aperture into an elliptical shape, reducing the stripping aperture size This angling increases the likelihood of blade contact with the fibre surface, potentially introducing defects Utilizing tools equipped with fibre alignment guides can effectively mitigate this issue.
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Figure 13 — Stripping tool held at an angle to fibre
Figure 14 — Stripping tool held correctly
After stripping the fibre, it is crucial to prevent the exposed glass from coming into contact with surfaces or contaminants, ensuring a clean, dust-free environment Avoid placing the fibre on benches or tables, and store it properly to prevent contact with the stripped end A recommended method is to clamp the jacketed section while keeping the stripped end free Prolonged exposure of the bare fibre should be avoided, and potting should be conducted promptly after removing the primary buffer.
Regular cleaning of stripping tools is essential to prevent the accumulation of coating material, which can result in improper stripping and damage to fibers A small cleaning brush is effective for this task While alcohol-moistened wipes are occasionally recommended for tool cleaning, they may cause snagging on the blade, posing a risk of damage.
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The aperture should also be inspected and calibrated at regular intervals to check for blade wear and damage
Figure 15 illustrates a new blade aperture that must be clean and match the diameter specified on the tool body, such as the 305 àm tool shown, which is nearly within specification In contrast, Figure 16 depicts a tool with offset butting blades, which can damage the optical fibre by reducing the stripping aperture below the required size This misalignment risks the blades contacting the glass fibre surface during stripping, potentially causing flaws and weakening the glass.
Tool blade wear can lead to distortion and misshaping, resulting in a blade aperture that may exceed the specified tool size, as shown in Figure 17.
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Regular inspection of the aperture for damage is essential, as sharp 'burrs' on the tool blades can diminish the stripping aperture and potentially harm the glass surface.
Incorporating an alignment feature in tooling is essential for centralizing stripped fiber, as tools with only a stripping aperture are not suitable for removing primary coatings due to their tendency to angle easily, which can result in fiber damage.
Certain cable buffer layer combinations, such as those with multiple primary coatings like Fibre, can be stripped more efficiently by removing multiple layers simultaneously, which also minimizes debris on the fibre However, the specific combination, size, and composition of the coatings may cause the fibre to bend during the removal of multiple buffers Therefore, it is essential to test the suitability of this technique on the specific type of fibre being used.
Automated buffer stripping tools can significantly reduce operator-induced process variables A compact machine specifically designed for primary buffer stripping utilizes heat between two plates to soften the coating, facilitating easier removal This automated process not only simplifies the operation, reducing the need for skilled labor, but also centralizes the fiber, minimizing the risk of bending.
This machine operates with a blade aperture similar to handheld tools, which can lead to fiber damage if the incorrect blade size is selected To prevent this, it is essential to choose a blade aperture that is larger than the underlying fiber diameter, with an appropriate clearance of approximately 20 µm.
Alternative techniques
The Fibre Optic Harness Study indicates that chemical stripping methods inflict less damage on fibres compared to other techniques Soaking the fibre in hot concentrated sulphuric acid for at least 10 seconds is considered the most effective way to remove soft acrylate-type coatings After this process, the fibre should be rinsed with de-ionised water and dried However, due to significant health and safety concerns, chemical stripping is likely too hazardous for general use and would be limited to specialized production environments.
An alternative method for chemical stripping involves immersing the fiber in solvents like dichloromethane, methylethyl ketone (MEK), or acetone, with commercial paint strippers also being an option These solvents effectively soften or swell the buffer, facilitating its removal through wiping Once the buffer is adequately softened, a tissue is used to grip the buffer, allowing for the targeted section to be pulled away easily.
The effectiveness of this technique must be evaluated for the specific type of fiber or cable used It is essential that the coating is softened sufficiently to allow for easy removal in a single section.
Chemical buffer removal methods can cause 'wicking' of the chemical into the fiber and cable, potentially softening or lifting the buffer in critical areas that require complete protection To ensure the effectiveness of these chemical techniques, it is essential to eliminate this issue.
Using a flame to burn off the primary coating of optical fibers is a potential removal method, but it poses significant risks, including the embrittlement of the fiber and the creation of soot or debris that necessitates repeated cleaning with an IPA-soaked tissue Consequently, this technique is not advisable for primary buffer removal due to the potential hazards associated with combustion products.
The coating removal technique utilizes a fusion-splicing machine to pass the fiber through a fusion arc, effectively eliminating tough primary coatings However, this method raises concerns about potential embrittlement of the fiber and alterations to its refractive index, along with a spark hazard, making it unsuitable for general use.
The hot coil technique utilizes heat to effectively eliminate any residual primary coating, addressing issues of "grow out" in aerospace applications Developed in the late 1980s and early 1990s, this method was specifically designed to tackle problems associated with connectors featuring jewel alignment mechanisms and cables coated with soft silicone It was determined that "grow out" resulted from fine silicone residues remaining on the fiber after conventional mechanical removal methods were employed.
The hot coil method involves heating a coil of element wire to around 650 °C, achieving a dull orange color The fibre, which has been mechanically stripped, is then passed through the heated coil at a consistent rate and subsequently withdrawn.
The hot coil method is an effective technique for removing silicone, particularly when the majority of the silicone has already been eliminated through other methods Surface analysis tests, specifically using the XPS technique, were conducted on silicone-coated fibers after their primary coating was removed using various approaches, as illustrated in Figure 18.
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The study reveals varying levels of silicon across different samples, indicating that the cleanest fibers exhibit a high silica return and a low silicone return Among the methods tested, the hot-coil technique ranks second in effectiveness, following sulfuric acid.
The hot coil method is effective for removing silicone but must be used carefully to avoid damaging the fibre and reducing its strength Controlled insertion and removal speeds are crucial for optimal results, as excessive heat can deform or burn the secondary coating, limiting the length of fibre that can be cleaned While hot coil stripping is a viable option for general termination processes with a silicone buffer layer, it requires skilled handling Ideally, it is best to avoid using fibres with silicone primary coatings bonded directly to the glass surface.
Figure 18 — XPS surface analysis results on stripped silica fibres
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Removal of troublesome coatings
The two main coatings that fall into this category are Polyimide and Silicone
Polyimide primary coatings are specifically engineered to adhere firmly to fibers for optimal protection and performance, making them intentionally challenging to remove Typically applied as a tight tolerance coating, stripping polyimide is not usually necessary However, in certain situations, removal becomes unavoidable, and mechanical hand tools often prove ineffective for this task.
Alternative techniques for cable processing include dipping the fiber into hot water at 125 °C, as recommended by a cable manufacturer specializing in this type of cable.
The process involves using a concentrated sulphuric acid and hydrogen peroxide solution at 135 °C, where the fibre must be rinsed in de-ionised water afterward At this high temperature, the reaction time is typically between 10 to 30 seconds, while at room temperature, the process is significantly slower This solution is highly corrosive to organic materials, necessitating strict safety precautions, including the use of protective clothing and goggles to prevent contact with skin or clothing Additionally, tests on avionic fibre optic cables have shown that immersion in this solution can severely damage the Kevlar®, secondary coating, and silicone layers.
A crude flame technique involves continuously passing the fiber through the center of a flame until the polyimide blackens, followed by wiping the burnt coating with a solvent-soaked tissue This process may require several repetitions, but it is known to embrittle the glass fiber and is generally not recommended.
Some avionic fibres feature a silicone primary buffer layer, but these coatings are not preferred for future avionic cable specifications Silicone coatings are difficult to remove, leaving residue that can weaken bond strength in connectors While mechanical hand tools can effectively remove most of the silicone, additional cleaning methods are necessary to eliminate any remaining residue.
Evidence of strength reduction when stripping primary buffer coatings
Different primary buffer stripping processes have been examined, highlighting the varying levels of damage they inflict on the glass fiber surface, as demonstrated in Figure 19.
The plot illustrates the cumulative failures of stripped fibres during tensile strength testing The strongest results, located on the far right, are from fibres stripped using a dichloromethane-swollen primary buffer Following this, fibres stripped by an automated machine, which heats the buffer and mechanically removes it, show slightly lower strength Despite the overall strength of the fibres, most data points indicate a 'lower limit' due to slippage from their bonded grips during testing, suggesting that residue may persist after the automated stripping process.
The results indicate that mechanical hand tools with butting blades and features designed to centralize the fiber and minimize bending cause less damage compared to simple hand tools equipped with a shearing blade aperture All tools utilized a stripping aperture of 300 µm, while the fiber diameter measured 280 µm.
1) Sulphuric acid carbonises the polyimide by removing the OH group, then the hydrogen peroxide oxidises the carbon layer, leaving exposed glass
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Figure 19 — Graph showing the strength of fibre after stripping with different tooling
The stripping process significantly impacts the quality of the termination, necessitating the minimization of flaws and defects in the fiber to ensure a clean surface for optimal adhesive bonding A well-controlled chemical stripping process is ideal, but if health and safety concerns arise, an automated process should yield satisfactory results Optimizing stripping parameters is crucial for effective buffer removal, with current evidence suggesting that high temperatures and short dwell times are most effective Post-stripping fiber cleaning may be necessary, although mechanical hand tools, while convenient, can cause more damage to the fiber Selecting tools with butting blades and fiber alignment features can mitigate this damage Additionally, removing multiple buffers may be less harmful to the fiber compared to removing only the primary buffer, although this approach may accelerate tool wear depending on the buffer materials' size and hardness.
Defects in terminated fibre sections can become potential failure points in connectors, as stresses tend to concentrate at the tips of these defects, which may also expand due to moisture interaction Therefore, an effective stripping process is crucial; it should minimize both the size and number of defects, ensuring a clean fibre surface for optimal bonding.
To clean or not to clean
Mechanical contact with exposed fibre surfaces can weaken them by introducing microscopic flaws While cleaning typically involves mechanical processes that may cause damage, experimental results indicate that the strength reduction from careful cleaning is significantly less than that caused by using hand tools for coating removal Cleaning should be avoided if the fibre appears clean, especially if the primary buffer stripping process is known to leave no residue However, some buffers may not strip cleanly, leaving visible debris Additionally, certain stripping techniques that use chemicals or heat can leave microscopic residues In such cases, cleaning the fibre can enhance bonding during termination.
C um ul at ive Fai lur e P robabi lit y
(Chemically Stripped Fibre) lower limit Chemically Stripped Fibre
The Shearing Blade Hand Tool lacks an alignment feature, while the Butting Blade Hand Tool includes alignment features, providing a lower limit for precision Additionally, the Automated Stripping Machine offers a lower limit option and can be enhanced with a heat buffer and butting blade strip for improved performance.
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For effective cleaning, it is advisable to use a gentle approach with a non-aggressive solvent, with isopropyl alcohol (IPA) being the preferred choice Acetone, on the other hand, is too harsh and may damage the underlying buffer layers.
To clean the fibre, avoid scrubbing and instead use a soaked lint-free tissue folded lengthwise, applying gentle pressure while wiping from the base to the tip of the fibre Discard the tissue after use and do not reuse it If the fibre appears clean, refrain from further cleaning Ensure that the solvent has completely evaporated before inserting the cleaned fibre into a connector ferrule, allowing approximately 60 seconds for full evaporation at room temperature.
Care should be taken to avoid solvent ‘wicking’ up the remaining buffer layer This could compromise the protection of the fibre.
Adhesives
General
The adhesive must effectively secure the fibre within the connector throughout its operational lifespan, preventing any relative movement between the fibre and the ferrule during this period.
Adhesive types
There are a large number of adhesive technologies available for attaching the fibre to the connector ferrule, examples being:
Anaerobics (cure in the absence of air);
Table 1 outlines the key advantages and disadvantages of various adhesive types, enabling readers to recognize the primary limitations associated with many adhesive systems available in the market.
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Table 1 — Advantages and disadvantages of adhesive types
Good peel and shear strength
Requires UV light source accessible to adhesive
Absolute max 150 °C Often leaves a ‘tacky’ surface layer
Anaerobics Rapid cure at room temperature
Won’t cure in presence of air
Cyanoacrylates Rapid cure at room temperature
Epoxies Good gap filling capabilities
Quick cure at high temperatures
Can thin during cure cycle
Quantities of each part need to be accurately dispensed
Urethanes Excellent toughness and flexibility
Anaerobic adhesives require the exclusion of air to cure, which would complicate curing in a production environment and make curing in service on the aircraft extremely difficult
Current adhesive systems like Cyanoacrylates, Urethane, and Acrylate exhibit low resistance to temperature and solvents, limiting their application in aircraft installations to controlled environments The widespread use of thermally cured epoxies for terminating optical fibers highlights their superior strength and performance It is essential to select an epoxy adhesive with suitable thermo-mechanical properties for optimal results.
The key adhesive properties to be investigated when testing or selecting an adhesive are:
Themomechanical stability – viscoelastic properties (Tg);
Relaxation/creep effects also related to Tg;
Most of the work performed on adhesives within the Fibre Optic Harness Study was appropriate glu
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The importance of glass transition temperature (Tg)
The adhesive parameter Tg, or glass transition temperature, is crucial in understanding the properties of epoxy adhesives, which are viscoelastic and exhibit characteristics between viscous liquids and elastic solids At lower temperatures, these adhesives are glassy with a stiffness modulus of approximately \$10^9 \, \text{Nm}^{-2}\$, while at higher temperatures, they become more rubber-like with a modulus around \$10^7 \, \text{Nm}^{-2}\$ The glass transition region between these states is significant, and many technical papers emphasize the importance of high Tg values as a key control parameter for producing reliable terminations.
Epoxy cure schedules
The curing process of a two-part epoxy is influenced by several factors, including the mixing of its components, which initiates a chemical reaction While this adhesive can cure at ambient temperatures, achieving a complete cure is unlikely without optimal conditions, even though it will eventually harden.
Applying heat to adhesive significantly accelerates and enhances the curing process The effectiveness of the adhesive is influenced by both the temperature and the duration of heat application.
The amount of adhesive dispensed significantly influences the optimum cure schedule, as larger quantities of mixed adhesive cure more rapidly due to the exothermic reaction that generates heat This reaction accelerates the curing process, with more adhesive leading to greater heat production However, in fibre optic terminations, the small amount of adhesive used is unlikely to substantially affect the cure schedule, though it remains an important factor to consider when determining and mixing adhesive quantities.
The final performance of an epoxy adhesive is likely to be affected by a number of different parameters, such as:
Having the correct ratio of each part of adhesive
Having a suitable quantity that exhibits a well-controlled and known exothermic reaction
Ensuring the two parts of epoxy are completely mixed together
Tests conducted on suitable glu, mixed for identical durations and subjected to different curing temperatures and schedules, revealed that the Tg value ranged from 103 °C to 134 °C, as shown in Table 2 below.
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Table 2 — Effect of cure schedule on Tg
Cure schedule 1 Cure schedule 2 Measured Tg
100 60 120 180 118 a Chosen cure schedule for the Fibre Optic Harness Study work
The recommended curing schedule should take into account not only the adhesive's performance but also the interactions between the fiber and the adhesive, as well as the adhesive and the termination.
When selecting an optimal cure schedule, it is essential to consider factors such as minimizing stress on fibers during curing due to thermal shock Achieving a balance between the final glass transition temperature (Tg) and the practicality of the curing process is crucial For instance, while a 24-hour cure at 100 °C may yield an excellent Tg, it is impractical for aircraft repairs In contrast, a shorter cure time of 1 hour may only slightly reduce the Tg value, making it a more feasible option for performing terminations.
Based on this work and when using appropriate glu The Harness Study recommended a cure schedule for 'standard' termini as shown in Figure 20
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Shorter cure schedules can be achieved by increasing the temperature applied to terminations Hot air guns, commonly used in aerospace for applying heat shrinkable identifications to cables, can reach nozzle exit temperatures of 200 °C and are often suggested for quick curing However, this rapid curing method is not advisable for high integrity aircraft terminations due to potential risks.
The adhesive can boil, introducing bubbles into the adhesive which will reduce its strength and cause areas of stress changes, thus increasing the likelihood of failure
Accelerated curing of the adhesive transforms it from a viscous to a glass-like state more quickly, which heightens the risk of fiber damage during the curing process and increases the likelihood of the adhesive detaching from one of the surfaces being bonded.
Using a hot air gun can lead to inconsistent temperature control, as the temperature fluctuates significantly based on the proximity of the termination to the heat source This variability can impact the final glass transition temperature (Tg) and the overall completeness of the cure.
The curing temperature must remain below the cable's non-operating maximum temperature, typically between 125 °C and 150 °C for avionic cables Exceeding this limit can lead to permanent damage, potentially compromising the performance and reliability of the cable termination.
Usability
When selecting an adhesive, it is crucial to ensure it not only meets performance requirements but is also user-friendly Factors such as special handling needs, shelf life, pot life, viscosity, mixing procedures, and qualification must be taken into account to identify the most suitable adhesive for a specific application The following sections will briefly address these considerations.
The shelf life of an adhesive must be adequate to meet termination requirements, which can vary significantly based on the environment In a production facility with high termination volumes, a shorter shelf life may suffice, whereas an airline operator, needing adhesive for occasional fiber repairs, requires a longer shelf life.
The pot life of an adhesive refers to the duration it remains usable after mixing, influenced by the quantities mixed and the storage temperature due to the exothermic reaction The bi-pack TM sachet style is advantageous as it features pre-measured quantities separated by a clip mechanism Typically, an adhesive packaged in a sachet and stored at ambient temperatures (around 20 °C – 25 °C) has a usable life of approximately 4 hours post-mixing.
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To minimize stress on the fiber during insertion into the ferrule, it is crucial to ensure that the adhesive is free from air bubbles To achieve this without lengthy waiting times, using out-gassed adhesive is recommended Additionally, if using sachet-style packaging, it is advisable to supply the sachets in a vacuum-packed format.
Qualification
Currently, there are no aerospace-qualified adhesives or established qualification standards for their release Given the intricate nature of adhesive composition and curing processes, this field would greatly benefit from focused efforts and collaboration.